Typically, design codes require that second order P-Delta effects be considered
when designing concrete frames. These effects are the global lateral translation
of the frame and the local deformation of members within the frame.
Consider the frame object shown in Figure A-1, which is extracted from a story
level of a larger structure. The overall global translation of this frame object is
indicated by ∆. The local deformation of the member is shown as δ. The total
second order P-Delta effects on this frame object are those caused by both ∆ and
δ.
The program has an option to consider P-Delta effects in the analysis. When
P-Delta effects are considered in the analysis, the program does a good job of
capturing the effect due to the ∆ deformation shown in Figure A-1, but it does
not typically capture the effect of the δ deformation (unless, in the model, the
frame object is broken into multiple elements over its length).
Consideration of the second order P-Delta effects is generally achieved by
computing the flexural design capacity using a formula similar to that shown in
the following equation.
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