A little Gap between STAAD Analysis and AISC ASD

Hi there guys

I've been calculating with STAAD Pro. & by myself and I realized that there are a little difference between two answers.

When you analyze steel structure, usually you apply the AISC Code (ASD). I did two experiment, first I used STAAD Pro. analysing stress on brace which is a member of trussed structure, and then I calculated by myself with the Code book. In this case, according to the AISC, I could get the allowable stress data by formula like 0.45*Fy. but after I used STAAD I was suprised that the answer was not same as my own. I assume that it was because I didnt set up the RATIO in parameter option. After I set up RATIO data 0.45, I finally got the right answer.

Here's my question : Does STAAD Pro. get the AISC ASD Code right in case like this? Or do I have to set some kinda RATIO in parameter section to get the right answer? It seems like I need to set up RATIO.. but I would like to hear from you guys.

 

Thanks to all,

Heying

  • Without more details, it is diffcult to explain why there is difference. Such as, what is the cross section (Wide flange, channel, angle, etc.) What is the member length? What are the design parameters - LY, LZ, FYLD, etc. What are values of the member end and intermediate section forces - axial, bending moments, etc.

    The RATIO parameter is needed only if you want to change the right hand side of the interaction equations of the code in question (such as H1-1, H1-2, H1-3, etc. in Chapter H of the AISC 9th edition specifications) from 1 to something else, such as say 1.33 or 0.8 for example.

    If you can post your hand calculations and STAAD model and state the member number and load case for which you find a mismatch, we can take a look. A better option is to contact our technical support group (selectservices.bentley.com)



  • Dear Kris,

     

    Thanks for your replying and I'm attaching my calculation&STAAD Pro. file.

    Material is Steel and I just set up FYLD and UNB, UNT. It is just a experimental model. I checked the brace(#42) about load case 2,  and the stress data is 280.700, 269.559, 258.419, 247.279, 236.139 at each distance from the start node. but After I calculated with AISC code the answer was 111.69585N/mm2, from [250N/mm2*0.45] (250N/mm2 is yield strength of steel) The formula I used is 0.45*Fy, which is for pin-connected members.. and I found the corner 1,2,3 on the stress table, what do they mean?

    I checked the default ratio is 1.0, and I thought I need to change it for each member, like ratio for column member and for beam would not have same ratio.

     

    Also I'm wondering if STAAD Pro. recognizes by itself which one is compression member or tension member. For example, when I modeled a Column, Does STAAD anlyze the member as a compression member? By that means STAAD Pro has its own algorythm about discriminate the members..

    Here I attach the STAAD file.

     

    Thanks a lot,

    Heying

    3D_2Bay_Frame-r2.std
  • Heying,

    This is technically too detailed to discuss in this forum. I am providing you with some suggestions at this time. But if you have further questions, it would be best to contact our technical support group.

    Though all the input can be created using the STAAD GUI, it is also worth knowing the commands to ensure that the data written into the STAAD input file doesn't contain any errors.

    To implement the suggestions described below, use the STAAD editor.

    1. Under load case 1, you have

    SELFWEIGHT Y -1 LIST 1 TO 6 11 TO 13

    Your structure has 41 members and only 9 of them appear in this list. If you want the weight of all members considered, change it to

    SELFWEIGHT Y -1

    When no list is provided, it defaults to ALL

    2. Lines 63 to 65 are

    PARAMETER 1

    CODE AISC

    CHECK CODE ALL

    Following this, you have

    PARAMETER 2

    CODE AISC

    UNB 6 MEMB 11 TO 13 24 TO 27 52 53

    UNT 6 MEMB 11 TO 13 24 TO 27 52 53

    UNB 10 MEMB 28 TO 39

    RATIO 0.45 MEMB 40 TO 51 54 55

    FINISH

    It means that the code check (line 65) is done before the parameters UNB, UNT and RATIO are read. You have to reverse the sequence as described below.

    Also, why are you assigning RATIO as 0.45? As I said last time, RATIO is used to modify the right hand side of the interaction equations. It is generally not done except for increasing the allowables by 33% for wind and seismic as per the AISC code. The other reason for using it is to do a member selection with some reserve margin of capacity to offset any potential increase in stresses. If it is left out, the equations will be checked against an allowable value of 1.0.

    So, you need to change lines 63 to 71 to

    PARAMETER 1

    CODE AISC

    UNB 6 MEMB 11 TO 13 24 TO 27 52 53

    UNT 6 MEMB 11 TO 13 24 TO 27 52 53

    UNB 10 MEMB 28 TO 39

    TRACK 2 ALL

    CHECK CODE ALL

    FINISH

    Save the file and run the analysis. Go to the output file. You will find that member 42 fails in slenderness with a ratio of 1.159. The type of axial force associated with the failure is compressive. Slenderness failure means that the maximum KL/r (in this case, KL/rz is equal to 231.88) exceeds the allowable of 200 for compression.

    If you want to instruct the program to skip the slenderness check, add the line

    MAIN 1.0 MEMBER 42

    just before the CHECK CODE ALL statement. Run the analysis again and view the output file. You will then run into another problem due to the axial compressive stress exceeding the Euler stress as per the code. The only solution for that is to use a larger section.

    One other thing. You have specified a command LOAD LIST 1 to 6 just prior to the PARAMETER command. This tells the program to design the members for all 6 cases. Normally, only the combination cases are checked. If you want to do this, you need to change it to

    LOAD LIST 5 6



  • Also, the answers to your questions are :

    Your question:

    Also I'm wondering if STAAD Pro. recognizes by itself which one is compression member or tension member.

    Answer :

    Yes. It is based on the nature of the axial force in the member for the load case being checked.

    Your question:

    For example, when I modeled a Column, Does STAAD anlyze the member as a compression member? By that means STAAD Pro has its own algorythm about discriminate the members..

    Answer :

    A member may be in axial compression for some load cases and axial tension in others. Depending on that, the rules used in the design may vary from load case to load case. Those rules are laid out in the design code under chapters like D, E, F, etc. for AISC 9th edition. Finally, the rules of Chapter H - called interaction equations - are used to determine the interaction ratio and thus, see if the member passes or fails.



  • Thanks Kris,

    it helped me a lot. really appreciate it.

    have a great day!