ASCE7-16, 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!!

 

[RobertHale]

In this case, I think the overstrength factor only applies to the supporting beams and columns. Presumably for this irregularity, you have a strut element to transfer the horizontal seismic force but the requirements of 12.10 do not require the overstrength factor on this element for this SDC. This requirement is less about ductility and energy dissipation and more about making sure you don't "kneecap" your lateral load resisting system. If the supporting elements fail then you compromise everything above.

Robert Hale, PE, SE