Mechanics of a Concrete Coupling Beam

[pittguy12]

Studying for the lateral portion of the SE exam and I am working through the concrete coupling beam portion of ACI 318-08.

Can someone give me a good description of how the mechanics of this system are supposed to work? I do not typically do a lot of multi-story building design and so I have not run into this yet.

I gather that these are used to bridge between to segments of a concrete shear wall in order to increase its overall lateral capacity. I would suppose then that the design shear strength Vn referenced in eq 21-9 of the code is referring to the vertical shear that would be induced in the beam at each end from a horizontal lateral force acting on the adjacent walls. Is this assumption correct? Beyond this, does the design of these elements then come down to proper detailing as specified in the rest of section 21.9.7?

Thank you...

 

[jdgengineer]

You are basically on the right track. The purpose of the coupling beams is to couple two wall piers together to act as one coherent piece. This will significantly increase the stiffness of the wall and help with energy dissipation in an earthquake. In order to tie the two pieces together, there are typically very large forces on the coupling beams and the coupling beams need to resist these forces in a ductile manner.

For the sake of the argument, a coupling beam is intended to be designed so that flexural failure governs, as flexural failures are much more ductile than shear failures, and therefore will provide the building extra resiliency in the event of a large earthquake. If a conventional coupling beam was used (Ln/h >= 4) the coupling beam would be designed similar to a moment frame beam in the sense that the shear capacity of the member would need to be larger than the moment capacity of the member. Therefore the design shear force would be 2 Mp / L + Vg.

When a diagonally reinforced coupling beam is used, the diagonal reinforcing is sized so that flexural failure will govern. See attached for derivation...

The coupling beam will then be detailed so that that compression reinforcing doesn't buckle, and that the concrete remains confined.

Hope that helps... (although I'm no expert on coupling beams)