Application of overstrength factor in Buildings in SDC B

[Yousef ZAA]

Good day Engineers,
For Buildings with SDC B that have vertical irregularity of type 4 (Discontinuous Shear Walls-Having Shear walls supported on beams and columns), Table 12-3-2 in ASCE7-16 refers to section 12.3.3.3 which says that elements supporting discontinuous walls should be designed with an overstrength factor, reading this and knowing that the diaphragm have shear forces dumped on it from the discontinuous walls, does that mean that the diaphragm should be designed with an overstrength factor? when the code says (element supporting discontinuous walls) does that mean only the gravity supporting system, i,e: beams and columns only or the diaphragm is under that terminology? Reading C12.3.3.3 the commentary in ASCE7-16 t clearly says that the purpose behind the overstrength here is to protect the GRAVITY LOAD CARRYING SYSTEM. I know that the overstrength factor has to do with forcing the vertical lateral resisting system to be ductile and dissipate energy while the diaphragm is elastic, but the way I'm reading the code this action is mandatory with SDC C, D, E & F, not necessarily in SDC B!!

 

[driftLimiter]

This type of load to the diaphragm is referred to as a transfer load.

Review ASCE 7-16 Section 12.10.1.1
Here the required forces for the diaphragm are presented. There is a paragraph near the end of this section that describes what to do with transfer forces. The gist of it is that yes you need to use overstrength for the in plane shear as well.

You are correct that the overstrength factor is designed to force the lateral system into ductile response, but yielding in the diaphragm is not the preferred failure mode in most applications, so this provision is designed to provide adequate diaphragm strength.

You will have to verify for yourself, but my read of this suggests these requirements apply to most structures in SDC B as well.

 

[SandwichEngine]

I'm not aware of anything in the code saying that you don't have to use the seismic overstrength factor for every building, no matter the seismic design category. Check out ASCE 7 table 12.2-1. Covers lower SDC buildings and doesn't have any language that precludes the overstrength factor in lower SDC jobs.

However in practice, seismic loads will rarely control over wind loads before you get to SDC D, which may be why some don't bother.

Understanding the seismic overstrength factor was something I researched a lot several years ago. In my understanding, it is the writers of the code trying to codify a concept that while simple, isn't easy to codify. That system is called capacity design. The idea is sort of like the fuse box in your car. No matter what you do with any electrical system the smallest portion of that circuit has to be the fuse. Likewise in a building, the idea is to create a fuse element that will fail in a ductile fashion, and make sure it is the weak link in the structural system. Pretty easy to say but hard to write equations that capture the idea. That's where the seismic overstrength factor comes in. It's trying to use a formula to make sure you beef up places that would have a tendency to fail in a brittle fashion, such as brace connections.

I find that thinking about the previous concept helps me understand items in the code. For the car circuit example, imagine there were a bunch of specifications about the particular electrical component you're designing. One part says that your circuitry must have a minimum size that's way larger than what it needs. If you're just looking at that one code point, it can be hard or impossible to understand. If you're thinking about the fact that you're trying to force the fuse to fail, not something deep in the dash board, the requirement makes sense, you interpret it correctly instead of looking for an exception, and you move on.