Développement d'un fusible ductile pour les diagonales de contreventement faites de profilés 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.