To perform a dynamic analysis, the static model must first be created and error checked. The model is also usually run through static analysis before the dynamic analysis begins, but this is not required unless nonlinear supports or hanger selections are included in the model. If nonlinear supports are present, the static analysis must run and the results made available before the dynamic analysis can be performed.
The dynamic analysis techniques used by CAESAR II require strict linearity in the piping and structural systems. Dynamic responses associated with nonlinear effects are not addressed. An example of a nonlinear effect is slapping, such as when a pipe lifts off the rack at one moment and impacts the rack the next. For the dynamic model, the pipe must be either held down or allowed to move freely. Nonlinear restraints used in the static analysis must be set to active or inactive for the dynamic analysis. CAESAR II allows you to set the nonlinear restraints to any configuration found in the static results by specifying the value of Static Load Case for Nonlinear Restraint Status on the Control Parameters tab. You usually select the operating case to set the nonlinear restraint configuration. For example, if a +Y support is active in the static operating case and the operating case is used to set the status of the nonlinear supports for dynamics, CAESAR II installs a double-acting Y support at that location for the dynamic analysis. The pipe does not move up or down at that point regardless of the dynamic load.
Another nonlinear effect is friction. Friction effects must also be linearized for use in dynamic analysis. By default, CAESAR II excludes the effects of friction from the dynamic analysis. If requested, CAESAR II can approximate the friction resistance to movement in the dynamic model by including spring stiffness normal to the restraint line of action. For a Y restraint with friction, the friction stiffness is added in the X and Z directions. You define the stiffness of these springs as a function of the friction load calculated in the static analysis. CAESAR II calculates the friction stiffness by multiplying the resultant force on the restraint from the selected static case results, the friction coefficient, and the Stiffness Factor for Friction defined on the Control Parameters tab. For example, if a normal force on the restraint from the static analysis is 1000 lb and the friction coefficient (mu) is 0.3, then the total friction load is 300 lb. If Stiffness Factor for Friction is 500, then springs having a stiffness of SQRT(1000^2 + 300^2)*0.3*500=156605 lb./in are inserted into the dynamic model in the two directions perpendicular to the line of action of the friction restraint. Converting friction damping into stiffness is not mathematically legitimate but serves as a good engineering approximation for dynamic friction in a wide variety of situations.