Design and analysis of a seismic resilient steel moment resisting frame equipped with damage-free self-centering column bases

Many recent research studies focused on the development of innovative seismic resilient structures by chasing the objectives of minimising both seismic damage and repair time, hence allowing the definition of structures able to go back to the undamaged, fully functional condition, in a short time. In this context, the present study investigates an innovative type of self-centring damage-free steel column base (CB) connection and its beneficial effects when used within steel moment-resisting frames (MRFs). The proposed connection consists of a rocking column equipped with a combination of friction devices, providing energy dissipation capacity, and post-tensioned bars with disk springs, introducing restoring forces in the joint. Contrary to conventional steel CBs, the proposed connection exhibits moment–rotation behaviours that can be described by simple analytical equations, allowing the definition of an easy-to-apply design procedure. Numerical models of the connection, developed in OpenSees, are validated against experimental results and successively implemented within a four-storey case study steel MRF. Incremental Dynamic Analyses are performed to derive the samples of the demand for the engineering demand parameters of interest while accounting for the record-to-record variability. Fragility Curves show the effectiveness of the proposed solution in reducing the residual storey drifts and in protecting the first-storey columns from damage, hence providing significant advantages in terms of repairability, and hence resilience of the structure with a negligible increase on the overall cost. The results show that the damage-free behaviour of the CBs is a key requirement when self-centring of MRFs is a design objective

Seismic performance of self-centering hybrid coupled wall systems: Preliminary assessments

Hybrid Coupled Wall (HCW) systems consist of reinforced concrete walls connected with steel coupling beams. HCWs benefit from the superior lateral stiffness of the reinforced concrete walls, while the coupling mechanism reduces the moment demand at the base of the walls. The present study investigates the seismic performance of a new HCW system equipped with friction-damped self-centering coupling beams and examines the efficiency of the new system in reducing residual deformations. The coupling beams of the intended HCW system consist of self-centering links, which can be easily repaired after severe earthquake events. The self-centering system utilized in this study features the following advantages distinguish it from conventional self-centering solutions: (i) it eliminates the coupling beams elongation problem (ii) it facilitates the application of pre-fabricated self-centering components to mitigate uncertainties raised by post-tensioning the connections on site. In this paper, the seismic behavior of the proposed lateral load-bearing system is investigated under several ground motion records and intensities. It is demonstrated that the applied self-centering mechanism has the capacity to minimize earthquake-induced residual deformations and repair time without increasing the damage level expected for the concrete walls in conventional HCWs

Seismic Response of a Steel Resilient Frame Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damage-free behaviour of the system. How-ever, to date, little attention has been paid to their column bases, which represent fundamental com-ponents in order to achieve resilience. In fact, column bases designed by current conventional ap-proaches lead to significant seismic damage and residual drifts leading to difficult-to-repair structures. The present paper evaluates the seismic performance of steel MRFs equipped with an innovative dam-age-free, self-centring, rocking column base joints. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behav-iour of the connection. Non-linear dynamic analyses have been carried out on a set of ground motions records to investigate the effectiveness of the column base in protecting the first storey columns from yielding and in reducing the residual storey drifts. Incremental Dynamic Analyses are used to investigate the influence of the record-to-record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters of the frame and of the column base connection. The results show that the damage-free behaviour of the column bases is a key requirement when self-cen-tering of MRFs is a design objective

A Review of Friction Dissipative Beam-to-Column Connections for the Seismic Design of MRFs

The use of friction-based beam-to-column connections (BCCs) for earthquake-resistant moment-resistant frames (MRFs), aimed at eliminating damage to beam end sections due to the development of plastic hinges, has been prevalent since the early 1980s. Different technical solutions have been proposed for steel structures, and some have been designed for timber structures, while a few recent studies concern friction joints employed in reinforced concrete structures. Research aimed at characterizing the behavior of joints has focused on the evaluation of the tribological properties of the friction materials, coefficient of friction, shape and stability of the hysteresis cycles, influence of the temperature, speed of load application, effects of the application method, stability of preload, the influence of seismic excitation characteristics on the structural response, statistical characterization of amplitude, and frequency of the slip excursion during seismic excitation. Studies aimed at identifying the design parameters capable of optimizing performance have focused attention mainly on the slip threshold, device stiffness, and deformation capacity. This review compiles the main and most recent solutions developed for MRFs. Furthermore, the pros and cons for each solution are highlighted, focusing on the dissipative capacity, shape, and stability of hysteresis loops. In addition, the common issues affecting all friction connections, namely the characteristics of friction shims and the role of bolt preload, are discussed. Based on the above considerations, guidelines can be outlined that can be used to help to choose the most appropriate solutions for BCCs for MRFs

Optimised Strategies for Seismic-Resilient Self-Centring Steel Moment Resisting Frames

In recent years there have been significant advancements in the definition of innovative minimal damage structures chasing the urgent requirements of more resilient societies against extreme seismic events. In this context, a type of seismic-resilient moment resisting frames (MRFs) is based on the use of Self-Centring Damage-Free (SCDF) devices in column bases and beam-to-column joints. However, when these devices are widespread across the whole structure, the details’ complexity increases significantly with respect to conventional solutions, thus limiting their practical application. To overcome this drawback, current research works are focusing on the definition of optimum locations for SCDF devices such that their effectiveness is maximised. Within this context, the present study investigates optimum locations for a limited number of SCDF devices to be used within mid- and high-rise MRFs. An 8-storey structure is selected for case study purposes and nineteen configurations are investigated considering different positions of SCDF joints. Numerical models of the selected configurations are developed in OpenSees and Incremental Dynamic Analysis are performed. The seismic responses of the case-study structures equipped with different layouts of SCDF devices are evaluated and compared. Some considera-tions in terms of optimal distributions of SCDF devices are made with the aim of maximising the efficiency of the solution and the seismic performance of mid- and high-rise MRFs

Performance-based assessment of seismic-resilient steel moment resisting frames equipped with innovative column base connections

Low-damage and self-centring column base connections have been proposed in the last two decades as innovative solutions able to provide the seismic resilience in Moment Resisting Frames (MRFs). Although many works have demonstrated the benefits deriving from the adoption of these systems, only a few research studies investigated the significant parameters influencing their self-centring capability. This paper investigates the influence of the frame layout (i.e., sto-reys and bays number) on the seismic performance of perimeter MRFs equipped with damage-free self-centring column bases previously studied by the authors. Nine case-study perimeter steel MRFs are designed and modelled in OpenSees. Incremental Dynamic Analyses are per-formed monitoring both global and storey-level Engineering Demand Parameters, including peak and residual interstorey drifts. Fragility curves are successively used to evaluate the self-centring capability of the structures. The present study provides insights on the use of the adopted con-nections for the residual drift reduction of MRFs and defines the boundaries of the investigated parameters for their application. Results highlight that the self-centring behaviour is particularly sensitive to the number of storeys and tends to reduce with the increasing height of MRFs equipped with the proposed connections

Low-Damage Friction Connections in Hybrid Joints of Frames of Reinforced-Concrete Buildings

Seismic-resilient buildings are increasingly designed following low-damage and free-from-damage design strategies that aim to protect the structure’s primary load-bearing systems under ultimate-level seismic loads. With this scope, damping devices are located in accessible and easy-to-inspect sites within the main structural frames where the damage concentrates, allowing the primary structure to remain mostly undamaged or easily repairable after a severe earthquake. This paper analyses the effects of friction-damping devices in structural joints of RC buildings endowed with hybrid steel-trussed concrete beams (HSTCBs) and standard RC columns. The study proposes innovative solutions to be adopted into RC moment-resisting frames (MRFs) at beam-to-column connections (BCCs) and column-base connections (CBCs). The cyclic behaviour of the joint is analysed through 3D finite element models, while pushover and non-linear time history analyses are performed on simple two-storey and two-span MRFs endowed with the proposed devices. The main results show that the BCC endowed with curved slotted holes and Perfobond connectors is the most effective in preventing the damage that might occur in beam, column, and joint, and it is adequate to guarantee good dissipative properties. For CBCs, the results showed that the re-centering system with friction pads is the most effective in containing the peak and residual drifts, preventing the plasticization of the column base

Lateral capacity and seismic characteristic of hybrid cold formed and hot rolled steel systems

This thesis addresses the application of hybrid cold-formed steel (CFS) - Hot-Rolled Steel (HRS) structures, as a new lateral force resisting system for light weight steel framed buildings in seismic regions. The study considers hysteretic behaviour, as well as maximum lateral load resisting capacity through comprehensive testing and advanced numerical analyses. The study identifies the advantages and disadvantages of the proposed hybrid system and provides in depth knowledge about performance characteristics of this innovative structural system, in order to facilitate the use of this system in earthquake-prone regions. The project is divided into three main parts: experimental, numerical and analytical studies. A comprehensive literature review is performed as a part of this study, in order to discover the existing gaps in the current knowledge regarding the structural performance of CFS structures and the methods for lateral performance enhancement. The literature review suggests that although CFS walls are not new, and have been used as non-structural components for many years, their application as main load-bearing structural frames is relatively new. That is, appropriate guidelines that address the seismic design of CFS structures have not yet been fully developed in the literature. In addition, the lateral design of these systems is not adequately detailed in the available standards of practice. There have been several attempts to improve the seismic performance of such structural system by different bracing or sheathing configurations. However, there is minimal background information available on hybrid systems such as hot rolled-cold formed structures. In this study, a series of CFS-HRS hybrid shear walls are constructed in order to investigate the lateral behaviour of the walls with different configurations to obtain the optimum combination of HRS and CFS. Different configurations are considered to provide the most efficient load transfer pattern from cold formed steel part of the wall to the Hot Rolled section, which is responsible for withstanding the lateral loads. The CFS part is aimed to transfer lateral loads to HRS part without any internal local failure. The ideal failure condition is the HRS yielding. Therefore, the optimum rigidity of the HRS part is of great importance to prevent any local failure happening prior to reaching the maximum lateral capacity of the HRS. For each experimental specimen, the hysteretic envelope curve is plotted, and different characteristics are evaluated. Since the failure mode of such systems is very complicated, the test results will provide the possible failure modes to be utilised for any further investigation or any optimisation analysis in numerical and analytical studies. In addition, Non-linear finite element (FE) analysis is employed using the ABAQUS software [1], in order to investigate the seismic performance of the proposed hybrid shear walls in multi-storey light steel frames. The nonlinear analysis accounts for different structural characteristics, including material non-linearity, geometric imperfection and residual stresses. The numerical models are verified based on experimental test results. The principal objective of this part of the study is aseismic optimisation of the proposed hybrid system and finding the corresponding dimensions and configurations to improve the strength and stiffness to achieve the objective. Using the hybrid wall panel system, a 4-storey building in an earthquake prone region is designed as per the relevant codes of practice. For the designed 4-storey building, the CFS part of the panel only bears the gravity loads, while a hot rolled steel collector transfers the lateral load to the HRS part acting as the main lateral load resisting system. Finally, the building is designed using different lateral load resisting systems and the results are compared with those from the proposed hybrid system in terms of cost. Furthermore, based on the real failure mode shapes obtained from test specimens, a Finite Strip Method program is developed to evaluate the elastic buckling mode shapes of a single stud with an arbitrary section detail. The code is helpful for design of CFS studs as explained in Chapters 3 and 5

PERFORMANCE EVALUATION OF TWO NEW SEISMIC RESISTANT DIAGRID FRAMING SYSTEMS

The diagrid system offers a visually appealing and structurally efficient structural system for gravity load bearing. The architectural elegancy and high structural redundancy of the diagrid structure makes it a desirable choice for tall building design. However diagrid structure is prone to high inelastic deformation demand during strong earthquakes. To address this issue of limited ductility and energy dissipation capacity in conventional diagrid framing, two new types of seismic resistant diagrid structural systems termed highly energy-dissipative ductile (HED) diagrid and hybrid diagrid framing systems are proposed in this research and their seismic performance is assessed. The proposed HED diagrid framing system provides a competitive design option in high seismic regions with its high ductility and improved energy dissipation capacity enabled by incorporating replaceable shear links interconnecting the diagonal members at their nodes. A parametric study has been conducted to investigate the effect of different design parameters on the seismic performance of this system. A new type of composite brace comprised of glass fiber reinforced polymer (GFRP)-tube confined concrete, steel core and post-tensioned tendons, is developed for self-centering diagrid members. The hysteretic behavior of a self-centering chevron assembly comprised of two inclined composite braces is subsequently examined. Constitutive modeling of GFRP-tube confined concrete with high confinement volumetric ratio is conducted with experimental data calibration under monotonic and cyclic compression. The constitutive model is implemented into a finite element analysis platform OpenSees to enable nonlinear analysis of complex structures utilizing this type of confined concrete elements. The self-centering chevrons are implemented in the lower stories of the hybrid diagrid framing system to form base diagonals with large stiffness, enhanced ductility and energy dissipation capability and enable a rocking behavior for the diagrid system. The structural characteristics and seismic behavior of these two new seismic resistant systems are demonstrated with a prototype 21-story building subjected to nonlinear static and dynamic analysis. The findings from nonlinear time history analysis verify that satisfactory seismic performance can be achieved by these structural systems subjected to design basis earthquakes in California, specifically showing re-centering behavior while all main structural elements remain elastic in both systems

Repairable Precast Buildings and Bridges

A new moment-resisting precast connection is developed in the present work through experimental and analytical studies to accelerate construction of bridges and buildings, to improve their seismic performance, and to quickly repair them through replacement of exposed reinforcement. The new precast joint detailing incorporates (1) detachable external reinforcing steel bars restrained against buckling, which is referred to as buckling restrained reinforcement (BRR), to develop plastic bending moments, (2) a steel pipe connecting the precast members through a pin connection to resist plastic shear forces, and (3) detachable mechanical bar splices to assemble and disassemble BRR at any time specifically after server event as a quick repair method. Feasibility and performance of a new type of BRR that can be used as detachable external reinforcing steel bars, were experimentally investigated. Furthermore, a simple design method for BRR with or without a section modification (e.g. dog-bone) is presented to further help engineers with the design of external reinforcement and energy dissipaters. The seismic performance of the proposed repairable precast connections was investigated through cyclic testing of four half-scale beam-column specimens detailed based on a nine-story building designed for Los Angeles, which is a high seismic region. A reference cast-inplace beam-column specimen was also included for comparison. The test results showed that the repairable precast connections can withstand more than fourteen times the design level earthquake with insignificant damage and ability to be repaired afterward. Furthermore, a comprehensive analytical study including pushover and nonlinear response history analyses was performed to investigate the seismic performance of three-, six-, and nine-story repairable precast and cast-in-place (CIP) buildings, and repairable precast bridge columns. It was found that the stiffness of the proposed precast system is 60% of conventional cast-in-place structures but the displacement capacity of the proposed precast structures can be four times higher than that in the conventional structures. The increase in the displacement demands of the precast systems due to lower initial stiffness is usually within the design limits and there is no need to increase the member sizes. Based on the construction, and experimental and analytical studies, it can be concluded that the proposed precast connection detailing is expected to improve the seismic performance of bridges and buildings, to expedite the construction, and to eliminate the need of structure total replacement after severe earthquakes since the repair is done by replacement of exposed bars