Tension only bracing for R=3 steel structure, SDC C?

[Delmarva_Struct]

I am designing an R=3 steel structure with the SDC = C. I would like to install tension-only x-bracing to resist the seismic and wind loads.

Per the process listed in AISC 341, for an R-3 SFRS, the brace is primarily checked for compression buckling as well as tension yielding. What is the process to design tension-only braces for this system? is it possible? I would like to use 3x3x3/8 angles preferably.

This will be a single-story structure. The tension bracing will be a 2-tier x-bracing system between two columns.

 

[SandwichEngine]

Metal building guys use tension only bracing for R=3 systems all the time. They usually live in the "not detailed for seismic" category in ASCE 7. Because it's tension only, you don't check the braces for compression at all. Just assume they'll buckle out of plane and provide zero strength.

 

[Delmarva_Struct]

Thanks! The AISC 341 has only one example(Part 3) for R=3 systems and they used a double-angle brace with checks for compression buckling.

I guess the fundamental idea is that you can use R=3 for any type of SFRS (Tension only OCBF, OCBF, SCBF, OMF, etc.) as long as you are in SDC B and C i.e moderate/low seismic zone. This is based on the fact that even "normally" designed and detailed buildings have some degree of seismic resistance.

 

[straub46]

An R=3 system is not the same thing as an OCBF, SCBF, OMF or any of those listed. It's a specific system that's permitted to be used in SDC C and less per ASCE 7-10. All the other seismic systems you list have specific detailing and capacity checks that need to be done, which are illustrated in AISC 341.

If you're designing an R=3 system, you don't probably don't even need to open 341, assuming you're talking about a "Not Detailed for Seismic" system. If not, then you need to brush up on the R factors, Cd, Omegas, and the special detailing and capacity requirements of your selected system.

Go Bucks!

 

[Delmarva_Struct]

I understand. OCBF, SMF, etc. have their own R, Cd, and Omegas and have seismic detailing listed in AISC341.

I am using R=3, which means the system is "not detailed for seismic". Once I have decided to use R=3, I can choose tension-only diagonal bracing to resist the seismic forces and these will be designed and detailed based on the AISC specification. I intend on using 3x3x3/8 2-tier diagonal bracing. I guess it's not right to call it "OCBF" or "SMF" etc. because I am using R=3.

 

[MotorCity]

If R=3, put away AISC 341.

 

[Delmarva_Struct]

Understood! Thanks all.

 

[structSU10]

I agree with straub - the code tells you that you dont need to follow AISC 341 requirements only AISC 360 (IBC section 2205) so that book stays on the shelf for an R=3 system.

 

[phamENG]

For those who aren't aware, the Seismic Design Manual has a tab labeled "R=3" and gives a nice example of a SFRS not detailed for seismic resistance. That's probably why Delmarva was looking in that book and mistakenly referring to the 'front matter' as a part of the spec. But I agree, it's a pretty narrow slice, example-wise, and just points back to 360 for everything.