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RE: Maximum base shear

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(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