Use member area loads to apply your diaphragm loading.... That doesn't attempt to model the diaphragm at all (so you don't get any stiffness), but you will get load distribution via trib area.

Actually I think the issue is with me, I'm still in a hand calculation mind set... Is there really any need for a "flexible diaphragm" if one is using software? Couldn't the diaphragm just be modeled with a plate using an appropriate in plane stiffness to account for nail slip etc? In which case the diaphragm would behave like a flexible diaphragm to the same extent that the real diaphragm would? The concept of a flexible diaphram is just an approximation of reality anyhow. Perhaps I have made a mountain out of a mole hill...

Using plates in RISA 3D to model a diaphragm messes up your beam designs and behavior.

You can use plate elements to model the diaphragm. But, at that point it would be a semi-rigid diaprhagm (or semi-flexible). The key points to be aware of would be the following:

1) You will have to adequately mesh the plate elements in order to get accurate results. Therefore, you will be spending a lot more time modeling the structure than if you ignored the diaphragm completely.

2) If you don't want the diaphragm to affect the plate elements, then you should make the plates weightless (i.e. set the density of the material to zero) and use the option for "plane stress" plates.... Though this may not be available on older versions of the program.

So if I just get RISA-Floor, will life be good in terms of flexible diaphragms?

Josh, thank you. The semi-flexible diaphragm modeled with plate elements seems to work just fine when the "plane stress" option is used. I did not get too deep into it but the semi-flexible VS flexible issue seems inconsequential when the plate parameters represent diaphragm materials that would typically justify a flexible diaphragm assumption in the first place. Modeling the lateral forces with no diaphragm at all in this case just does not work. It does take a bit more time to model and solve, if RISA-Floor makes it painless I'm probably just going to get on board to speed things up.

RISAFloor should make it easier, but I wouldn't call it painless.

With some of the new analysis requirements (esp AISC's Direct Analysis Method), flexible diaphragms can lead to false stability issues (because the diaprhagm stiffness isn't modeled). That doesn't happen with the semi-rigid (or semi-flexible) diaphragm modeling using plate elements. But, it can happen whenever you use the idealized "flexible" diaprhagm assumptions... even when that diaphragm came from Floor.

JAE,

RISA 3D now has the option to disconnect the diaphragm for no composite action.