Response Modification Coefficient

[mjohan]

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

 

[mjohan]

The walls are light gauge steel studs.

 

[JAE]

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.

 

[Taro]

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).

 

[mjohan]

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...)

 

[JAE]

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.

 

[Taro]

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).