Stiffness Factor for Friction - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
13

Available for: Modal, Harmonic, Spectrum, Range, and Time History

Typical units: fixed to 1/inch

Specifies the friction stiffness factor as described below. Enter a value greater than zero to consider friction stiffness in the analysis. Enter 0.0 to ignore friction in the analysis.

Dynamic analyses in CAESAR II act only on linear systems, so any non-linearities must be linearized prior to analysis. Modeling of friction in dynamic models presents a special case, because friction impacts the dynamic response in two ways. Static friction (before breakaway) affects the stiffness of the system by providing additional restraint. Kinetic friction (after breakaway) affects the damping component of dynamic response. Due to mathematical constraints, damping is ignored for all analyses except time history and harmonics, for which it is only considered on a system-wide basis.

The software considers friction using this friction stiffness factor. The software approximates the restraining effect of friction on the pipe by including stiffnesses transverse to the direction of the restraint at which friction was specified. The stiffness of these frictional restraints is calculated as:

Kfriction = F * µ * Fact

Where:

Kfriction = Stiffness of frictional restraint inserted by the software.

F = The load at the restraint taken from the selected static solution. This load is the total resultant load acting on the restraint, including friction:

F = (Fx2+Fy2+Fz2)1/2

µ = Friction coefficient at restraint, as defined in the static model.

Fact = Friction stiffness factor. Units are fixed in 1/inch, so you must convert to other units. If stiffness Kfriction is defined in N/cm and force F is defined in N, then you must convert Fact to 1/cm:

Kfriction = F * µ * Fact * (1 in/2.54 cm).

This factor should be adjusted as necessary in order to make the dynamic model simulate the actual dynamic response of the system. The factor does not correspond to an actual dynamic parameter but is an adjustment factor to modify system stiffness. Entering a friction factor greater than zero causes these friction stiffnesses to be inserted into the dynamic analysis. Increasing this factor correspondingly increases the effect of the friction. Values such as 1000 are typical. Entering a friction factor equal to zero ignores any frictional effect in the dynamic analysis.