The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Methods of Structural Analysis according to AS 4100 – Part 3: Braced frame vs Sway frame in the moment amplification method for elastic analysis

Written By: Lexatus - Sep• 15•13


In the previous post, we discussed about the Elastic analysis at first-order with moment amplification. We have seen how this method can be a good alternative to a second-order analysis. Basically, we need to increase the moment by a certain quantity including in this way the second-order effects.

We have also introduced the moment amplification factors. The factors are two:

– δb => moment amplification factor for a braced member (Clause;

– δs => moment amplification factor for a sway member (Clause

This method can always be used?

No, there is a limitation. When the factor is more than 1.4 (that means we are increasing by 40% the first-order bending moment), a full second-order analysis has to be applied. In this case, instead of referring to Clauses or, we need to proceed in accordance with Appendix E.

Why the difference between braced member and say member?

There is an important difference between the two cases:

– for a braced frame, the PΔ effects may be neglected, hence the moment amplification factor to use is δb, that takes into account only the ratio between the axial load and the buckling load. To put it simple, in a braced frame we only look at local instability Pδ;

– for a sway frame, PΔ effects cannot be neglected, hence we need to look at both PΔ and Pδ (local instability is always present).

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Eng. Onorio Francesco Salvatore

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