The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Methods of Structural Analysis according to AS 4100 – Part 1: General

Written By: Lexatus - Sep• 14•13


This post provides guidance on calculating design action effects as required by AS 4100. The methods of analysis recognized by AS 4100 are:

(a) first-order elastic analysis with moment amplification (Clause 4.4.2 of AS 4100)

(b) second-order elastic analysis (Appendix E of AS 4100)

(c) plastic analysis with moment amplification (Clause 4.5 of AS 4100), and

(d) advanced analysis (Appendix D of AS 4100).

These four methods consider the interaction of load and deformation that produce second-order effects. From an AS 4100 perspective, second-order effects (known as PΔ and Pδ effects) can increase the design bending moment for members subject to bending and axial force.

In first-order analysis, it is assumed that the member remains elastic under the action of the design loads for all strength limit states. As such, method (a) without moment amplification – i.e. first-order elastic analysis – does not consider these second-order effects and may be used for members with bending moments only, axial tension or compression force only and, for braced members, combined bending moments and tension forces.

Second-order effects, which are caused by changes in the geometry of the member, are not accounted for in first-order analysis. Consequently, some modification must be made for second order effects and AS 4100 includes methods for making a suitable adjustment to the calculated design actions.

Second-order elastic analysis does account for the effects of design loads acting on the structure and its members in their displaced and deformed configuration. With respect to AS 4100, no adjustment is required to the calculated design actions with a second-order analysis. Second order effects may be substantial in some frames.

All of the methods of analysis are discussed in detail in the commentary to AS 4100.


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Eng. Onorio Francesco Salvatore

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