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Response Modification Coefficient

Response Modification Coefficient

Response Modification Coefficient

(OP)
What would you classify the R value for a 4 story building, laterally supported utilizing X-braces?  I am using the IBC 2000 code.

TIA,
Mark

RE: Response Modification Coefficient

(OP)
The walls are light gauge steel studs.

RE: Response Modification Coefficient

It depends on your framing materials and your desired degree of detailing.

If you have a steel framed building, in Table 1617.6, you would fall under "Building Frame Systems" (item 2).

Now, with X braces, you could either be type C, Special concentrically braced frames (R = 6); or type D, Ordinary Concentrically braced frames (R = 5)

The special frames require a higher degree of detailing vs. the ordinary.

RE: Response Modification Coefficient

I get the impression from the second post that this is not a structural steel braced frame system.  Do you have metal stud walls with diagonal strap tension bracing?  If so, the R factor would be lower.  The closest thing in the IBC would be Type 1L in Table 1617.6, "Light frame walls with shear panels-all other materials" (R=2).  In the UBC, the system is Type 1.3 in Table 16-N, "Light steel-framed bearing walls with tension-only bracing" (R=2.8).

RE: Response Modification Coefficient

(OP)
Thank you both for the response.

Yes, Taro, the bracing is tension bracing on light gauge studs.  

I also thought that R = 2 was the appropriate value.  I just couldn't believe the seismic base shear would increase by 3!!  After seeing some X-braces in the field, however, it does not seem like a bad idea at all.  (Many braces were loose, poorly connected, etc...)

RE: Response Modification Coefficient

Taro / mjohan

Can you have tension ONLY braces for seismic in this condition?  I thought that a minimum percentage of braces have to be split between tension/compression.

RE: Response Modification Coefficient

JAE,
Yes, the bracing is light gage flat strap bracing that carries tension only.  That is why the R factor is so low.  Better performance would be achieved by using structural panel sheathing (R=6).

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