Diaphragm using plate modeling in RISA 3D without RISA Floor

[acwooten]

Hello,

I am working on a multi-story, all concrete structure that I've modeled using plates in RISA 3D. I'm at the point where I need to apply my lateral loading and I'm unsure of the best approach to capture the diaphragm action of the concrete floors. Specifically for seismic, I know I can determine the total base shear and use ASCE 7-10, 12.8.3 to distribute the seismic loads to each floor based on that floors tributary height, but that would take awhile. Is there a good way to let RISA apply those loads? A co-worker suggested taking all basic load cases that contribute to the seismic weight and just multiplying the Cs value by their weight and applying it in the X and Z directions. But it seems like that would not distribute to the diaphragms correctly.

Thanks in advance!

 

[JoshPlumSE]

It really shouldn't take too long to calculate the story shears at each level yourself. Honestly, it's a good idea to do this manually even if the program is capable of doing it. Just to check that the program numbers look correct.

If you want to use the program to "test" your hand calc, then you could temporarily add in rigid diaphragms so that RISA-3D will do the calculation itself. That will just give you the shear for each story level. Just be sure to save your model BEFORE you do this so that you don't have extra diaphragms or loads floating around.

My tendency is to suggest that each story shear be converted into a plate surface load in the lateral direction (Story shear / area of plates at that level), then apply this individually for each floor. It may not be perfect, but it should be pretty good.