MECHCHECK

I/O Options Entry

MECHCHECK - Massless Mechanism Check

Description

The MECHCHECK command can be used in the I/O options section to perform a massless mechanism check on the model.

Format

MECHCHECK

Comments

Running normal modes analysis or linear static analysis by adding MECHCHECK helps detect the massless mechanisms. In Normal Modes Analysis, the massless mechanism shows up as rigid body modes. In linear static analysis, a small amount of stiffness is added to the massless mechanism to run the model and detect such mechanisms.

A massless mechanism occurs due to degrees-of-freedom that do not possess both stiffness and mass. In Normal Modes Analysis, to find such degrees-of-freedom, a small amount of mass is added to the mass matrix so that the massless mechanism shows up as a rigid body mode (the stiffness matrix K is still zero but the mass matrix M becomes non-zero). This is the reason why the lower bound on the EIGRL bulk data entry is set to blank or zero when MECHCHECK is used. In linear static analysis, a small amount of stiffness is added to the massless mechanism, to run the model and detect such mechanisms.

The analysis results with this card cannot be used since the model is changed internally. It can only be used to find the massless mechanism. The message “RESULTS FROM THIS SOLVER RUN ARE INACCURATE AS IT IS IN MECHCHECK MODE” is also displayed in the .out file when MECHCHECK is included in the I/O Options section of the solver deck.

4.	MECHCHECK results (in the .out file) include a list of rigid body modes in the following format:

ANALYSIS RESULTS:

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ITERATION #

Rigid Modes in Eigenvalue Loadcase: #

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Mode Grid No. Type x-comp y-comp z-comp

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1 ## Tran ## ### ###

2 ## Rotn ## ### ###

… …. … etc … … …

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