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Re: Seismic requirements in NYC

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If they are using UBC '88, '91 or '94, Rw = 8 appears alright. Notice that UBC '97 and IBC do not give this factor. Instead they use R, which is significantly less than Rw. The seismic load (before R or Rw factor) comes out about the same in both procedures, but the way it is integrated in to design level forces through force combinations is quite different.
Rw is supposed to be compatible with ASD combinations and service state loads, whereas R is supposedly compatible with LFD combinations and ultimate limit state. The approximate factor between R and Rw is 1.5 (only my observation, no scientific thought implied).
If the seismic load is really insignificant compared to design wind loads, I'd assume a lower value of R (or Rw in your case) and do away with seismic detailing. However, your jurisdiction (or the checker assigned to you) may have something to say about this.
----- Original Message -----
To: seaint
Sent: Tuesday, September 19, 2006 9:18 PM
Subject: Seismic requirements in NYC

Hello All,


I am designing the bracing connections for a mid-rise (approx. 15-story) residential building in Manhattan, NYC.  The lateral force resisting system consists of braced frames and concrete shear walls. The EOR has indicated an Rw = 8 on the drawings and references the NYC building code.  From what I gather, the NYC building code follows UBC 1990.  I know that if this were somewhere else with IBC as the code, I would be thrown into seismic detailing of connections because R>3, but I do not see this requirement in the NYC code.  The EOR has provided design brace forces on the elevations.  Is there any such seismic detailing requirement or can I just design the bracing connections for the loads given?  I understand that this question might be better directed to the EOR, but I wanted to know the correct answer before asking him. 




K. DeLabar