Overstrength Factor when R=3 1

[lweiss44]

Does anybody know of a reference that requires or suggests the use of the overstrength factor (or other multiplier) when R=3 is used for a structural steel system? Clearly the AISC Seismic Provisions do not apply when R=3, but is there another Code reference with a direct requirement, or perhaps a publication that suggests its use, due to similar considerations? Point in case would be where R=3.25 and the Seismic Provisions are used - then the of the overstrength factor is required (where specifically indicated). Thanks.

 

[WillisV]

The only places O/S factors are specifically required where an R=3 system is specifically allowed to be used (SDC C or less) would be for drag struts/collector beams if your SDC is a C per IBC 2006 12.10.2.1 or for elements supporting discontinuous walls or frames having certain vertical or horizontal irregularities in SDC B or C as discussed in IBC 2006 12.3.3.3.

 

[JAE]

WillisV - just to understand - it appears that the use of R=3 for steel structures "not specifically detailed for seismic resistance" appears to be only allowed for SDC B and C per ASCE 7-05, table 12.2-1.

The two sections you referred to above are dealing diaphragms so these are essentially "outside" the definitions of a system not detailed for seismic? Is that the way the logic flows?

In other words - with an R=3 from Section H of table 12.2-1, you can ignore all the seismic detailing requirements for the lateral force resisting system (i.e. the beams, columns, braces, etc.)

but for diaphragms, they typically aren't part of the steel lateral system so despite the use of R=3 you still have to use the OS factor in diaphragm design in some cases. Is this how you see it?

 

[WillisV]

JAE - that is not exactly how I think about it. Essentially you have ASCE 7 mandated over strength requirements for all types of systems and materials, and then you have the material-specific over strength requirements.

The two ASCE 7 sections I pointed out are applicable to wood, concrete, etc. as well as steel. Note that though it is in the 12.3 diaphragm section, 12.3.3.3 actually applies the overstrength factor to the beams and columns of the system.

For R=3 steel systems, it just so happens that there are no ADDITIONAL overstrength factors in the material code (AISC 360), while for other types of steel systems with higher R factors there ARE additional overstrength factors in the material code (AISC 341).

Since the O.P. asked about R=3 systems, I kept the answer related specifically to those systems, but there are other ASCE 7 mandated overstrengths that are not specifically related to the material as well, such as for example foundations for Cantilever Column Systems in 12.2.5.2.


One minor note to your post as well is that you can certainly use an R=3 system in SDC A, but in that case overstrength factors never come into play.