Experimental tests on steel members with variable I welded section

Steel members with variable I cross sections are often used in current practice for the design of pitched-roof portal frames. Tapering of the members can be applied both for rafters and columns where in plane bending moment reaches significant values. When important gravitational loads are applied at the level of the roof, considerable bending moment develops nearby the connection between rafter and column. Beside this, the effect of the axial compressive force shall be accounted for as well, hence the stability issues become more complex. Thus the ultimate capacity should be influenced by the applied lateral restraints, which are provided in practice by side-rails, purlins and in some particular cases by fly braces. Analytical evaluation of the ultimate capacity of the tapered web members subjected to both bending moment and compression, which are also laterally restrained, represents a real challenge for structural engineers when applying the current EN1993-1 rules. The interaction formula of EN1993-1, as it is stated in the code, can be applied only for uniform members. There are some provisions that might be applied for non-uniform members, namely the general method, but its application requires advanced finite element analysis. The paper presents the results obtained experimentally, that allows emphasizing the actual behaviour and ultimate capacity of steel welded members with variable I cross sections. The tests were done on elements with different web thickness and different typology of lateral restraints (e.g. no restraints, lateral restraints and torsional restraints). In the end a complete comparison between numerical, analytical and experimental results will be also presented. Although it was observed that failure mechanism for the selected specimen varies in function of the web thickness and lateral restraints, the difference in the ultimate capacity is rather small.