Are the applied surface loads cumulative?

No, in RAM Structural System only the top applied surface load counts. The underlying loads are not deleted however, so if you delete the top load you can see the original load underneath. If too many load layers are applied to a model, a polygon error can occur when processing the loads. For this reason it is always best to remove any existing surface loads before modeling new layers.

Also note, this is different than the behavior in RAM Concept. In that program, overlapping surface loads are cumulative. Consequently, when Direct Gravity loads are imported from RAM SS into Concept, they are converted into equivalent separate polygons that do not overlap.

Is the structure self-weight included in the loads?

That depends on the settings under RAM Manager - Criteria - Self-Weight. Here the user can automatically include beam, column, wall or deck self-weight. Note, open web steel joists self-weight is never included.

For composite beam design, the self weight is always considered part of the Construction Dead Load. Hence, if all the self weight options are turned on, and there is no other load present during construction, the user applied CDL might be zero.

On the right hand side of the dialog box are the settings for self-weight as it applies to the building mass which is used for seismic loads, dynamic analysis and for P-Delta calculations.

In order for the steel gravity beam and column self weight mass to be considered in RAM Frame, it is imperative that those modules be run first, using the design-all process. So long as the RAM Manager indicates a green light next to each of those modules, RAM Frame should have the latest member self weight data available.

Note, in RAM Frame, under Loads - Masses, the program calculated diaphragm mass totals can be overridden with User Specified values, normally using calculated masses is advised. There is a similar dialog box for the total Gravity Loads which is used to determine program generated notional loads.

How are partition loads handled.

The Partition load is an additional Live Load; it is treated as an unreducible Live Load and will not be reduced. It is in addition to the loads specified as Live Load. Partition loads are defined variously by the Codes, some as Dead Load and some as Live Load. For those codes that define Partition loads as an unreducible Live Load, those should be specified here. For those Codes that define Partition loads as a Dead Load or as part of the regular Live Load, those should be included as part of the Dead Load or Live Load accordingly.

Unlike construction live load, the partition live load is not a portion of the total live load entered. You can apply 0 Live Load and still apply 15 psf Partition Live load, for example.

Partition loads are not automatically included in the seismic mass. The total Mass DL should be increased to account for partition weight as required by the code for seismic loads.

How is the self weight of Concrete Beam determined?

The program calculates rectangular beam unit self-weight based on the area of the beam times the "Unit Weight of Self weight". The other "Unit Weight" parameter is only used in calculating the elastic modulus, E, of the member.
The Concrete slab can independently be included in the self weight, so in cases where there is a concrete slab and rectangular concrete beams the weight of the concrete times the thickness of the slab and the width of the beam is double counted.

 
To alleviate this problem, "T" shaped beam sections are handled differently. With T beams, it's only the area of the stem below the slab that is applied as the beam self weight.

.

Why are my Roof Live loads ignored in the design?

RAM Structural System currently considers Snow OR Roof LL, but not both at the same time. In RAM Manager under Criteria - Members loads there is a toggle to select which the program should consider. Set the toggle to “Consider snow loads, Ignore roof live loads” when snow loads are modeled.

Note: Live Reducible, Unreducible and Storage type loads are always considered, it is only the Live - Roof type loads that are excluded when the option to consider snow loads is turned on.

Are my snow loads automatically added to the building weight for seismic load determination?

No, the program only uses the assigned Mass Dead Loads plus whatever self-mass options are turned on under RAM Manager - criteria - self weight when determining the total building mass or weight used in Seismic load determination (and in P-Delta calculations). The user should increase the Mass DL of applied surface loads to account for the weight of the snow load (or a percentage of the weight as required.

Note, the provided templates for load combinations do correctly consider snow load acting simultaneously with Dead, Live and Seismic loads, however.

This also applied to Storage Live Loads, even if the magnitude entered for the Storage Live load is large (e.g. > 125 psf) no portion of the storage live load is automatically considered in the seismic mass. The user must increase the Mass DL (or manually alter the masses in Ram Frame) when part of a storage live load needs to be added to the seimsic mass.

How can I apply a drift snow load?

Within the snow loads, only the top load counts. Since only the top snow load counts, the drift snow load should typically taper down from the max value to the flat-roof snow load as a minimum. The program gives a warning when any portion of the sloping plane of snow load is 0 or less magnitude.

In general, it’s best to define snow drift loads with M1 and M2 set to the highest value, and M3 set to the flat roof level. Then the loads can be applied in rectangular or trapezoidal areas as required. In the image below, the total snow load on the left is 50 psf tapering down to 30 psf on the right. This would be used in conjunction with a flat roof snow load of 30 psf applied first to the whole roof.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Defaults Utility

The Ram Manager Tools - Ram Defaults utility opens a dialog box where the user can preset nearly all of the criteria settings used throughout the RAM Structural System modules. For a more complete description of the options refer to the indexed wiki topics below.

Changes to the default criteria will only affect new models created using File - New. Existing models (or models that are created using File - Save-as) already have a full set of criteria associated with the saved file and those are not affected by changes to the defaults utility.

The exception to this rule is the settings for dxf output. Changes to the dxf layers, for example, will affect the next new dxf file created on that machine, regardless if the model being exported is new or old.

Alternatively, some users make a simple model with all of the desired criteria settings as a type of template file. Each time a new project is starting the template model is opened and then a File - Save-as is performed to start the new file using all of the template file settings. One advantage of this approach is that the template file can include additional information like load definitions, deck types or concrete section info. Another advantage of this approach is that you could have different template models for different types of structures (maybe one for hospitals in California and another for industrial buildings in Arizona where different types of codes and loads are applicable).

Default Categories

• RAM Manager Defaults
• RAM Modeler Defaults
• RAM Steel Defaults
• RAM Concrete Defaults
• RAM Frame Defaults
• RAM Foundation Defaults
• DXF Options Defaults
• Other Defaults

What is the Working directory?

A RAM Structural System model file (e.g. filename.rss) is literally a WinZip file and within any .rss file you should find many component files of the same name with a wide range of extensions including (but not limited to) .b3d, .ram and .uid. These files are extracted from the model file and put into the working directory when opened in RAM Manager (in older versions of the program all of these files were simply saved together in the model directory).

For this reason the working directory should always be on the local drive and in a place in which the “user” has administrative rights. It’s also important for the user to have the rights to add, modify and delete files in the model directory.

Note: the working directory is set the first time the program is run, to confirm or edit the working directory location go to C:\ProgramData\Bentley\Engineering\RAM Structural System and double click the Ramis.ini file (c:\windows\ramis.ini prior to version 14). This should bring up the Ramis.ini file in Notepad (or other text editor). Look for the line that begins

working=path to working directory.

You can edit this path to the working directory here. Spaces are allowed as are references to mapped drives, but for the reasons mentioned above, a local drive should be used.

Also in the [Directories} section you will find the paths for other installation directories like the Tables folder where the program looks for all the tables and the default reports directory for saved reports.

For more information on the contents of the Ramis.ini file, refer to the RAM Defaults Guide [TN] . 

When I open a model I get a warning, “This file appears to be currently opened by: User Name…”.

If the listed user really is in the file, they need to exit the program normally to release the model so that you can open it (RAM SS models do not allow for concurrent use).

If that user is not currently in the file or unavailable, you should navigate to the directory where the model is saved and look for a file of the same name with the file extension “.usr”. This is a lock file that was created when the user last accessed the model. The .usr file is normally deleted when the model is closed, but if the program terminated irregularly, the file might persist. Simply delete the .usr file and the .rss file can then be opened.

When I open a model I get a warning, “A temporary backup file has been found for this database (path to file), which indicates that the program abnormally terminated…”. 

If you select the “Most Recent Database” you are telling the program to reload the files from the working directory which should be the same as they were just before the crash. In such cases, we suggest that you immediately use the file – save-as feature to turn this into a new model file.

Clicking “Backup Database” deletes the working files and restores the files from the saved model whenever it was last saved. “Cancel” leaves everything alone. If your not sure when the last save occurred, click cancel and then check the modified date of the model file through an explorer window before returning to pick either option.

Starting with version 14.06 an additional backup file is created for models that crash and are then re-opened. A copy of the rss file with the extension .ssr is created if you reopen a model that is already expanded to the working directory indicating a previous crash or incomplete closure. Like the .backup file, the .ssr file can be renamed with a .rss extension to restore that version of the file.

When I open a model I get a warning, “Could not delete RAM model: filename. Working files in directory \path to working directory\, please use the explorer to delete these files.”

There are two situations where this message might appear. The first is a restriction to the working directory or the model directory. The user needs to have the rights to add, modify and delete files from both locations (preferably administrative rights). See above for more on the working directory.

The second problem is a corrupt or partial model file. If the model file is missing any of the critical component files it will not open correctly and the same message will appear. To investigate, rename the model file, changing the extension form .rss to .zip and double-click it to see what inside. You should see a series of files all with the same name but with a wide range of extensions including (but not limited to) .b3d, .ram and .uid. When a model file does not contain a complete set of component files it is unusable. It is a mystery what causes a file to become this way, but it seems that something must have interrupted the save process. A virus scanner may also be responsible.

At this point the backup file should be used, see below.

When attempting to load a module, I receive a message that a file with a db.sdf extension already exists.

This problem is caused by a problem with Microsoft SQL Server 2005 Compact Edition, a component installed with RAM Structural System. To fix it, select Control Panel from the Windows Start menu, and open Add or Remove Programs (for Windows XP users) or Programs and Features (for Windows Vista users). Then locate Microsoft SQL Server 2005 Compact Edition, and attempt to repair it. Windows XP users can do this by clicking the Change button and choosing the Repair option. Windows Vista users can do this by right-clicking on the listing and choosing Repair from the contextual menu.

If repairing the component fails, download an installer for SQL Server 2005 Compact Edition from Microsoft's website and install it. It can be found at:
http://www.microsoft.com/downloads/details.aspx?FamilyId=85E0C3CE-3FA1-453A-8CE9-AF6CA20946C3&displaylang=en

Something is wrong with my model file, is there an automatic backup?

Yes, in the directory where the model file is saved, there should be another file of the same name with the extension, “.backup”. Rename this file something.rss (you won’t be able to use the same name of the original model file unless you move or rename it first). The backup file should be a complete version of the model from the previous time it was saved. If your not sure when that was, simply check the modified date.

Note: when backing up your own files, it is only the .rss model file that you need to save. When restoring backup files or old files from a backup CD or tape, make sure the files are not read-only. If you attempt to open a RAM model that is read-only, you will get another warning, “Failed CopyRssFileToWorkingDir:…”, “Failed DeleteRssInWorkingDir:…”, or “File error 2,6”.

How can I reduce the size of my files?

A RAM Structural System model (file.rss) is already a compressed zip file, but the size of that file can become quite large, especially when the analysis and design results are saved with the file. In RAM Manager under the File menu is an option to ZIP the model. When zipping models using that command you will have the opportunity to purge the model of some unnecessary results (which can be regenerated later just by running the file again). The optional results are:

• Include the RAM Frame Wall Forces results
• Include other RAM Frame Analysis results
• Include RAM Concrete Analysis results
• Include the RAM Concrete Column Interaction Surfaces
• Include the RAM Concrete Shear Wall results.

See Also

RAM Defaults Guide [TN]

RAM Table Editing [FAQ]

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Why does my flat slab (or flat plate) model have one-way shear reinforcement or fail the shear check?

When engineers design flat slabs by hand, they often IGNORE the one-way shear checks. They decide that punching is all that is appropriate. (This is often decided without much consideration – it just “seems right”).

Concept does not make this decision, as nowhere does the code advise to ignore one-way shear checks in a flat slab or flat plate. In fact, ACI 318-02 rule 11.12.1.1 specifically requires a one-way shear check in flat plates. Nonetheless - you should decide what the possible failure mechanism is and so what is appropriate. It may, or may not, be appropriate to ignore the one-way shear results.

In RAm Concept, one-way shear checks are independent of the punching shear check. Transverse shear reinforcement is detailed by the program, if required for one-way shear capacity. Stud rail reinforcement is detailed, if required for punching shear capacity. Transverse steel reinforcement is not considered in the punching shear check. Stud rails are not considered in the one-way shear check.

One of the biggest factors in thick slabs (especially in mats) is the fact that the program designs for shear at each cross section, while the design codes generally allow the designer to ignore shear some distance from the face of the support (typically "d"). In cases where shear reinforcement is only required in the last section at the face of the support it can be ignored.

The results have a lot more shear reinforcement than expected.

This is likely to be a shear core issue. Refer to “About shear core” on page 99 and “Shear core in slabs” on page 100 of Chapter 22, “Defining Design Strips”.

For a post-tensioned beam, the reason could be that Concept is deducting a fraction of the (bonded) duct from the web width per the appropriate code rules.

Concept calculates the number of duct by dividing the Strands per tendon by the Max strands per duct (as specified in the Materials) and rounding up to the next integer.

Refer to the following sections for an explanation of Concept’s shear web calculation:

• For AS 3600, “Section 8.2 Shear Design” on page 415
• For BS 8110, “Section 3.4.5 Design shear resistance of beams” on page 432.
• For IS 456, “Section 22.4 Design shear resistance of beams” on page 460.

Note: There is no ACI318 rule concerning deduction of ducts.

What does this audit text mean: “Depth “d” is zero - replacing with “column” effective depth. Depth is still zero - giving up.”?

The is likely a combination of two things:

• there is net compression force and a small moment, and as such the bending designer does not provide any reinforcement
• the minimum designer has been turned off If this is the case, you should consider turning the minimum designer back on.

A similar condition tends to occur near the inflection point of a span. In these locations the moment can be very small and a flexural capacity check can pass with very little top reinforcement, but if there is no tension reinforcement at the tension face, or of there is a lot more reinforcement on the compression face, this skews the calculated depth to the tension reinforcement (d) and drastically reduces the shear capacity.

Can I model user shear reinforcement?

Transverse reinforcement cannot be modeled as user reinforcment. It is always designed by the program. We are looking to add user defined transverse reinforcement in the future.

Does the reinforcing weight in the Estimate Report include transverse reinforcement?

Yes, transverse reinforcement is included in the reinforcment weight in the Estimate Report.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Error or Warning Message

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Explanation

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How to Avoid

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1. Is there a way to obtain the average stress at a node where several plate elements meet?
2. In the plate element stress results, what do the terms TRESCAT and TRESTAB stand for? How are they calculated?
3. I am modelling a concrete slab using plate elements. I am looking for the moments in the slab at the center of each element. I noticed that the output gives the bending moments per unit width. What is the per unit width? Would that be the thickness of the plate element?
4. I have modeled a 40" x 40" column base plate with (4) 12" dia. pipe columns on it (equally spaced in both directions). How do I tell STAAD that the base plate will be on a concrete pedestal (f'c = 4.0 ksi)?
5. How do you assign properties to solids?
6. I would like to change the direction of the local Z axis of an element so that it points in the opposite direction. How do I do it?
7. Is it possible to apply a concentrated force on the surface of an element? The point where the load acts is not one of the nodes of the element, as a result of which I cannot use the JOINT LOAD option.
8. How can I find the maximum shear stress on my plate element model?
9. The plate element results contain a term called TMAX. Is TMAX the best representation of the total stresses resulting from the torsion on the element?
10. In the post processing mode - Results menu - Plate Stress Contour, there are two options called Max Top and Max Bottom. Are these direct stresses or flexural stresses?
11. Can STAAD.Pro be used in designing a mat foundation?
12. When modelling plate elements, should the individual elements satisfy any minimum requirements for the ratio of the length of their side to their thickness?
13. I have a model where supports are defined at the nodes of some of the plate elements in the structure. If I divide the support reaction values by the thickness, length etc., of the side of the elements adjacent to the support, shouldn't the values match the ELEMENT NODAL STRESSES?
14. Why shear stresses are not included in the Von Mises stresses for solid elements?
15. How to change the local axis orientation X and Y for plates ?
16. What is the difference between Plate element , Shell element and Surface?
17. How to model an orthotropic plate so that it would have a smaller stiffness in one direction than in another?
18. Is it possible to obtain the element stresses/ moments at a certain location within a plate ?
19. I have modeled a T beam using quad plates. I am trying to find out the bending stress in the flange. How can I get that ?

1. Is there a way to obtain the average stress at a node where several plate elements meet?

Presently, STAAD only reports the nodal stresses of the individual elements meeting at the node. In the Plate page of the post-processing mode, the lower table on the right hand side of the screen contains the element nodal stresses. However, the average value from all such plates connected to the node is not reported.

Averaging is straightforward at joints that are only connected to one plane of plate elements and the loading is a normal pressure. However transverse shear will jump at a line if a line load is applied; so a single average would be inappropriate for that stress only. Similarly for stresses across a line of beams or walls or a line of bending moments; etc.

If two walls and a floor meet at a joint there are 3 planes that should be treated separately. Also averaging should be separate for the same surface on either side of a wall to account for the stress discontinuity. At the common joint there would be 12 sets of stresses (4 plates on each of 3 surfaces).

So averaging can be interrupted due to certain loadings, plates in other planes, and other members.

Further complexity occurs for contours and corner stresses if a shallow curved surface is being averaged. Most likely the inplane stresses should be averaged separately from the bending stresses, without coordinate transformations, since the flat plate faceted surfaces are trying to simulate a smooth surface.

The above considerations are not easily automated. We hope to implement at least some simple cases in future.

2. In the plate element stress results, what do the terms TRESCAT and TRESTAB stand for? How are they calculated?

TRESCA is 2.0 times TMAX. TMAX is the maximum inplane shear stress on a plate element. TMAX = 0.5 * max[abs((s1 - s2)) , abs((s2 - s3)) , abs((s3 - s1))] where s1 and s2 are the inplane principal stresses and the 3rd principal stress, s3, is zero at the surface. TRESCAT is the value for the top surface of the element. TRESCAB is on the bottom. Top and bottom are in accordance with the direction of the local Z axis.

Example problem 18 in the examples manual shows the calculation of TMAX

3. I am modelling a concrete slab using plate elements. I am looking for the moments in the slab at the center of each element. I noticed that the output gives the bending moments per unit width. What is the per unit width? Would that be the thickness of the plate element?

For Mx, the unit width is a unit distance at the center of the element, parallel to the local Y axis.

For My, the unit width is a unit distance at the center of the element, parallel to the local X axis. 

Diagrams explaining the various plates forces, moments and streses are available at section 1.6.1 of the Technical Reference manual.

4. I have modeled a 40" x 40" column base plate with (4) 12" dia. pipe columns on it (equally spaced in both directions). How do I tell STAAD that the base plate will be on a concrete pedestal (f'c = 4.0 ksi)? My first guess is to assign supports at the mesh intersections:

SUPPORTS 1 TO 529 ELASTIC MAT DIRECT Y SUBGRADE 4

Any suggestions?

Your guess is a good one. You can model the support as an elastic mat foundation. To do that, you first need to know the subgrade modulus of concrete. One of the methods by which the modulus can be computed is using the following equation:

Ks = Es / B ( 1 - PoissonRation * PoissonRatio )

( Reference: Foundation Analysis and Design ( Fifth Edition ) by Joseph E. Bowels Page 503 , Equation 9-6a )

In addition, if you want to make sure the concrete pedestal takes only compressive force, then specify the SPRING COMPRESSION command for those joints in the direction KFY.

An example of this is

SUPPORTS
1 TO 529 ELASTIC MAT YONLY SUBGRADE 987

SPRING COMPRESSION
1 TO 529 KFY

If you have any anchor bolts attached to the baseplate, they can be modeled as spring supports (tension only).

An example of this is

SUPPORTS
1000 TO 1004 FIXED BUT MX MY MZ KFY 5467

SPRING TENSION
1000 TO 1004 KFY

5. How do you assign properties to solids?

You do not have to assign any properties for solid elements. For solids, the only information required is their geometry (node numbers and their coordinates), and material constants (E, Poisson, etc.). You may refer to example problem 24 in the examples manual if you want details.

6. I would like to change the direction of the local Z axis of an element so that it points in the opposite direction. How do I do it?

From the Select menu at the top, select the Plates Cursor. Then select the element for which you want the Z axis direction changed. From the Commands menu, select Geometric Constants followed by Plate Reference Point and give the coordinates of this point. Choose the Local Axis direction to point towards or away from the Reference Point. The Assign option should be set to "To Selection". Click on OK.

7. Is it possible to apply a concentrated force on the surface of an element? The point where the load acts is not one of the nodes of the element, as a result of which I cannot use the JOINT LOAD option.

Yes, it is possible to do this. In Section 5.32.3 of the STAAD.Pro Technical Reference Manual, if you look at the syntax of the element pressure loading, you will find the following :

element-list PRESSURE direction x1 y1 x2 y2

In this syntax, (x1,y1) and (x2,y2) represent the corners of the region (on the element) over which the PRESSURE load is applied. However, if you omit the terms (x2,y2), the load will be treated as a concentrated force acting at the point (x1,y1), where x1 and y1 are measured as distances, from the centroid of the element, along the local X and Y axes, of the point of action of the load.

Thus, if you want to apply a 580 pound force along the negative global Z direction at a distance away from the centroid of (1.3,2.5)feet along the local X & Y axes of element 73, you can specify the following commands

UNIT POUND FEET
LOAD 1 CONCENTRATED LOAD ON WALL
ELEMENT LOAD
73 PR GZ -580.0 1.3 2.5

8. How can I find the maximum shear stress on my plate element model?

Since there are several types of shear stress results we can get from STAAD, the expression "maximum shear stress" needs to be clarified. So, let us first see what the choices are :

SXY - For any given element, this is the in-plane shear stress on the element and acts along the plate local X-Y axes directions.

TMAX - This is the maximum inplane shear stress on the element and is a composite of SXY and the stress resulting from torsion MXY.

SQX - This is the out-of-plane shear stress on the X face at the centroid of the element.

SQY - This is the out-of-plane shear stress on the Y face at the centroid of the element.

All of these results can be obtained in a report form, with additional options like sorting done in ascending or descending order for a user-defined set of elements and a user-defined set of load cases. As an example, do the following for getting a report of TMAX sorted in the order from maximum to minimum for all plates for load cases 4 and 5.

Go to the post-processing mode. Select all plates. From the Report menu, select Plate Results - Principal stresses. Select TMAX, and set the sorting order from High to Low. Switch on "Absolute values" also to perform sorting based on Absolute values. Click on the Loading tab, and select just cases 4 and 5. Click on OK. A report will be displayed. Click the right mouse button inside the table, and select Print.

9. The plate element results contain a term called TMAX. Is TMAX the best representation of the total stresses resulting from the torsion on the element?

Among the various stresses resulting from the torsional moment MXY, the only stress which is considered in TMAX is the shear stress. There are other stresses such as warping normal stresses which do not get represented in TMAX.

TMAX is the maximum inplane shear stress on an element for a given load case. It represents inplane shear stresses only. It contains contributions from the direct inplane shear stress SXY as well as the shear stress caused by the torsional moment MXY. Example 18 in the examples manual shows the derivation of TMAX from SXY and MXY.

While on the subject of shear stresses, one must note that the plate is also subjected to out-of-plane shear stresses SQX and SQY, which do not have any representation in TMAX.

10. In the post processing mode - Results menu - Plate Stress Contour, there are two options called Max Top and Max Bottom. Are these direct stresses or flexural stresses?

These are the principal stresses SMAX and SMIN. Principal stresses are a blend of axial stresses (also known as membrane stresses SX and SY), bending stresses (caused by MX and MY) and inplane shear stresses (SXY). Since the bending stresses have distinct signs for the top and bottom surfaces of the element, the principal stresses too are distinct for top and bottom. The derivation for principal stresses is shown in example 18 of the STAAD Examples manual.

11. Can STAAD.Pro be used in designing a mat foundation?

The answer to the question is Yes. The following are the major steps involved in the modelling and design of mat foundations using STAAD.

1) The mat foundation has to be modelled using finite elements. If the length and width of the mat are atleast 10 times larger than its thickness, plate elements can be used. If not, one may use 8 noded solid elements. The remainder of the structure involving the beams, columns and slabs also has to be modelled along with the mat. If beams share a common boundary with the mat and slabs, to ensure the proper transfer of load between the beams and the mat & slabs, the mat & slabs have to be divided into several elements, the beams have to be divided into several members, and the elements and members must share common nodes.

2) Generally, the supports for the mat are derived from the subgrade reaction of the soil. Using this attribute, and the influence area of each node of the mat, the spring constant for the supports may be derived. STAAD contains an automatic spring support generation facility for mat foundations. One may refer to Section 5.27.3 of the STAAD.Pro Technical Reference Manual for details on
this type of support generation.

3) Soil spring supports generally tend to be effective against resisting compressive forces only. They are ineffective in resisting uplift. This type of a unidirectional support requires those springs to be assigned an attribute call SPRING COMPRESSION.

4) The loads on the mat and the rest of the model have to be specified. Then, the structure has to be analyzed. This will generate the plate stresses and corner forces needed to design the mat.

5) You can then use the program's concrete design ability to design the individual elements which make up the mat. The only tedious aspect of this is that the program can presently design individual elements only. The task of taking the reinforcement values from each element and assembling the reinforcement picture of the overall mat has to be done by you manually.

We suggest you take a look at example problem number 27 in the STAAD.Pro examples manual for guidance on analysing mat foundations. In that example, the aspects explained in steps 1,2, 3 and 4 above are illustrated. Example problems 9 and 10 discuss concrete design of individual plate elements.

Note : A better option is to use STAAD.foundation software for design of mat. Mat modeled and analyzed in STAAD.Pro can be imported into STAAD.foundation will all results data and it can be subseqently designed in STAAD.foundation. One can also model and design mats directly in STAAD.Foundation.

12. When modelling plate elements, should the individual elements satisfy any minimum requirements for the ratio of the length of their side to their thickness?

No, they do not have to. However, for the overall slab or wall, if the span in either direction is less than 10 times its thickness, then the slab or wall becomes more like a solid than like a plate; and thick plate theory may not be adequate. In that case, 8-noded solid elements may be necessary.

13. I have a model where supports are defined at the nodes of some of the plate elements in the structure. If I divide the support reaction values by the thickness, length etc., of the side of the elements adjacent to the support, shouldn't the values match the ELEMENT NODAL STRESSES? I am aware of the fact that element stresses are in the local axis system of the element, and support reactions are in the global axis system, and am making the required transformations before making the comparison.

 The element nodal stresses are obtained as the value of the stress polynomial at the coordinates of those joints. Stresses in an element are most accurately determined only at the center of the element (in the middle of the joint displacement locations used in calculating that stress). The stress values calculated at the nodes will only be approximate (only the displacements of the joints from this one element are used in calculating the stress). Stresses at a joint would be improved if the stresses from the other elements at the joint (on the same surface) were averaged. Consequently, the comparison you suggest is not feasible.

A better alternative would be to compare the forces at the node rather than the stresses at the node.

The output for the command

PRINT ELEMENT FORCES

consists of the 3 forces and 3 moments at each of the nodes of the elements, reported in the global axis system. Thus, the output will consist of FX,FY,FZ,MX,MY,MZ with the 3 forces having units of force (not stress) and the 3 moments have units of moment (not moment per unit width). If you add up the values at the nodes of those elements which are connected to the support, those values must be equal to the support reaction.

Another consideration is the way in which element loads are evaluated and used. Staad computes the equivalent forces at the corner joints (same total force, center of force, and direction). The remainder of the analysis and results are as if you had applied the loads as joint loads rather than as element loads. Two exceptions, temperature loads are applied internally to the element and plate releases will affect the load distribution to the joints.

Say you have a wall with uniform pressure. Half of the load on the elements along the base will be applied directly to the base, the other half is applied to the line of joints at the top of these elements. So the internal transverse shears are too high at the top of the element. The transverse shears are OK at the center and too small at the base. The same will be true for the element force output of transverse forces. However, the reactions will have the entire force. A finer mesh in general, and near the base in particular, will improve the element stress and load distribution.

14. Why shear stresses are not included in the Von Mises stresses for solid elements?

Have a look at the attached figure which is from the following link: http://en.wikipedia.org/wiki/Von_Mises_yield_criterion

There are 2 equations for calculating Von Mises stresses.

The first equation uses the STAAD.Pro terms SXX, SYY, SZZ, SXY, SYZ and SZX.

The second equation uses the terms S1, S2 and S3. These are the principal stresses. There are no shear terms in this equation. This is the one implemented in STAAD.Pro.

The Von Mises stress should be the same from both equations.

15. How to change the local axis orientation X and Y for plates ?

Although it is very easy to flip the direction of the local Z axis for the plates by using the Commands > Geometric Constants > Plate Reference Point option, unfortunately there is no easy way to change the local X and Y for plates as needed. For example, if one needs to re-orient the local x for the plate to be parallel to the global X direction, one has to redefine the incidence of the plates by going to the STAAD editor and changing the ELEMENT INCIDENCE data manually. The local X would be a vector from the center of the plate, drawn in a direction from node1 to node2 and is hence dependent on the element incidences data. In other words the element incidences need to be such that node1 to node2 should be the direction of the global X. The rule which governs how the plate local axes are oriented, is explained under section 1.6.1 of the Technical Reference Manual..

16 . What is the difference between Plate element , Shell element and Surface?

Plate element and shell element

 Both terms represent the same thing in the STAAD context, which is, a 3-noded (triangular) or a 4-noded (quadrilateral) element to which a thickness has to be assigned as a property.

In STAAD, this element has both attributes - membrane (in-plane effect) and bending (out-of-plane effect). The bending effect can

be shut off by declaring it as ELEMENT PLANE STRESS. The in-plane effect can't be shut off.

Plate Element and Surface

 If you want to model a structure which contains a wall, slab or panel type component, you have two choices in STAAD :

a) Model that panel using a collection of individual elements. This is called a finite element mesh. This is an assembly of the 2d triangular and/or quadrilateral elements described above.

b) Model that as a single physical object called a Surface.

 Option (a) is achieved using the mesh generation facilities in STAAD.

 In option (b), (surface object), what happens under the hood is that, during the analysis, STAAD transforms the surface into a finite element mesh. The type of mesh (number of elements,

type of elements, size of elements, etc.) that is generated from the surface is based on the parameters that you provide at the time of defining the surface.The details of the mesh thus

 generated are to a large extent, masked from the user. Results are presented for that surface, not for the individual elements that it is made up of.

 In other words, a surface is merely an object that represents a collection of elements. When the program goes through the analysis phase, it subdivides the surface into a plate elements . From analysis point of view , both plate and surface are the same thing . The difference is in the interpretation of results . For plates , the stresses are reported while for Surface the force is reported .

 17. How to model an orthotropic plate so that it would have a smaller stiffness in one direction than in another?

You can create the orthotropic 2D material, which would have a different Young's Modulus in X and Y direction (hence different stiffness in both directions). To do so, please go to General -> Materials vertical tab, then in the Material table on the right bottom corner select Orthotropic 2D tab and click Create button. In the opened window provide the required information and then assign this material to the plates.

18. Is it possible to obtain the element stresses/ moments at a certain location within a plate ?

There is a print command that lets you print the element stress at any location within a plate. The syntax for the print command is  

PRINT ELEMENT JOINT STRESSES AT f1 f2 LIST elements-list where f1 and f2 represent the distances along local X and local Y of the plate measured from the plate origin in current units.

For example if the current length units is set to “feet” and to one wants to print the moments/stresses at 0.5 ft from the center of a plate # 47, the print command would be  

PRINT ELEMENT JOINT STRESSES AT 0.5 0.5 LIST 47

You may refer to the section 5.42 of the Technical Reference for more details.

19. I have modeled a T beam using quad plates. I am trying to find out the bending stress in the flange. How can I get that ?

STAAD.Pro reports the bending moments for plates and not bending stresses. However one can find the bending stress without much effort from the data available. First one has to pick a plate close to where the stress is required. Mx (or My ) moments are reported as part of the Plate Center Stress table within the Plate page in the Postprocessing mode. The moment is reported per unit width. One may calculate the Section modulus S ( = 1/6x w x t2 , where w = width of flange plate over which moment is reported, t = thickness of flange plate ). The bending stress would then be M/S.

Alternately STAAD.Pro does report the combined stress for plates. This is available within the Combined Stress tab inside the Plate Center stress table. For the selected plate, note the Top combined stress ( Comb Sx or Comb Sy as appropriate ). The combined stress is the P/A + M/S value. From within the Shear Membrane and Bending tab of the same table, note the SX or the SY which would be the P/A component. One can then find the M/S component by taking out the P/A part from the combined stress. 

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

What's the difference between a plan and a layer?

A layer is an organizational concept. A layer is a collection of related objects and results and each object and result resides on one and only one layer. For example, all slab elements are on the Element layer. Plans, on the other hand, are a display and editing concept. Each plan is a filtered view of all of Concept’s layers. A plan can be set up to edit a particular layer, but the plan does not “own” the layer. All changes that are made to the layer using the plan will be visible in all other plans, because all plans are viewing the same set of layers. See Chapter 3, “Understanding Layers” and Chapter 4, “Using Plans and Perspectives” for more information.

I have two items at the same location, how do I select just one of them?

If you have two objects of the same type at the same location in Concept it will cause a warning during meshing. For example, if you copy and paste a column below without unselecting the original column before pasting you will end up with a double column below.

The warning in that case will be something like this:

---------------------------
Analysis Error
---------------------------
An error has been found.  Two column elements below the slab are at the same location.  Delete column element #1 or 2 (below the slab) at (1,1).
---------------------------

Turning on the display of the column numbers will help in locating the double column.

To select just one of the objects, try double clicking right at the location of the double object. The will select the last or "top" object only. It works best when no snap modes are active. You can also use the visible objects control to turn off unrelated object types (like the slab) first.

Once you have selected just one object you can move or delete it without affecting the duplicate. 

If you want to select the other, "lower", object instead, start by selecting both at once with a fence, then hold down shift and double click again to de-select the top object leaving only the lower object(s) selected.

Below is an illustration of a problem with two walls below the slab that overlap along part of the length which also causes an error with meshing:

Why do I see nothing in a perspective display?

The perspective “camera” may be looking in the wrong direction. Click Zoom Extent ( ) or Show Print Viewpoint ( ). It could also be that the plotted elements do not exist yet (such as the design strip cross sections which are created during the calc all process)

Why don’t the tendon profiles change in the perspective view?

The accurate tendon curvature and final elevations are calculated by the program in the early phases of the analysis. For anything other than a completely flat slab, the tendon perspective won’t reflect the correct tendon profile until the calculation is run again.

How can I create a layer to view some specific results which are not already included?

Go to Layers menu - New plan.

Give the new plan a label that is meaningful to what you plan to plot.

Pick the parent layer, e.g. the load combination or rule set of interest.

If there are any other Visible Objects you want shown, adjust the Visible objects settings and click OK.

Once the new blank layer is created go to View - Plot and pick the desired plot options.

When I print or export the Report, why are some Plans excluded?

For every plan, table and layers there is an "Include" flag for whether that view should be included in the full design report or not. Make sure the desired plans and parent layers to be printed have the “Include” flag set to Yes, otherwise those plans will not be part of the report.

You can always print or export those plans individually.

What can I do if the graphics don't look right on my machine?

Ram Concept uses DirectX for graphics. so first verify that your graphics adapter supports this technology. More often than not, updating the graphics adapter driver is the solution to graphics problems in Ram Concept. Here's an example of what you might see with an out of date driver:

There could be extra blue lines like a spider-web connecting the edges of the slab or problems with line weight.

See Also

Ram Concept Plotting Results

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Is pattern loading possible?

Yes, but the user must graphically define the load patterns. Refer to the program manual for further details:

• Section 10.9 - About load patterns
• Chapter 20 - Creating Pattern Loading

Are area loads additive?

Yes, the program considers the cumulative total of overlapping surface loads.

Note, it is possible to assign loads with negative magnitude which sometimes makes surface load modeling simpler.

Also note, this is opposite to the behavior of surface loads in RAM Structural System where only the top load counts. When you import surface loads from RAM SS into Concept, they are automatically adjusted to be equivalent.

Can I make a line load that's triangular or non-uniform?

Yes, when entering the magnitude of the line load, enter two values separated by a comma. For example, if you enter the following properties:

The load you create will linearly vary from 2 k/ft at the first point to 1 k/ft at the end.

You can similarly enter surface loads that slope using three magnitude values separated by commas.

Why do I get a Analysis Error that says, "Rule Set "Service Design" is being used by load combination that appear to have load factors set for different purpose.  This is likely an error."

Ram Concept has a hard coded set of load factors for each design code and for each rule set. You can find more about the default load factors in the manual in chapters 54-64 in the second sub-sections for "default load combinations."

When the load combinations are manually altered by the user and the load factors are changed, or when new combinations are added that do not use the expected load factors, then such an error will occur.

If the changes are intentional then the warning can be ignored.

To reset the load combinations to program presets use the Criteria menu option - Rebuild Load Combos...

We recommend saving the file, printing out the current load combinations, then using the Rebuild Load Combinations tool to reset values and finally compare the values when deciding what is best.

See Also

RAM Concept Lateral Self Equilibrium Analysis

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Error or Warning Message

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Explanation

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How to Avoid

Option 1 XXXX(Delete this heading if only one option exists.)XXXX

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Center of Rigidity

In order to review the center of rigidity for a structure, a special Centers of Rigidity load case must be created under Loads – Load cases in RAM Frame. Once the analysis of that load case has been performed, a “Centers of Rigidity” report becomes available under the reports menu.

The Centers of Rigidity report lists the centers of rigidity of each diaphragm at each of the levels that contain lateral members. The report also lists the center of mass and Story Lateral Stiffness of each diaphragm.

The center of rigidity can be thought of as the location through which a lateral load would cause lateral deformation of the diaphragm without causing rotation. The concept is analogous to that of a cross section’s “shear center”.

Below are the results for a 30’x10’ structure, 10 stories tall, with walls on 3 sides.

Isometric view of “C” shaped wall model, center of mass at (15,5)

For buildings with torsional irregularity, the lateral load may always be on the same side of the center of rigidity even after considering a 5% accidental eccentricity of the load. In this case, the torsion will always be “adding” to the shear on one side of the building and “subtracting” shear from the other side. To ignore these effects, an additional lateral load case with the same magnitude as the generated load, but with a location exactly at the center of rigidity can be added. This, pure shear load case, can then be included in the load combos to yield a conservative design for the frame elements that otherwise would benefit from the eccentricity. Some codes may require such a conservative approach, but most building codes recognize the reality of the eccentricity and the effect it has on the structure. In those cases, the code may simply want an increase in the accidental torsion to be considered. This can be changed under Loads – Masses, within the Analysis mode of RAM Frame.

Returning to the example above, an interesting phenomenon occurs in regard to the “Yr” location. Even though the levels are identical, the center of rigidity shifts from level to level. The higher the level, the more eccentric the center of rigidity. In other words, a load must be applied from the long wall in order to bend the channel without twisting it.

A load applied to the diaphragm in line with the long wall (y=10’), actually produces a large diaphragm rotation. For a load applied to the center of the diaphragm (y=5’) the effect is even worse

Channel Loaded Through Center of Rigidity	Channel Loaded Through Long Wall

The Center of Rigidity location is not directly used in determining the loads or in the finite element analysis. The exception to this rule is in the determination of some wind loads where the eccentricity affects the applied loads directly.

The center of rigidity is always calculated using a rigid diaphragm analysis, even where semi rigid or flexible diaphragms are called out.

Why is more than one center of rigidity shown on a level?

The program calculates and reports a center of rigidity for each rigid or semi-rigid diaphragm on every level.

See Also

RAM SS Walls FAQ

Structural Product TechNotes And FAQs

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

STAAD.Pro Model

Relevant STAAD.Pro model (.std file) can be downloaded from the following link:

STAAD.Pro model for IS 800:2007 verification example

Critical Design Forces

For Sectional Properties and Critical design forces, go through the following post:

Section Classification of Laterally unsupported ISMB section

Moment Capacity Check 

STAAD.Pro Design Output

This contains wiki on the IS:800 2007 code.

• Section Classification of Laterally unsupported ISMB section

• Slenderness Check of Laterally unsupported ISMB section

• Tension Capacity Check of Laterally unsupported ISMB section

• Compression Capacity check of Laterally unsupported ISMB section

• Shear Capacity Check of Laterally unsupported ISMB section

• Moment Capacity Check of Laterally unsupported ISMB section

STAAD.Pro Model

Relevant STAAD.Pro model (.std file) can be downloaded from the following link:

STAAD.Pro model for IS 800:2007 verification example

Critical Design Forces

For Sectional Properties and Critical design forces, go through the following post:

Section Classification of Laterally unsupported ISMB section

Interaction Check 

STAAD.Pro Design Output

STAAD.Pro Model

Relevant STAAD.Pro model (.std file) can be downloaded from the following link:

STAAD.Pro model for IS 800:2007 verification example

Critical Design Forces

For Sectional Properties and Critical design forces, go through the following post:

Section Classification of Laterally unsupported ISMB section

Deflection Check 

STAAD.Pro Design Output

DIRECT  ANALYSIS PER THE AISC 360-05 AND ITS IMPLEMENTATION  IN STAAD

Stability of a Structure

The AISC 360-05 Chapter-C specifies that the stability shall be provided for the structure as a whole and for each of its elements.

That means the stability needs to be maintained for the individual members, connections, joints and other building elements as well as the structural system as a whole.

The code recommends using any method that ensures the stability of the structure as a whole and for individual building elements, and meets with all the following requirements are permitted.

1. Flexural, shear and axial member deformations and all other deformations that contribute to displacements of the structure.

2. Second-order effects (both P-∆ and P- Ϩ effects)

3. Initial geometric imperfections

4. Stiffness reduction due to inelasticity

5. Uncertainty in stiffness and strength

AISC suggests adopting the Direct Analysis Method (DAM) which satisfies all the aforementioned conditions.

Stability Design Approaches :

From stability consideration of a structure, AISC chapter C suggests the three approaches for determining the required flexural and axial strength of a member in the structure.

(1)   Effective Length Factor method  (ELM ) (C.2.2a ) : Unless the First –Order to Second Order drift ratio is not greater than 1.1 , this method  demands the determination of actual “K” value of compression members . It is a conventional method which has been adopted by engineers for designing steel columns for a long time. Determination of the Effective Length factor “K”of a member is the cornerstone of this method. The K value accounts for the contribution of boundary conditions to the axial load carrying capacity of a steel column. Since the ELM approach is based on several assumptions on geometry, boundary condition, and material properties of columns, sometimes this approach may be very conservative and inappropriate for the design of compression members.

(2)   First Order Analysis per C2.2b : This method  suggests to perform the first-order elastic analysis using nominal geometry and nominal stiffness . Although the method is derived from the DAM ,  it is only applicable when the sidesway amplification factor B2 <1.5

Detailed explanation is covered in chapter C2.2 of AISC 360-05.

Following are the few limitations of this method

(a) Structure supports gravity loads primarily through nominally vertical    columns, walls or frames.

(b) Second-order effects must be limited.

(c)    Inelastic effect must not be significant.

 (3)   Direct Analysis Method (DAM)  :

The Appendix-7 of the AISC 360-05 introduced the DAM which is a new method addressing all the necessary stability requirements suggested by the code.   Performing the rigorous Direct Analysis is an advanced approach of stability analysis which considers both geometric and material non-linearity and is far more accurate when compared with the other approximate methods.

Three basic parameters addressed by the DAM.

(1)   Consideration of  the P-∆and  P- Ϩ effect

 To address the geometric non-linearity, this method strictly demands the consideration of P-∆and P- Ϩ effect in a member and the overall structure .

The AISC chapter C2.1 specifies using the Second Order analysis to address those effects.

 The AISC 360-05 code states that any second order method that includes the P-∆and P- Ϩ  effect may be used , but the following two methods are mostly used .

   (a)   Moment Magnification factor method per C-1b

This is a second order analysis done by magnifying the moments determined in the first order elastic analysis. This is an  approximate method which is also popularly known as B1 - B2 method as the code specifies the equations eqn- C2-2 and C2-3  to determine the amplification factors for a member’s internal deformation (B1) and for the drift (B2)  respectively and use them to calculate the second order flexural and axial strength of the member by eqn- C2-1a and C2-1b .

 (b)    Direct , Rigorous Second order analysis .       Due to the iterative process involved in determining the actual value of forces and displacements on account of the second order effect, it is mostly performed by the computer programs.

(2)   Geometric Imperfection .  Any column used in real life situation never follows the ideal column straightness. Presence of crookedness, initial deformities or out of plumbness are very much feasible. 

To account for these pragmatic considerations, AISC came up with the concept of notional load.

Notional Load  is a pseudo lateral load to imitate the initial crookedness and out of plumbness   of a member . The magnitude of Notional Load at each level is   Ni = 0.002Yi  , where Yi is the  gravity load acting  on the ith level . The 0.002 factor  is equivalent to the allowable tolerance for  initial out of plumbness of each story ( 1/500 times of story height) .

(3)   Stiffness reduction due to the material Non-Linearity .  

Stiffness of the members needs to be reduced to account for the inelastic effects due to residual stress and the uncertainty in strength and stiffness. Inelastic  effect which is caused by residual stress   include stresses due to temperature , as some elements   of the hot rolled cross-section cools faster than others, and also due to the effects of straightening that must be done to meet ASTM A6 tolerances. Areas with residual stress yields prior to the overall yielding of the section, causing some elements to soften in-elastically prior to reaching their design strength. The loss of stiffness due to residual stresses also increases the frame and member deformations.  And this effect is addressed in the DAM by the reduction of Axial Stiffness (EA ) and Flexural Stiffness (EI) .

The reduced Axial Stiffness is EA* = 0.8 EA

The reduced Flexural Stiffness is EI* = 0.8 EI τ_b

The calculation of τ_b which is dependent on the level of axial stress is elaborated in chapter 7.3.3 of the AISC 360-05 .

However, τ_b can be assumed 1.0 if the additional notional load of 0.001 times of gravity load is applied.

The  other advantages of DAM :

(a)   Plain, direct and simple approach.

(b)    Eliminates the ambiguity and the intricacy involved in calculation of effective buckling length factor as required by ELM. An engineer needs to assume K=1 in the DAM.

(c )   Can be used for all types of steel structures like Braced frame , moment frame and combined  frame system.

(d)   Convenient and safe design approach with stability consideration.

(e)    Performs accurate and exact analysis considering both the geometric and material non-linearity.

The scope  of the DAM  in Staad .

Staad.pro performs the direct analysis based on the rigorous second order analysis method.  However, the moment magnification factor approach is not implemented in Staad. 

Staad forms the (K+Kg) matrix which accounts for the Geometric non-linearity, is the combination of the Global stiffness matrix and the Global Geometric Stiffness matrix. For the material non linearity, program reduces the axial and flexural stiffness in accordance to the code guidelines.

The DAM in Staad, which addresses the overall geometric and material non-linearity effect, performs the P-Delta analysis in the background -rigorous second order analysis-.

This is an iterative process as the Flexural stiffness coefficient τ_b is dependent on the axial force developed in the member.

Procedures to define and perform DAM in Staad are as follow.

Defining DAM parameters .

Go to COMMANDS -> LOADINGS -> DEFINITIONS -> DIRECT ANALYSIS.

Now in the dialogue box,

(1)   FLEX PARAMETER: This parameter represents τ_b involved in reduced flexural stiffness calculation (0.8* τ_b *EI) .The default value of τ_b is 1.0 but user can define the initial approximate τ_b value (Refer to chapter 7-3-(3)).

(2)   FYLD PARAMETER: Its default value is 36 ksi but user can input the desired yield strength value.

(3)   AXIAL PARAMETER : As the code does not specify any variable reduction coefficient for reduced axial stiffness , the constant 0.8 is taken  for reduced axial stiffness determination ( 0.8*EA)

(4)    “Notional Loads”: Although the default factor is 0.002, but the program allows user to specify different notional load factor values.

Please note that as the Flexural stiffness coefficient τ_b is dependent on the magnitude of the axial stress developed on the member, it also implies that the material starts showing inelasticity as the member force increases

If the defined notional load factor is greater than 0.00299, the program sets the iteration limit to 1 and does not perform any further iterations.

Running  the Direct Analysis .

Before running the direct analysis, the load combinations are to be created where the notional loads against the corresponding gravity loads need to be added for every frame levels.

Being a non- linear analysis, the combination should be done by the REPEAT LOAD option. Once the notional load is called in the combination case, Staad automatically applies the lateral loads at each level as the   factored proportion of the gravity load that is being applied on that level.

Once the modeling is completely done, go to COMMANDS ->PERFORM DIRECT ANALYSIS.

A dialogue dox will appear which allows the user to select the Design Method.

If the ASD is selected then as per chapter 7.3 of the AISC 360-05  , Staad  automatically multiplies the loads internally by 1.6 and the results are subsequently divided by 1.6 to obtain the design forces.  The user must ensure of defining the correct ASD load combination.

Additionally, user can specify the tolerance of Tau and displacement values.

 The program runs iterations in each step, changing the member characteristics until the maximum change in any τ_bis less than the specified Tau tolerance. If the maximum change in any τ_bis less than 100 times the τ tolerance and the maximum change in any displacement degree of freedom is less than the specified displacement tolerance; then the solution has converged for this case.

The beauty of the Direct Analysis feature in Staad is that, program performs the iterations for both the geometric and material nonlinearity to capture the real effect. Effect of the actual geometric non-linearity is determined by the P-Delta analysis and the material inelasticity by iteration of τ_b value.

Validation Staad DAM with Benchmark problem ,Case-2 (page-16.1-435) in the AISC 360-05

Input data

B=d = 5 inch , L = 500 inch , P =4 Kips ,  H = 2 Kip , E = 29732 Ksi  , Py = 50 ksi         

In Staad

After performing the Direct Analysis in Staad , the maximum moment and maximum displacement reported are  1290.392 Kip-in and 72.588 inch repectively .

The Hand-Calculation

REFERENCES

(1) AISC 360-05, Specification for Structural Steel Buildings

(2) AISC Stability Analysis handouts

(3) Structural Steel Design by Jack .C Mormac

(4) Steel Design by William .T. Segui

.

Balance and Hyperstatic Loading

The following equation is important in understanding the difference between Balance and Hyperstatic Loading:

Balance = Primary + Hyperstatic

The balance loading considers all the loads that the tendon puts on the concrete. These include the force results (axial, shear, and moments) in the slab and the restraint (reactions) of the supports.

The primary forces and moments are those that would be expected to be in the concrete due to the post-tensioning if the concrete deformation associated with the post-tensioning was totally unrestrained. This can be calculated at any cross section based purely on the cross section geometry and the tendon location.

The hyperstatic forces and moments are the difference between the unrestrained (primary) and the restrained (balance) results.

In summary,

      Balance:  due to tendons and restraint

      Primary:  due to tendons

      Hyperstatic:  due to restraint

The effects of both tendons and restraint must be considered for each design rule: service, strength, etc. Typically, the effects are considered differently in service and strength design.

In service design, the concrete stresses based on the applied forces and moments are calculated. This is a simple M/S + P/A calculation. This calculation must include both the effect of the tendon and the restraint. As a result, the balance loading is traditionally used for service design.

In strength and ductility design, the capacity is based on a cross section analysis using the tendons that intersect the section. In this case, the primary tendon force is used as part of the internal moment resisting arm. It would not be appropriate to use the balance loading in this calculation, because it already includes the primary force. If it was used, then the primary force would be included twice in the equilibrium solution. By including the hyperstatic forces in the cross section design and the primary tendon force as an internal cross section force, the entire post-tensioning effect is included in the solution.

Since the strength and ductility designs use a cross section analysis with the intersecting tendon force, it is important that a tendon intersects each left, right, and center strip in the model. Problems can occur in models with banded tendons, if a tendon does not intersect the left or right strips. In these cases, the middle strip will receive significant hyperstatic axial compression and, as a result, will have significant bending strength from that alone. Also, if there is no resultant tension reinforcement in the cross section, the effective depth cannot be calculated, which can lead to both shear design and ductility design problems. For PT slabs designed per ACI, it is common to use full-width design strips, which should ensure that each cross section is intersected by a tendon. In PT slabs designed using other design codes, there may be a requirement to design the slab with column and middle strips. However, these codes should also require some distribution of tendons in both column and middle strips. In these cases, each cross section (left, right, and center) should be intersected by a tendon based on the code required tendon distribution.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

1. How does Staad "direct" a spring to determine if it is in compression or tension?
2. What is the difference between a LOAD COMBINATION and a REPEAT LOAD?
3. In a structure which has wall panels in addition to other things, is it possible to temporarily disable some of the outer panels and analyse just the rest of the structure consisting of interior beams, columns, and slabs?
4. What's the difference between ELASTIC MAT and PLATE MAT for spring support generation?
5. The output from my STAAD run contains the warning message :THIS STRUCTURE IS DISJOINTED
6. Warning message in output file: This structure is disjointed, accompanied by several instability warnings
7. I am using STAAD.Pro 2002. When the STAAD Analysis and Design engine is running, you can't minimize the box that shows the activities in progress - it is always on top. I like to start the "run analysis" then go on to work on something else while it is running. It is not very convenient having the box on top of everything. How can i fix that?
8. I need to specify a cable member. Can I give it the proper properties using user define tables?
9. My STAAD outout file contains the warning message : **WARNING** THE POISSON'S RATIO HAS NOT BEEN SPECIFIED FOR ONE OR MORE MEMBERS
10. Read/write Error in Unit No. 17
11. I need to analyze a frame whose members have been rotated about the local z axis. Is there anyway to model this situation using STAAD? Can you input a point and define the orientation of the local axis of that point? Or is there some other way to model this situation?
12. Is IX the St. Venant's Torsional Constant or is it the Polar Moment of Inertia?
13. How does one get a report indicating whether the applied loads are in equilibrium with the support reactions?
14. How can I check whether the story drift of the floors are within allowable limits?
15. I have a structure with supports marked as elastic foundation. I am getting an error message "Error 0550" when I run the analysis. What could be the cause of this?
16. I want to use the Master/slave command to model a rigid diaphragm in STAAD.Pro 2001. The problem is two columns separated by 40 ft. One column goes from joints 1 to 2 and the second column goes from joints 3 to 4. I wish to have a rigid link between joint 2 and joint 4. The program gives an error of multiple structures. What additional connections do I have to do to tie these joints together?
17. I am going to incorporate into my model, rock anchors which will be installed down the center of the pipe piles. In case you are not familiar with these, they are a thread rod attached to the top of the pile, go down the center and are anchored into the rock and are used to take the uplift forces in the pier. I was planning on using the post tensioning command. Is this correct?
18. When I try to analyse a model which contains the ELASTIC MAT command for generation of soil spring supports, I encounter an error message : Error 0550* support joints are collinear
19. Do you have any thumb rule/ formula for estimating the time required for solving a structure involving plates elements?
20. I have to analyse a structure for temperature load. The temperature difference is 600 deg C. (from 0 deg C to 600 deg C). Which value of alpha I shall use, (i.e. alpha for 0 deg or alpha for 600 deg) for the analysis? Can I use an alpha value for the mean temperature?
21. Can I carryout a machine foundation analysis using STAAD PRO (Embedded Block foundation and Pile foundation)?
22. UNEXPECTED COMMAND IN LOAD DATA CHECK SPELLING AND ORDER OF DATA IN CASE NO. 1
23. What is the procedure used by STAAD.Pro in computing the stiffness matrix of a tapered beam?
24. While using the "Master Slave" command to define a rigid diaphragm in a framed multi-storeyed structure, what criteria should one adopt to determine the "Master node"?
25. I am analyzing a simply supported beam, which is 20 feet long. There is a concentrated force acting at the mid-span point of the beam. In one case, I model it as a single member and apply the load at midspan using the member load option. In another case, I model it as 2 separate members, each 10 ft long, and apply the load at the central node using the JOINT LOAD option. The member cross section is a W12X26 from the American steel table. When I look at the deflection at the 10 ft point, using the PRINT SECTION DISPLACEMENT command for case (a) and the PRINT JOINT DISPLACEMENT command for case (b), the values do not match. Why?
26. Can I design a stringer using STAAD.Pro?
27. In the member end forces output, why are two values being reported for axial forces? Also, why is it that sometimes the numerical values of these two are the same and sometimes they are not?
28. When I try running the analysis on STAAD.pro 2001, I get the error message error message as "Fatal Error: Cannot start analysis Engine". However, when tried with another machine with Win 95/98. The same software is working fine without giving any error.
29. I have a continuous beam, and all members have the same E and same properties. I run it once with one set of member properties. I double the depth of the members and run it a second time. The results of the second run (midspan moment, reactions, etc.) do not match those from the first run. Shouldn't the results stay the same as the EI remains constant for both the beams?
30. During the analysis of a large model, the analysis engine stops with the error message Read/write ERROR in Unit No. 17
31. I want to ignore the stiffness of certain members during analysis but want the loads applied on them to be transferred to the structure. Can I use the INACTIVE MEMBER command for this ?
32. How to add/change/modify sections to the Section Database?
33. ERROR-STAAD BUILT IN MEMORY BLOCK HAS OVERFLOWED
34. When I save my file in STAAD.Pro, all REPEAT commands are changed by STAAD to the expanded format. The same thing happens when I do a File > Save As. I would like to retain my input using the REPEAT commands. Is there any way to do that ?

1. How does Staad "direct" a spring to determine if it is in compression or tension?

For the purpose of defining the sense of the force in the SPRING TENSION/SPRING COMPRESSION facility, the following rules are adopted in STAAD :

A support reaction force is considered TENSILE if it is opposite to the positive direction of the axis under consideration. Another way of putting it is that, for this condition, the displacement along that axis of the support node is in the same direction as the positive direction of that axis.

A support reaction force is considered COMPRESSIVE if it is along the positive direction of the axis under consideration. Another way of putting it is that, for this condition, the displacement along that axis of the support node is in the direction opposite to the positive direction of that axis.

These rules are applicable for global axis supports, as well as inclined axis supports.

Hence, use the center of the circular pipe as the REFERENCE POINT for the INCLINED supports. The local X axis for the inclined supports will then point from the perimeter towards the center of the circle. The supports around the circumference can then be assigned COMPRESSION only springs. .

2. What is the difference between a LOAD COMBINATION and a REPEAT LOAD?

The difference lies in the way STAAD goes about calculating the results - joint displacements, member forces and support reactions. For a load combination case, STAAD simply ALGEBRAICALLY COMBINES THE RESULTS of the component cases after factoring them. In other words, for example, in order to obtain the results of load 10, it has no need to know what exactly constitutes load cases 3, 4 and 5. It just needs to know what the results of those cases are. Thus, the structure is NOT actually analysed for a combination load case. With a REPEAT LOAD case however, the procedure followed is that which occurs for any other primary load case. A load vector {P} is first created, and later, that load vector gets pre-multiplied by the inverted stiffness matrix.

3. In a structure which has wall panels in addition to other things, is it possible to temporarily disable some of the outer panels and analyse just the rest of the structure consisting of interior beams, columns, and slabs?

STAAD has an INACTIVE MEMBER command. This command can be used with beam/columns as well as plate elements. The members and elements subjected to this condition will have their stiffness, as well as any applied loading on them, ignored. This means, only the remainder of the structure will be treated as being active. So, what you can do is use the INACTIVE MEMBER command in conjunction with CHANGE to create a multiple analysis model.

In this manner, a single STAAD input file can represent various stages of construction of the structure. Take a look at example 4 in the Examples manual. Although that example illustrates the procedure using just beams, the same can be used with plates included in the structure.

4. What's the difference between ELASTIC MAT and PLATE MAT for spring support generation?

With the ELASTIC MAT you enter a list of joints from which STAAD will attempt to form a perimeter which encloses an overall area. This is done with a convex hull algorithm. Lastly, areas are assigned to each joint. If the convex hull rules are met, the algorithm works well. However for mats with irregular edges or holes, the algorithm may not do what the user expects and one may end up with springs with unreasonable spring constant values.

Since many mat foundation problems have plates defining the entire mat, we have added the PLATE MAT option where you enter a list of plates that entirely define the mat. Roughly 1/4th of the area of each plate is assigned to each joint in the plate in the same manner as uniform pressure or self weight is distributed.

So if you have the foundation support entirely defined by plates, then use the PLATE MAT option. Otherwise use the ELASTIC MAT option. With this option please observe the rules listed in the Tech Ref Manual. Avoid convex angles. You may have to subdivide the region into several sub-regions with several ELASTIC MAT commands. Add "PRINT" to the end of the command to see the areas assigned to each joint where a support is generated.

5. The output from my STAAD run contains the warning message :

THIS STRUCTURE IS DISJOINTED. IGNORE IF

MASTER/SLAVE OR IF UNCONNECTED JOINTS.

Is STAAD telling me that my structure is in several pieces?

This is referred to in STAAD as Multiple structures. The outline below explains the process for identifying the disparate components and merging them so they form a single structure. One of the consequences of having multiple structures, namely, structural instabilities, is also discussed. 

6. Warning message in output file: This structure is disjointed, accompanied by several instability warnings.

The error message, "this structure is disjointed..." is caused by multiple structures in the model. Multiple structures exist when one portion of the model does not have any way of transferring forces to another portion. It is possible to create a model that visually appears to be a single structure, but upon closer inspection it is revealed that the model contains two or more unconnected structures. Instability warnings may also appear in the output file, due to the fact that the structure appears to be properly supported when, in fact, it is not.

What do we mean by member connectivity? What determines whether two members are connected so that one can transfer loading to another? Consider two lines that intersect in 3D space, with the starting point of one line lying on the longitudinal axis of the other line. The fact that the end of one member lies on the axis of another is not sufficient to ensure connectivity. Connectivity is insured only when intersecting members are divided into segments that meet at a single, common node. ("Common node" means a single node number. Two separate node numbers, both having the same X,Y,Z coordinates is called a duplicate node situation. You can test for duplicate nodes in the model by pulling down the Tools menu and selecting the Check Duplicate command, then select the Nodes command from the Check Duplicate sub-menu).

7. I am using STAAD.Pro 2002. When the STAAD Analysis and Design engine is running, you can't minimize the box that shows the activities in progress - it is always on top. I like to start the "run analysis" then go on to work on something else while it is running. It is not very convenient having the box on top of everything. How can i fix that?

If you click on the top left corner of that box, you will find an option called "Do not Stay on Top". Switch that on. You can then make that window recede behind other windows.

8. I need to specify a cable member. Can I give it the proper properties using user define tables?

The only property that a cable requires is the cross section area. So any property type which can enable the program to obtain the area would be acceptable. A few examples are :

As a PRISmatic section with the diameter specified using the term YD :

MEMBER PROPERTY
1 PRIS YD 1.0

or

As a PRISmatic section with the Area specified using the term AX

MEMBER PROPERTY
1 PRIS AX 0.35

or

As a PIPE section with the outer and inner diameters specified using the terms OD & ID

MEMBER PROPERTY
1 TA ST PIPE OD 1.1 ID 0.0

If you wish to specify it as a user defined section using a user table, that would be acceptable too.

9. My STAAD outout file contains the warning message :

**WARNING** THE POISSON'S RATIO HAS NOT BEEN SPECIFIED FOR ONE OR MORE
MEMBERS/ELEMENTS/SOLIDS. THE DEFAULT VALUE HAS BEEN SET FOR THE SAME.

What does this message mean?

The Poisson's ratio is one of the fundamental material properties required to perform the analysis of a structure. It is generally used to obtain the value of G (Modulus of Rigidity) using the relationship

E=2G(1+Poisson)

where E is the Young's modulus

If you have failed to specify the Poisson's ratio, STAAD attempts to "guess" the value of that term based on the value defined for E. For example, if E is in the neighbourhood of 29000 ksi (steel), Poisson's will be chosen to be about 0.30. If E is in the neighbourhood of 3150 ksi (concrete), Poisson's will be chosen to be about 0.17.

If you would like to see what value has been chosen by the program, you may specify the command

PRINT MATERIAL PROPERTIES

after all the CONSTANTs have been provided. The Poisson's ratio will be reported in the output file along with some of the other CONSTANTs.

It is best to specify a value explicitly instead of having the program estimate a value on its own.

10. After I launch the analysis, the program completes some of the processes, and then comes up with the message :

*** STAADPro ERROR MESSAGE ***
** Read/write Error in Unit No. 17
++ Calculating Joint Displacements. 16:24:21

What does this error mean, and what can I do to avoid it?

This is very likely due to the fact that you have run out of disk space, particularly on the drive which is being used for the SET TEMP environment variable in NT and 2000 operating systems. Since this is usually the C: drive, you need to increase the free space available on the C drive, or whichever is being used for SET TEMP.

 11. I need to analyze a frame whose members have been rotated about the local z axis. Is there anyway to model this situation using STAAD? Can you input a point and define the orientation of the local axis of that point? Or is there some other way to model this situation?

We presume you mean that the member is rotated about the local "X" axis and not the local "Z" axis. When you use STAAD's default coordinate system, the local "X" is the longitudinal axis of the member, and local Z is generally the major axis of the member. So, changing the orientation of a member involves rotation about the local "X" axis, and not the local "Y" or local "Z" axes.

There are a couple of ways to change the orientation.

a) By specifying an angle using the BETA command. This is explained in Sections 1.5.2, 1.5.3 and 5.26.2 of the Technical Reference Manual. You may also refer to example 1 of the Examples manual for a sample problem which shows the usage of the command.

b) Using the REFERENCE POINT method. This too is explained in Sections 1.5.2, 1.5.3 and 5.26.2 of the Technical Reference Manual.

In the STAAD.Pro GUI, you may click the right mouse button, select Labels, and switch on Beam Orientation to get a visual representation of the directions the local X and Y axes point to.

Graphically, you can specify the BETA angle from 2 places :

If you go to the General - Property page on the left side of the screen, you will find the Properties dialog box on the right side and it contains a tab called Beta Angle through which the value can be specified.

If you go to the Commands menu on top of the screen, choose Geometric Constants - Beta Angle.

Graphically, you can specify the REFERENCE POINT by going to the Commands menu on top of the screen, and choosing Geometric Constants - Member Reference Point.

12. Is IX the St. Venant's Torsional Constant or is it the Polar Moment of Inertia?

IX is generally known just as the Torsion Constant. For a circular cross section, the torsion constant equals the Polar moment of inertia. For non-circular cross sections, it usually is less than the polar moment of inertia. If you have the AISC publication T114, you will see it referred to in that document using the expression J. If you have the textbook "Roark's Formulas for Stress & Strain, 6th edition, Warren C.Young, McGraw-Hill", you will find it being referred to using the expression K in section 9.2 of the book.

13. How does one get a report indicating whether the applied loads are in equilibrium with the support reactions?

There is an option which can be provided along with the PERFORM ANALYSIS command for getting this information. It is called PRINT STATICS CHECK. It is explained in Section 5.37 of the STAAD.Pro Technical Reference Manual. The sample below shows how this is done.

LOAD 3 LATERAL FORCES
JOINT LOAD
4 6 8 10 FX 12.5
LOAD COMBINATION 10
1 1.2 2 1.4 3 0.9
PERFORM ANALYSIS PRINT STATICS CHECK
PRINT ANALYSIS RESULTS
UNIT NEWTON MMS
START CONCRETE DESIGN
CODE BRITISH
FYLD 425 ALL
FC 35 ALL
DESIGN COLUMN 45 57
END CONCRETE DESIGN
FINISH

If you prefer to use the graphical method for specifying this option, this is how it can be done.

In the Modelling mode, click on the Commands menu from the top of the screen. Select Analysis | Perform Analysis. A Perform Analysis dialog box will be displayed. Set the radio button on Print Statics Check. Click on OK.

Save the file and run the analysis. Then view the output file. (This can be done from File | View | Output file | STAAD Output). If you scroll down to the region where the PERFORM ANALYSIS command is specified, the equilibroum report will be available after that command.

14. How can I check whether the story drift of the floors are within allowable limits?

If you have STAAD.Pro 2001 Build 1005 or Build 1006, you can specify a command called

PRINT STORY DRIFT

in your input file. Run the analysis. Then check your output file, The drift for each story will be reported. You will have to manually verify that this is within your allowable limits.

15. I have a structure with supports marked as elastic foundation. I am getting an error message "Error 0550" when I run the analysis. What could be the cause of this?

There are a few reasons why you will run into this error :

1) The ELASTIC MAT command is meant for generating support spring constants for the support nodes of plate elements. The feature does not work in the case where the support spring generation is requested at nodes which aren't connected to any plate.

STAAD attempts to find the tributary area of each node for which the elastic mat command is specified. It determines this area on the basis of the "influence area" of the nodes. If those nodes are attached to a non-plate entity such as a beam, the corresponding nodes do not have an "influence area".

2) The plates might be inclined to the global horizontal plane (XZ). Presently, the feature works satisfactorily only if the plate is truly horizontal. An inclined plate or a vertical plate cannot presently be handled by this feature.

16. I want to use the Master/slave command to model a rigid diaphragm in STAAD.Pro 2001. The problem is two columns separated by 40 ft. One column goes from joints 1 to 2 and the second column goes from joints 3 to 4. I wish to have a rigid link between joint 2 and joint 4. The program gives an error of multiple structures. What additional connections do I have to do to tie these joints together?

You would need to split the columns into 2 members each.

There is a limitation that a master joint cannot have supported directions or be connected to a member or element that is connected to a support; similarly, the slave joints may not have supported directions or be connected to a member or element that connects to a support.

17. I am going to incorporate into my model, rock anchors which will be installed down the center of the pipe piles. In case you are not familiar with these, they are a thread rod attached to the top of the pile, go down the center and are anchored into the rock and are used to take the uplift forces in the pier. I was planning on using the post tensioning command. Is this correct?

If the prestressing force is applied after the piles are driven into the soil and the pile cap is cast, the load type you may want to apply in analysing the model is the one involving the MEMBER PRESTRESS command. That is because, there is a small anomaly in STAAD in the definition of the terms PRESTRESS and POSTSTRESS. The way STAAD defines these terms and the way they are conventionally defined are opposite to each other. You will find an explanation of this anomaly in Section 1.16.5 of the STAAD.Pro Technical reference manual under items 4 and 5.

If you want an example which demonstrates the usage of these commands, please refer to example problem 6 in the STAAD.Pro Examples manual.

18. When I try to analyse a model which contains the ELASTIC MAT command for generation of soil spring supports, I encounter an error message :

Error 0550* support joints are collinear.

If you look under the "Notes" item of Section 5.27.3 of the STAAD.Pro
Technical Reference Manual, you will find that the program attempts to put
together a closed surface from the joint-list that accompanies each ELASTIC
MAT command. When you specify the commands in the following manner :

SUPPORTS
1 ELASTIC MAT DIRECT Y SUBGRADE 259
2 ELASTIC MAT DIRECT Y SUBGRADE 259
3 ELASTIC MAT DIRECT Y SUBGRADE 259
4 ELASTIC MAT DIRECT Y SUBGRADE 259
5 ELASTIC MAT DIRECT Y SUBGRADE 259
6 ELASTIC MAT DIRECT Y SUBGRADE 259
7 ELASTIC MAT DIRECT Y SUBGRADE 259
8 ELASTIC MAT DIRECT Y SUBGRADE 259
9 ELASTIC MAT DIRECT Y SUBGRADE 259

it fails to find a closed surface, because a single joint does not form a
closed surface.

Based on the type of model you have, you can replace those multiple ELASTIC
MAT commands with a single command as follows :

SUPPORTS
1 TO 625 ELASTIC MAT DIRECT Y SUBGRADE 259

19. Do you have any thumb rule/ formula for estimating the time required for solving a structure involving plates elements?

Run times depend on many factors. For static analysis without master/slave, the run time for large problems is dominated by the triangular factorization time.

Triangular Factor. Time = '[ (Fac) * (6 * no. of joints / 1000) * (bandwidth /100)**2 ] / 3600 in hours.

The bandwidth is printed with problem statistics.
Fac is a computer dependent factor.
Use Fac = 0.20 if you have 1.5GHz with 1 GB memory and 9600 SCSI drives.
Use Fac = 0.70 if you have 1.0GHz with 256 MB memory and 7200 drives.
Use Fac = 1.50 if you have 0.5GHz with 128 MB memory and 5400 drives.

As an example:

500Mhz, high bandwidth
Triangular Factor. Time = '[ (1.50) * (6 * 23000 / 1000) * (9000 /100)**2 ] / 3600 = 466 hours = 19.4 days

1000MHz Faster computer, more supports, lower bandwidth:
Triangular Factor. Time = '[ (0.70) * (6 * 22000 / 1000) * (5000 /100)**2 ] / 3600 = 64.1 hours = 2.67 days

20. I have to analyse a structure for temperature load. The temperature difference is 600 deg C. (from 0 deg C to 600 deg C). Which value of alpha I shall use, (i.e. alpha for 0 deg or alpha for 600 deg) for the analysis? Can I use an alpha value for the mean temperature?

The following answer was supplied by Ray:

Since STAAD is linear for material behavior, an alpha that results in the desired strain and displacement would be best. You can enter any positive value for alpha. The units of the alpha value must be the same as the temperature change units since STAAD does not explicitly enter temperature units.

21. Can I carryout a machine foundation analysis using STAAD PRO (Embedded Block foundation and Pile foundation)?

The answer is Yes. The piles have to be modelled as columns. If the machinery sits on a slab, that will be modelled using plate elements. The supports for the model are going to be the resistance (based on subgrade modulus) offered by the soil, which may be modelled as springs. The dynamic loads due to the machinery will be modelled as forcing function loading, either as discrete time-force pairs as shown in example 16, or as a sinusoidal loading as shown in example 22.

22. Attached is a portion of my STAAD output file which contains an error messsage as shown :

30. LOAD 1 (1.4DL + 1.7LL)
31. SELF Y -1.4
32. MEMB LOAD
33. 11 TO 16 UNI Y -2.8
34. 11 TO 16 UNI Y -5.1
35. START CONCRETE DESIGN

**WARNING - UNEXPECTED COMMAND IN LOAD DATA
CHECK SPELLING AND ORDER OF DATA IN CASE NO. 1
COMMAND= START CONCRETE DESIGN
CHECK RESULTS CAREFULLY, LOADS MAY HAVE BEEN LOST
***STAAD.Pro WARNING***
ANALYSIS NOT PERFORMED - SO ABOVE COMMAND CAN NOT BE PROCESSED.
*********** END OF THE STAAD.Pro RUN ***********

Can you explain what it means?

You are following up your load instructions with a concrete design statement. (Notice that after 11 TO 16 UNI Y -5.1, you are specifying START CONCRETE DESIGN). There is no instruction for performing the analysis. A concrete design cannot be done until after the analysis has been performed. So, you need to specify the command PERFORM ANALYSIS in between those 2 lines, as in,

11 TO 16 UNI Y -5.1

PERFORM ANALYSIS PRINT STATICS CHECK

START CONCRETE DESIGN

23. What is the procedure used by STAAD.Pro in computing the stiffness matrix of a tapered beam?

1. Define the static deflection curves of a beam due to unit displacement at each of its 12 degrees of freedom while maintaining the displacements at the remaining 11 degrees of freedom as 0. These curves are usually referred to as shape functions.

2. Using the principle of virtual work, calculate each term of the stiffness matrix. This is an integral involving E (modulus of elasticity), I (moment of inertia for the applicable axis), and the product of the second derivative of the two shape functions which are relevant to the specific stiffness matrix term being generated. The integration is performed over the full length of the member.

Simpson's method is used in performing the integration. STAAD uses 100 integration points over the member length. The moment of inertia is calculated at the start and end of each integration point.

24. While using the "Master Slave" command to define a rigid diaphragm in a framed multi-storeyed structure, what criteria should one adopt to determine the "Master node"?

For an ordinary static analysis, any of the joints in a master/slave system can be the master joint.

A second point to understand about the Staad.Pro 2001 input is that the command

SLAVE ZX MASTER j JOINT joint-spec

should be used for ordinary floor rigid diaphragms (assuming they are parallel to the ZX axes). This command sets the ZX plane to behave rigidly for inplane actions; while out-of-plane shear and bending will behave flexibly.

The command

SLAVE RIGID MASTER j JOINT joint-spec

should be used if the master/slave connections behave rigidly in all directions and rotations.

For a natural frequency calculation and dynamic analysis, the master joint should be the joint nearest to the c.g. of the masses associated with the master/slave joints.

If the c.g. is well off the structure as in an L shaped structure with narrow wings, you may want to add a joint at the c.g. just for the purpose of being the master; [remember to connect that joint to another joint with a dummy (low E value) member if it is not already connected].

25. I am analyzing a simply supported beam, which is 20 feet long. There is a concentrated force acting at the mid-span point of the beam. In one case, I model it as a single member and apply the load at midspan using the member load option. In another case, I model it as 2 separate members, each 10 ft long, and apply the load at the central node using the JOINT LOAD option. The member cross section is a W12X26 from the American steel table. When I look at the deflection at the 10 ft point, using the PRINT SECTION DISPLACEMENT command for case (a) and the PRINT JOINT DISPLACEMENT command for case (b), the values do not match. Why?

The difference is due to shear deformation.

When STAAD computes the displacement at a node using the stiffness method, the bending stiffness coefficients in the stiffness matrix include the contribution from

1. flexural deformation

2. shear distortion if the shear areas are part of the user specified member property input.

As a result, the joint displacements consist of the pure bending component plus the shear deformation component.

When you ask for intermediate section displacements along a member span, STAAD calculates these using the moment area method from

* the joint translation and rotation (which includes the shear deformation component as explained above)

* flexural deformation of the member (the shear deformation component is not considered here)

In other words, in the implementation of the moment area method, only the term (a) contains shear deformation where as term (b) does not. This is an error in the program, albeit a small one.

If shear deformation is eliminated from all of the above calculations, this discrepancy will be removed. Shear deformation is part of the above calculations only when the shear areas AY and AZ are non-zero values. When member properties are specified using methods such as defining sections from the built-in steel tables, or by specifying them as PRISMATIC sections with a DEPTH and WIDTH value, STAAD internally calculates the shear area before proceeding with the stiffness matrix assembly.

The user may nullify the effect of shear deformation by doing one of the following :

* Provide the properties using the PRISMATIC attribute, and specify just AX, IX, IY and IZ, as in,

1 PRI AX 21 IX 35.4 IY 45.3 IZ 85.75

In this case, since AY and AZ are not specified, shear deformation will not be calculated.

* Provide a very large value for the shear areas AY and AZ, thereby minimizing the shear deformation, as in

1 PRI AX 21 IX 35.4 IY 45.3 IZ 85.75 AY 1E10 AZ 1E10

In this case, the large shear area will result in negligible shear deformation.

26. Can I design a stringer using STAAD.Pro?

To answer your question, we would like to offer some insight into how the capabilities of a structural engineering software are related to the type of sections you wish to analyze and design.

Typically, almost all structural engineering programs have 2 parts to them :

* Analysis - Computing forces and moments in beams, columns, plates, etc., nodal displacements, support reactions.

* Design - Checking the adequacy of a section for a beam or column to carry the forces induced into that member from the applied loads on the structure.

For analysis, these programs do not care about the shape of the cross section. Regardless of what the shape is, the programs simply look for 4
pieces of information : Area, Moments of Inertia about the 2 principal axes, Torsional Constant. If shear deformation calculation is desired, the shear areas should be provided too. However, to help you avoid the chore of specifying these 4-6 values, these programs also allow you to specify the section as one of the standard shapes built into the program, and then internally calculate these 4 quantities using the property calculation rules for that shape. So, if the shape you wish to have analyzed is one such standard shape, you can simply define it in that way, as for example, the flange width, flange thickness, web depth and web thickness for an I shape.

However, if the shape of your section does not conform to one of the built-in shapes that the program supports, you will have to type in these
4-6 property values using a property type usually called PRISMATIC.

For design however, shape does matter. That is because, all design codes are written to allow design of only certain specific shapes, due to the fact that buckling of elements of the cross section plays a major role in determining the capacity of the section. Usually, these shapes are limited to I shapes, T shapes, Channels, Angles, Double angles, Z shapes, Rectangular tube shapes, Circular pipe shapes, etc.

So, the answer to your question is :

Analysis - Yes.

Design - Depends on the shape of the section, and the code according to which it should be designed.

27. In the member end forces output, why are two values being reported for axial forces? Also, why is it that sometimes the numerical values of these two are the same and sometimes they are not?

There are two values because member end force output consists of the forces and moments at the start node as well as at the end node of the member.

At the start node, a positive value of the axial force indicates axial compression, and a negative value indicates axial tension. At the end node, a positive value indicates axial tension, and a negative value indicates axial compression.

Generally, if the values at the start and at the end are not the same in magnitude, it is due to a load acting along the local X axis of the member. A typical example of this is a column (vertical member) subjected to selfweight loading. The difference in magnitude of the axial forces at start and end should be equal to the load acting along the local X axis of the member.

28. When I try running an analysis in STAAD.Pro, I get an error message "Fatal Error: Cannot start analysis Engine" .

The error may be caused by the fact that the paging file size which has been set in Windows in your machine is not large enough to run the program.

1. You can determine whether this is true by doing the following. Make sure you are logged in with Administrative privileges before you attempt any of the following steps.

Using Windows Explorer, locate the file "SProStaad.exe" which should be present in the folder "\Sprov8i\staad\sprostaad".

Run that file by double clicking on it. If it comes up with a message that the paging file size is not large enough, that is an indication of the problem mentioned in step 1 above.

2. If the above is true, you can remedy the error by doing the following:

In Windows, go to Control Panel - System - Advanced Settings

Select performance options. The total paging file size for all drives will be listed in a window. You may want to increase it.

Exit those settings dialog boxes, and try running the program again. Chances are it should work this time.

If that does not resolve the issue, please uninstall STAAD.Pro and reinstall the software.

29. I have a continuous beam, and all members have the same E and same properties. I run it once with one set of member properties. I double the depth of the members and run it a second time. The results of the second run (midspan moment, reactions, etc.) do not match those from the first run. Shouldn't the results stay the same as the EI remains constant for both the beams?

The difference is due to shear deformation. Instead of specifying properties as YD and ZD (which would trigger a shear deformation calculation), provide the values as

AX, IX, IY and IZ

and check the results. They should stay the same.

Or, if you are running STAAD.pro 2004 or later, specify the command

SET SHEAR

on the second line of your input file. This will forcibly switch off shear deformations from the calculations.

Now, changing just YD or ZD should have no effect on the results.

30. During the analysis of a large model, the analysis engine stops with the error message

*** STAAD.Pro ERROR MESSAGE ***
** Read/write ERROR in Unit No. 17

*** ERRORS IN SOLVER ***

Even though the hard disk may have plenty of free space, the problem might be caused by the fact that the disk is formatted with the FAT32 format. This format has a file size limit of 4.2GB. So although there is enough disk space, the single file size limit is exceeded for the STAAD solved matrix file (.L17).

You can determine the type of formatting in effect for your drive by going through the following simple steps.

• On your Windows Desktop, go to My Computer.
• Select the drive which serves as the repository for your SET TEMP settings on your machine. If there is only one hard drive on the machine, it will be that drive. 
• Right click on the drive and select  Properties
• Make sure the file system is NTFS
• Make sure there is enough Free space

If the above also indicates FAT32, you need to find a computer that has its disk formatted with the NTFS format (which does not have a file size limit). Most new Windows XP based computers have the NTFS format.

31. I want to ignore the stiffness of certain members during analysis but want the loads applied on them to be transferred to the structure. Can I use the INACTIVE MEMBER command for this ?

Loads defined on inactive members are not considered in analysis. If you are trying to ignore the stiffness of the members, you may consider defining a dummy material for these members with low E value so that the stiffness contribution from these members become negligible. That way loads applied on these members are still going to get transferred to the rest of the structure.

32. How to change /modify the Section Database?

Or

How to add new sections to the Section Database?

 Please download the  video file from the following link  which demonstrates this feature :

 http://communities.bentley.com/products/structural/structural_analysis___design/m/structural_analysis_and_design_gallery/258994.aspx

 An arbitrary section ISMB 200 has been added to the Indian Section Data Base.

33. When users run the analysis, it gives an error message... "ERROR-STAAD BUILT IN MEMORY BLOCK HAS OVERFLOWED".

If you are trying to analyse a large model, you may need to increase the memory alloted to STAAD.

From the Edit menu, choose Edit Input Command File and go to the STAAD.Pro input editor. Please add
the following statement as the first line in your model, on the line before the command STAAD SPACE—

MEMORY 85

STAAD SPACE

Run the analysis. This input will allow up to 85,000,000 4-byte words for STAAD's internal memory
block (default is 80 million).

If you still get the error message, increase the memory block till the error message is eliminated.

34. When I save my file in STAAD.Pro, all REPEAT commands are changed by STAAD to the expanded format. The same thing happens when I do a File > Save As. I would like to retain my input using the REPEAT commands. Is there any way to do that ?

The REPEAT and REPEAT ALL commands were used at times when the graphic user interface was not
so advanced and the primary mode of data input used to be by typing commands in the input file. But these days, the graphic user interface in Staadpro is so developed that there is no need for the users to go to input file and type in information. Consequently the commands like REPEAT and REPEAT ALL lost importance and have now been replaced by GUI features like Translational Repeat , Circular Repeat etc.

However they are still recognized as valid commands by the engine so that if your file contains those commands, Staad would be able to read the geometry information. When the file is saved using the GUI, the REPEAT commands get replaced. This improves the performance of the software because it no longer has to remember blocks of information that used to appear within the REPEAT commands.

However if you want to retain the REPEAT commands in the input file, keep a copy of the full set of commands like
JOINT COORDINATES, MEMBER INCIDENCES and  ELEMENT INCIDENCES which predominantly use the REPEAT options,  and place them after the FINISH command at the end of the input file. When you are done with
the full job and would not be saving the model using the GUI any more, you can finally replace the JOINT COORDINATES, MEMBER INCIDENCES, ELEMENT INCIDENCES sections with the copied blocks containing the REPEAT commands placed at the end of the file . Then run the analysis again and you will have your analysis results and at the same time your input file will contain the REPEAT commands. 

When you do a File > Save As, the file is saved by the GUI and hence the REPEAT commands are again replaced. If
you would like to save the file by a different name and retain the REPEAT commands, the best thing to do is to do the Save from outside the STAAD environment. You may use Windows Explorer. You can simply copy the original .std file and then rename it using Explorer to get your second file.

35. What is a Reference Load Case?

Reference Load is an array of load data. It is not a primary load case and STAAD will not analyse the Reference Load Case. The Reference Load Case forms an input in the primary load case.

The advantage of using the reference load case in models having non-linear analysis like a P-Delta or Member Tension/Compression is that it can be used to specify load combinations in a primary load case and thereby make the program analyse the combinations directly without analysing the individual component cases specified as Reference Load Cases. This can result in saving of time and computer resources significantly.

36. Can we specify time-history loads in a  Reference Load Case?

No, we cannot. We can only specify load items directly in a reference load case. Components like Wind Load, Seismic Load, Time History Load, Temperature Load etc., which needs the program to generate the load items cannot be specified within a Reference Load Case.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

Steel Unbraced Lengths in RAM Elements

General

The format for steel unbraced lengths (Lb pos, Lb neg, L33, L22, and LTorsion) changed in V11. The help context in the member design parameters worksheet discusses the format. This can be accessed by clicking on a cell in the Members – Steel Design Parameters worksheet and hitting F1 on your keyboard.

Old Implementation

Prior to V11, unbraced lengths were entered as a single value in the steel design parameters. If no value was entered, the unbraced length was assumed to be equal to the physical length of the member (j node to k node). If a non-zero value was entered, that value was used as the unbraced length for every station considered during the design of the beam. However, parameters such as Cb were always calculated using the physical length of the member. This limitation could be unconservative.

V11 Implementation

In V11, unbraced lengths need to be entered such that the sum of the unbraced lengths is equal the physical length of the member. For example, if a beam is 20’ long and the unbraced length is 5’, the unbraced length should be entered as 5;5;5;5. This allows the program to calculate parameters such as Cb for the actual unbraced segment rather than the physical length of the member. A tool button was introduced to rapidly generate the unbraced lengths.

However, this new method introduced some limitations into the program. For example, it is not possible to enter an unbraced length that is longer than the physical length of the member. This might be a necessity if you are modeling a bent or simulating a curved member. In addition, it is not possible to assign multiple Cb values to correspond to the unbraced lengths that were assigned. In other words, only a single Cb value can be entered and it will be used for all segments. If Cb is left blank, it will be calculated for all segments.

V12 Implementation

The V12 implementation functions as described below.

i. The engineer doesn’t type anything in the spreadsheet cell (either directly or with the tool) and thus the value remains the default “0”:  RE uses the physical length of the member as Lb and Cb will be automatically calculated based on the physical length of the member.

ii. Just one value is entered in the cell

     a. If the value is less than the member’s physical length, RE uses that value for all pertinent code checks (just like before v11.0) and Cb = 1. Besides printing Cb (as 1) in the design report, RE adds a note at the bottom of the report saying “Cb not calculated for the Lb specified. It is conservatively prescribed as 1”. However if there’s a value in the Cb cell other than zero “0”, then that value is used in all segments and the note is not printed.

     b. If the value is equal to the physical length of the member, then it would be exactly the same as i.a above.

     c. If the value is larger than the member’s physical length, RE uses that value for all pertinent code checks and Cb = 1. Besides printing Cb (as 1) in the design report, RE adds a note at the bottom of the report saying “Cb not calculated for the Lb specified. It is conservatively prescribed as 1”. However if there’s a value in the Cb cell other than zero “0”, then that value is used in all segments and the note is not printed.

iii. Multiple values are entered in the cell (either directly and separated with semicolon or using the tool):

     a. If the sum of the values is less or equal to the physical length of the member, RE calculates the difference for the last segment and Cb is automatically calculated for each segment. The sum is validated for unintended, non-summing entries.
 
     b. If the sum of the values is larger than the physical length of the member, RE does NOT allow it. In other words, one single unbraced length for the member, could be larger or smaller than the distance from node to node, but multiple unbraced lengths, must match the member’s physical length.

Note: Old models (prior to v11.0) opened in v12.0 will retain the original unbraced length assignments. These will fall under option ii above. The engineer must keep in mind that the final results may still vary because before, Cb was calculated with the physical length (which now only applies to option i above) and now with Cb = 1.

Similar behavior is applicable to L33, L22 and Ltorsion.

Adjusting the unbraced length of Built-Up members (double angles) to accommodate Intermediate Connectors locations.

In the Steel design criteria for AISC 360 there are two user variables that control this:

1. Intermediate Connectors - This option is used to define the use of two equations in the design of built-up members subjected to compression. The available options are:
   1. Intermediate connectors that are snug-tight bolted, to use the equation E6-1.
   2. Intermediate connectors that are welded or pretensioned bolted, to use the equation E6-2.
   3. Intermediate connectors in shear and bolt values based on bearing values (but pretensioned), to use the equation E6-2..
2. a (Connectors) - The distance between connectors in built-up members. It is used to calculate the modified slenderness of the built-up member following Section E6 specially 2L sections.

See AISC 360 Section E6 for further details.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs  

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!