There are two ways to specify the initial p-delta analysis in ETABS V7 and V8 as followsInitial P-Delta analysis may be specified in ETABS via Define>P-Delta Options and using either of the following two methods:
- Non-iterative Based on Mass
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- , in which load is automatically computed
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- from the mass at each level
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- . This
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- is an approximate
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- method which does not require an iterative solution
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- , providing for faster computation. P-Delta is considered by treating the structure as a simplified stick model
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- , a process which is most effective with a single rigid diaphragm at each
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- level. Local buckling is not captured as effectively.
The
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- benefit of this non-iterative method is that P-Delta may be considered in load cases
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- which do not specify gravity load. When gravity load is specified, we generally recommend the Iterative Based on Load Cases
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- method.
- Iterative Based on Load Cases
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- , in which load is computed from a specified combination of static load cases
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- , then known as the P-Delta load combination
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- . This is an iterative method which considers P-Delta on an element-by-element basis.
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*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.
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- Local buckling is captured more effectively. An example application may be when load includes the dead load case and a fraction of a live load case.
When the iterative method is selected, two additional options become available :- Convergence Tolerance (Relative), Iteration is used to make sure that equilibrium is achieved at each step of the analysis. Use this parameter to set the relative convergence tolerance that is used to compare the magnitude of force error with the magnitude of the force acting on the structure. Using a smaller value ensures better equilibrium, although the default value is usually adequate.
- P-Delta Load Combination, in which users may specify the single load combination for the initial P-Delta analysis
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- .
As an For example, suppose that the a building code requires the following load combinations to be considered for design:
Code Block
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(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 A P-Delta load combination of 1.2 DL + 0.5 LL is typically conservative when considering 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 a structure. Combinations (3) and (4) will accurately capture this effect, while (5) and (6) should be conservative. Combinations (1) and (2) have no lateral load, therefore P-Delta effect should not be of concern. Please notice this is a non-linear static case and should be performed only after the model runs without any numerical issues for simple linear cases and results of basic variables such as deformations are within realistic or expected values.
P-δ effect
ETABS accounts for P-δ effect, which is associated with local deformation relative to the chord between member ends. In mots practical cases , this effect can be captured using design factors. There is usually no need to divide columns. More specific information about P-δ can be found here: P-Delta effect
Notes
- ETABS uses the same stiffness for all static load cases, response-spectrum analysis, and time-history analysis.
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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