Analysis of frames with concentric bracings considering their evolutive behaviour

Clauses 6.7.1(1), In Eurocode 8, the design concept for frames with concentric bracings is that diagonals in

6.7.2(2) tension are the reliable dissipative zones, while diagonals in compression buckle and do not contribute significantly to stiffness and resistance.

The problem is that the reality is evolutive. In a first stage, the compression action effect in the diagonal can increase up to the buckling strength ,Npl Rd; however, in the following cycles the strength of that diagonal in compression decreases, due to permanent deformation from the first buckling stage, so that the value of the buckling strength, Nph Rd, cannot be reached anymore. This decrease in resistance is sharp, progresses with loading cycles and cannot easily be estimated. The stiffness and strength of the structural system accordingly decrease to the reference situation envisaged in EN 1998-1. The question is to propose a design approach which is safe-sided in estimations of the evolutive behaviour. This reality is behind the Eurocode 8 clauses which deal with diagonals in compression, although these are in principle outside of the reference design model.

Clause 6.7.2(3) For all types of bracing, design considering explicitly the contribution of the compression diagonals using a justified non-linear procedure is allowed, as stated in clause 6.7.2(3). For the sake of simplicity, Eurocode 8 proposes simple alternatives applicable with a standard

Fig. 6.12. Moment redistribution to reduce design moments

A/Sd (compression)

A/Sd (compression)

Fig. 6.13. Decoupled diagonal bracings linear elastic analysis. These alternatives entail special requirements, which are discussed later.

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