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,
When you have a two bay braced frame with tension-only members under lateral load some of the braces drop out as they go into compression and the end result is asymmetric deflection and the brace on the leading end takes a larger force.
Showing the deflected shape with consideration for both vertical and lateral deflections helps to explain why. Here's an example:
Seth,
I removed the podium level and below to test your theory. I did not change anything else. At bottom level, one of the braces takes all the load, while the other one has zero axial load. see below. under load case +y (wind), I get a negative shear at level 2 brace. See image below.
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.
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