Answer: Mass- and stiffness-proportional damping, normally referred to as Rayleigh damping, is commonly used in nonlinear-dynamic analysis. Suitability for an incremental approach to numerical solution merits its use. During formulation, the damping matrix is assumed to be proportional to the mass and stiffness matrices as follows:
where:
Relationships between the modal equations and orthogonality conditions allow this equation to be rewritten as:
where:
Here, it can be seen that the critical-damping ratio varies with natural frequency. The values of η and δ are usually selected, according to engineering judgement, such that the critical-damping ratio is given at two known frequencies. For example, 5% damping ( ξ = 0.05 ) at the first natural frequency of the structure ( ω i = ω 1 ), and at ω j = 188.5 (30 Hz). According to the equation above, the critical-damping ratio will be smaller between these two frequencies, and larger outside.
If the damping ratios ( ξ i and ξ j ) associated with two specific frequencies ( ω i and ω j ), or modes, are known, the two Rayleigh damping factors ( η and δ ) can be evaluated by the solution of a pair of simultaneous equations, given mathematically by:
SAP2000 allows users to either specify coefficients η and δ directly, or in terms of the critical-damping ratio either at two different frequencies, f (Hz), or at two different periods, T (sec).
When damping for both frequencies is set to an equal value, the conditions associated with the proportionality factors simplify as follows:
