Overstrength Factor for Wood Shear Walls & Diaphragms

[vcueto80]

Have a one-story, rectangular light-frame building (wood) in a Seismic Design Category D region. Since I am in Category D I want to detail for seismic loads(Have 3x framing in shearwalls per SDPWS section 4.3.7 item 3c, >350plf for ASD). In section 12.10.2.1(ASCE 7-05) for collector elements the Overstrength Factor can be ignored. Then in Table 12.2-1 Seismic Force-Resisting Systems(ASCE 7-05) page 120 for Light-framed walls the Overstength Factor is 2.5 (flexible diaphragm).

I have read in a previous post “When to use Overstrength Factor,” since ASCE 7-05 does not really state when to use the load combinations in section 12.4.3.1. Somewhere I read the OS factor is for critical connections.

My question is why can the collector element use a different load combination which does not include the OS factor and the shear wall use an OS factor since both are critical and tied to each other??

 

[slickdeals]

The collector beams are designed for the overstrength factors, while the shear walls are not.

A collector "collects" and transfers the load to the lateral load system and hence, is deemed a critical member in the load path. As a result, it gets an overstrength factor. Connections are the same way as well. Connections are designed to remain elastic during a seismic event so that the member yielding/buckling can dissipate the energy. You don't want your connection to fail before the member.

 

[vcueto80]

According to ASCE 7-05 section 12.10.2.1 the collector element for light-frame shear walls does not use an overstrength factor. But for shear walls an OS factor is given in Table 12.2-1