The 22 February 2011 Mw 6.2 Christchurch (Lyttelton) earthquake was a particularly severe test for both modern seismically designed and existing non‐ductile reinforced concrete (RC) buildings. Some ...16.2 % of 833 buildings with RC systems within the Christchurch central business district (CBD) were severely damaged. There were 182 fatalities, 135 of which were the unfortunate consequences of the complete collapse of two medium‐rise RC buildings. As with the post‐Northridge 1994 earthquake, the design performance of “modern” structures is being scrutinized – with the inevitable question: is “life safety” but irreparable damage still a valid performance target? This brief paper presents a summary of RC building damage from a broad performance‐based earthquake engineering perspective. Several preliminary lessons, not all of them surprising, and the issues that have arisen will be discussed using case study buildings, with suggestions for urgently needed research areas.
Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other ...events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.
The research presented here seeks to address the key parameters influential on out-of-plane (OOP) instability of rectangular walls, which was observed in the 2010 Chile and the 2011 New Zealand ...earthquakes in some well-confined modern walls. For this purpose, a finite element model, previously verified for its capability to reliably simulate different failure modes of this type of structural walls, was adopted. The parametric matrix included a series of specimens designed to be tested by the authors and other specimens already tested by other researchers. The effect of slenderness (unsupported height-to-thickness) ratio, reinforcement ratio, length and axial load on the OOP response of these specimens is scrutinized using the predicted response of the boundary region longitudinal reinforcement at the elevation corresponding to the maximum OOP displacement in each case. For a given slenderness and longitudinal reinforcement ratio, development of a critical average tensile strain over a certain height can lead to formation of OOP instability in walls. Therefore, the effect of these parameters on the strain history, strain gradient along the wall height, as well as the stress–strain response of the longitudinal bars throughout the cyclic loading are evaluated and the variation of the OOP response attributable to the effect of each parameter is discussed. This parametric study indicated that the initiation of OOP displacement in doubly reinforced walls after unloading from a peak displacement and during reloading in the opposite direction is in line with the significant reduction of compressive stiffness (yielding in compression) in the boundary region longitudinal bars along a specific height (at least 60% of the wall height). Based on the findings of this study, an experimental campaign was designed and four of the parametric models were experimentally tested under in-plane cyclic loading. In addition to the parameters noted above, the effects of shear strength, boundary conditions and eccentricity of material properties across the wall thickness on the OOP response of the benchmark parametric model are also briefly discussed. The experimental observations as well as the numerical versus experimental comparisons are presented in a companion paper.
Structural frames made of prefabricated laminated timber beams and columns connected by unbonded post-tensioning and additional mild steel reinforcement have recently been proposed for multi-story ...timber buildings. The benefits of post-tensioning to assemble prefabricated timber elements are rapid erection, simple connections, and high seismic resistance. It has been shown that prefabricated post-tensioned timber members can be designed to have excellent seismic resistance, with the post-tensioning providing re-centering capacity after major earthquakes, while energy is dissipated through yielding of replaceable mild steel devices.
This article summarizes the results of an investigation into the seismic response of full-scale timber beam-column joints, subjected to extensive experimental study and numerical modelling, performed as part of a larger research program on timber structures at the University of Canterbury, New Zealand. The beams and columns were fabricated from laminated veneer lumber (LVL). The tested joint was designed as part of a moment-resisting frame for a six-story building located in a high-seismic region. The timber members and the re-centering/dissipating elements were optimized to produce the intended dissipating-recentering behavior and commercially available prestressing arrangements were used to verify their applicability in a timber structure.
The results of the experiments on both interior and exterior beam-column joints are presented, for both post-tensioned-only and hybrid models. The post-tensioned-only solutions exhibited nonlinear elastic behavior with full re-centering while the hybrid systems provided significantly greater levels of energy dissipation compared with the post-tensioned-only solution. The joints were also tested with a number of additional features such as steel armoring plates at the interface, reinforcement in the form of long screws embedded into the column and combination of the two. The armored the column faces with steel plates exhibited higher stiffness and smaller deformation in compression perpendicular to the grain. The joint region within reinforced column reduce the subsequent drop in prestressing forces in the system. Combination of armoring and reinforcements protect the joint region through minimizing joint shear panel deformation. In general, the tested arrangements exhibited high levels of ductility and negligible residual deformations with no significant damage of the structural elements. The numerical models accurately predict the behavior of different types of beam-column joints tested and can be used to design similar connections. These full-scale tests also demonstrate the practical feasibility of post-tensioned timber frames for multi-story timber buildings.
In recent decades, significant research efforts have been devoted to developing fragility and vulnerability models for mainshock-damaged buildings, i.e., depending on the attained damage state after ...a mainshock ground motion (state-dependent fragility/vulnerability relationships). Displacement-based peak quantities, such as the maximum interstory drift ratio, are widely adopted in fragility analysis to define both engineering demands and structural capacities at the global and/or local levels. However, when considering ground-motion sequences, the use of peak quantities may lead to statistical inconsistencies (e.g., fragility curves’ crossings) due to inadequate consideration of damage accumulation. In this context, energy-based engineering demand parameters (EDPs), explicitly accounting for cumulative damage, can help address this issue. This paper provides an overview of recent findings on the development of aftershock-fragility models of mainshock-damaged buildings. Particular focus is given to state-of-the-art frameworks for fragility analyses based on cumulative damage parameters. Moreover, a literature review on damage indices and energy-based concepts and approaches in earthquake engineering is reported to better understand the main advantages of the mostly adopted energy-based parameters, as well as their limitations. Different refinement levels of seismic response analyses to derive fragility relationships of mainshock-damaged buildings are also discussed. Finally, the benefits of adopting energy-based EDPs rather than, or in addition to, peak quantities in state-dependent fragility analyses are demonstrated on a reinforced concrete frame building. Specifically, a refined lumped plasticity modeling approach is adopted, and sequential cloud-based time-history analyses of a Multi-Degree-of-Freedom (MDoF) model are carried out. The results highlight that energy-based approaches for fragility analysis effectively capture damage accumulation during earthquake sequences without inconsistencies in the obtained statistical models. On the other hand, estimating global or local structural capacity in terms of cumulative EDPs is still challenging. Further experimental data are needed to better calibrate the quantification of energy-based damaged states.
The severe socio-economic impact of recent earthquakes has further highlighted the crucial need for a paradigm shift in performance-based design criteria and objectives towards a low-damage design ...philosophy, in order to reduce losses in terms of human lives, repair/reconstruction costs, and recovery time (deaths, dollars and downtime). Currently, displacement-based parameters are typically adopted to design/assess the seismic performance of the structures, by limiting the maximum displacement or the maximum interstorey drift ratio (IDR) reached by the structure under different earthquake intensities. However and arguably, displacement-based quantities are characterized by inherent weaknesses, since, for instance, they are not cumulated parameters, thus not able to capture directly the effects of multiple cycles, deterioration and damage cumulation. Therefore, in the last decades, energy-based approaches were investigated and developed in order to establish alternative engineering demand parameters for the assessment of post-event damage through a dynamic energy balance. Towards the main goal of developing an integrated Displacement and Energy-Based Design/assessment procedure (DEBD) for actual use in practice, this research work proposes an innovative approach based on the use of inelastic spectra correlating the energy components with the corresponding maximum displacement response parameters of the structure. In practical terms, the proposal is to further integrate and develop the well-known Direct Displacement-Based Design, by directly adopting the hysteretic energy as an additional design parameter. The energy inelastic spectra are developed through an extensive parametric analysis of Single-Degree-of-Freedom (SDoF) systems, with different nonlinear hysteretic models. In such an approach, the maximum seismic energy demand imparted to a structure can be directly predicted and controlled, whilst distinguishing the various components of the energy balance, including the hysteretic one. The effects of near-field and far-field earthquakes are also investigated. Results show that in the first case the seismic demand is concentrated in the peak of a few large cycles that absorb the demand energy induced by the high component in peak ground velocity in the second case the higher equivalent number of plastic cycles tends to become critical for structures with inadequate structural details and prone to suffer by cumulative cycles and overall plastic fatigue mechanisms.
This article presents the results of shake-table experiments of a 2/5 scale 3-storey reinforced concrete (RC) frame building designed with non-ductile detailing and no capacity-design considerations, ...tested with and without column lap-splices in the upper two storeys in two experimental series comprising different input motions. The specimen consisted of two 2-span frames in parallel jointed together by transverse beams and floor slabs. It included a set of “older” detailing comprising of: (1) plain round bars, (2) 180° end hooks in beam longitudinal reinforcement, (3) no transverse reinforcement in the joints, and (4) lap splices in potential plastic hinge zones (first test series only), which, alongside a strong-beam and weak-column mechanism, create a very fragile fuse in the panel zone region of the exterior/corner beam-column joints. In the first experimental series, the structure experienced predominantly elastic responses during the first two tests, and a top-storey rocking mechanism due to column lap-splices failure during the third test. In the second experimental series, the modified specimen with welded lap-splices experienced minor damage during the first test whereas it suffered severe shear failures in the corner beam-column joints of the first storey during the second test, associated to inter-storey drift ratios and joint rotations of up to 3.8% and 0.035 rad, respectively. The large global and local demands suffered by the specimen during this last test were associated to resonant effects, as found with the Short-Time Fourier Transform (STFT) spectrogram analysis of the building response and of the input motion.
A survey of 217 multistory buildings carried out following the 22 February 2011 Christchurch Earthquake further confirmed many of the known vulnerabilities vertical nonstructural components (VNSC) ...have to seismic movements. VNSC include facades, claddings, external infills, and internal partitions. In order to improve the seismic performance of such components, this paper will outline issues identified during damage reconnaissance following the Canterbury earthquake sequence. Displacement-based demands have been obtained using nonlinear response history analyses of a set of code-compliant, post-1990s reinforced concrete buildings when subjected to recorded ground motions of the major Canterbury earthquakes. The corresponding expected damage to VNSC based on informative guidance given in design codes and damage limits obtained experimentally are compared against the damage observed in post-earthquake assessments.
Overview of Connection Detailing for Post-tensioned Dissipative Timber Frames Miliziano, Alessandra; Granello, Gabriele; Palermo, Alessandro ...
Structural engineering international : journal of the International Association for Bridge and Structural Engineering (IABSE),
04/2020, Volume:
30, Issue:
2
Journal Article
Peer reviewed
The concept of post-tensioned rocking structures was successfully applied to timber in 2005, in prestressed laminated timber (Pres-Lam) technology. When applied to moment-resisting frames, the ...technology relies on unbonded post-tensioning tendons passing through hollow beams and the supporting column to provide beam-column connections with moment capacity. Supplemental dissipation devices can be included in the design, resulting in a recentring/dissipative system. Since seismic movements are accommodated through a control rocking mechanism between the beam and the column, the detailing of the beam-column joint can significantly affect the seismic performance of the structure. Among the main concerns, the shear transfer system should be able to transfer shear forces without interfering with the rocking motion, and ignoring the face friction and joint deformation should not compromise the drift activation of the gap opening. Attention should also be paid to minimizing post-tensioning losses while providing access for potential retensioning. As a result of the continuous and rapid development of Pres-Lam frame systems, a wide range of alternative solutions for connection detailing has been tested in the laboratory or implemented in existing buildings. This paper presents an overview of the beam-column detailing implemented in the five Pres-Lam frame buildings currently operative in New Zealand.
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