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re: Modeling Rubber in SAP2000

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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. <snip>

This is also why it is a good idea not to use .499999999 as v. The matrix inversion won't fail, but you'll have a very large stiffness on your K matrix diagonal, relative to the rest of your structure. The roundoff errors, even using double precision, can cause inaccuracies in your model. Not being a user of the popular SE analysis packages, I don't know what kind of checking results you get (NASTRAN will output a value of the largest/smallest eigenvalue ratio. The "old" standard was 1000:1 - anything higher and your model was generally poor wrt element uniformity and you should consider a better model technique)

Jordan Truesdell, PE

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