ASCE 7-10 Redundancy Factor for 2-story X
ASCE 7-10 Redundancy Factor for 2-story X
(OP)
A colleague and I were discussing this the other day. For the redundancy factor, ρ (rho) per ASCE 7-10, §12.3.4.2, option (b) states that if the SFRS consists of two bays of seismic force-resisting framing on each side of the structure, you can use a ρ = 1.0. Consider a steel OCBF configured in a 2-story X (e.g. figure C-F2.2 (a) of AISC 341-10). Can that be considered two bays? I've always assumed it cannot, because I usually think of one "bay" as being bounded by columns on each end. He posits that it can, because you have two braces, which act in both tension and compression. I looked through the commentary and couldn't find any further guidance. What are your opinions?






RE: ASCE 7-10 Redundancy Factor for 2-story X
RE: ASCE 7-10 Redundancy Factor for 2-story X
To meet the rho = 1.0 rule, removal of any one brace will not reduce the story strength more than 33%. That implies at least 4 elements taking load in a given plane. Realistically, removal of one tension brace when two tension and two compression braces are present might actually violate the 33% rule also depending on the relative tensile vs. compressive capacities. The two story X is basically one bay of an inverted V configuration on the first story and a V configuration on the second story.
RE: ASCE 7-10 Redundancy Factor for 2-story X
You are correct in the sense that this is one way to determine whether or not you can use a ρ=1.0, but this option could require a fair amount of analysis depending on the building layout. Thus, for more regular structures, the code offers a second option to determine rho. The second option (the one I'm referring to in my question) requires no calculations or analysis to determine the value of rho.
That's a lot of words to essentially say you didn't quite answer my question. My question is this: assuming one is trying to use part (b) without further analysis of that code section, would a two story X frame count as two bays?
RE: ASCE 7-10 Redundancy Factor for 2-story X
RE: ASCE 7-10 Redundancy Factor for 2-story X
RE: ASCE 7-10 Redundancy Factor for 2-story X
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: ASCE 7-10 Redundancy Factor for 2-story X