Developpement d'un fusible ductile pour les diagonales de contreventement faites de profiles w pour la conception parasismique de charpentes lourdes en acier

In the last two decades, the concept of ductile fuses appeared as a consequence of the adoption of capacity design principle in code seismic design provisions. For steel frames, these new design rules have generally resulted in cost increases compared to past practice. The impact has been particularly severe for steel concentrically braced frames when bracing members are selected according to their factored compressive resistance. In that case, brace connections, beams, columns and other connections must be designed to resist lateral load effects that correspond to the probable tensile and compressive resistances of the braces, which are much higher than those induced by the specified seismic loads. The reduction of the seismic design forces associated to ductile inelastic response are therefore partially or completely offset.;The role of ductile fuses shaped along bracing members is to keep the brace yield tensile resistance strength close to the brace compressive resistance, so that extra costs can be mitigated. Several effective ductile models have been developed for light single-storey steel buildings. Hence, it is relevant to transpose the concept for heavy steel frames where W-shapes are commonly used for the bracing members. The objectives of the project were to examine the feasibility of this variant and establish design guidelines for the system.;Finite elements analysis was carried out to assess the performance of fuses shaped by locally reducing the cross section area of the member in such a way that the brace compressive strength would not be affected. However, as was observed in previous similar studies, local buckling is likely which may detrimentally impact on the brace behaviour. A buckling restraining mechanism was progressively developed and the analysis of the successive failure modes enabled to set criteria to achieve the required performance, especially the system ductility. The bending moment that develops upon overall buckling of the brace turned out to be more harmful for the fuse due to the resulting additional compressive stresses induced in the fuse. It is therefore desirable that the confinement be designed to also resist part of the bending moment acting in the fuse zone.;Restraining local buckling still represents a challenge when implementing ductile fuses in bracing members. However, no comprehensive design rules have been established yet to determine the required strength and stiffness that must be provided by the confinement. As a consequence, an approach similar to the one used for buckling restrained braces has been chosen. The out-of-plane force caused by the local buckling of the fuse is resisted by steel plates assumed to form an equivalent beam system. Along this line, the contribution of these components to flexural stiffness has been idealized to obtain a practical and systematic design method. Although the approach that was adopted is conservative, the dimensions of the confinement system as designed remains generally small, with plate thicknesses usually smaller than that of the flat elements of the brace cross-sections. The procedure is iterative but it has been automated by means of an Excel-VBA software.;A full-scale test program was performed to validate the design assumptions. Two brace specimens with two short fuses shaped near the end connections were tested, as well as a one brace with a longer fuse located at one end only. In both cases, the fuses showed satisfactory performance, as the tensile capacity significantly reduced while the compressive remained nearly unchanged. The restraining system components seemed to work well in bending so that local instabilities were avoided. The fuses showed a higher ductility than expected, and their capacity matched those calculated according to the Canadian design standard, including the effect of the reduction of the cross section area and strain hardening effects.;The study also showed that the fuse would be advantageous for braces with a slenderness ratio exceeding 60, as the disparity between the tensile and compressive capacities becomes significant. The tensile capacity reduction is generally higher than 15 % when the slenderness ration is about 70, and can raise up to 30 % for the range above 100.;This research project focused on the performance of isolated bracing members, which would be applicable to single-storey buildings. Further research is needed to assess the impact of ductile brace fuses on the behaviour of multi-storey structures for which the system has been initially proposed.