Which R factor to use for a 1996 steel building

[petrosoft]

I was asked if a 6000 square foot, one story steel building could take the added weight of solar PV panels on the roof. I have the plans and am trying to decide if the current ICC code would be more relaxed for the seismic requirements than originally calculated. Can anyone shed light here? I usually work with wood frame construction.

The criteria are Seismic Design Catagory D
SDs = 1.00
SD1 = 0.6

Roof Dead Load = 14.6 psf

The building was originally designed with V = 0.183 W

It has a moment frame in the transverse direction and crossed 5/8" rods in the longitudinal direction.


What Response Modification Factor, R,would you use under current code? I'm hoping that the PV panels (4 psf) would not add undue seismic loading to the building.

Thanks---Peter

 

[dougantholz]

First: determine if the local building code has adopted the IEBC (International Existing Building Code) by ICC. If so, look at those rules.
Second: if the IEBC is not law, then you should use chapter 34 or 35, I can't remember which of the IBC (Labeled existing buidlings).
Third: don't worry about what R factor should be used, look at the mass. Generally if you increase the building or any components mass by more than 5%, you must evaluate those components per TODAY's building code.

What I would suggest doing, is once you break the threshold of analyzing the building per today's standard, you go to the architect/owner and tell them. It is often a deal breaker to have to seismically upgrade a building for some proposed modification.

 

[petrosoft]

Thanks for that advice. I assume that the seismic loading will be increased by 4/15 psf = 27% under the panels. However, I want to check if the current code could reduce the seismic coeficient in any way and reduce it below the design coeficient of 0.183. I'm trying to figure out which R factor to use for the cross braced 5/8" rods. I guess the moment frame is an ordinary steel moment frame with an R of 3.5 since it has no apparent notches in the beams. Would you agree?

Peter

 

[ajh1]

The safest assumption for the moment frames is as you say, ordinary moment frames. In the other direction, the R factor is a function of what the vertical bracing system is. If it is rod bracing in the walls, then it would be OCBF (R=3.25). Roof rods fall under the overall definition of "horizontal diaphragm".

 

[KBVT]

The spectral acceleration values have been reduced in many areas of the country so you actually receive a lower seismic base shear coefficient under the current code. This may offset the increased structural mass due to the additional panels on the roof.