Current Revision posted to Structural Analysis and Design Wiki by Sye on 5/14/2013 8:00:43 PM
STAAD.Pro Steel Design [FAQ]
FAQs,STAADPro,SteelDesign
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Document Information
Document Type: FAQ
Product(s): STAAD.Pro
Version(s): All
Original Author: Bentley Technical Support Group
While I can look at the model with Stress Ratio values annotated next to the steel members that I have asked to be checked, when I do the member query (double clicking on the members) I don't see the Design Property and Steel Design boxes anymore. Why is that?
Design Property and Steel Design tabs are not displayed for members which have not been designed. Are you sure you are clicking a member for which the design has been done? Sometimes, when ratios are annotated on the screen, the picture may become quite cluttered with data and in an effort to double click on a designed member, one may end up clicking on a member for which design has not been performed. So, first check that the member you are double-clicking has indeed been designed. If you are certain that STAAD has done the design and evidence of that exists in the analysis output file and in the postprocessing Unity Check tables, but still you are not able to see these tabs in the dialog box which comes up when you double click on the member, please send us your .std model and our support representatives will look into that. Our email address is support@reiusa.com
STAAD is checking deflection for beams or girders for all the load combinations in my model. Is there a way to tell STAAD which load combination to check?
You have to use the LOAD LIST command to achieve this. Supposing you want to check deflection for combination cases 81 and 82. And assume that L/Deflection has a limit of 240. The command sequence required to achieve this is
LOAD LIST 81 82
PARAMETER
CODE AISC
DFF 240 ALL
CHECK CODE ALL
However, after these commands, you have to reset DFF to a very small number so that deflection does not become a criteria for any further design operations. That is because, once a parameter is specified in STAAD, it stays that way till it is changed again. So, after the above, you need to specify
PARAMETER
CODE AISC
DFF 1 ALL
The steel design output indicates a slenderness failure (KL/r exceeds allowable). Why? The axial force on the member is very small.
The code has requirements which say that the KL/r ratios for a member should not exceed certain allowable limits. For members subjected to tensile forces, the code suggests one limit, and for members subjected to compressive forces, there is another limit.
This check does not consider the amount of the axial force. It only looks at the sign of the force to determine if it is a tensile force or compressive force.
In most codes, this is the first check STAAD does on a member. If the member fails the check, no further calculations are done for that member.
So, STAAD performs these checks by default. However, the code does not offer any guidelines on what must be the minimum magnitude of the axial force for the member to become a candidate for this check.
So, in STAAD, two parameters are available - one called MAIN and another called TMAIN if you wish to bypass this check (TMAIN is available for some codes only). MAIN=1 is for bypassing the slenderness check in compression, and TMAIN=1 is for bypassing the slenderness check in tension.
I set my deflection limit to L/360, but the maximum deflection indicated in the summary of node displacements in PostProcessing shows a deflection of 1.5 inches. Isn't this above the limit that I set?
During steel design per the AISC ASD code, there are two types of deflection checks you can perform with STAAD. They are
- Check for local deflection. This is usually applicable to members which are connected at both their ends to other members.
- Check for the relative displacements between the nodes such as for a cantilever beam.
LOCAL DEFLECTION is defined as the maximum deflection between the 2 ends of the beam relative to a straight line connecting the 2 ends of that member in its deflected position.
If you go to
Help - Contents - Technical Reference - Commands and Input Instructions - Printing Section Displacements for Members
you will find a diagram indicating this is in figure 5.41.
To obtain more information on the difference between the 2 methods of deflection checking, please go to
Help - Contents - Technical Reference - American Steel Design - Design Parameters (which comes after Allowables per AISC code)
It will bring up section "2.4 Design Parameters"
At the end of the parameters table, you will see several notes. Please read Notes items 1 through 4 for the description of the two methods.
As you can see there, the default condition, which is also represented by a value of zero for the CAN parameter, is to perform the LOCAL DEFLECTION check.
Your question indicates that what you are looking for is a check of the nodal deflections. The cantilever style check STAAD offers is probably the solution for your problem. If so, specify the CAN parameter with a value of 1.
THE VALUE OF E FOR MEMBER NNN DOES NOT SEEM RIGHT. What does this mean?
The steel design output for several members is accompanied by the following warning message :
WARNING : THE VALUE OF E FOR MEMBER 21 DOES NOT SEEM RIGHT.
WARNING : THE VALUE OF E FOR MEMBER 22 DOES NOT SEEM RIGHT.
WARNING : THE VALUE OF E FOR MEMBER 23 DOES NOT SEEM RIGHT.
During steel design, there is a check for ensuring that the Modulus of Elasticity (E) specified for the member is within the range that is normal for steel. This is because, E is a crucial term that appears in many equations for calculating section capacities and the program wants you to know if the value appears to be abnormal.
In STAAD, you specify E either explicitly under the CONSTANTS command block or through the DEFINE MATERIAL block, as in the examples below.
Example 1 :
UNIT KIP INCH
CONSTANTS
E 29000 ALL
DENSITY 0.283E-3 ALL
Example 2 :
UNIT METER KNS
DEFINE MATERIAL START
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
END DEFINE MATERIAL
CONSTANTS
MATERIAL STEEL MEMBER 101 TO 121
So, if you are specifying an E value which is significantly different from that for steel, such as say, Aluminum, and then later asking the member to be designed according to a steel code, as in the following example, the above-mentioned warning message will appear.
UNIT FEET POUND
DEFINE MATERIAL START
ISOTROPIC ALUMINUM
E 1.44e+009
POISSON 0.33
DENSITY 169.344
ALPHA 1.28e-005
DAMP 0.03
END DEFINE MATERIAL
CONSTANTS
MATERIAL ALUMINUM MEMBER 21 TO 30
..
..
PARAMETER
CODE AISC
CHECK CODE MEMBER 21 TO 30
The KL/r value that STAAD reports for the Y axis for a single angle does not match what I get from my hand calculation. Can you explain why?
For single angles, the local Y and Z axes are the principal axes as shown below:
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The KL/r value is computed using ry and rz which are based on the principal axis system. Chances are that your handculation uses the geometric axes.
I have a large model with several hundred members which have been assigned steel sections. I am doing a code check and I want to find out which of those members have failed. Can I get a list of just those members without having to scroll through hundreds of pages of steel design output?
There are 2 methods for finding just those members which have failed the steel design checks.
- From the Select menu, choose By Specification - All Failed beams. The members which fail the check will be highlighted. You can then isolate them into a New View to examine them in greater detail. Double click on those members or use Tools - Query - Member to access a dialog box with tabs called Steel Design and Design Property to see the cause of the failure along with allowable and actual stresses and critical conditions.
- In the Post processing mode, go to the Beam page along the left side of the screen. One of the sub-pages will be Unity Check. A table will appear along the right side of the screen. One of the tabs of that table is Failed Members. Select this tab, and click on each row of the table to look at each such member individually.
I am running STAAD.Pro 2003. In the TRACK 2 output for the American LRFD code, I find some terms that I am not familiar with. Can you tell me what those are?
The terms reported in the TRACK 2 output for American LRFD are :
AX = Cross section Area.
AY : Area used in computing shear stresses along local Y axis.
AZ : Area used in computing shear stresses along local Z axis.
PY : Plastic Section modulus about local Y axis.
PZ : Plastic Section modulus about local Z axis.
RY : Radius of gyration about local Y axis.
RZ : Radius of gyration about local Z axis.
PNC : Axial compression capacity.
pnc : Axial compressive force used in critical condition.
PNT : Axial tensile capacity.
pnt : Axial tensile force used in critical condition.
MNZ : Nominal bending capacity about local Z axis.
mnz : Bending moment about local Z axis, used in critical condition.
MNY : Nominal bending capacity about local Y axis.
mny : Bending moment about local Y axis, used in critical condition.
VN : Shear capacity.
vn : Shear force associated with critical load case and section location.
DFF : Permissible limit for checking length to deflection ratio.
dff : Actual length to deflection ratio.
I am running STAAD.Pro 2002. In the TRACK 2 output for the AISC ASD code, I find some terms that I am not familiar with. Can you tell me what those are?
The terms reported in the TRACK 2 output for AISC ASD are :
AX = Cross section Area
AY : Area used in computing shear stresses along local Y axis
AZ : Area used in computing shear stresses along local Z axis
SY : Elastic Section modulus about local Y axis
SZ : Elastic Section modulus about local Z axis
RY : Radius of gyration about local Y axis
RZ : Radius of gyration about local Z axis
FA : Allowable axial stress. If failure condition involves axial tension, this is the allowable axial tensile stress. If failure condition involves axial compression, this is the allowable axial compressive stress.
fa : Actual axial stress.
FCZ : Allowable bending compressive stress about local Z axis.
FTZ : Allowable bending tensile stress about local Z axis.
FCY : Allowable bending compressive stress about local Y axis
FTY : Allowable bending tensile stress about local Y axis.
fbz : Actual bending stress about local Z axis, used in the design condition
fby : Actual bending stress about local Y axis, used in the design condition.
FV : Allowable shear stress.
Fey : Euler stress for buckling about local Y axis.
Fez : Euler stress for buckling about local Z axis.
DFF : Permissible limit for checking length to deflection ratio.
dff : Actual length to deflection ratio.
I am using STAAD.Pro 2003 and I want to use physical members to do a steel design. I know how to manually create physical members by selecting the individual members, right-clicking the mouse and choosing Form Member. But if I have hundreds of these members, can I do it faster?
In STAAD.Pro 2003, you can use the Auto-Form member option to let the program automatically create physical members for you. From the Member Design page in the Steel Design Mode, go to Member Design | Physical Members | Auto Form Members. The rules it uses to create physical members are as follows:
- All elements must form a single continuous line. But they do not have to form a straight line. Thus curved members may be formed.
- There must be a free end. Whilst curved members are allowed, they cannot form a closed loop.
- All elements should have the same beta angle.
- All elements must point in the same direction. Check with the orientation labels if necessary. Use the reverse element command on elements that point the wrong way.
- None of the elements can be part of another member.
- The section properties must be consistent at each element end. Elements can taper along their length, but where one element ends and the next starts, they must have the same section reference.
- All elements must be made from the same material.
- Vertical segments are converted into columns first.
I want STAAD.Pro to perform a steel design based on the LRFD 3rd Ed rather than the 2nd Edition. The output always says "LRFD 1994". How do I tell it what code to use?
If you wish to use LRFD 3rd Edition Code, you can write CODE LRFD3 when providing the design parameters.
The 3rd edition of the American LRFD steel code has been implemented along with the 2nd edition. In general, the principles outlined in the code for design for axial tension, compression, flexure, shear etc., are quite similar to those in earlier versions of the code. The major differences are in the form of incorporation of the Young’s modulus of steel in the various equations for determining various limits like slenderness and capacities.
Consequently, the general procedure used in STAAD for design of steel members per the AISC-LRFD code has not changed significantly. Users may refer to Section 2 of the STAAD.Pro Technical Reference manual for these procedures.
Those who wish to use the 1994 edition of the code can still do so by specifying the code name as:
CODE LRFD2
An example of commands used for performing design based on the new and old codes are as shown.
Example for the LRFD-2001 code (3rd Ed)
UNIT KIP INCH
PARAMETER
CODE LRFD
or
CODE LRFD3
FYLD 50 ALL
UNT 72 MEMBER 1 TO 10
UNB 72 MEMB 1 TO 10
MAIN 1.0 MEMB 17 20
SELECT MEMB 30 TO 40
CHECK CODE MEMB 1 TO 30
Example for the LRFD-1994 code (2nd Ed)
UNIT KIP INCH
PARAMETER
CODE LRFD2
FYLD 50 ALL
UNT 72 MEMBER 1 TO 10
UNB 72 MEMB 1 TO 10
MAIN 1.0 MEMB 17 20
SELECT MEMB 30 TO 40
CHECK CODE MEMB 1 TO 30
I am not sure how STAAD deals with the specifications of the unsupported length for top flange compression.
For example, if I have a truss whose top chord is laterally supported at every other node (i.e. two member lengths being unsupported), then should I highlight every two members (of the top chord) seperately and then tell the program to take their combined length as being unsupported, or should I highlight the entire top chord and then specify the correct unsupported length.
The value you specify for UNL is what STAAD uses for the expression Lb which you will find in Chapter F of the AISC ASD & LRFD codes. Starting from Version 2001, UNL has been replaced with UNT and UNB for these codes. If the Lb value for the top flange is different from that for the bottom flange, you have to specify the corresponding values for UNT & UNB.
So if the bracing points are at every alternate node, first determine the distance between the alternate nodes. Then assign that value for both beams which exist between those nodes.
For example, if you have
Member 5 connected between nodes 10 and 11, and is 6.5 ft long
Member 6 connected between nodes 11 and 12, and is 7.3 ft long
and both the top and bottom flanges are braced at nodes 10 & 12, you can assign
UNIT FEET
PARAMETER
CODE AISC
UNT 13.8 MEMB 5 6
UNB 13.8 MEMB 5 6
To assign these parameters using the GUI, while in the Modelling mode, select the Design page from the left side of the screen. Make sure the focus is on the Steel sub-page. On the right side, select the proper code name from the list box on the top. Click on the Define Parameters button along the bottom right side. In the dialog box which comes up, select the tab for UNT and UNB, specify the value, and assign it to the appropriate members.
I would like to perform code checking on a 8" x 2 1/2" x 10 Gage channel per the AISI Coldformed steel code. But this channel is not listed in the sections available in your database. Can I assign it using a user provided table?
At present, sections whose data is specified using a "User Provided Table" (see section 5.19 of the Technical reference manual for details) cannot be designed or checked per the AISI code. However, the following approach may be used to get around this limitation.
You may add your section to the STAAD AISI section database, so that your section becomes a permanent part of the database. This can be done using the following method.
From the Tools menu, select Modify Section database. The various steel databases available in the program will be listed in a dialog box. You will find ColdFormed (US) at the end of this list. Expand this list, and choose Channel with Lips or Channel without Lips as the case may be. On the right half of the dialog box, the Add option will become activated. Select that, and you will now be provided with an interface through which you can add your channel to the list. Save and Close it.
You can now go to the Commands menu, and choose Member property - Steel Table - AISI Table to obtain visual confirmation that this new section is permanently included among the list of channel sections. You should now be able to assign this new section to the members through the usual property pages and menus.
Increasing the NSF value in Steel Design does not change the Failure Ratio for a member, Why?
In the design input parameters, I set NSF to .85 for my steel design. The design output result showed a failure ratio of 1.063 on Member 1. I then proceeded to change the NSF parameter to 1.0. This time, the design output result showed the same failure ratio of 1.063. It seems that nothing has changed. I increased the net section factor by 0.15, but the stress ratio hasn't changed?
The NSF value has an effect only on allowable axial tensile capacity, and the actual tensile stress.
If axial tension, or axial tension plus bending, are not what determine the critical condition, changing the value of NSF will not have any impact on the failure ratio. For example, if the critical failure condition for a member is compression, changing NSF will have no impact.
Check to see what the critical condition is. It will show up in the form of expressions such as:
AISC H1-1 or Slenderness, etc.
I ran my STAAD model and got an error message which stated that "This version does not design prismatic sections". What does this mean?
In the earlier versions of STAAD (STAAD-III), the code check for prismatic sections was done using allowable stresses which are arbitrarily chosen as 0.6 x Fy. However, this assumption of 0.6Fy was not based on any code specific requirements. The word PRISMATIC is meant to indicate a section of any arbitrary shape. But neither the AISC nor LRFD codes provide guidelines for design of arbitrary shapes. Section capacities are dependent upon aspects such as the width to thickness ratio of flanges and webs, lateral torsional buckling etc. From that standpoint, using an allowable stress of 0.6Fy for PRISMATIC sections was not always conservative.
A way around this limitation (lack of specific guidelines) would have been to use the rules of a known shape, such as a Wide Flange, for designing prismatic shapes. That would require knowledge of equivalent flange and web dimensions. When the properties are defined using the PRISMATIC option, there is no means to convey information such as dimensions of flanges or webs to the STAAD design facility. Hence, the design of PRISMATIC shapes is not supported in STAAD/Pro. You may get around this problem by defining the properties using the GENERAL section in a User Provided Table. For a GENERAL section, STAAD provides the means for providing dimensions of the components that are critical from the standpoint of computing allowable stresses. The allowable stresses for a GENERAL section are computed using the rules of a wide flange shape (I shape). As a result, the allowable stress value will be dependent on attributes such as dimensions of the cross section, length of the member, etc.
The KL/ry reported for a double angle does not match my hand calculations. I am designing the section per the AISC ASD 9th edition code.
For singly symmetric shapes such as Tees and Double Angles, the KL/r value for the Y axis is calculated by STAAD using the rules for flexural torsional buckling as explained in page 3-53 of the AISC ASD manual. It is not calculated as Ky multiplied by Ly divided by ry.
I am using the composite beam design capabilities. But the output does not show any evidence of this design. Why?
There are 2 sets of data associated with analysing and designing a composite beam.
Step 1 : Define the member properties as a composite beam. To do this, one has to use the "TA CM" option as explained in Section 5.20.1 of the STAAD.Pro Technical reference Manual. For example, if member 1 is a composite beam made up of a 3.0 inch thick slab on top of a W18X35, and the grade of concrete is 4.0ksi, one would have to specify
UNIT INCH KIP
MEMBER PROPERTIES
1 TA CM W18X35 CT 3.0 FC 4.0
Step 2 : Parameters for steel design. This is what you find in Section 2.9 of the STAAD.Pro Technical reference Manual. These are the attributes which are to be used in the actual design equations, using the expression PARAMETER, as in,
PARAMETER
CODE AISC
BEAM 1 ALL
TRACK 2 ALL
FYLD 50 ALL
CMP 1 ALL
DR1 0.3 ALL
WID 60 ALL
FPC 4 ALL
THK 4 ALL
SHR 0 ALL
DIA 0.75 ALL
HGT 4 ALL
RBH 2 ALL
CHECK CODE ALL
The most important thing to note here is the usage of the parameter CMP. Unless it is set to 1.0, STAAD does not design the beam as a composite section. The beam will be designed as a pure steel beam section in the absence of the "CMP 1" parameter.
How does one change the value of the yield strength of steel?
FYLD is one of the items specified as parameters for steel design. The STAAD Technical Reference manual and International Design Codes manual contain information on specifying parameters for steel design.
There are example problems in the STAAD Example manual demonstrating how parameters are specified for design. The example below shows some typical post-analysis commands.
PERFORM ANALYSIS PRINT STATICS CHECK
PRINT MEMBER FORCES LIST 5 7
PRINT ELEMENT STRESSES LIST 10 TO 16
UNIT KIP INCH
PARAMETERS
CODE AISC
UNT 1.0 ALL
UNB 20.0 ALL
LY 60 MEMBER 36 40
LZ 60 MEMBER 36 40
FYLD 46.0 MEMBER 47 50
CHECK CODE ALL
FINISH
If you prefer to use the graphical method, this is how you can specify it. From the left side of the screen, select the Design page. Make sure the sub-page says Steel. On the right hand side of the screen, go to the top, and choose the appropriate code.
Select the members on the structure for which you wish to assign the FYLD parameter.
Then, on the bottom right hand side of the screen, you will find a button called Define Parameters. Click on that button. Select the FYLD tab. Specify the value, and click on Assign.
In STAAD/Pro 2000 and STAAD.Pro, I no longer see the UNL parameter for the AISC ASD and LRFD codes. Instead, I see the parameters UNT and UNB. Why?
In versions of STAAD prior to STAAD/Pro 2000, the mechanism for specifying the unsupported length of the compression flange was through the means of the UNL parameter. However, the drawback of this command is that if the value for the top flange is different from that of the bottom flange, there wasn't any means to communicate that information to STAAD.
Consequently, 2 new commands were introduced, namely, UNT and UNB.
UNT stands for the unsupported length of the TOP flange of the member for calculating the capacity in bending compression and bending tension.
UNB stands for the unsupported length of the BOTTOM flange for calculating the capacity in bending compression and bending tension.
To avoid the confusion that may arise from having 3 separate parameters to specify 2 items of input, we no longer mention the UNL parameter. However, to enable the current versions of STAAD to analyze input files created using the older versions of STAAD, the UNL parameter continues to work the way it did.
These 2 new parameters are to be used in place of UNL. If UNT/UNB is specified in addition to UNL, UNL will be ignored. If neither UNT nor UNB are specified, but UNL is specified, the value of UNL will be used for both top and bottom flange.
The steel design output for a tube section checked per the AISC ASD code indicates an SY and SZ substantially different from the values which are reported in the AISC publication. Why?
In steel design per the AISC ASD code, the elements of the cross section (flange, web etc.) have to be put through some tests per Chapter B of the code. These tests are required to classify the cross section into one of 3 types - Compact, Non-compact, Slender.
If a section is classified as slender, the allowable stresses on the section have to be determined per the rules of Appendix B of the code. For slender "stiffened elements", which is the type a tube falls into, the effective section properties have to be calculated and those effective properties must then be used in computing the actual stresses.
The extent of the cross section deemed effective depends on the bending moment on that section. It is very likely that for the critical load case, the effective properties are less than the gross section properties, which is why you see the reduced Sz and Sy in the output.
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.
Utilizing DFF in STAAD only helps one check the local deflection. What if I want to check the drift of a column / beam frame?
If my joint displacement printout says that joint of a column/beam joint has moved 1.42 inch in the global X, then my drift ratio is 18x12/1.42 = 152.11, but the "dff" says 1072 for the same column, then where is the dff being measured?
When the DFF parameter is specified, the deflection checks during steel design are performed on the basis of so called "local axis deflection", not the nodal displacements in the global axis. For this reason, it is not possible to include storey drift checks into the steel design calculations at present.
If you want additional information on local axis deflection, please refer to example # 13, and Section 5.42 of the STAAD Technical Reference Manual.
Can I get STAAD to check deflection in both axes?
Yes. However, rather than check the deflection for each axis independently, STAAD finds the resultant deflection "d" and compares the "L/d" (length to deflection ratio) against the allowable limit specified by you through the DFF parameter.
Will STAAD explicitly state that the beam has passed the deflection criteria?
When STAAD performs steel design (code checking as well as member selection), it checks several conditions required by the code. The one which gives rise to the highest unity check is the one determined as critical. If the deflection criteria ends up being the worst condition, you will see it being reported as the critical condition.
You can verify whether a member has passed the deflection check by looking at the terms "DFF" and "dff" in the steel design output. "DFF" is the value you input. "dff" is the value the program calculates as the actual "L/d" ratio. If "dff" is larger than "DFF", the member is deemed safe for deflection.
What are the design parameters which control deflection check?
1) DFF : This is the value which indicates the allowable limit for L/d ratio. For example, if a user wishes to instruct the program that L/d cannot be smaller than 900, the DFF value should be specified as 900. The default value for DFF is 0. In other words, if this parameter is not specified as an input, a deflection check will not be performed.
2) DJ1 and DJ2 : These 2 quantities affect the "L" as well as the "d" in the calculated L/d ratio. They represent node numbers that form the basis for determining L and d.
By default, DJ1 and DJ2 are the start and end nodes of the member for which the design is being performed, and "L" is the length of the member, namely, the distance between DJ1 and DJ2. However, if that member is a component segment of a larger beam, and the user wishes to instruct STAAD that the end nodes of the larger beam are to be used in the evaluation of L/d, then he/she may input DJ1 and DJ2 as the end nodes of the larger beam. Also, the "d" in L/d is calculated as the maximum local displacement of the member between the points DJ1 and DJ2. The definition of local displacement is available in Section 5.42 of the STAADPro Technical Reference Manual, as well as in Example problem # 13 in the STAADPro Examples Manual.
A pictorial representation of DJ1 and DJ2, as well additional information on these topics is available under the "Notes" section following Table 2.1 in Section 2.8 of the STAADPro Technical Reference Manual.
If you use the design parameter TRACK 2.0, you will see a term called "dff" in the STAAD output file. This terms stands for the actual length to deflection ratio computed by STAAD. If "dff" is smaller than "DFF", it means the member has violated the safety requirement for deflection, and will be treated as having failed.
THIS VERSION DOES NOT DESIGN TAPERED POLE SECTIONS (MEMBER 1). What does this error message mean?
I am using tapered tubular section properties in my model. When I try to design those members using the AISC code.
The AISC code currently does not have the rules for designing tubular sections which are 6 sided, 8 sided, 12 sided, etc. That is why you cannot currently design them per the AISC code.
There is a code from ASCE called the ASCE publication # 72. That document contains the rules for designing these shapes. Those rules are implemented in STAAD's transmission tower code, and if you have purchased that code, you should be able to design them.
I am using STAAD to do steel design per the AISC code. For 2 members with similar cross sections, one passes, the other fails. Fact is, the one which fails has almost no load on it. The other is significantly more stressed but still passes. Is something wrong in the steel design calculations that STAAD is doing?
You will notice that, for the member which failed, the cause of the failure is reported using the phrase "L/R-EXCEEDS". This means that the member has failed the slenderness check.
When STAAD performs steel design on a member per the AISC code, it adopts the following sequence :
It first sets the allowable KL/r in compression to 200 and the allowable KL/r in tension to 300.
For the member being designed, it goes through all the active load cases to see if the member is subjected to axial compression and/or axial tension.
Next, it compares the actual KL/r against the allowable KL/r. If this check results in a FAILure, the member is declared as FAILed, and design for that member is immediately terminated. The requirement to check this condition is in Section B of the AISC specifications.
If the member passes the KL/r check, only then does the program go on to do the remainder of the checks such as axial compression + bending, shear, etc.
It must be noted that failure to satisfy the KL/r check is a reflection of the slenderness of the member, not the capacity of the section to carry the loads which act on it. Even if the axial load or bending moment acting on the member is a negligible quantity, the fact is, failure to satisfy KL/r will result in the member being declared as unsafe as per the code requirement.
If you do not want the KL/r condition to be checked, you can switch off that check using a parameter called MAIN. Set MAIN to 1.0 for a specific member and it won't be checked for slenderness. See Table 2.1 of the STAAD.Pro Technical Reference Manual for details.
What do the following parameters mean?
NSF 0.85 ALL
BEAM 1.0 ALL
KY 1.2 ALL
RATIO 0.9 ALL
LY 18 ALL
LZ 18 ALL
CHECK CODE ALL
NSF 0.85: This parameter is called Net Section Factor. One of the criteria used in determining the capacity of a section in Axial Tension is fracture of the net section. The capacity is calculated as NSF X Gross Area X Ultimate Tensile Strength of steel in tension
BEAM 1.0: This means the design or code checking of the member will be done by determining the safety of the member at a total of 13 points along the length of the member. Those 13 points are the start, the end, and 11 intermediate points along the length. If this parameter is not set, design will be performed by checking the safety at only those locations governed by the SECTION command.
KY 1.2: The KY value is used to determine the KL/r for the Y axis -
Ky multipled by Ly divided by Ry.
RATIO 0.9: The code requires one to check the safety of a member by verifying several interaction equations for compression, bending, tension, etc. The right hand side of these equations is usually 1.0. The RATIO parameter allows one to set the right hand side to the value of the RATIO parameter, in this case 0.9.
LY 18: The LY value is used in calculating the KL/r for the Y axis -
Ky multipled by Ly divided by Ry.
LZ 18: The LZ value is used in calculating the KL/r for the Z axis -
Kz multipled by Lz divided by Rz.
CHECK CODE ALL : For ALL members, the safety of the section is determined by evaluating the ratio of applied loading to section capacity as per the code requirements.
When one does the AISC code check or member selection, what are the calculations the program is performing?
The checks done as per the AISC ASD 9th edition code are :
- Slenderness - Checks for KL/r limits per Chapter B
- Local Buckling per Chapter B
- Axial Compression + Bending per Section H
- Axial Tension + Bending per Section H
- Shear per Section F
When I run code checking [as per BS5950] of the steel prismatic members which were defined in the User Provided Table, I get the following message in my output file:
CHECK CODE ALL
DESIGN NOT PERFORMED WITH PRISMATIC PROPERTIES
USER-TABLE MAY BE USED TO DESIGN PRISMATIC SECTIONS
The program is not designing the steel members defined as "Prismatic" in the UP Table, whereas all other members defined otherwise as Tee, Channel etc are being designed. Also I couldn't understand the meaning of the last line "User-Table may be used to design prismatic sections".
Since PRISMATIC sections by definition are those whose section shape is not one of the standard shapes like a W, C, Angle, etc., there are no readily available rules in the code to follow. Due to this reason, prismatic shapes are presently not designed per the BS code nor the ACI code.
You may get around this problem by defining the properties using the GENERAL section in a User Provided Table. For a GENERAL section, STAAD provides the means for providing dimensions of the components that are critical from the standpoint of computing allowable stresses, such as flange, web, etc. The allowable stresses for a GENERAL section are computed using the rules of a wide flange shape (I shape). As a result, the allowable stress value will be dependent on attributes such as dimensions of the cross section, length of the member, etc.
I am using STAAD to perform steel design on a member per the AISC ASD code. I want the column to be designed based on an unbraced length of 20 ft. I have set the UNT and UNB values to 20 ft, but STAAD appears to consider only a 10 feet length in its KL/r calculations. How do I correct this problem?
The parameters UNT and UNB are for specifying the unsupported length of the compression flange for the purpose of computing allowable stresses in bending compression.
If you want to specify the unbraced length for the purpose of computing allowable stresses in axial compression, use the parameters LY and LZ. See Table 2.1 of the STAAD.Pro Technical Reference Manual for details.
How do I get a design parameter, say the RATIO parameter, to be applied only to certain load cases?
You would need to use the "LOAD LIST" command. For example, if you only were interested in the 1st, 3rd and 5th load cases for the RATIO parameter you would need to write:
LOAD LIST 1 3 5
RATIO 0.5
In your input file.
I run the analysis of a 3-D bridge truss model and requested a CODE CHECK of the members. The results of this code check do not correspond to my hand calculation results.
The results of this code check show some very strange numbers in as far as code ratio using AISC- H1-1 formulation is concerned. Reference result output for members number 62 to 74 for example. Other ratios do not seem right either.
If you look at the AISC equation H1-1, you will find that there are 2 terms in the denominator, called
(1-fa/Fey)
and
(1-fa/Fez)
If the value of fa equals or exceeds Fey or Fez (Euler stresses), the respective terms become zero or negative, which is not a desirable event. In such a situation, STAAD replaces that negative number with the value 0.0001. The consequence of this is that, that part of the interaction equation becomes magnified by 10000, which will cause the overall value of the left hand side of equation H1-1 to increase significantly.
The above scenario is what occurs in the case of several of the members in the list 62 to 74. If you want to obtain proof of this, you may do the following. Change the value of the TRACK parameter from 1 to 2, and you will get a more detailed design output. That output will include the values of fa, Fey, Fez, etc.
To remedy the problem, you need to use a larger cross section so that "fa" becomes smaller, or use one with a smaller KL/r value so that Fey and/or Fez become larger.
What is the LX parameter used for?
The LX is the parameter used in calculating the axial compression capacity for flexural torsional buckling
The KL/r value that STAAD reports for a single angle member does not match my hand calculation. Design is per the AISC ASD 9th edition code.
A single angle is subjected to 2 buckling modes :
- Column buckling. This is determined using the simple expressions (Ky.Ly/ry) and (Kz.Lz/rz), where ry and rz are the radii of gyration about the principal axes.
- Flexural torsional buckling : This mode of buckling uses an equivalent KL/r, which is computed on the basis of equation (4-4) on page 5-311 of the AISC ASD 9th edition code. Generally, this mode of failure produces a higher KL/r than the ones from the column buckling mode.
You should check whether the flexural torsional buckling mode governs in your case. The KL/r calculated for the flexural torsional mode, if it happens to the largest of the 3 values, is reported only with a TRACK 1.0 detail of output. It does not get reported for TRACK 0 or TRACK 2 level of detail of output. In other words, if you want to see the KL/r in the flexural torsional buckling mode, use the parameter TRACK 1.0.
What are the SSY and SSZ parameters for AISC ASD based steel design?
SSY and SSZ are terms which dictate how sidesway criteria should be used in computing the Cm coefficients. For both of them, a value of 0.0 means sidesway is present for the corresponding axis, and, a value of 1.0 means sidesway is not present for the corresponding axis.
When SSY is set to 0.0, Cmy is set to 0.85 as per page 5-55 of AISC ASD.
When SSZ is set to 0.0, Cmz is set to 0.85 as per page 5-55 of AISC ASD.
When SSY is set to 1.0, Cmy is calculated as per the equations on page 5-55 of AISC ASD.
When SSZ is set to 1.0, Cmz is calculated as per the equations on page 5-55 of AISC ASD.
If the CMY parameter is specified (and the value is a valid one), that value is used, regardless of what the value of SSY is.
If the CMZ parameter specified (and the value is a valid one), that value is used, regardless of what the value of SSZ is.
The KL/ry reported for a T shape does not match my hand calculations. I am designing the section per the AISC ASD 9th edition code.
For singly symmetric shapes such as Tees and Double Angles, the KL/r value for the Y axis is calculated by STAAD using the rules for flexural torsional buckling as explained in page 3-53 of the AISC ASD manual. It is not calculated as Ky multiplied by Ly divided by ry.
Can you provide me with some help on how I can include deflection check as one of the criteria in steel design?
Deflection of a beam or a column can be included as one of the criteria during code checking or member selection with most steel design codes in
STAAD. The ratio of length to maximum deflection of a beam (L/d ratio) will be calculated by STAAD. STAAD will then check that quantity against the allowable limit which the user specifies under the PARAMETERS option.
What are the design parameters which control deflection check ?
1. DFF : This is the value which indicates the allowable limit for L/d ratio. For example, if a user wishes to instruct the program that L/d
cannot be smaller than 900, the DFF value should be specified as 900. The default value for DFF is 0. In other words, if this parameter is not
specified as an input, a deflection check will not be performed.
2. DJ1 and DJ2 : These 2 quantities affect the "L" as well as the "d" in the calculated L/d ratio. They represent node numbers that form the basis for determining L and d.
By default, DJ1 and DJ2 are the start and end nodes of the member for which the design is being performed, and "L" is the length of the member, namely, the distance between DJ1 and DJ2. However, if that member is a component segment of a larger beam, and the user wishes to instruct STAAD that the end nodes of the larger beam are to be used in the evaluation of L/d, then
he/she may input DJ1 and DJ2 as the end nodes of the larger beam. Also, the "d" in L/d is calculated as the maximum local displacement of the member between the points DJ1 and DJ2. The definition of local displacement is available in Section 5.42 of the STAADPro Technical Reference Manual, as well as in Example problem # 13 in the STAADPro Examples Manual.
A pictorial representation of DJ1 and DJ2, as well additional information on these topics is available under the "Notes" section following Table 2.1 in Section 2.8 of the STAADPro Technical Reference Manual.
What are the results one gets from STAAD for the deflection check?
If the steel design parameter called TRACK is set to 2.0, the L/d ratio calculated for the member can be obtained in the STAAD output file. The value is reported against the term "dff". Notice that the expression is in lower-case letters as opposed to the upper-case "DFF" which stands for the allowable L/d.
If "dff" is smaller than "DFF", that means that the displacements exceeds the allowable limit, and that leads to the unity check exceeding 1.0. This is usually a cause for failure, unless the RATIO parameter is set to a value higher than 1.0. If "DFF" divided by "dff" exceeds the value of the parameter RATIO, the member is assumed to have failed the deflection check.
What are the limitations of this check?
Since the "d" in L/d is the local deflection, this approach is not applicable in the case of a member which deflects like a cantilever beam.
That is because, the maximum deflection in a cantilever beam is the absolute quantity at the free end, rather than the local deflection. Check whether STAAD offers a parameter called CAN for the code that you are designing to. If it is available, set CAN to 1 for a cantilever style deflection check.
Since the deflection which is checked is a span deflection and not a node displacement, the check is also not useful if the user wishes to limit story drift on a structure.
In the output for steel design, what does the term "dff" represent?
"dff" is the value of actual length divided by local deflection. The actual length value is the distance between the nodes DJ1 and DJ2 which default to the actual end nodes of the member. The deflection used is the maximum local deflection between the points DJ1 and DJ2. You can get the Max. Local Displacement value by looking at the output of the PRINT SECTION DISPLACEMENT command. The definition of DFF, DJ1 and DJ2 may be found in Table 2.1 of the Technical Reference Manual for STAAD/Pro.The word PRISMATIC is meant to indicate a section of any arbitrary shape. But the AISC code does not provide guidelines for design of arbitrary shapes.
Section capacities are dependent upon aspects such as the width to thickness ratio of flanges and webs, lateral torsional buckling etc. From that standpoint, using an allowable stress of 0.6Fy for PRISMATIC sections was not always conservative.
In STAAD-III, I was able to get a steel design for members defined using the PRISMATIC property attribute per the AISC ASD code. I cannot do this in STAAD/Pro. Why?
In the earlier versions of STAAD, the code check for prismatic sections was done using allowable stresses which are arbitrarily chosen as 0.6 Fy. However, this assumption of 0.6Fy was not based on any code specific requirements.
The word PRISMATIC is meant to indicate a section of any arbitrary shape. But the AISC code does not provide guidelines for design of arbitrary shapes. Section capacities are dependent upon aspects such as the width to thickness ratio of flanges and webs, lateral torsional buckling etc. From that standpoint, using an allowable stress of 0.6Fy for PRISMATIC sections was not always conservative.
A way around this limitation (lack of specific guidelines) would have been to use the rules of a known shape, such as a Wide Flange, for designing prismatic shapes. That would require knowledge of equivalent flange and web dimensions. When the properties are defined using the PRISMATIC option, there is no means to convey information such as dimensions of flanges or webs to the STAAD design facility. Hence, the design of PRISMATIC shapes is not supported in STAAD/Pro.
You may get around this problem by defining the properties using the GENERAL section in a User Provided Table. For a GENERAL section, STAAD provides the means for providing dimensions of the components that are critical from the standpoint of computing allowable stresses. The allowable stresses for a GENERAL section are computed using the rules of a wide flange shape (I shape). As a result, the allowable stress value will be dependent on attributes such as dimensions of the cross section, length of the member, etc.
In the context of design, what is meant by the term Ratio?
In steel design, the Pass/Fail status of a member is determined according to various conditions. According to most design codes, the member has to be checked for failure against axial compression and axial tension, slenderness, compressive & tensile stresses caused by axial compressive force + bending moments, failure caused by shear stresses, etc. For each of these conditions, determination of whether the member is safe or unsafe is done by checking whether the actual values due to the loading exceed or are less than the allowable values. The amount by which the member is stressed for each of these conditions is quantified in the form of the Ratio. For example, take the case of equation H1-1 of Section H of the AISC-89 specifications. The number obtained by computing the left hand side of that equation is the Ratio corresponding to that equation.
I have multiple sets of design in the same STAAD file and I am only able to see the results for the final set in the Postprocessing mode (GUI). How can I view the results for all design sets in the GUI ?
The postprocessing Beam >Unity Check page can report the design results only for the final set of design. This is a limitation in STAAD.Pro as the program architecture does not allow that results of multiple design sets to be made available at the same time graphically. The analysis output file is the only place where you can view results for all design sets. The only way to view the results of a previous design cycle graphically is
- to go to the editor and comment out the subsequent design sets and rerun the analysis
- reverse the order for the design data blocks so that the set, for which the GUI data is needed, becomes the last set.
HowistheshearstresscalculatedinSTAAD.ProforAISCdesigncode?
TheshearstresscalculatedbySTAADisthemaximumshearstressbydefaultwhichisbasedonthestandardformulaVQ/Ib,where
V=Shearforce
Q=Momentofareaofthepartofthecrosssectionthat
isabove(orbelow)theplanewhereshearstressisbeingcalculated,about
theneutralaxis
I=MomentofInertia
b=Widthofthesectionattheplanewherethestressis
beingcalculated
SothetermIb/Qisreportedastheshearareathatcorrespondstothisshearstresscalculation.
IfrequiredonecangetSTAADtocalculatetheaverageshearstressinsteadofthemaximum.ThereisaSHEdesignparameterthatcanbeusedtoinfluencehowSTAADcalculatestheshearstress.Whentheparameterissetto0(default),stressiscalculatedasmentionedabove.Howeverwhenthisparameterissetto1,averageshearstresswillbecalculatedbasedontheformulaV/Ay(orAz)whereAyorAzaretheshearareaforthecrosssection.
See Also
Product TechNotes and FAQs
Structural Product TechNotes And FAQs
External Links
Bentley Technical Support KnowledgeBase
Bentley LEARN Server
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