I am analyzing a a frame of primarily individually defined round HSS sections, sections are defined as below.
When the frame is post-processed with the AISC 360-10 code Method ASD, it appears that even when the code states TORSION 0, the program still runs a torsion check for sections that were individually defined round HSS (though the standard Pipe sections did not have a torsion check completed). My understanding is that no torsion checks should be completed when the torsion parameter is set to 0 - is this wrong?
Additionally, the results for the torsion checks are wildly (and exponentially) larger than expected for members that had been individually specified (refer to image of Design Output below for a 356mm diameter hollow section with 35mm wall thickness). The other check outputs for bending, axial and shear are as expected in terms of ratio outputs, very low. However, the allowable Torsion Capacity is much lower than expected with 6.52e-03 kNm for the size of section, which leads to a ratio of 7.245 for torsion, this is then amplified using the combined ratio H3 clause and results in an even higher UR value of approx 57.
It appears that STAAD is using an odd torsional J value or Ix in calculating the Torsional Capacity for specified round HSS sections. Can this value be amended? Or is there a way to get STAAD to ignore the Torsion checks for specified round HSS?
For HSS sections, torsion check is automatically carried out. The design parameter TORSION only affects the torsion check for non-HSS sections. Regarding the second part of your question, the HSS torsional constant ( C ) is considered as per the values provided in AISC section tables. You may check the values by going to Tools > Section Database > Rolled Steel and choosing the appropriate section table ( like HSS Round) from within the AISC database (v14.1). Editing the value for the parameter C is possible too using the Edit Record option but ensure that you are using the correct value.
How about round HSS which user defined as tapered tube, how the torsional constant (C) was calculated? AISC 360 have a user note to calculate this but i am not sure that it is used in Staad.Pro? I think Staad.Pro probably used incorrect torsional constant for the user defined round HSS and this is the reason why the utilization ratio was so high.