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D1.F.5.2 Slenderness Effects and Analysis Considerations

Slenderness effects are extremely important in designing compression members. The ACI 318-14 code specifies two options by which the slenderness effect can be accounted for. 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 moments and forces, and the effect of the duration of loads. Another option is to approximately magnify design moments.

STAAD.Pro uses both these options. To perform the first type of analysis, use the command PDELTA ANALYSIS instead of PERFORM ANALYSIS. This analysis method will accommodate the requirements as specified in Section 6.7 of the ACI 318-14Code (Section 10.10 in previous editions), 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. If it is desired, STAAD.Pro can also accommodate any arbitrary moment magnification factor (second option) as an input, in order to provide some safety due to the effects of the duration of loads.

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, moment-magnification is based on empirical formula and assumptions on sidesway.

Considering all this information, it is our belief that a PDELTA ANALYSIS, as performed by STAAD.Pro , is most appropriate for the design of concrete members. However, you should note that to take advantage of this analysis, all combinations of loadings must be provided as repeat load cases and not as load combinations. This is due to the fact that load combinations are just algebraic combinations of forces and moments, whereas a repeat load case consists of one or more primary load case which are revised during the PDelta analysis based on the deflections. Also note that the you must provide the proper factored loads (e.g., 1.4 for DL etc.). STAAD.Pro does not factor the loads automatically.