(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
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
damping, this value could be higher.
The "R" factor relates to structure ductility and the "I" factor reduces the
"R" factor for critical structures. For a perfectly elastic structure, the
could be assumed as 1.0.
Thus conceivably a structure could respond above 2.5(0.66)(1.0) = 1.65g.
From: Alex Nacionales [mailto:alexcnac(--nospam--at)skyinet.net]
Sent: Saturday, May 06, 2000 5:56 PM
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
Ca = .44(1.5)
This says that .243G is a maximum for buildings.
Could this g force be exceeded by an intensity 7
Thanks in advance.
Alex C. Nacionales