Incremental Analysis

Single Increment

This is the default option. In this option 100% of the loads will be applied to the structure at the start of the analysis. The analysis will then run for the number of cycles specified or until convergence is achieved. No intermediate results will be stored. If convergence has not been achieved after a significant number of cycles it is possible that the structural model capacity has been exceeded and/or the model represents a mechanism. The non-convergence can be investigated by looking at the changes in the residual forces during analysis.

Multiple increment

In this option, the total applied loads will be divided into a number of equal or unequal increments. The first increment will be analysed, and then the load will increase in steps one increment at a time. An analysis case is generated with the total accumulated load for each load stage. Results are stored for each analysis case if convergence is achieved.

Automatic Increment

In automatic increment analysis, the program is searching for the structural capacity of the model, by successively increasing the load factor up to a maximum specified. The structural model capacity can be defined as the load at which analysis ceases to converge, or, the load at which predefined excessive deflections occur. This option can also be used to investigate the load factor at which serviceability criteria (deflections or rotations at key nodes) are met.

In this option, the magnitude of each load increment (except the first increment) is automatically selected by the program. The first load increment is specified as a percentage of the total imposed loads. Analysis cases are generated by the program with the loading increasing until specified limits are exceeded or the capacity of the model is found. These conditions can be limits on deflection or loading and are defined in the Relaxation Analysis Task Control dialog. The automatic load increment scheme is as follows:

1. Do the first increment analysis (the increment level is known) and record the number (N) of cycles being used.
2. Determine the limit of cycles allowed for the following increment analyses, 5×N is used.
3. Determine the magnitude of load increment.
   1. If the previous three or more increment analyses are all converged within the limited cycles, the current increment will be 1.328×Last_increment
   2. If the previous two increment analyses are all converged within the limited cycles, the current increment will be 1.236×Last_increment
   3. If only the last increment analysis is converged within the limited cycles, the current increment will be 1.15×Last_increment
   4. If the analysis cannot converge within the limited cycles, the current increment will be reduced by a factor of 0.618, if it still cannot converge, the increment will be reduced by this factor again.
   5. If the increment determined above is larger than the maximum increment allowed, the maximum increment will be used.
   6. If the increment determined above is smaller than the minimum increment specified, the analysis will be terminated.
4. Do nonlinear analysis under the current load level (previous loads plus the current load increment, previous nodal displacements are carried over from the previous loads (if converged) to speed up the process )
5. Check displacement and/or load factor limit, if satisfied, stop the analysis, otherwise Go to Step 3.

The program uses the convergence information from each run to pin-point the load at which convergence fails, or increases the load until the specified limits are exceeded. This is deemed to be the structural capacity of the model.