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American Concrete Design
Section 3
3-6
3.5 Slenderness Effects and Analysis
Consideration
Slenderness effects are extremely important in designing
compression members. The ACI-318 code specifies two options by
which the slenderness effect can be accommodated (Section 10.10
& 10.11 ACI-318). 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 has been written to allow the use of both the 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
10.10 of the ACI-318 Code, 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 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, is most appropriate for the
design of concrete members. However, the user must note, to take
advantage of this analysis, all combinations of loadings must be
provided as primary load cases and not as load combinations. This
is due to the fact that load combinations are just algebraic