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Based on the second-order inelastic analysis, this paper presents a practical method of design for steel–concrete composite beam-columns that satisfies code performance requirements. The concept of second-order inelastic analysis for structures is to consider both geometric and material nonlinearities and their imperfections so that the real structural behavior can be captured and the assumption for effective lengths is not required. The refined plastic hinge approach proposed herein traces the gradual material yielding and simulates full plasticity of a cross-section. The definitions of the initial and full yield surfaces (which are used to initiate the yielding and to indicate the full plastic stage) for steel–concrete composite cross-sections, based on the cross-section analysis, are presented. The proposed method is verified in examples of isolated composite columns and frames.

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ABSTRACT:The experimental and analytical investigations on bare and composite rectangular hollow sections (RHS) used as members of trusses are presented in this paper. The load resistances of the trusses consisted of steel and concrete-filled RHS tubes are compared to quantify the beneficial effects due to the in-filled concrete. The maximum loads on the trusses are also computed by the design method in Eurocode 3 (Steel member) and Eurocode 4 (Composite member) and compared with the test results. The results showed that the use of effective length method in linear analysis and design method is less convenient and accurate than the second-order analysis. The second-order analysis and design method not only gives a more accurate prediction than the linear analysis, but it also provides an efficient design as the assumption of effective length is not required to guess. Most experiments were focused on the behavior of single member with ends restrained against lateral movement. In this paper, the end movements of the members were restrained by other connecting truss members. The end movements of the member induce the P-Δ effect its inclusion is important in the analysis and design.Several commonly used design codes provide different design methods on composite members such as Eurocode 4 [5], BS5400 [6] and CoPHK [7]. These codes contain various design methods for several types of composite columns and these methods include the first order linear analysis and effective length method for member buckling strength check. The accuracy of those design methods depend heavily on the precision of determination of effective length factor which is not quite possible to estimate since the idealized assumption for simple end conditions like pin and rigid ends are unrealistic in most practical structures. In this paper, two effective length factors were used to predict the design load, and the results will be compared with test results.As an alternative to the first order linear analysis with effective length assumption, the second-order analysis and design method for steel tube and concrete-filled steel tube members is recommended in many design codes such as Eurocode 4 [5] and CoPHK [7] as a preferred design method, especially when the elastic critical load factor is small. In the second-order analysis, the nonlinear effects such as P-δ, P-Δ effects and initial imperfection can be directly included in the analysis, and the estimation of the effective length is no longer required, and the member section capacity can be directly used for buckling strength check without modification factors. The individual member check is replaced by the section capacity check in a single equation in place of the approach