Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis Solutions
	Subarea:	Dynamic Analysis
	Original Author:	Bikash, Bentley Technical Support Group

What is Calculate Rayleigh command?

This command, Calculate Rayleigh may be used to calculate the Rayleigh method approximate frequency of the structure. Thus, this command typically follows a load case.

This command is specified after all other load specifications of any primary load case for which the Rayleigh frequency is calculated. This Rayleigh frequency calculation is based on the Rayleigh iteration method using 1 iteration. If a more accurate, full-scale eigensolution is required, the MODAL CALCULATION command.

Frequency Calculation (Modal Calculation)

General Format of while defining Load Cases for which Rayleigh Frequency is calculated.

LOADING 1
SELFWEIGHT X 1.0
CALCULATE RAYLEIGH FREQUENCY
LOADING 2
SELFWEIGHT Z 1.0
CALCULATE RAYLEIGH FREQUENCY
LOADING 3 WIND LOAD

In case 1, the structure is being displaced by SELFWEIGHT applied statically along global X. For most frames which are supported only at their base, this produces a deflected shape resembling the lowest mode shape along global X. Hence, this frequency will resemble that for the lowest X direction mode.

Similarly, the frequency calculated for load case 2 ought to be similar to that for the lowest Z direction mode because the selfweight is applied statically along global Z.

The output will consist of the value of the frequency in cycles per second (cps), the maximum deflection along with that global direction and the joint number where that maximum occurs.