1. I want to analyze my model for a response spectrum in X, Y and Z directions. I am specifying all the 3 direction factors as 1.0 ...Is my correct approach correct?
2. In the output file of a response spectrum analysis, there is the section that shows the mass participation factors in the x, y, and z directions. Then it shows the 'base shear' in all 3 directions. What is the reference point for this? I mean, does Staad select a 'base' or is the value just the sum of all forces in that particular direction?
3. This is a question dealing with response spectrum analysis. I know that if a force is applied in the response spectrum load case, it will be converted to a mass that will in turn affect the modal response. My question is, will that input force still be applied as a static force in the analysis? Or, would I have to apply the force in a different load case to account for it?
4. I am currently working on a seismic model which requires a response spectrum input.
   What I am finding is that regardless of the value I specify for damping, the displacements and forces appear the same. Is this right?
5. I am attempting to run a spectral analysis of a structure. I have a design spectrum in terms of %ground acceleration and i am trying to determine how the scale factor works. Could you direct me on what the scale factor is? Also should I be entering the % ground acceleration into the chart or the actual acceleration? 
6. I need to know how to input the missing mass correction command as too much mass is missing even after increasing the number of modes.
7. As I understand The Rayleigh method is used for natural Frequency calculations (first mode only) in the command CALCULATE NATURAL FREQUENCY & also in the command DEFINE UBC LOAD or 1893 load. Whereas the matrix method of iteration (like Staddola method) is used in the Response Specterum method of analysis . Does this mean the values which we got by define 1893 load or Calculate natural frequency are wrong?
8. The natural frequencies obtained during a Response Spectrum Analysis do not match the values calculated using the CALCULATE RAYLEIGH FREQUENCY command. Why is that?
9. Are the joint displacements (Inch, Radians) reported in example 11 the static displacements? If these are amplitudes (for example, at joint 5 for load case 2, X trans = 1.94384 inch) doesn't that mean this joint is failing in amplitude since it is very high?
10. In a response spectrum analysis, the base shear does not match the summation of shears I get at the base of the columns. Which one should I use as base shear ?
11. I am doing a response spectrum analysis and getting an error "No unsupported masses entered ...". What is the problem ?
12. Please find attached a file, which is the model with the Euro Spectrum. In the box “Design Ground Acceleration” I put 0.075 which is the ag value. Question: Is this value the correct value to input in this box or it should multiplied by g = 9.81m/sec2
13. Is it possible to save the floor spectrum data that is calculated by STAAD.Pro in a text or .xlsx file ?

Should I specify Response Spectrum in ALL directions in the same load case

2. In the output file of a response spectrum analysis, there is the section that shows the mass participation factors in the x, y, and z directions. Then it shows the 'base shear' in all 3 directions. What is the reference point for this? I mean, does Staad select a 'base' or is the value just the sum of all forces in that particular direction?

Each mode has a base shear that comes from the modal displacement at each joint with mass in the direction being excited by the base acceleration and the input spectral acceleration and the modal frequency. These modal base shears are combined by SRSS or any other method in STAAD that you select. In effect, all supported joint directions form the base where the displacement of every mode is zero.

3. This is a question dealing with response spectrum analysis. I know that if a force is applied in the response spectrum load case, it will be converted to a mass that will in turn affect the modal response. My question is, will that input force still be applied as a static force in the analysis? Or, would I have to apply the force in a different load case to account for it?

Response spectrum analysis is a dynamic analysis based on ground motion spectral acceleration. The acceleration usually varies with the period. Since there is no direct input for masses, what you are entering as forces are weights, and STAAD extracts masses from those weights. Hence, the same weight value should be entered in all 3 global directions for general space structures in order to get the natural modes and frequencies correctly.

The response spectrum result will be an absolute unsigned value for each output quantity which represents the maximum value for that quantity. Because of this, the 6 force/moments at each end of a beam will all be positive. Also given the member forces/moments on one end, you cannot compute those results on the other end because the values are considered independent much the same as if these were peak values in time history that all occurred at different times.

If you want static loading results combined with spectrum results, then use load combinations, possibly with the SRSS option.

4. I am currently working on a seismic model which requires a response spectrum input.
What I am finding is that regardless of the value I specify for damping, the displacements and forces appear the same. Is this right?

The damping factor that one specifies in the input has no effect at all if the combination method is SRSS. For the SRSS scheme, the effect of damping is built into the spectrum values (period vs. acceleration or period vs. displacement) that the user specifies. In other words, if the damping factor is f1, the acceleration that the user should provide ought to be A1 corresponding to period T1. If the damping factor is f2, the acceleration ought to be A2 for the same period T1. In other words, for the SRSS method, the effect of damping has to be reflected on the spectral acceleration or spectral displacement that is being input. The damping coefficient by itself does not have a direct impact on the results. It's effect is indirect.

With the CQC method, it is a different story. Damping will generally have an impact on the results, because, the damping factor is an explicit term in the equation used in CQC.

5. I am attempting to run a spectral analysis of a structure. I have a design spectrum in terms of %ground acceleration and i am trying to determine how the scale factor works. Could you direct me on what the scale factor is? Also should I be entering the % ground acceleration into the chart or the actual acceleration?

The spectral accelerations entered, after multiplication by the scale factor, must be in the current length units of the STAAD input. For example, if the spectral acceleration is in g's (%ground acceleration) and the current units are inches, then the scale factor must be 386.088; or 32.17 for feet; or 9.80665 for meters. The scale factor is simply the conversion factor from the units of the spectral acceleration to the current units of the STAAD input data.

6. I need to know how to input the missing mass correction command as too much mass is missing even after increasing the number of modes.

The answer to your question is available in Section 5.32.10.1 of the STAAD.Pro Technical Reference manual. Just use the keyword MIS along with the SPECTRUM command. For example,

SPECTRUM CQC X 1.0 ACC DAMP 0.05 SCALE 32.2 MIS

Please refer to example 11 in the Examples manual for information on the commands required for doing a response spectrum analysis.

7. As I understand The Rayleigh method is used for natural Frequency calculations (first mode only) in the command CALCULATE NATURAL FREQUENCY & also in the command DEFINE UBC LOAD or 1893 load. Whereas the matrix method of iteration (like Staddola method) is used in the Response Specterum method of analysis . Does this mean the values which we got by define 1893 load or Calculate natural frequency are wrong?

It is not true that the lowest frequency is the one which is associated with significant amount of participation of the masses of the model. That may be true of structures which look like a cantilever. But if the spatial distribution of masses is extensive, there is no guarantee that the fundamental mode is the most critical mode.

The statement that the Rayleigh frequency is associated with the first mode of the structure too is not correct.

A structure has several modes of vibration. If the structure were free to vibrate, the modes of vibration will follow the ascending order of strain energy. Consequently, if Y is the weakest direction of the structure, a Y direction mode will be the first mode. If the next weakest direction is Z, then the second mode will be a Z direction mode. Structures have local modes, where a small region within the model vibrates while the rest of the model remains stationary. It is entirely possible that a local mode is the lowest energy mode.

In many cases, torsional modes happen to be the lowest modes. Local and torsional modes are associated with negligible mass participation. You should look at the mode shapes of all the modes to get a sense of all the major vibration modes.

Since when using the Rayleigh method, one tends to load the structure in a manner which generally resembles a large mass participation mode, there is no sense in comparing the Rayleigh frequency with the lowest frequency from the eigensolution. Instead, you have to try to compare the displaced shape of the model used in the Rayleigh calculations with the various modes from eigensolution until you find a mode shape which resembles the displaced shape. When you do find a match, you will find that the Rayleigh frequency will be similar in value to the frequency of the matching mode.

If you do not like the frequency being used in the IS 1893 load generation, which is Rayleigh based, there is an option in STAAD for the user to provide his/her own value of the frequency. This is done using the PX and PZ options, as in the following example.

ZONE 0.05 K 1.0 I 1.0 B 1.0 PX 0.4 PZ 0.8

The values you provide for PX and PZ will be used in place of the one calculated by the Rayleigh method.

8. The natural frequencies obtained during a Response Spectrum Analysis do not match the values calculated using the CALCULATE RAYLEIGH FREQUENCY command. Why is that?

In STAAD, there are 2 methods for obtaining the frequencies of a structure.

1. The Rayleigh method using the CALCULATE RAYLEIGH FREQUENCY command
2. The elaborate method which involves extracting eigenvalues from a matrix based on the structure stiffness and lumped masses in the model.

The Rayleigh method in STAAD is a one-iteration approximate method from which a single frequency is obtained. It uses the displaced shape of the model to obtain the frequency. Needless to say, it is extremely important that the displaced shape that the calculation is based on, resemble one of the vibration modes. If one is interested in the fundamental mode, the loading on the model should cause it to displace in a manner which resembles the fundamental mode. For example, the fundamental mode of vibration of a tall building would be a cantilever style mode, where the building sways from side to side with the base remaining stationary. The type of loading which creates a displaced shape which resembles this mode is a lateral force such as a wind force. Hence, if one were to use the Rayleigh method, the loads which should be applied are lateral loads, not vertical loads.

For the eigensolution method, the user is required to specify all the masses in the model along with the directions they are capable of vibrating in. If this data is correctly provided, the program extracts as many modes as the user requests (default value is 6) in ascending order of strain energy. The mode shapes can be viewed graphically to verify that they make sense.

Thus, the answer to the question is : If you want to use the Rayleigh method, make sure you provide the right type of loading. If the load you apply causes an arbitrary displaced shape which has no resemblance to the vibration mode you are interested in, the frequency value you get will be wrong.

9. Are the joint displacements (Inch, Radians) reported in example 11 the static displacements? If these are amplitudes (for example, at joint 5 for load case 2, X trans = 1.94384 inch) doesn't that mean this joint is failing in amplitude since it is very high?

For spectrum load cases, they are the absolute maximum displacement that those degrees of freedom will ever experience under the dynamic loading which that spectrum represents. In this example however, the numbers are so large only because the spectrum used is rather unrealistic. The spectral acceleration for mode 1 is 2.8g, which is unlikely even in the most intense earthquake.

10. In a response spectrum analysis, the base shear does not match the summation of shears I get at the base of the columns. Which one should I use as base shear ?
Related query : In a response spectrum analysis, summation of the support reactions does not match the base shear reported. Which one should I use as base shear ?

The one that the software reports as the base shear, is the correct value to use. The column shears ( or support reactions ) reported are all individual maximums and may not occur at the same instant of time. There is a high probability that at the instant when the base shear is maximum, some of the column shears ( or support reactions ) will be less than their individual peak values. Moreover the method used for modal combination, gets rid of signs and hence column shears ( support reactions ) like any other response spectrum output, would be devoid of any sign. Hence one cannot add these up to arrive at the base shear.

11. I am doing a response spectrum analysis and getting an error "No unsupported masses entered ...". What is the problem ?

There are 3 things you need to check

1. You have not defined any seismic masses as part of your seismic load case. You need to add these as loads and the software internally converts these to masses. You may refer to section 5.32.10.1.1 of the Technical Reference Manual for details. There is an example 11 provided with the software which can also be referred to.

2. You may have included Self weight as part of the seismic mass definition but the density of the material you have used may have been set to 0. 

3. All nodes in your model are restrained from vibrating due to supports. You may need to generate additional nodes so that there are some free masses in your structure that can vibrate.

12. Please find attached a file, which is the model with the Euro Spectrum. In the box “Design Ground Acceleration” I put 0.075 which is the ag value. Question: Is this value the correct value to input in this box or it should multiplied by g = 9.81m/sec2 ?

 In STAAD Eurocode 8:2004 implementation, if you have specified 0.075 in the box designated to specify the Design Ground Acceleration, the engine takes that value as 0.075g for calculations. It automatically multiplies that value with the acceleration due to gravity.

13. Is it possible to save the floor spectrum data that is calculated by STAAD.Pro in a text or .xlsx file??

Click at the top left corner of the floor spectra table as shown in the attached screenshot. It will highlight all the contents of the table. Press Ctrl-C on your keyboard to copy.

Open a blank Notepad file. Type Ctrl-V for paste.

.

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!

In the output file of a response spectrum analysis, there is the section that shows the mass participation factors in the x, y, and z directions. Then it shows the 'base shear' in all 3 directions. What is the reference point for this? I mean, does Staad select a 'base' or is the value just the sum of all forces in that particular direction?

Each mode has a base shear that comes from the modal displacement at each joint with mass in the direction being excited by the base acceleration and the input spectral acceleration and the modal frequency. These modal base shears are combined by SRSS or any other method in STAAD that you select. In effect, all supported joint directions form the base where the displacement of every mode is zero.

This contains STAAD.Pro wikis related to "Analysis" topics.

DIRECT ANALYSIS

Where are the custom configuration settings for Ram Elements and the various modules saved? 

Throughout Ram Elements there are numerous locations where the user can customize various settings. These are typically noted with a check box indicating "Current values as default" like the one at the bottom of the Analysis dialog box:

Those settings and others like print settings, recent file lists and more are saved in various config files typically located in a path like this (Windows 7):

C:\Users\User.Name\AppData\Roaming\Bentley\Engineering\RAM Elements.en\

Where the User.Name is the current user login name.

If you want to restore the program presets you can delete files within this folder (or the whole folder) and then restart Ram Elements. This might be necessary if the current settings are somehow corrupted and causing the program to malfunction (a module might not launch at all, or the window might be stretched to the point where it's unusable, for example).

The following batch file has been created to automate the removal process.

See Also

RAM Elements - View Control FAQ

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!

This is a question dealing with response spectrum analysis. I know that if a force is applied in the response spectrum load case, it will be converted to a mass that will in turn affect the modal response. My question is, will that input force still be applied as a static force in the analysis? Or, would I have to apply the force in a different load case to account for it?

Response spectrum analysis is a dynamic analysis based on ground motion spectral acceleration. The acceleration usually varies with the period. Since there is no direct input for masses, what you are entering as forces are weights, and STAAD extracts masses from those weights. Hence, the same weight value should be entered in all 3 global directions for general space structures in order to get the natural modes and frequencies correctly.

The response spectrum result will be an absolute unsigned value for each output quantity which represents the maximum value for that quantity. Because of this, the 6 force/moments at each end of a beam will all be positive. Also given the member forces/moments on one end, you cannot compute those results on the other end because the values are considered independent much the same as if these were peak values in time history that all occurred at different times.

If you want static loading results combined with spectrum results, then use load combinations, possibly with the SRSS option.

Challenges involved in modeling with surface elements in STAAD.Pro

The fact that one does not need to generate a finite element mesh when modeling with surface elements makes it an attractive modeling option for many engineers modeling their structures using STAAD.Pro. While there are advantages of using surface elements under some scenarios, before one uses surface elements for modeling, one should be aware of the challenges that come with it. As mentioned, an advantage of using surface element is that the minute details involved in the process of converting a physical object like a wall or slab into an analytical model consisting of a plate element mesh, is not something that the user has to bother about. However, in many situations, not knowing these details can lead to errors, some of which are impossible to detect because the underlying elements cannot be seen graphically.

For example in a structure, there may be beams elements and plate elements sharing common edges with the surface elements. For proper connectivity to be established in between these entities, each has to be divided or meshed such that the number of divisions match at the common edges or in other words these entities should share common nodes. Unless that happens, the connectivity would not be proper which would lead to incorrect load transfer between these entities. Although STAAD.Pro internally meshes the surface element into a plate mesh however the adjacent entities are not automatically split or meshed accordingly to establish proper connectivity and the onus is on the engineer to ensure that. For proper connectivity to be established, one needs to go in and set the number of divisions for each edge of a surface such that it matches with adjacent plate mesh or with segments of beams at the periphery. One would also need to ensure that columns adjacent to surfaces, usually modeled using beam elements, are also split to match with the surface divisions. Matching these divisions for regular rectangular geometry may not be that complicated but in a real life structure where slabs could be of any shape, have openings, walls may have cutouts and may have discontinuities, establishing proper connectivity could be a real challenge. So the idea of modeling with surfaces just because it is simpler to model it that way, is not necessarily true.

In addition, although one can influence the edge divisions along an edge of a surface but one cannot control the internal meshing fully as irregularity in geometry may result in generation of a triangular mesh. In such cases it is difficult to estimate the number of elements that could be generated during the meshing process. Not knowing this beforehand can lead to unwanted consequences such as a massive increase in the size of the model, to a point where the program simply cannot handle such a massive volume of data. The following wiki deals with one such scenario

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/22310.error-total-jointelement-limits-exceeded-while-creating-surface

There is however one situation where the user has to use surface elements. That is when he/she wants to perform reinforced concrete design of a shearwall per the ACI, British or Indian codes. STAAD.Pro can perform a shearwall design if and only if that wall is modelled using the surface entity.

It may be worth mentioning here that modeling shear walls with plate elements is also very common. The following wiki explains how shear walls modeled with plates can be subsequently designed

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/23103.modeling-shear-walls-using-plates

So this is what it boils down to. When one needs to model a floor slab, roof or a wall that does not need to be designed as a shear wall, they are better off using plate elements. When a shear wall needs to be modeled and also designed as per one of the codes mentioned above, the user needs to make a choice as to whether to use plates or surfaces after weighing in all the pros and cons. In our view, the only drawback of using plates is that one needs several of them to model a wall or a slab and that increases the size of the input data. But even then one has a lot more control on the model size when using a plate mesh. In addition, fixing modeling errors are a lot easier for a model using a plate mesh.

If there are instability messages, are there any simple checks to verify whether my analysis results are satisfactory?

There are 2 important checks that should be carried out if instability messages are present.

a) A static equilibrium check. This check will tell us whether all the applied loading flowed through the model into the supports. A satisfactory result would require that the applied loading be in equilibrium with the support reactions.

b) The joint displacement check. This check will tell us whether the displacements in the model are within reasonable limits. If a load passes through a corresponding unstable degree of freedom, the structure will undergo excessive deflections at that degree of freedom.

One may use the PRINT STATICS CHECK option in conjunction with the PERFORM ANALYSIS command to obtain a report of both the results mentioned in the above checks. The STAAD output file will contain a report similar to the following, for every primary load case that has been solved for :

***TOTAL APPLIED LOAD ( KG METE ) SUMMARY (LOADING 1 )
SUMMATION FORCE-X = 0.00
SUMMATION FORCE-Y = -817.84
SUMMATION FORCE-Z = 0.00

SUMMATION OF MOMENTS AROUND THE ORIGIN-
MX= 291.23 MY= 0.00 MZ= -3598.50


***TOTAL REACTION LOAD( KG METE ) SUMMARY (LOADING 1 )
SUMMATION FORCE-X = 0.00
SUMMATION FORCE-Y = 817.84
SUMMATION FORCE-Z = 0.00

SUMMATION OF MOMENTS AROUND THE ORIGIN-
MX= -291.23 MY= 0.00 MZ= 3598.50


MAXIMUM DISPLACEMENTS ( CM /RADIANS) (LOADING 1)
MAXIMUMS AT NODE
X = 1.00499E-04 25
Y = -3.18980E-01 12
Z = 1.18670E-02 23
RX= 1.52966E-04 5
RY= 1.22373E-04 23
RZ= 1.07535E-03 8

Go through these numbers to ensure that 

i) The "TOTAL APPLIED LOAD" values and "TOTAL REACTION LOAD" values are equal and opposite.
ii) The "MAXIMUM DISPLACEMENTS" are within reasonable limits.

 What does a zero stiffness warning message in the STAAD output file mean?

The procedure used by STAAD in calculating displacements and forces in a structure is the stiffness method. One of the steps involved in this method is the assembly of the global stiffness matrix. During this process, STAAD verifies that no active degree of freedom (d.o.f) has a zero value, because a zero value could be a potential cause of instability in the model along that d.o.f. It means that the structural conditions which exist at that node and degree of freedom result in the structure having no ability to resist a load acting along that d.o.f.

A warning message is printed in the STAAD output file highlighting the node number and the d.o.f at which the zero stiffness condition exists.

Examples of cases which give rise to these conditions :

Consider a frame structure where some of the members are defined to be trusses. On this model, if a joint exists where the only structural components connected at that node are truss members, there is no rotational stiffness at that node along any of the global d.o.f. If the structure is defined as STAAD PLANE, it will result in a warning along the MZ d.o.f at that node. If it were declared as STAAD SPACE, there will be at least 3 warnings, one for each of MX, MY and MZ, and perhaps additional warnings for the translational d.o.f.

These warnings can also appear when other structural conditions such as member releases and element releases deprive the structure of stiffness at the associated nodes along the global translational or rotational directions. A tower held down by cables, defined as a PLANE or SPACE frame, where cable members are pinned supported at their base will also generate these warnings for the rotational d.o.f. at the supported nodes of the cables.

Solid elements have no rotational stiffness at their nodes. So, at all nodes where you have only solids, these zero stiffness warning messages will appear.

These are warnings and not errors because :

The reason why these conditions are reported as warnings and not errors is due to the fact that they may not necessarily be detrimental to the proper transfer of loads from the structure to the supports. If no load acts at and along the d.o.f where the stiffness is zero, that point may not be a trouble-spot.

What is the usefulness of these messages :

A zero stiffness message can be a tool for investigating the cause of instabilities in the model. An instability is a condition where a load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through, and may result in a lack of equilibrium between the applied load and the support reaction. A zero stiffness message can tell us whether any of those d.o.f are obstacles to the flow of the load.

1. I tried to model a concrete foundation using solid elements, basically following the approach described in example 24 of the examples manual. However, I got this warning stating "more than 12 DOF with zero stiffness". Can anyone please advise me what are the technical reasons for this and how I can possibly handle this warning?
2. When we generate the spring supports for a raft foundation using the command "list-of-joints Elastic mat Dir YONLY SUB 10000" we get instability warnings in FX, FZ and MY directions at certain joints. Why?
3. I have analyzed a structure and find that there are instability messages in the .anl (output) file
4. If there are instability messages, does it mean my analysis results may be unsatisfactory?
5. If there are instability messages, are there any simple checks to verify whether my analysis results are satisfactory?
6. What does a zero stiffness warning message in the STAAD output file mean?

See Also

RAM Instability In Finite Element Analysis

Structural Product TechNotes And FAQs

CALCULATING MODES AND FREQUENCIES

In STAAD, there are 2 methods for obtaining the frequencies of a structure.

• The Rayleigh method using the CALCULATE RAYLEIGH FREQUENCY command

• The elaborate method which involves extracting eigenvalues from a matrix based on the structure stiffness and lumped masses in the model.

BASIC PRINCIPLE

The Rayleigh method in STAAD is a one-iteration approximate method from which a single frequency is obtained. It uses the displaced shape of the model to obtain the frequency. Needless to say, it is extremely important that the displaced shape that the calculation is based on, resemble one of the vibration modes. If one is interested in the fundamental mode, the loading on the model should cause it to displace in a manner which resembles the fundamental mode. For example, the fundamental mode of vibration of a tall building would be a cantilever style mode, where the building sways from side to side with the base remaining stationary. The type of loading which creates a displaced shape which resembles this mode is a lateral force such as a wind force. Hence, if one were to use the Rayleigh method, the loads which should be applied are lateral loads, not vertical loads. For the Eigen solution method, the user is required to specify all the masses in the model along with the directions they are capable of vibrating in. If this data is correctly provided, the program extracts as many modes as the user requests (default value is 6) in ascending order of strain energy. The mode shapes can be viewed graphically to verify that they make sense.

Eigenvalue extraction method

This method is based on extracting eigen values and eigenvectors using the stiffness and mass matrices of the structure. If the stiffness and mass data are specified correctly, this is a far more reliable method than the Rayleigh method. In modal analysis we solve:

To obtain the natural frequency  using the RAYLEIGH METHOD, you have to specify the command CALCULATE RAYLEIGH FREQUENCY in the load case in which the load data which produce a mode-shape type deflected shape are specified.

To use the Eigen value method, specify the command MODAL CALCULATION REQUESTED in the load case in which the load data for the mass matrix are specified. For an example which illustrates this method, please go to Help-Contents-Application Examples-British Examples-Example 28.

Also, the Rayleigh cases should not have the MODAL CALC or dynamics in the same case.  Remember to leave these Rayleigh cases out of the Load List for design.

This page contains STAAD.Pro dynamic analysis solutions

Checking local deflection of a member for a Response Spectrum Load case

Finding out dynamic structural responses for few specific Modes - MODE SELECT

NOTE: MASS MODEL FORMED WILL BE USED IN SEISMIC/RESPONSE/TIME HISTORY LOADING

Calculating Modes & Frequencies

Question:

Please find attached files and let me know how to calculate the Accidental Eccentricity and the Dynamic Eccentricity  as reported in the STAAD Pro output. 

Explanation: 

 The Building Length (Along X) : 9.00 m

The Building Width (Along Z)  : 8.00 m

The Accidental eccentricity:

As per ASCE-07 code, where diaphragms are not flexible, the design
shall include the inherent torsional moment (Mt)
resulting from the location of the structure masses
plus the accidental torsional moments (Mta) caused by
assumed displacement of the center of mass each way
from its actual location by a distance equal to 5
percent of the dimension of the structure perpendicular to the direction of the applied forces.

Hence, the Multiplying factor for Accidental Torsion Moment is taken as 0.05.

Along X = (Length of the Structure perpendicular to direction of applied load along Z)* 5%

              = 9*.05 = 0.45

Along Z = (Length of the Structure perpendicular to direction of applied load along X)* 5%

              = 8*.05 = 0.40

The Dynamic Eccentricity:

The multiplying factor (DEC) for Natural Torsion Moment is taken as 1.5.

The Center of Mass for each floor diaphragm is reported in the output file

The C.M of the diaphragm 

X                Z

4.481     4.016

The Center of Rigidity of the same rigid floor is reported in the output file if  PRINT DIA CRcommand is invoked.

The C.R of the diaphragm 

X                            Z

4.33                4.133

The natural torsion is automatically included in the analysis for DEC <= 1.0 i.e. no additional inherent torsion is applied.

However, if DEC > 1.0, a twisting moment with modified eccentricity of (DEC-1) will act at CM.

For more information, please refer to the chapter 5.32.10.1.8 in the technical reference manual.

So, the Net Dynamic Eccentricity = Static Eccentricity*(DEC-1) -------------   Multiplying factor (DEC >1)

            Along X                        =  (C.M-C.R)*(1.5-1)

                                                 =  (4.481-4.33)*(.5)

                                                 =  0.074 = 0.07

            Along z                         =  (C.M-C.R)*(1.5-1)

                                                 =  (4.016-4.133)*(0.5)

                                                 = -0.058 = -0.06 

 Staad Output :

PROBLEM: Calculation of the design base shear from Rayleigh time period and to verify proper distribution of load to all the different joints in the structure. The structure modeled in STAAD. Pro is shown below:

What is the difference between a Pdelta and a Nonlinear Analysis ? I tried both on my structure and did not observe any significant difference.

In a Pdelta analysis you are accounting for the second order effects resulting from displacements ( normally Large delta and Small delta ). However if the structure or loading is such that the analysis results in large displacements, then in addition to accounting for the effect of these secondary forces, one should also account for the fact that the stiffness of the structure may change significantly as it deflects. Nonlinear analysis allows you to account for such changes in stiffness with application of load. Depending on the nature of the problem, the loads should be applied in steps while the stiffness is adjusted multiple times within a load step to arrive at a converged solution.

In your structure, nonlinear effects may not be that significant and hence you may not have noted a significant difference between a second order and a nonlinear analysis. If the structure stiffness would have changed significantly, you would have seen the effect in results.       

1.  In Chapter 3 of the Technical Reference Manual (American Concrete Design), under Section 3.5, in the 4th paragraph, you state that a PDELTA ANALYSIS is the appropriate way to account for slenderness effects during column design.
2. Can I do a Pdelta analysis using STAAD for Response Spectrum load cases?
3. When the PDELTA ANALYSIS command is specified with the CONVERGENCE (m) parameter, the output file contains a message with a convergence error. What does this number represent ?
4. When doing P-Delta analysis for the design, what does the message "Load Case = 101 Convergence Error (SRSS) = 3.6603E-02 INCH" in the output file mean?
5. What is the difference between PDelta and Nonlinear Analysis

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

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The definition of center of rigidity (CR) is as follows:

The center of rigidity is a point at a particular story as the location of application of lateral load at that point will not produce rotation of that story.

The above definition is valid when slab is modelled as a rigid diaphragm. A Diaphragm Constraint causes all of its constrained joints to move together as a planar diaphragm that is rigid against membrane deformation. For semi-rigid diaphragm, the inplane deformation on a floor points could be different due to inplane-deformation of slab. Therefore, there is no unique solution to center of rigidity i.e. the Center of rigidity is indeterminate quantity.

The center of rigidity is just a reporting item. The inherent eccentricity due to change is mass centroid and stiffness centroid is automatically include in the analysis regardless the choice of diaphragm. Therefore, the reported value for center of rigidity for semi-rigid diaphragm has no meaning and this reported CR does not affect the analysis results.

Center of rigidity is the stiffness centroid within a floor-diaphragm plan. When the center of rigidity is subjected to lateral loading, the floor diaphragm will experience only translational displacement. Other levels are free to translate and rotate since behavior is coupled both in plan and along height. As a function of structural properties, center of rigidity is independent of loading. Certain building codes require center of rigidity for multistory-building design-eccentricity requirements.

For a given floor-diaphragm, CR is calculated through the following process in case of the floor defined as Rigid Diaphragm:-

Case-1: applies a global-X unit load to an arbitrary point such that the diaphragm rotates RXY

Case-2: applies a global-Z unit load to an arbitrary point such that the diaphragm rotates RYZ

Case-3: applies a unit moment about global-Y causing rotation RYY

If there are “N” no. of rigid diaphragms present in the model, there will be “3N” no. of load cases generated for which static analysis will be performed.

Center of Rigidity (X, Z) is then computed as:

X= RYZ / RYY

Z = - RXY / RYY

In STAAD, the unit loads are applied at CM

The global coordinates of CR at each floor is given by (CRx, CRy and CRy), where CRx = CMx + X, CRy = CMy and CRz = CMz + Z

Where, centre of mass (CM) at each floor level and find its coordinates (CMx, CMy, and CMz)

NOTE: Center of rigidity is only applicable to rigid diaphragms because in-plane slab deformation is variable across laterally loaded semi-rigid diaphragms. During computation, an arbitrary coordinate is selected and loaded, and then center of rigidity is derived, as a function of stiffness, according to the displacement at this specific point. If a diaphragm constraint is not applied, displacement at any point will also depend upon variable local membrane deformation. As a result, no unique solution is available for center of rigidity since formulation assumes that all joints translate together in planar motion.

This page contains a list of TechNotes related to STAAD.Pro

Member Tension And Combination Load Cases [TN]

Pushover Analysis of Steel Structure using STAAD (TN)

Reference Loads An Introduction [CS]

STAAD.Pro Wall Analysis For Dams [TN]

STAAD.Pro EN 1993-1-1:2005 Implementation: Elastic Verification of Cross Sectional Resistance

STAAD.Pro EN 1993-1-1:2005 implementation: Calculation of Torsional Capacities of Cross Sections as per SCI P057

STAAD.Pro EN 1993-1-1:2005 Implementation: Code Check for Torsion

STAAD.Pro EN 1993-1-1:2005 Implementation: Example 2 verification as per SCI P364

STAAD.Pro EN 1993-1-1:2005 Implementation: Example 3 verification as per SCI P364

STAAD.Pro EN 1993-1-1:2005 Implementation: Example 4 verification as per SCI P364

Discussion on Seismic Detailing Concept of RC Structures (IS:13920-1993)

Limitations of the curved beams in STAAD.Pro ( TN )

“Missing Mass” in Dynamic Analysis

IS:1893 (Part-I)-2002 Response Spectrum Philosophy

Linear static, P-Delta, small P-Delta, stress stiffening, and geometric non-linearity in STAAD.Pro

Direct Analysis per AISC 360-05 and its Implementation In STAAD (TN)

Computation of Center of Rigidity

 Question:

Even after creating proper beam plate connectivity, why Staad still reports  instability warning in my model?

(Please visit the site to view this file)

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

The STAAD in-plane plate has one "spurious displacement mode" in addition to the 3 in-plane rigid body modes (and 9 elastic displacement modes). Normally, conditions such as edge beam members, supports which suppress rotation about the element local Z axis, other out-of-plane elements which are connected to the element in question, and irregular grids suppress this spurious mode. However, if none of these conditions exist, STAAD automatically creates a fictitious weak spring at one of the nodes to correct this instability, so there is no actual problem, just warning messages.

The structure in your case is one where none of the compensating conditions exist. In other words, the X-Z planar nature of your model, and support conditions or the column tip on which plates rests allows the rotation about the global Y axis meaning that there is nothing to suppress the spurious displacement mode, which is why STAAD is forced to create the weak spring. 

You can also introduce an additional  dummy beam inside the plate connecting the common node. See the attached modified.std model

(Please visit the site to view this file)


This will get rid of the warning message.

I am running an analysis and getting an error “Multiple Floor spectrum Generation block not allowed”. What does it mean ?