PSI - Issue 28

Romanin Luca et al. / Procedia Structural Integrity 28 (2020) 162–170 Author name / Structural Integrity Procedia 00 (2019) 000–000

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A first step towards the modelling of a welded structure up to the weld scale has been conducted. A numerical feasibility analysis has been conducted to assess the performance of a critical joint during an earthquake. The dimensioning performance parameter is not resistance but stiffness. In special applications, high tolerances are needed to guarantee functionality. The joint is required to maintain its tolerances even after the event of an earthquake, the plates are thus designed to remain elastic. Because of elasticity, the joint is decupled from the structure behavior and loads could be applied directly. In the hypothesis that welding distortions are already minimized and shims are employed to meet final tolerances, a doubt arises on permanent deformations that could take place after the first load application due to the hardened material in the weld region. A stress relief heat treatment could be necessary before installation to eliminate residual stresses and avoid the phenomenon. Being a one of a kind workpiece, a numerical study is convenient to assess if the hysteresis effect is negligible. In welded T-joint analyzed, the loads are transmitted from the bolts to the web through a partial joint penetration weld. The flange thickness is 100 mm while the web is 28 mm, the joint is designed to maintain geometrical tolerances even in the occurrence of exceptional loads as it could be an earthquake event. The analysis has been performed numerically and has been divided in two phases. The first phase deals with the welding simulation in order to obtain the residual deformation and stress field. The second phase introduces a cyclic external load to calculate deformations for each cycle. Numerical analysis has been conducted modelling a symmetrical 2D section of the joint. The results are compared in terms of residual stresses and deformations. In the welding region the material has already yielded because of material shrinkage, an appropriate material model has thus to be chosen to model accurately the behavior of the load application. A review of the material model available for cyclic loads and especially for welding has been conducted in the next section.

Fig. 1. Cross section of the welded joint analyzed.

Several works have been dedicated on earthquake assessment of steel structures. The vast majority is dedicated on beam global models neglecting the role of the connection as the most critical point, which failure could compromise all the beam. As an example, Kanyilmaz (2015) used a global approach neglecting the local nature of steel connections. He found good agreement with experimental results using fiber-based beam elements with distributed plasticity. The model gives good results until displacements demands are high due to damage in steel connections and local buckling phenomena. The modelling of steel connection is thus necessary to establish when a plastic hinge is formed. Studies conducted after the infamous Northridge and Kobe earthquake revealed that a percentage of the damage observed has been a consequence of substandard workmanship and improper inspection as noted by Bruneau, Uang, and Sabelli (2008). In addition, weld access holes and fillet welds used instead of full penetration welds did not allowed the exploitation of the full ductility of the joint. A practical solution is to use pre-qualified joint which have been physically tested to resist a seismic event such as the American FEM 350 “Recommended Design Criteria for Moment Resisting Steel Frames”, AISC 358 “Prequalified Connectionsf or Special and Intermediate Moment Resisting Frames for Seismic Applications” or the European EQUALJOINTS+ project.