Frequency Calculation (Modal Calculation)

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Frequency Calculation by STAAD.Pro

There are two method one can calculate Frequency of Structures.

1) Modal Calculation the more exact eigenvalue extraction method and
2) Raleigh Frequency Method, the approximation method

Today, here we will discuss about Modal Calculation. Modal Calculation to be used to obtain full scale eigensolution. In addition, the Dynamic results (using loads as masses) will include Mode Shapes and Frequencies.

The Modal Calculation command (MODAL CALCULATION) can be in any of the primary load cases. Modal Calculation should be applied to ONE applied to only ONE load condition.  

This command must be included in the load case that defines the weights and weight moment of inertias for eigensolutions. The users input LOADS (of course with direction since the loads are vector quantities) that get converted into masses (lumped). Loads get converted internally dividing by‘g’, the gravitational force.

The algebraic summation of loads computed by addition and subtraction (if there were +ve and -ve values) of loads before reaching to final loads and thereafter masses. Therefore, once again it does matter if +ve and -ve loads are input. All loads should have of identical directions (+ve or -ve) to avoid to get resultant loads. Mass is the derived from this algebraic resultant.

As much as possible, absolute values to be provided for the weights. STAAD is programmed to algebraically add the weights at nodes. Therefore, all loads should have identical directions (+ve or -ve) to avoid to resultant loads.

SELFWEIGHT X 1.0
SELFWEIGHT Y 1.0
SELFWEIGHT Z 1.0
ELEMENT LOAD
YRA 9 11 PR GX 200
YRA 9 11 PR GY 200
YRA 9 11 PR GZ 200