Click the Define menu > Preset P-Delta Options command to access the Preset P-Delta Options form. Use the form to apply the options to all linear load cases unless specified otherwise in the P-Delta/Nonlinear Stiffness area of the Load Case Data - Linear Static and Linear Direct Integration History forms.
Set the P-Delta analysis parameters by making selections in the following areas of the Preset P-Delta Options form.
Automation Method: An initial P-Delta analysis in this program considers the P-Delta effect of a single loaded state upon the structure. Specify the load using one of the following options:
None option. Use this option to not consider P-Delta effects, including removing previously considered effects.
Non-Iterative -- Based on Mass option. The load is computed automatically from the mass at each level as a story-by-story load upon the structure. This approach is approximate, but does not require an iterative solution. This method essentially treats the building as a simplified stick model to consider the P-Delta effect. It is much faster than the iterative method. It does not capture local buckling as well as the iterative method. This method works best if the model has a single diaphragm at each floor level, although it also works for other cases as well. The reason we provide this method is to allow consideration of P-Delta in cases for which gravity loads have not been specified in the model. If gravity loads have been specified in the model, in general, we recommend use of the Iterative Based on Loads option.
Iterative -- Based on Loads option. The load case is computed from a specified combination of static load patterns. This is called the P-Delta load case. For example, the load case may be the sum of a dead load case plus a fraction of a live load case. This approach requires an iterative solution to determine the P-Delta effect upon the structure. This method considers the P-Delta effect on an element-by-element basis. It captures local buckling effects better than the non-iterative method. We recommend the use of this iterative method in all cases except those for which no gravity load is specified in the model.
Relative Convergence Tolerance: This area is active if the Iterative -- Based on Loads option is selected in the Automation Method area of the form. See Iterative Solution, Convergence Tolerance for more information.
Iterative P-Delta Load Case: This area is active if the Iterative -- Based on Loads option is selected in the Automation Method area of the form. Specify the single load case from a combination of load patterns to be used for the initial P-Delta analysis of the structure. As an example, assume that the building code requires the following load combinations to be considered for design:
(1) 1.4 dead load
(2) 1.2 dead load + 1.6 live load
(3) 1.2 dead load + 0.5 live load + 1.3 wind load
(4) 1.2 dead load + 0.5 live load - 1.3 wind load
(5) 0.9 dead load + 1.3 wind load
(6) 0.9 dead load - 1.3 wind load
For this case, the P-Delta effect associated with the overall sway of the structure can usually be accounted for, conservatively, by specifying the P-Delta load case to be 1.2 times dead load plus 0.5 times live load. This will accurately account for this effect in load combinations 3 and 4 above, and will conservatively account for this effect in load combinations 5 and 6. This P-Delta effect is not generally important in load combinations 1 and 2 because there is no lateral load.
It is also possible to accurately account for the P-Delta effect associated with the deformation of the members between their ends in the ETABS analysis, but we do not recommend that you do this. Instead, we recommend that you account for this effect using factors in your design. The ETABS design postprocessors assume that this has occurred and includes those factors, where appropriate, in the design.
To account for the P-Delta effect associated with the deformation of the members between their ends in the ETABS analysis, first break up all of the columns into at least two objects between story levels. Then run each of the six load cases above separately with a different P-Delta load combination for each. Again, it is recommended that this effect be included using factors in your design, as is performed in the ETABS design postprocessors.
Add button. Add a load pattern to the load case definition as follows.
Select the load pattern name from the Load Pattern drop-down list.
Type in an appropriate scale factor in the Scale Factor edit box.
Click the Add button.
Modify button. Modify a load pattern as follows.
Highlight the name of the load pattern to be modified in the Load Pattern display box (not the drop-down list).
Select a new load pattern from the drop-down list and, if necessary, change the scale factor by typing directly in the Scale Factor edit box.
Click the Modify button.
Delete button. Delete a load pattern from the load case definition as follows.
Highlight the name of the load pattern to be deleted in the Load Pattern display list (not the drop-down list).
Click the Delete button.