STAAD member load definition error

I have been using STAAD Pro V8i (release 20.07.05.15) for multiple light structure analyses, and I have recently noticed an inconsistency in the software with regards to member load definition.

It is apparent that my version of STAAD is performing calculations backwards for uniform member loads defined in local member coordinates, and uniform member loads defined in global coordinates. See the attached screenshots and .std files for example. The example building is 30.35m wide.

Using loads defined in global coordinates (test.std), the displayed load direction is correct, but the calculated vertical reactions are incorrect (simple hand calculation says (30.35m) * (5.16 kN/m) / (2) = ~78.3 kN)

If I define the loads in local member coordinates (test2.std), displayed load directions are again correct, but the calculated reactions appear to be calculated as though the load was defined in global coordinates (they agree with the hand calculation above).

I am currently downloading the most recent version of STAAD to see if the problem will be fixed.  I'm really just looking for someone more experienced to validate these findings.  If I am correct, then I have many analyses to review/fix, and perhaps Bentley can fix the bug. 

UPDATE:  I get the same, seemingly incorrect results, in the newest version of STAAD.  Any ideas?  Please let me know if I can clarify anything with the problem I'm having.

Test2.std
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  • Based on your Load Case names, it appears that you are trying to represent snow load.  Snow load is defined to act on the horizontal projection of sloping roofs, so STAAD.Pro provides the projected direction options to handle the trig automatically for you.  The correct option to choose for your example is PY.

    In your model named Test.std, you used the GY option, which means that the load is aimed in the Global Y direction, but it is based on the slope length of the member, rather than the projected length.  That is why you were not able to match the STAAD reaction with your hand calc...the hand calc was assuming the projected length, but STAAD was basing the calc on the slope length.

    Your model named Test2.std would actually be a step in the wrong direction as far a s snow loads are concerned, because it is directing the snow load perpendicular to the top chord members, which is not correct.  This would unfairly reduce the predicted thrust at the bottom of the arch, and would not be the correct way to represent snow.

    If you revise the direction to PY in either of your models, you will match your hand calc exactly.

    Cheers,

    Chris

     

     



Reply
  • Based on your Load Case names, it appears that you are trying to represent snow load.  Snow load is defined to act on the horizontal projection of sloping roofs, so STAAD.Pro provides the projected direction options to handle the trig automatically for you.  The correct option to choose for your example is PY.

    In your model named Test.std, you used the GY option, which means that the load is aimed in the Global Y direction, but it is based on the slope length of the member, rather than the projected length.  That is why you were not able to match the STAAD reaction with your hand calc...the hand calc was assuming the projected length, but STAAD was basing the calc on the slope length.

    Your model named Test2.std would actually be a step in the wrong direction as far a s snow loads are concerned, because it is directing the snow load perpendicular to the top chord members, which is not correct.  This would unfairly reduce the predicted thrust at the bottom of the arch, and would not be the correct way to represent snow.

    If you revise the direction to PY in either of your models, you will match your hand calc exactly.

    Cheers,

    Chris

     

     



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