Function
- Enter the load cases and the control parameters for iterations used in the P-Delta analysis of a structure. In order to consider the P-Delta effects in the processes of general static and dynamic analyses, we need to define the load condition to formulate the geometric stiffness matrix. We need to also specify the convergence requirements.
Call
From the Main Menu select [Analysis] tab > [Analysis Control] group > [P-Delta]
Input
Control Parameters
Number of Iterations
Enter the number of iterations for the P-Delta analysis
Convergence Tolerance
Enter the convergence tolerance.
P-Delta Combination
Select the loads that will be applied to determine the geometric stiffness matrix used in the P-Delta analysis.
Enter different Load Cases with the corresponding Scale Factors.
Select the dead loads (self-weight, dead load, etc.) generally acting on the structures.
- To enter a new or additional load case
Enter the Load Case and Scale Factor; then click the Add Button.
- To modify the scale factor for a previously entered load case
Select the load case, modify the Scale Factor and click the Modify Button.
- To delete a previously entered load case
Select the load case to be deleted from the load case list at the bottom of the dialog box and click the Delete Button.
P-Delta Analysis and Buckling Analysis may not be used simultaneously. The use of P-Delta Analysis is limited to truss, beam (including tapered beam) and wall elements.
To delete all the entered data, click the Remove P-Delta Analysis Data Button.
Q&A
Q1. Which load cases do I have to add in “P-Delta Combination”? The result of the load combination which doesn’t include vertical load case was changed after performing P-delta analysis.
A1. The vertical (or gravitational) load cases should be added in “P-Delta Combination” so that the program can find the modified stiffness matrix of the structure based on the geometric stiffness matrix due to P-Delta load combination. Even if the lateral load case is added here, the results will remain the same because the lateral load will not make any axial forces in case of a cantilever column.
For example, if we make a test model as below, all the three cases give the same results for P-Delta analysis.
According to the information as mentioned above, in midas, P-Delta analysis will find the geometric stiffness matrix of structure due to the entered load cases in “P-Delta Combination”. This geometric stiffness matrix will be added to the elastic stiffness matrix to generate the modified stiffness matrix of structure, which will be applied to the analysis for all the load cases including lateral loads.
Additionally, when the user wants to see the 2nd order effect for multiple load combinations which have different load factors, the user has to modify the load case factor in “P-Delta Combination” and perform analysis again.