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RE: Maximum base shear
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Maximum base shear
- From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
- Date: Tue, 9 May 2000 12:02:27 -0400
(Re-sent. My previous draft reply was accidentally sent before completed.) The easy answer is "yes". First it must be recognized that code values are "best estimates" not fixed maximums. Earthquake engineering is not that precise. Then, we must break the code formula down into components. The Ca value is the only variable that relates to ground acceleration. The other factors relate to how a structure behaves under dynamic ground motion. I think that you could say that the "design ground acceleration" is Ca = 0.44(1.5) = 0.66g. (As per another thread regarding the "seismic margin" included in the NEHRP values, it is expected that a structure could withstand 1.5 times as much ground motion without collapse; but I haven't studied how this might affect the 1997 UBC values.) The 2.5 factor represents an assumed upper bound for amplification due to the natural frequency of the structure. For a structure with less than 5 percent damping, this value could be higher. The "R" factor relates to structure ductility and the "I" factor reduces the effective "R" factor for critical structures. For a perfectly elastic structure, the "R/I" factor could be assumed as 1.0. Thus conceivably a structure could respond above 2.5(0.66)(1.0) = 1.65g. -----Original Message----- From: Alex Nacionales [mailto:alexcnac(--nospam--at)skyinet.net] Sent: Saturday, May 06, 2000 5:56 PM To: seaint(--nospam--at)seaint.org Subject: Maximum base shear UBC 97 formula 30-5 page 2-14 states that the base shear need not exceed the value of the following; V=2.5 Ca I W/R for zone 4, at less than 2 km from a fault and using maxumum values of Ca and I, and R=8.5 for SMRF(Concrete) This was 12 in UBC 88 Ca = .44(1.5) V=Wx2.5x.44(1.5)(1.25)/8.5 V=.243W This says that .243G is a maximum for buildings. Could this g force be exceeded by an intensity 7 earthquake? Thanks in advance. Alex C. Nacionales
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