STRUCTURAL ANALYSIS & APPLIED LOADS STRUCTURAL ANALYSIS & APPLIED LOADS

To perform a structural analysis or emit a set of applied structural loads, one must enter the Structural Menu. This is accomplished by issuing the command:


     STRUCTURAL, -OPTIONS

where the available option is:



     -INITIALIZE

This command places the user in a sub-menu where he can define a situation and perform an analysis. If the option is selected, then this will delete all previous structural results; otherwise, the results will be added to previous results so that all of them are available later for Structural Post-Processing. In general, there are three types of commands available in this menu: commands which define the load cases to be used, commands which define the portion of the system which will be used, and commands which produce the results. When the solution has been completed, one should issue END_STRUCT to return to the main menu. There are no reports produced directly in this menu. The results that are produced are the system deflections and element loads, but to obtain a report of them, one must enter the Structural Post-Processing Menu.

Three general types of results can be obtained in this menu: structural analysis results (system deflections and element internal loads), loads applied to the structure, or vibration modes. In the first two cases, both the load cases and the portion of the system to be considered must be defined before the commands to compute the results are issued. With vibration modes, a single command suffices. The remainder of this discussion is applicable only to structural analysis and applied loads.

Before proceeding, however, it is best to make a distinction between load cases and load sets. A load set is either one of the intrinsic load sets that the program generates, or a user defined load set. Load sets are combined to form load cases. It is these cases that are used to obtain the results. When emitting applied loads, or when performing a structural analysis, it is the load cases which will be used.

It is beneficial to think of the solution as being performed in one of four types:

Here, the types refer to the type of loadings which will be applied to the structure. MOSES is different from most programs in that the structural dynamics is included directly in the analysis via generalized degrees of freedom. Thus, if generalized degrees of freedom are included during an analysis, the deformation inertia is automatically included when the load case is generated. The result is a true load case which accurately describes the static as well as the dynamic behavior. The type of solution is controlled by the type of loads generated.

When a structural analysis has been performed, the results can again be combined in the post-processor. If the problem is linear, combining the deflections in the post-processor or combining the loads prior to solution produce the same results. For a nonlinear problem, however, this is not the case.

For the linear problem in the frequency domain, either options 1 or 2 defined above are available. Of these two methods, the frequency domain allows the user more flexibility, while the time domain can offer some savings in computational effort. The frequency domain method must be used if one wishes to compute the cumulative fatigue damage for the system.

If one wishes to use nonlinear elements, the structural system of equations will be nonlinear. In this case, the frequency domain solution method will not really yield the proper results since the solution will be for unit wave. Here, it is better to combine loads to form the total load on the system prior to solution. Thus, one cannot correctly assess cumulative fatigue damage using nonlinear elements.

When the structural analysis is complete, one should issue:


     END_STRUCT