BOLTED CONNECTIONS FOR EASILY REPAIRABLE SEISMIC RESISTANT STEEL STRUCTURES

Recent years have brought significant advances in the design capabilities and construction practices of steel structures. These were partially caused by technological development and a direct effect of the research community efforts towards the mitigation of the earthquake induced damage. Making the traditional structural systems more resilient is one of the directions taken but, more and more, solutions with reduced post-earthquake repair costs are preferred. Steel structures are particularly malleable in the modern spirit of integrating devices which render the structure as “low-damage” or “easily repairable”. The recent earthquakes of Japan and New Zealand have demonstrated the feasibility and the advantages of such structural typologies. The current work presents an investigation on two steel structural solutions, including thus both moment resisting and braced frames, which have the potential of being easily used in practice, with minimal alteration of the design and erection procedures and improved post-earthquake economic benefits. The thesis focuses on (i) bolted connections of detachable short links for eccentrically braced frame and second, and (ii) on bolted friction connections for moment resisting frames. The main objective is to facilitate the application of these structural solutions in practice by enhancing the knowledge of their relevant bolted connection design and behavior

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

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

An innovative dual concentrically-braced moment-resisting steel frame for increased seismic resilience

Improving seismic resilience of buildings is one of the current challenges in structural engineering. In the context of steel structures, design of conventional systems in accordance to current codes aims at preventing collapse and ensuring life safety under the design earthquake. However, in major seismic events, these systems have experienced extensive damage in the main structural membersand large residual drifts, causing downtime and significant socio-economic losses. This thesis presents the development and validation of an innovative dual steel frame that reduces structural damage and residual drifts for enhanced seismic performance. The proposed system consists of a moment-resisting frame with concentric braces equipped with seismic dampers. These are stainless steel pins with high post-yield stiffness, placed in series with the bracing members. Replaceable elements are inserted in the beams to absorb plastic deformations that would concentrate in the beam-column connections. The seismic performance of the proposed dual frame is evaluated using experimentally-validated finite element models of a prototype steel building. The numerical results show that, under the design and maximum earthquakes, residual storey drifts are minimised due to the high post-yield stiffness of the seismic dampers and the elastic deformation capacity of the moment frame. Structural damage is concentrated in the replaceable seismic devices, indicating the potential for a quick recovery after a strong earthquake. The collapse potential of the proposed frame is also investigated. The fracture capacity of the seismic dampers is experimentally evaluated using two full-scale geometries in a configuration reproducing the damper-brace connection. Criteria for predicting ductile fracture under ultra-low cycle fatigue are calibrated using coupon specimens and complementary finite element analyses, and validated performing explicit simulations of the full-scale tests. The collapse of the dual frame is studied by means of incremental dynamic analyses explicitly simulating the ductile fracture of the seismic dampers. The results show that the dual frame has a superior seismic resistance against collapse as a result of the large energy dissipation and fracture capacities of the seismic dampers.EPSR

Seismic Behavior and Design of the Linked Column Steel Frame System for Rapid Return to Occupancy

The Linked Column Frame (LCF) is a new brace-free lateral structural steel system intended for rapid return to occupancy performance level. LCF is more resilient under a design level earthquake than the conventional approaches. The structural system consists of moment frames for gravity that combines with closely spaced dual columns (LC) interconnected with bolted links for the lateral system. The LC links are sacrificial and intended to be replaced following a design level earthquake. The centerpiece of this work was a unique full-scale experiment using hybrid simulation testing; a combination of physical test of a critical sub-system tied to a numerical model of the building frame. Hybrid simulation testing allows for full scale study at the system level accounting for the uncertainties via experimental component and having the ability to model more conventional behavior through numerical simulation. The experimental subsystem consisted of a two story LCF frame with a single bay while the remainder of the building was numerically modeled. Two actuators per story were connected to the specimen. The LC links have been designed to be short and plastically shear dominated and the LCF met the design intent of 2.5% inter-story drift limits. For evaluating the LCF response, hybrid testing was performed for ground motion at three different intensities; 50%, 10% and 2% probability of exceedence in 50 years for Seattle, Washington ground motions. The system overall had exhibited three distinct performance levels; linearly elastic, rapid return to occupancy where only the replaceable links would yield, and collapse prevention where the gravity beam components also became damaged. Results demonstrated a viable lateral system under cyclic and seismic loading, offering a ductile structural system with the ability to rapidly return to occupancy

Design and analysis of dual EBFs equipped with prequalified connections

The aim of this work is to present the results of the design of Dual-Eccentrically Braced Frames (D-EBFs) according to the Theory of Plastic Mechanism Control. These frames are equipped with four prequalified connection typologies, analysed in the framework of the RFCS founded EqualJoints Plus research project. They are first designed disregarding the presence of the joints and after that accounting for it. The seismic performances of the structures are investigated by both pushover and non-linear dynamic analysis