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Steel frame wind bracing systems are usually made of hot rolled profi les connected to frame elements directly or through a gusset plate. The behaviour of angle bracing members is generally complex since controlled by tension or compression, bending and torsion. The common practice is to transform the problem of complex behaviour into the buckling strength of a truss member. This paper deals with an analytical formulation of the force-deformation characteristic of a single angle brace subjected to compression. A strut model takes into consideration the effect of brace end connections and softening effect of its force-deformation characteristic. Two different boundary conditions, typical for engineering practice, are dealt with. Experimental program of testing the behaviour of angle brace in portal sub-frame specimens is described. Results of experimental investigations are presented. They are used for the validation of developed model. Conclusions are formulated with reference to the application of validated brace model in the analysis of braced steel frameworks.

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The paper summarises the current progress in methods of advanced analysis for design of frames with semirigid joints. The methods presented in the paper belong to general second-order refined plastic-hinge methods that allow for the combined effects of joint stiffness degradation and distributed plasticity along the member length as well as across the member sections. The advanced analysis for steel frame design, proposed by the authors, is based on the spring-in-series model. The effect of joint semi-rigidity and partial strength is taken care of by specifying certain values of the initial stiffness, ultimate moment and the shape factor of the moment-rotation characteristic for the spring representing the joint. The effect of imperfections affecting the performance of imperfect structural members in compression is modelled by the application of a simplified tangent modulus concept combined with the reduction of the initial value of the elasticity modulus. The effect of residual stresses is taken care of by specifying certain values of the shape parameter for the moment-rotation characteristic of the spring representing the gradual yielding of the member. It is dependent upon the cross-section type and fabrication method (ie upon the residual stress pattern resulting from rolling or welding processes). A case study analysis is presented. Concluding remarks referring to the application of advanced analysis in design, pertaining to the study case considered, are drawn.