Rigid and Semi-rigid diaphragm in ETABS - Slab with beams

[Mina Kromer]

Dear Sir,

I noticed when applying a semi-rigid diaphragm to a slab with beams floor I got axial load in beams and slab from the vertical loading. But this axial force disappeared when I applied rigid diaphragm.

We used to ignore axial load in slabs and beams from vertical load even in manual analysis and design.

I am asking, what is the right, to ignore it or design for it, in case I needed to apply the semi-rigid diaphragm type ?

Another question, where these axial forces in slab and beams move when applying the rigid diaphragm to the slab, do it transfer to another type of stresses, for example, does it make an increase in moments and shear or what ?

I appreciate your reply.

Thank you

 

[KootK]

1) First and foremost, understand the cause of the anomaly that you're seeing before proceeding.

2) Can you tell if the axial forces are arising because software is treating the beams as flexurally composite with the slab?

3) If the beams are being treated as flexurally composite with the slab then:


a) The axial arising from composite can be ignored if you don't want to detail and design the beams to be composte.

Another question, where these axial forces in slab and beams move when applying the rigid diaphragm to the slab, do it transfer to another type of stresses, for example, does it make an increase in moments and shear or what ?

b) The flexurally induced axial forces in the tops of your beams (steel?) would migrate down into the top flanges and operate at reduced lever arm. For the same steel beam, this would increase deflection and flexural stress in the beam. It would theoretically increase beam shear stress as well but that likely doesn't matter as we don't usually count on the concrete to resist vertical shear when we design composite beams anyhow.