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re: Modeling Rubber in SAP2000

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The suggestion to model gross behavior using a springs is a good one. If the
rubber component you want to model has a known force-displacement curve,
then you can establish a spring stiffness. If it is nonlinear, you might
have to rerun an analysis a couple of times to get a correct secant modulus
for the predicted displacement response. This assumes you aren't trying to
design the component, rather just employ it in a larger system.

If you can't get a manufacturer's force-displacement curve, you can go to
handbooks that can get a reasonable approximation using dimensions, shape
factors, type of rubber, etc. The Malaysian Rubber Producers Handbook is
one - just a booklet, but it might be tough to find. I'm sure there are

Young's modulus and poisson's ratio are not really the right material
properties for a detailed analysis. PR = 0.5 will always blow up the kernel
matrix because of the divisor term (1-2*PR). Thats why SAP won't let you do
it. You need a whole different material constituitive law. Mooney-Rivlin is
frequently used. You would need the Mooney-Rivlin constants, at least C1 and
C2, for the specific formulation of rubber you have. Rubbers, like plastics,
are a lot messier for material characterizations than metal.

Chuck Ritter        JAR Associates, Inc.
ritter(--nospam--at)      401-884-3014 x234         401-294-3826 fax

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