Experimental Study of Sliding Friction Damper for High-Performance Steel Structures

Passive energy dissipation devices are widely utilized in buildings to minimize structural and non-structural damage under seismic loading. Commonly used devices include, among others, yield, viscous and friction dampers. While the use of yield dampers, such as buckling-restrained braces (BRBs), has been fairly well established in the earthquake engineering practice, arguably, uncertainties regarding the amount of cyclic hardening of the BRB yield core necessitate the BRB qualification testing for the design of the BRB’s non-dissipative elements. An additional concern may be the residual deformations along the building height in the aftermath of earthquakes. Similarly, viscous dampers may exert forces that are influenced by the dynamic load imposed on a building due to their sensitivity to temperature and imposed velocity. Maintenance of viscous dampers due to potential leakage may be another complexity. While friction dampers do not generally experience the aforementioned issues, their use in seismic applications is still evolving. One reason could be that their performance is strongly dependent on the selected friction pad material type. Ideally, friction pads should have a static friction coefficient between 0.3 and 0.4 under high-pressure (between 10 and 20 MPa). They should also exhibit stable force-displacement hysteretic response under cyclic loading. All-in-all, the selection of a suitable friction pad material for structural applications is non-trivial. In this regard, friction pads commonly used in braking applications of the automobile industry may be effective to address long-term effects associated with corrosion and delamination of friction pads. In order to investigate their applicability in the context of seismic engineering, a sliding friction damper prototype is developed and tested under various loading histories at the EPFL Structures Laboratory. The damper is designed for a maximum axial force of 450 kN and can accommodate a maximum axial displacement of ± 80 mm. The force-displacement response of the friction damper is investigated by testing two friction pad types under two different pressure levels at the sliding interface. Cyclic tests with constant and increasing amplitudes are conducted at different rates to depict their influence on the force-displacement response of the friction damper. Furthermore, a pulselike loading protocol is carried out in order to test the friction damper under conditions similar to those of an actual seismic event. The evolution of the static and dynamic friction coefficients is examined by monitoring both the sliding force and bolt preload. The experimental results suggest that under sliding motion, the examined friction pads provide a fairly constant sliding force, thereby exhibiting similar static and dynamic friction coefficients. Furthermore, the force-displacement hysteretic response of the friction damper is fairly stable and repeatable. Finally, similar static friction coefficients are obtained for different slip loads and sliding velocities, suggesting that the behavior of the tested friction pads is nearly pressure- and velocity-independent.

The Role of the Composite Floor System and Framing Action in the Seismic Performance of Composite Steel Moment-Resisting Frames

Hammad El Jisr

With the advent of performance-based earthquake engineering (PBEE), the need for reliable prediction of earthquake-induced collapse of structures is essential. Despite the significant progress that has been made towards this goal, there are several hurdles that still need to be overcome. Deterioration models, which are currently used for simulating structural collapse, are largely based on available test data that featured subassemblies with overly simplified boundary conditions. These experiments did not ac-count for the force redistribution occurring within structural systems after the onset of nonlinear geo-metric instabilities under cyclic loading. For the case of composite steel moment resisting frames (MRFs), which is the primary focus of this thesis, the influence of the axial restraint provided by the slab continuity and framing action on the seismic behavior of composite steel MRFs has been recognized in prior work. Nevertheless, these effects have neither been thoroughly investigated nor quantified at lateral drift demands associated with dynamic instability of composite steel MRFs under seismic loading.In this doctoral thesis, a unique experimental program of a two-bay composite steel MRF subsystem was conducted at full-scale. The experimental program, which was corroborated by continuum finite element analyses, aimed at comprehending the role of the underlying physical mechanisms, associated with the slab continuity and framing action, on the hysteretic behavior of composite steel MRFs with a particular emphasis at large deformations associated with collapse. The research findings suggest that the slab continuity limits the extent of local buckling and beam shortening within the dissipative zones of composite steel MRFs. While nonlinear geometric instabilities attributable to local buckling and concrete crushing are pronounced in composite steel beams at large inelastic deformations, the framing action and slab continuity preserve the overall stability of the structural system even in cases that ductile cracks form within a dissipative zone. The test results suggest that the presence of the transverse beams within the floor system should be factored in capacity design principles of composite steel MRFs. The experimental findings also suggest that methods for computing the effective slab width in composite steel MRFs should be reassessed. The available data along with information from prior experiments informed the development of practice-oriented engineering models for the seismic assessment of composite steel MRFs. Moreover, a macro-model was proposed for simulating the hysteretic behavior of composite steel beams under cyclic loading. The model, which was validated thoroughly with available experimental data, simulates explicitly the cyclic deterioration in strength and stiffness of composite steel beams and their respective beam-slab connections. The proposed macro-model was employed to benchmark the seismic col-lapse risk of prototype buildings with composite steel MRFs, as well as the repairability of beam-slab connections within the framework of PBEE.

EPFL2022

The Role of the Composite Floor System and Framing Action in the Seismic Performance of Composite Steel Moment-Resisting Frames

Hammad El Jisr

EPFL2022