There are two ways to specify the initial p-delta analysis in ETABS V7 and V8 as follows:
Non-iterative Based on Mass: 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 is identical to p-delta analysis in ETABS V6.
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 you have a single rigid diaphragm at each floor level though it also works for other cases as well.
The reason we provide this method is to allow you to consider P-Delta in cases where you have not specified gravity loads in your model. If you have specified gravity loads in your model, then in general, we recommend that you use the Iterative Based on Load Cases option.
Iterative Based on Load Cases: The load is computed from a specified combination of static load cases. This is called the P-Delta load combination.
For example, the load 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 that you use this iterative method in all cases except those where no gravity load is specified in your model.
*Iteration Controls: This area is active if you select the Iterative Based on Load Cases option in the Method area of the dialog box. The Maximum Iterations item and the Relative Tolerance -Displacements item are discussed in the subsection titled "Iterative Solution" in Chapter 33 of ETABS V7 manual. Note that the maximum number of iterations specified is the maximum number of additional analyses after the first analysis is run.
*P-Delta Load Combination: This area is active if you select the Iterative Based on Load Cases option in the Method area of the dialog box. Here you specify the single load combination to be used for the initial P-Delta analysis of the structure.
As an example, suppose 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 due to the overall sway of the structure can usually be accounted for, conservatively, by specifying the P-Delta load combination 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 since there is no lateral load.
It is also possible to accurately account for the P-Delta effect due to 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 this is what you have done and includes these factors, where appropriate, in the design. If you did want to try and account for the P-Delta effect due to the deformation of the members between their ends in the ETABS analysis then you should first break up all of your columns into at least two objects between story levels. Then you should 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 accounted for instead by using factors in your design as is done in the ETABS design postprocessors.
Notes
In ETABS, program uses only one combination which is specified in Analyze > Set Analysis > Set P-Delta Parameters. Program uses the same stiffness for all static load cases, response spectrum analysis and time history analysis. Unlike SAP2000 where you can specify more than one P-delta combinations, ETABS currently does not have this feature. Refer to the description below which documents how P-delta analysis is used in ETABS.
We plan to add multiple P-delta analysis for ETABS v10 release which will be identical to SAP2000 program.
Attachments
File Link | directory = tips/ | filename = p-delta_analysis_parameters.pdf | title = P-Delta Analysis Parameters(PDF Format, 0.02 MB)