Critical Buckling Loads

Multipliers

There are four multipliers that controls the magnitudes of various loads when they are applied to the model. These are located in the Load Multipliers section of the Multipliers tab of the Analysis Parameters dialog box. The value in the Pressure multiplier field will multiply the magnitudes of all pressures and surface forces on the model. The value in the Acceleration multiplier field will multiply the magnitudes of any acceleration loads on the model. The value in the Displacement multiplier field will multiply the magnitudes of any displacement boundary elements applied to the model. The value in the Thermal multiplier field will multiply the thermal loads in the model, where the thermal load is proportional to (coefficient of thermal expansion) * (nodal temperature - stress free reference temperature). The thermal multiplier does not multiply the applied temperatures.

Solver Options

Solution Options Section

There are two solvers available for a critical buckling load analysis. This can be specified in the Type of solver drop-down menu in the Solution tab of the Analysis Parameters dialog box.

• The Automatic option will pick the optimum solver based on the model size.
• The Sparse option is recommended for large models. The sparse solver also takes advantage of multiple threads/cores if available in the computer.
• The Inverse option may provide a faster solution for small models. If the Avoid bandwidth minimization check box is activated, the bandwidth minimization will not be performed. This usually makes the analysis run longer.

The Percent memory allocation controls how much of the available RAM is used to read the element data and to assemble the matrices. (When the value is less than or equal to 100%, the available physical memory is used. When the value of this input is greater than 100%, the memory allocation uses available physical and virtual memory.)

The drop-down Number of threads/cores control is enabled when the solver is set to sparse.. You want to use all the threads/cores available for the fastest solution, but might choose to use fewer threads/cores if you need some computing power to run other applications at the same time as the analysis.

Inverse Iteration Solver Section

If you are using the inverse solver, specify the convergence tolerance to be used in the Convergence tolerance for Eigenvalue field and specify how many iterations can be used to achieve this tolerance in the Maximum number of iterations field.

Sparse Solver Section

If the sparse solver is chosen, then the Sparse Solver section is enabled. The input for this section is as follows:

• The Type of sparse solver drop-down menu contains the sparse solvers currently available. If you choose a solver that is not available on an operating system, the processor will use the best one for the operating system. The sparse solvers available are as follows:
• • Default: Use BCSLIB-EXT on Windows and use inverse iterative solver on Linux.
  • BCSLIB-EXT (Windows only): Use the Boeing solver. The BCSLIB-EXT solver may write temporary files to the folder specified by the environment variable USERPROFILE. By default, this variable is set to the folder C:\Documents and Settings\Username where C: is the drive on which the operating system is installed. The error numbers -701 or -804 returned from the BCSLIB-EXT solver means that it ran out of hard disk space for storing the temporary files. If this occurs, change the USERPROFILE variable to a directory that can provide sufficient hard disk space. (See the Windows Help and Support for documentation on changing environment variables.)
• The Solver memory allocation field sets the amount of memory to use during the sparse matrix solution for the BCSLIB-EXT solver. In general, allocating more memory should result in a faster analysis. The other sparse solvers adjust the memory setting automatically; so no setting is required for them.
• The Sophisticated usage option is turned on by default. This option provides a robust solution for most models. Users might deactivate this option to reduce the solution time. However, if the nature of the model is ill conditioned, the solver may fail during the solution.
• The Number of buckling modes to calculate field sets the number of buckling modes to calculate. Usually, the first mode (the lowest buckling load) is of interest.
• The Upper buckling load cutoff factor and Lower buckling load cutoff factor fields will limit what buckling load multipliers are calculated. The number of requested buckling modes will start with the first buckling mode with a load factor larger than the lower cutoff. The analysis will end once a buckling mode with a load factor larger than the cutoff is calculated. This capability is useful when the lowest buckling mode is not of interest. For example, the lowest theoretical mode may occur if the loads are reversed, but in some cases you know that the loads cannot be reversed. In this situation, a small negative number for the lower cutoff prevents the calculation of the negative buckling loads.

Control Data in Text Output Files

After the analysis is complete, the analysis results can be output to a text file. The Output tab of the Analysis Parameters dialog box can be used to control the data that is output to this file.