Hello,
I have several double bay braced frames in the building. The diagonals are 16-ga and 18-ga CFS straps (tension only members), while the end posts are HSS 6x6. I did not model a collector beam as I have a rigid diaphragm which will be provided in real. I would expect the axial load in the two braces to be same (or almost the same), however, in several instances the loads are off by 10-20%. The frames are 9 stories tall and differences go away the higher up the building we go, in other words, magnitude of axial loads converge. Any ideas why this would be the case?
Thank you,
Is this the axial load from a lateral load case? If the frame is symmetric (same dimensions and section sizes) and the diaphragm rigid, then I would expect the results to be symmetric as well.
Note, most users would go ahead and model the beam as lateral here, it makes it easier to do things like pull up an elevation view.
The load is axial load in the brace due to wind load. The frame is symmetric and the diaphragm is rigid.
I would expect the loads to be the same as well, that is why I am not 100% sure of the results.
Do you have any suggestions on how to proceed?
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I see, your picture does not include the structure below the frames (podium). Since the frame sets upon a slab the relative rigidity of the two points influences the forces in the braces above. In other words, the diaphragm constrains the nodes to move together horizontally, but not vertically, and if the vertical deflections vary then the brace strains are different, hence the stress and force too.