Analysis of the Frame

Before analysing the portal building ensure that all the appropriate ultimate and service load case options on the General tab have been included. Activating an option to show any type of loading, for example, Draw Snow Load, will show the total number of load cases that have been generated for combinations of dead, live, wind, snow and crane loads. The frame should now be analysed. You can choose an elastic or plastic analysis.

Initially a pure elastic analysis based on the current section sizes, removing any plastic hinges that may have been previously formed.

Once all groups have been sized perform a plastic analysis. This detects the formation of plastic hinges, and calculates plastic moment redistribution. This is recommended even while sizing elastically to confirm that the frame is elastic as intended.

Plastic Analysis

During MasterPort plastic analysis, plastic hinges are allowed to form at the member nodes. Where a plastic hinge forms the moment is limited to the plastic moment capacity of the section.

The analysis procedure is as follows;

• The portal frame is analysed as normal, calculating the member forces and displacements.

• The major axis moments are compared to the plastic moment capacities of the sections.

• If any of the moments exceed the plastic moment capacity the portal frame is reanalysed with the residual moments redistributed back into the frame.

• When analysis is complete the user is notified of either;

the formation of any plastic hinges and the loading case in which they occurred.

or, no plastic hinges are formed, i.e the frame is fully elastic.

P-delta analysis

You can also allow a p-delta analysis to be carried out on the frame. For each loadcase, the initial analysis is carried out followed by the p-delta analysis. The p-delta analysis continues to analyse the frame until it becomes stable or alternatively gives a message stating that the frame is unstable, at which point the position of the first instability is declared as a node number and an instability direction for that node. Each loadcase is analysed in turn.