# TR.34.2 Modal Calculation Command

This command may be used to obtain a full scale eigensolution to calculate
relevant frequencies and mode shapes. It should not be entered if this case or any other
case is a `TIME LOAD` or `RESPONSE SPECTRUM` case. For Steady
State/Harmonic analysis this command must be included in the load case that defines the
weights and weight moment of inertias for eigensolutions

## General Format

MODAL CALCULATION ( REQUESTED ) ( MISSING MASS )

This command is typically used in a load case after all loads are specified. The loads will be treated as weights and weight moment of inertias for eigensolutions (see G.17.3 Dynamic Analysis and TR.32 Loading Specifications). You are advised to specify the loads keeping this in mind.

This case will be independently solved statically and dynamically. Static results using the loads will include joint displacements, member forces, support reactions, and other outputs computed from a normal static analysis without any dynamic effects included.

STAAD.Pro can include the stress stiffening effect (geometric stiffness) based on the axial member forces/plate in-plane stresses from a selected load case when calculating the modes & frequencies of a structure.

In addition, the Dynamic results (using loads as masses) will include Mode Shapes and Frequencies.

`MODAL CALCULATION`command can be included in any of the primary load cases, but only in one of them.

For Steady State/Harmonic analysis, enter the `MISSING MASS` parameter to include the missing mass procedure in the analysis.