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

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.

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

Filling materials may exist in all scales of rock fractures, not only influencing seismic wave attenuation, but controlling rock mass instability. The objective of this study is to experimentally investigate the seismic response of rock fractures filled with granular materials. This experimental study simplifies the complex interaction between seismic waves and filled rock fractures and considers the seismic-induced compressive responses of a filled single fracture and a set of filled parallel fractures and the seismic-induced frictional slip on a filled fracture. The seismic-induced compressive response of a filled single fracture is investigated by a split Hopkinson rock bar (SHRB) test. The SHRB test simulates the P-wave propagation across the filled fracture and verifies the analytical solutions predicted by the displacement and stress discontinuity model. The seismic wave attenuation across the filled fracture is due to wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The specific fracture stiffness and the specific initial mass of the filling materials are two key parameters in interrelating the physical, mechanical and seismic properties of the single fracture filled with dry granular materials. The SHRB test is modified to study the seismic-induced compressive responses of a set of filled parallel fractures and to verify the analytical solutions predicted by the modified recursive method based on the displacement and stress discontinuity model. The seismic wave attenuation across the filled parallel fractures is due to multiple wave reflections between the filled fractures, as well as wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The dynamic compaction of the filling materials induces the decreases of loading rate and dominant frequency when the P-wave propagates across each filled fracture in a fracture set. The seismic-induced frictional slip on a filled fracture is performed by a dynamic-induced direct-shear (DIDS) test. A generated plane P-wave propagates as a dynamic shear load and induces the frictional slip along a layer of filling materials. The dynamic shear stress is non-uniformly accumulated in the filling materials. The shear stress at the trailing edge controls the dynamic triggering of frictional slip owing to the highly compacted filling materials. The dynamically triggered frictional slip is quickly arrested by the re-connection of sand contacts after the instantaneous disturbance, while the statically triggered frictional slip is unrecoverable during a short duration and contains a main slip and a few short slips before and after the main slip. The seismic response of rock fractures filled with granular materials is decomposed into the seismic-induced compressive responses and the seismic-induced frictional slip in this study. Both seismic-induced responses are strongly related to the dynamic response of the filling materials. The analytical models and the experimental experiences are possible to be numerically combined to study random wave propagation across rock fractures filled with granular materials and to be applied to estimate seismic wave attenuation and rock mass instability.

EPFL2013

Displacement-based seismic design of symmetric single-storey wood-frame buildings with the aid of the N2 method

Katrin Beyer, Panagiotis Mergos

This paper presents a new methodology for the displacement-based seismic design of symmetric single-storey wood-frame buildings. Previous displacement-based design efforts were based on the direct displacement-based design approach, which uses a substitute linear system with an appropriate stiffness and viscous damping combination. Despite the fact that this method has shown to produce promising results for wood structures, it does not fit into the framework of the Eurocode 8 (EC8) provisions. The methodology presented herein is based on the N2 method, which is incorporated in EC8 and combines the non-linear pushover analysis with the response spectrum method. The N2 method has been mostly applied to reinforced concrete and steel structures. In order to properly implement the N2 method for the case of wood-frame buildings, new behavior factor–displacement ductility relationships are proposed. These relationships were derived from inelastic time history analyses of 35 SDOF systems subjected to 80 different ground motion records. Furthermore, the validity of the N2 method is examined for the case of a timber shear wall tested on a shake table and satisfactory predictions are obtained. Last, the proposed design methodology is applied to the displacement-based seismic design of a realistic symmetric single-storey wood-frame building in order to meet the performance objectives of EC8. It is concluded that the simplicity and computational efficiency of the adopted methodology make it a valuable tool for the seismic design of this category of wood-frame buildings, while the need for extending the method to more complex wood-frame buildings is also highlighted.

2015

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

EPFL2013