R value question

[SteelPE]

I am working with a client who is looking to add on to his existing building that was built in 2000. The existing building is 100’ wide x 400’ long and is built using a proprietary bearing system. After running through some quick calculations I have concluded that the code required wind loads are approximately the same as they were 15 years ago.

The problem I have is in regards to seismic loads. The client is adding another 50’ wide x 400’ long addition to the building (increasing the area of his building by 50%). The existing drawings indicate R=3.5 which I believed corresponded to a Load bearing wall system of concentrically braced frames (I think this is from the old BOCCA Code). What type of system would this correspond to today utilizing IBC 2009 and ASCE7-05? I leaning towards an ordinary steel concentrically braced frame system, as they utilize steel members within their panels, but I am not sure if this is correct.

Any insight on this is appreciated.

 

[PEFLWI]

ASCE 7 2010 uses an R of 3.25 for concentric braced frames. However, brace connections must be designed for omega level forces, where omega is 2. See table 12.2-1. See section 12.4.3.2 for load combinations with omega. Basically this means you will have to design the connections for twice the seismic load.

The other approach is to use an omega of 3 and most of the seismic detailing requirements can be ignored. You can only use the R=3 approach for Seismic Design Categories A, B or C.

 

[SteelPE]

The problem I have with that approach is the fact that I am increasing the seismic loads beyond what the system was designed for.

I guess I have two options:

1) Run an analysis on the existing structure with the new loads and hope reinforcing is not required (yeah right).

2) Keep the new structure independent from the existing.

The problem with item #2 is the diaphragm ration of the building which is 8:1 as I can not add bracing inside of the building.... and the client doesn't want to add bracing outside of the building.

Not going to be easy.