Assigning Diaphragm

[abdulqadeer29]

While purpose of assigning diaphragm is to transfer inertial forces during earthquake to vertical members . There is general practice which I am following for quite a long time,  that we most of the time don't consider inclined members like stair and ramp. i model them as horizontal slab.

I have recently observed that there is a difference in the result while considering them as inclined members.

My question is do we have to assign diaphragm to these inclined members (ramp or stair case)?

[Badar (BAZ)]

These members act as tension, or compression members; they act like a brace. I think, they shouldn't be assigned as diaphragm.

[abdulqadeer29]

These members act as tension, or compression members; they act like a brace. I think, they shouldn't be assigned as diaphragm.

when we model stair and ramp in Etabs, this increases the stiffness of the structure and also play a critical role in resisting torsion of the building.Should we proceed in this way??

or we take a slab instead of stair and control torsion of the building by adding shear walls??

[UmarMakhzumi]

You should always consider stairs and ramps in your structural model. There is also a discussion about it in Pauley and Priestley if I remember correctly. Also please make sure you design your stairs for the load they are attracting.

 

Thanks.

[Muneeb Badar]

Dear Abdul Qadeer,

 

There are two points

 

1- You can add the stair in the structure and ramps in the model just to have a bracing in the members like column and retaining walls. This will provide you stiffness as well as the effective length of the other members reduced which is the actual and practical situation.

 

As far as the addition of stairs and ramps for diaphragm concerned, you can not add these in the diaphragm. Basically as we all know diaphragm transfers the lateral forces to vertical members. The diaphragm design is basically the design of SLAB for inplane forces. If you have not design the slab for these inplane forces then you can not assume semi rigid or rigid diaphragm (In case of minor earthquake areas, normally the existing slab thickness will be enough and also not required by code to design diaphragm for minor earthquake areas). So diaphragm assumption will be valid if you provide a path to transfer the forces from Retaining wall to shearwall through slab at basement levels and from shear wall to slab at top levels. If there is an opening or there is a stair or ramp then you do not have any path to transfer the forces to the shearwall and you can not provide the reinforcement for inplane forces. In this case it is not good enough to model these ramps and consider them these are transferring the forces.

 

Thanks

 

Muneeb