Determination and evaluation of the seismic behaviour factor of high-post-yield stiffness concentrically-braced steel frames for improved seismic resilience

Abstract

Eurocode-8 aims to protect human life from a design-seismic event and damage restriction for a frequent seismic event. However, it doesn’t restrict the residual deformations and doesn’t consider confining the damage accumulation due to possible aftershocks. Therefore, traditional seismic-resistant frames undergo large residual deformations that may increase further due to aftershock sequences as the energy is dissipated through inelastic deformations in the primary framing elements. This thesis proposes a novel seismic-resistant steel frame that maintains building functionality after significant earthquakes and subsequent events by reducing the collapse probability, residual deformations, and damage accumulation while theoretically eliminating impractical repairs by concentrating the damage in replaceable elements. The frame has energy-dissipating chevron-type braces equipped with replaceable hourglass-shaped pins made of duplex stainless steel. Under design seismic loading, energy is dissipated through inelastic deformations confined in the replaceable pins while the other framing elements remain elastic. As a result of the inherent properties of the stainless-steel pins, the frame exhibits high post-yield stiffness, which reduces the residual drifts. Moreover, the system can hypothetically restore its normal seismic performance after replacing the damaged pins if the damage is confined in the pins and residual deformations are kept below the construction tolerance limits. The seismic performance objectives are set to prevent global collapse under a rare seismic event and secure human life under a design-seismic action. Furthermore, a target is established to offer a resilient seismic performance by eliminating the damage in the framing elements under a design-seismic event. Then, additional damage limitation objectives are set to limit peak and residual displacements under frequent and design-level earthquakes, respectively. Last, an objective is set so that the seismic performance of the frame is not curtailed due to aftershock sequences while limiting the damage accumulation in the pins. A novel Eurocode-8-based design methodology is developed for concentrically-braced systems with high post-yield stiffness and adopted for various archetypes. Then, the risk-consistent approach recently developed by (Vamvatsikos et al., 2020) (INNOSEIS) is adopted to determine Eurocode-8-compatible seismic behaviour and overstrength factors. Specifically, the behaviour factor is assessed through fragility analysis based on incremental dynamic analysis considering high and medium seismicity site-specific ground motion suites; each matches the seismic hazard at three European sites. In contrast, the overstrength factor is evaluated through nonlinear pushover analysis. Furthermore, the recently developed mainshock-consistent-aftershock sequences selection procedure developed by (Papadopoulos et al., 2020)is integrated with the INNOSEIS approach in a novel way to evaluate the effects of earthquake sequences effects on the seismic performance of the frame. Here, the behaviour factor is evaluated by adopting site hazard-specific sequences selected for a site in Terni, Central Italy, through damage-dependant fragility analyses based on nonlinear back-to-back dynamic analyses at multiple intensity levels. Afterwards, prediction models for the damage accumulation in the pins are developed by linear regression while employing cumulative energy or duration-based intensity measures. Then, the behaviour factor is evaluated in terms of damage accumulation using the most reliable prediction model. Therefore, a detailed numerical nonlinear model is constructed in OpenSees for the proposed frame integrating experimentally calibrated modelling features. The model incorporates geometrical and material nonlinearities capturing the strength and stiffness deterioration of the primary framing members while modelling the fracture of the pin-brace system. The detailed numerical model and the developed design methodology can be adopted for comparable systems with high post-yield stiffness, conceptually pinned connections, and possessing symmetric behaviour. It is validated that the seismic performance of the proposed frame, adopting a behaviour factor of 6.5 and a design overstrength factor of 3, prevents global collapse and assures life safety. Also, it ensures concentrating the damage in the pins under a design-seismic event. Moreover, it guarantees to keep the residual drifts below 1/300 under a design-seismic action and the peak drifts below 0.75% under a frequent seismic event. However, a reduced behaviour factor of 4 is recommended to keep the residual drifts below 1/500 and, for archetypes up to 12-storey, the peak drifts below 0.5%. Last, The performance is validated under mainshock-aftershock sequences for archetypes up to 12-storey, confirming that a behaviour factor of 6.5 guarantees to keep the damage accumulation index of the pins below 30%, demonstrating the superior fatigue capacity of the system under earthquake sequences. Here, the cumulative absolute velocity-based model is adopted as it efficiently predicts the damage accumulation index.