Initial P-Delta analysis may be specified in ETABS using either of the following two methods:
- Non-iterative Based on Mass, where load is automatically computed from the mass at each level. This is an approximate method which does not require an iterative solution, providing for faster computation. P-Delta is considered by treating the structure as a simplified stick model, a process which is most effective with a single rigid diaphragm at each level. Local buckling is not captured as effectively.
The benefit of this non-iterative method is that P-Delta may be considered in load cases which do not specify gravity load. When gravity load is specified, we generally recommend the Iterative Based on Load Cases method.
- Iterative Based on Load Cases, where load is computed from a specified combination of static load cases, then known as the P-Delta load combination. This is an iterative method which considers P-Delta on an element-by-element basis. 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 which become available include:
- Iteration Controls, where a menu presents options for Maximum Iterations and Relative Tolerance-Displacements. Maximum Iterations specifies the number of additional analyses to run after the first. Additional details are described in the ETABS Manual (Iterative Solution).
- P-Delta Load Combination, where users may specify the single load combination for the initial P-Delta analysis.
- Iteration Controls, where a menu presents options for Maximum Iterations and Relative Tolerance-Displacements. Maximum Iterations specifies the number of additional analyses to run after the first. Additional details are described in the ETABS Manual (Iterative Solution).
For example, suppose that a building code requires the following load combinations:
(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
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 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.
P-δ effect
ETABS may account for P-δ effect, which is associated with local deformation relative to the chord between member ends, though we do not recommend that this method be implemented because it will significantly increase computational time without providing the benefit of useful information. Instead, P-δ may be captured through either of the following methods:
- Apply design factors, which ETABS post-processing assumes to be done. These factors are therefore included in design, when applicable.
- Divide members into segments (at least two per column), then run each load case separately with a different P-Delta load combination for each.
Notes
- ETABS uses the same stiffness for all static load cases, response-spectrum analysis, and time-history analysis.
- P-delta combinations may be specified through Analyze > Set Analysis > Set P-Delta Parameters.
