Seismic retrofit

Résumé

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.), many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge ear

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Optimal placement of bracings for seismic retrofit of building frames

Mamta Parakh

2006

Chargement

Seismic Rehabilitation of Deficient Steel Braced Frames with Conventional and Innovative Retrofit Techniques

Valériane Matthey

Steel concentrically braced frames (CBFs) as lateral load resisting system are common in steel construction, for their high strength and architectural versatility. However, the brace connection details have to be well designed to ensure a ductility of the frame under seismic excitation. This Master Thesis studies the behavior of a steel CBF, located in Sendai (Japan). It experienced significant damage at brace connections after the 2011 Tohoku-Oki Earthquake in Japan. They are mainly due to the eccentricity created by the single-lap gusset plates along the long duration of the imposed ground motion hazard. The seismic response of the CBF is assessed by means of nonlinear analysis with the performance-based approach according to ASCE/SEI 41-13 (2014). As the braces do not meet any performance criteria, seismic retrofit is required. Three retrofit solutions are investigated and compared. Two of them are considered as conventional solutions. They consist to replace the braces and gusset plates, once with new Chevron braces and once with new X braces. The new braces are designed with the Capacity Design rules. The third solution modifies the CBF into a Rocking Braced Frame (RBF). This innovative high-performance system allows the column bases to uplift in order to dissipate the seismic energy and localize the damages in easily replaceable fuses to protect the primary members. The retrofit works are mainly located at the column bases. Dynamic analyses of the initial frame and its retrofits are computed for a seismic input recorded during the 2011 Tohoku-Oki Earthquake. The peak storey drift ratio (SDR), residual SDR and story shear forces are compared. The new Chevron CBF has the worst performance of the three retrofits. New X-bracing CBF and the RBF show a similar performance, the RBF being slightly better.

2020

Chargement

Concentrically Braced Frames (CBFs) built before the introduction of the capacity design concept do not usually meet modern seismic design requirements. They are vulnerable to seismic events and they could develop unpredictable failure mechanisms. An existing building on the EPFL campus which has these characteristics is selected as case study for this work. Retroﬁt measures are then deemed necessary for this building. Its most critical CBF is pinpointedandsubsequentlyretroﬁtted. Threeretroﬁtsolutionsareproposed. Acommonlyretroﬁt proposition using Conventional Buckling Braces (CBBs) is compared to two retroﬁt solutions applyingrecentlydevelopedhigh-performancebraceswithintentionalinitialeccentricitycalled Braces with Intentional Eccentricity (BIE) and Naturally Buckling Braces (NBBs). Compared to CBBs, these innovative bracing systems provide larger energy dissipation at small drift levels and offer larger post-yielding stiffness. In this work, these high-performance braces, that were only investigated in single brace conﬁgurations in previous studies, are examined in more realistic conditions. Initially, numerical models and equations deﬁning the physical behavior of BIE and NBBs are validated according to existing experimental test results. Moreover, a new design procedure able to deﬁne cross-section properties and initial eccentricity values is established. Subsequently, with the purpose of evaluating the three different retroﬁt solutions, a nonlinear model of the selected critical CBF is developed and studied through nonlinear response history analysis. The model consists of a 3-story V-braced CBF. The obtained results demonstrate that the beneﬁts resulting from both innovative bracing systems are successfully experienced by the structure when designed according to the established design procedure. Both global and local behaviors are positively affected, signiﬁcantly improving the seismicresisting performance of the analyzed structure. Those ﬁndings suggest that BIE and NBBs appear to represent a viable alternative to CBBs for the retroﬁtting of existing bracing systems

2020

Chargement