Plastic Analysis

The use of the plastic analysis option means that within the analysis, the software will calculate the positions of any plastic hinges which will be formed in selected members. The plastic moment of a section will be determined by the selected section, determining the section plastic modulus. The plastic hinge will only allow a moment up to and not exceeding the plastic moment at the point of the plastic hinge. In line with plastic theory, the first hinge will occur at the point of maximum bending moment.

The software uses an iterative approach when using the Plastic analysis. An initial elastic analysis is carried out to determine the point of maximum moment and a plastic hinge applied. The analysis is then re-run and the modified bending moments calculated. Further plastic hinges are determined, if selected and the process repeats until either a solution is found, or sufficient plastic hinges form such that the structure is no longer stable, in which case an analysis warning will be given, identifying the node at which the instability occurs.

The Plastic Analysis is specified on a per load case basis.

The Plastic Analysis includes two options when applying the Plastic Analysis to any load case. These options are: -

1.Include Incremental loading – the load is applied in increments and the positions any plastic hinges is determined at each load increment and the positions of any hinges is used in the next step in the loading. This assists in determining which hinges form in which order.

2.Find the plastic load factor – this option calculates the moment at the hinge position at which a hinge would form, compared to the moment at that point in the load case under consideration and takes a ratio of the two moments to return the plastic hinge formation moment as a ratio of the moment due to the load case. A ratio less than 1.0 indicates that the plastic capacity of the member is less than the moment which would occur at that point in the load case under consideration. Where the Load Factor ratio is greater than 1, then the plastic capacity is greater than the moment applied in the load case under consideration.

The plastic load factor is given in the Graphical Analysis Outputs. Refer to the Results chapter for further details.

To enable the Plastic Analysis to be applied to several load case, rather than having to go load case by load case, clicking on the Apply icon will open the Apply Plastic Analysis Setting Loading Cases pane. This pane is shown below.

To apply the Plastic Load analysis, select the load case in the upper pane. The load case will highlight blue. To multi-select load case, hold the Ctrl key and then click on each required load case in the upper right-hand pane. The load cases can be scrolled using the mouse wheel

In the bottom pane, options are providing to allow the quick selection of load cases. The selection will be shown in the upper pane, with the selected load case highlighted in blue.

Clicking the Apply icon will add all the plastic analysis to the selected load cases.

Clicking Close will close the Apply Plastic Analysis Settings Loading Cases window and go back to the Analysis Options Window.