Fenix is capable of performing P-Delta analysis for models with frame and/or shell elements. To enable P-Delta analysis, the feature must be turned on in three (3) locations. This allows the user much more control over how and when P-Delta is considered. The three locations are:
To enable P-Delta, first check the “Include P-Delta” check-box on the Model Definitions window, under the Analysis tab:
If this check-box is unchecked, P-Delta will be disabled regardless of whether other P-Delta settings are enabled. However, checking this box does not automatically enable P-Delta, as it must be declared in two additional locations.
The maximum number of iterations limits the total iterations performed for P-Delta analysis. In general, five iterations should be sufficient. However, some problems may require more. Be careful, though, for P-Delta may never converge if the induced forces continue to grow, meaning that stability is not maintained.
The P-Delta convergence tolerance is the decimal percent difference from the previous iteration that will signal convergence. For example, the default is 0.005, meaning 0.5% difference in total reactions from the previous iteration will indicate convergence and stop the iterations.
The second location where P-Delta analysis must be declared is on the Load Combinations. Unlike other software which may use a single load combination to determine the axial force for P-Delta analysis, Fenix uses the axial force from each load combination. Thus, you are required to tell Fenix which load combinations you want P-Delta to be enabled for. This allows you to disable P-Delta for load combinations where it may be unnecessary, such as gravity-only combinations. The fewer load combinations with P-Delta enabled, the faster the analysis will run.
P-Delta is an iterative analysis. Because Fenix performs P-Delta using the axial force from an individual load combination, it does not solve all elastic patterns independently and linearly combine later. Instead, it solves each load combination in full to determine the axial force at each specific load combination. Thus, the more combinations that have P-Delta enabled, the more time a model will take to analyze, as each load combination with P-Delta enabled will be iterated.
The last location required to enable P-Delta is on the individual frame and/or area element assignment windows. This allows the user to specify which elements should have P-Delta forces calculated. The fewer elements requiring P-Delta force calculations the faster the model will run. Thus, it may be a good idea to only enable P-Delta on column elements and keep it turned off for beam elements (if you are only interested in the lateral effects of P-Delta). Similarly, for shells, it may not be necessary to include P-Delta for floors, while keeping it enabled for walls.
Frame Elements
Area Elements
