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D8.E.4 Slenderness Effects and Analysis Consideration

Slenderness effects are extremely important in designing compression members. The IS:456 code specifies two options by which the slenderness effect can be accommodated (Clause 39.7). One option is to perform an exact analysis which will take into account the influence of axial loads and variable moment of inertia on member stiffness and fixed end moments, the effect of deflections on moment and forces and the effect of the duration of loads. Another option is to approximately magnify design moments.

STAAD.Pro uses of the first option. To perform this type of analysis, use the command PDELTA ANALYSIS instead of PERFORM ANALYSIS. The P-Delta analysis will accommodate all requirements of the second- order analysis described by IS:456, except for the effects of the duration of the loads. It is felt that this effect may be safely ignored because experts believe that the effects of the duration of loads are negligible in a normal structural configuration.

Although ignoring load duration effects is somewhat of an approximation, it must be realized  that the approximate evaluation of slenderness effects is also an approximate method. In this method, additional moments are calculated based on empirical formula and assumptions on sidesway (Clause 39.7.1 and,IS: 456 - 2000). The rules of Clause 39.7.1 have been implemented in STAAD.Pro. They will be checked if the ELY and ELZ parameters are specified.

Considering all these information, a P-Delta analysis, as performed by STAAD.Pro may be used for the design of concrete members.

Note: To take advantage of this analysis, all the combinations of loading must be provided as primary load cases and not as load combinations. This is due to the fact that load combinations are just algebraic combinations of forces and moments (i.e., analysis results), whereas a primary load case is revised during the P-delta analysis based on the deflections. Loads can be combined prior to analysis using the REPEAT LOAD command.
Note: You must specify the appropriate load factors (e.g., 1.5 for dead load, etc.) as STAAD.Pro does not factor the loads automatically.