I am modelling a multi storey building with 4 composite floors and I have encountered the following problems:
- using a composite deck I get VERY large deflections, of approximately 700mm, this makes me think it is not working as a Diaphragm. How can I resolve this?
- using diaphragm mode in STAAD, I get no axial forces in my steel beams, especially in those in the brace bays, this makes me think all the forces are absorbed by the more stiff concrete and none are transferred into the steel. This would be ok if I could export the forces in the diaphragm and use it to determine the forces in the steel. However, so far I haven't been able to find a way to get any output from the diaphragm.
What am I doing wrong? Any help would be greatly appreciated.
Agata
A composite deck is not equivalent to defining a rigid diaphragm. It assigns the composite properties to the beams based on the effective width of the slab. However it does not make the floor completely rigid. It also does not account for the torsion that a rigid diaphragm would be subjected to due to the eccentricity between CM and CR. So you should always use the floor diaphragm option which includes these effects in the analysis. You would not get axial forces in the members because the whole diaphragm is idealized as rigid ( for translation in X, translation in Z and rotation about Y ) and there is no relative movement in-plane between the members. The rigid diaphragm also ensures that the lateral shears are transferred to the lateral force resisting systems in proportion to their relative stiffness.