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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.

Note: The 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.

Mass Model

The mass model for dynamic load cases (i.e., time history, modal load case, or response spectrum), the mass model used by the program is determined in the following order of precedence:
  1. load specified in the first dynamic load case entered, otherwise
  2. if the first dynamic load case does include any load data, then all reference load cases defined as LOADTYPE MASS will be used, otherwise
  3. then all reference load cases defined as LOADTYPE GRAVITY will be used, otherwise
  4. then all load cases defined as LOADTYPE DEAD and LIVE. At least one load case must be defined as DEAD
Note: If the model includes any rigid floor diaphragms, then it must have one or more suitable reference load cases defined with the mass data (a requirement of the rigid floor diaphragms feature). In this case, the same data will be used to create the mass models for both the rigid floor diaphragms and the dynamic mass model.

Refer to G.17.3.2 Mass Modeling for additional details.

Parent topic: TR.34 Frequency Calculation
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