X-Bracing and the Seismic Overstrength Factor

[lmc45]

I used to work for a metal building company that used an omega equal to 1 for the x-bracing in sesismic categories A through C. Now, the company I work for uses omega equal to 3 for all applications of x-bracing. Which is correct?

 

[mudflaps]

Using 3 sure makes life, and spread sheets, simple. Sometimes it's harder to be correct than safe.

Old CA SE

 

[penpal97]

Omega = 3, just don't forget to design your connections by multiplying the forces by Cd = 3. While the members are allowed to yield, the connections have to remain elastic.

 

[slickdeals]

Isn't Cd a deflection amplification factor? What implications will it have on a connection design strength?

The concept behind seismic detailing is to pick a fuse member and then make sure that all other members around it are stronger than the fuse. This way the designated yielding member will be the one taking the brunt of the forces and providing energy dissipation.

 

[penpal97]

sorry....let me try again...

You should really just be using the coefficients in the ASCE 7-05 Ch 12. But lets say you don't feel like it ...

I am assuming when you designed the structure, you used an R = 3. If that is the case, then you should be using an Omega = 3 (to be conservative) for connections. You could probably use Omega = 2 and no one would complain. I do not recommend using an Omega = 1 unless....

If you designed the structure using an R = 1 and assumed the whole thing to remain elastic, then you could use an Omega = 1.

 

[ChipB]

Let's all move to Florida where all one needs to worry about is wind!

It sounds like the company you are working for now is classifying their buildings as "Steel Systems not Specifically Designed for Seismic Resistance"
R=3
Omega=3
Be careful b/c this is allowed in seimic design categories greater than C.

In essence, this is equivalent to penpal's last post. You've divided your seismic forces by 3 (R=3) then amplified your seismic forces by 3 (Omega=3).
You're taking the full seismic load.

In connections, your designing your brace to yield (Yield not Fail) with your connection being strong enough, and in some cases, flexible enough, to withstand the force which causes the brace to buckle and/or elongate. Tihs is the "fuse" slickdeals is talking about

 

[JAE]

ChipB - didn't you mean: "be careful b/c this is NOT allowed in seismic design categories greater than C."


??

 

[d9struct]

I realize that this question is a month old, but I think there needs some clarification. The question first posed asked about designing for Design Category C and for R=3 which refers to Table 12.2-1 from ASCE 7-05, SFRS-"H"(steel systems not specifically detailed for seismic resistance ...) Note, this is in Chapter 12 "Building Structure Systems" and not Chapter 15, "... Requirements for Non-Building Structures". However, there is a reference back to chapter 12 in 15.4.1a to allow you to select a system from either table but specifically refers you to the table in 15 for the appropriate R, CD, and Omega values. Since there is no "H" in the table from CHAPTER 15 it does leave it open to interpretation if "H" is allowed for a non-building structure, say an open structure supporting process equipment. It does give values for pipe racks and would not allow you to use a higher R value than 1.0 without utilizing ASCE 341-05.

Anyway, that's another topic. In many of the responses to the question, it was stated that you needed to design the connections using Omega=3, or essentially using R=1. While this may be prudent, it is NOT required by ASCE 7-05, AISC 360-05 (remember that AISC 341-05 is not required here). The reference from the table in Chapter 12 refers you to ASCE-7-05, 14.1.2. This section refers you to AISC 360-05 not AISC 341-05 for R=3.

The bottom line is that when you can use the system "H", you are designing your structure, connections, foundation for R=3. The omega is not applicable. Also, for Design Class C, ro = 1.0. While this may not be as conservative as using the Omega for your connections, it is what is currently allowed by the codes ( ASCE 7-05, IBC-2006, AISC 360-05.

The answer to the original question is R=3 is correct, but nothing stops you from using a smaller R value, except for the one footing the bill for the additional conservatism.

I do agree with the explanation concerning the use of a higher force for connections, but this is required for moderate and high seismic areas (Seismic Design Classes greater than C).

 

[ChipB]

JAE,
Sorry missed your post previously.
YES, that is what I meant! See? You're keeping me straight again!
Chip

 

[JAE]

Please do the same for me OK?

 

[lmc45]

D9Stuct:

What about where IBC 06 (Section 1605.1) refers you to Section 12.10.2.1 of ASCE 7 and it tells you that collector elements, splices, and their connections need to resist the load combinations with omega?