Overstrength factor for ordinary braced frame (R=3.25) or steel not specially detailed for seismic (

[sjtuchenyi]

I have a project. we could use ordinary braced frame (R=3.25) or steel not specially detailed for seismic (R=3). I know for R=3.25, we have to use over-strength factor to design brace, column, connections. but for R=3 system, do we need to use the load combo with overstrength factor? did not structural steel design for a while, please help

 

[phamENG]

Yes. Using R=3 just gets you out of the special detailing requirements from the seismic design manual.

 

[DaveAtkins]

I believe if you are in SDC A or B you do not need to use overstrength factors.

DaveAtkins

 

[phamENG]

Dave - looking through ASCE 7-10 12.4, I don't see an exception based on design category. Rho is permitted to be taken as 1.0 in SDC B and C (12.3.4.1) - maybe that's what you're thinking of?

 

[DaveAtkins]

There isn't an exception, but in ASCE 7-10 12.10.2.1, SDC A and B are not included in the seismic design categories which require overstrength factors for the design of collector elements.

DaveAtkins

 

[phamENG]

Alright...since you mentioned the possibility of designing an ordinary braced frame, I'm guessing you have a copy of the AISC Seismic Design Manual? I have the 2010 edition. Page 1-14 and -15 will help you shed light on when to use the overstrength in a steel building. To go with R=3, there is example 3.5.1 on page 3-22: Braced Frame Brace Design. That section is all about R=3 steel structures subjected to seismic loads.

 

[sjtuchenyi]

I did some research. in my case, I have seismic design category C. Here is my summary
by using the system with R=3, we are allowed to use AISC specification to design the system not the AISC 341. In the specification of B2, we can use load and load combo in applicable code, ASCE 7 2.3 or 2.4.
IBC 2205 also have these. But it sees like for collector, we need to include the over-strength factor per ASCE 7 2010, 12.10.2.1 for SDC C to F

 

[phamENG]

And batter piles, and pile splices, and elements supporting discontinuous walls, etc. Like I said, the Seismic Design Manual from AISC has a section with examples for your exact situation. It walks you through the code interpretation for most of the commonly encountered items. Remember, the SDM isn't AISC 341. 341 is in the SDM, but the sections in the front are design guides and information that is really useful even if you don't have to meet the specific requirements of 341. Look for the black tab that says "R=3".