Search

Summarized in this article are the major findings from a detailed damage investigation of an instrumented ductile reinforced concrete moment frame building subjected to a design-level earthquake. The building sustained widespread damage during the 2016 M-7.8 Kaikoura earthquake and was subsequently demolished. Prior to demolition, the structural system was extensively surveyed to document the severity and distribution of the damage sustained in the reinforced concrete frames and the flooring system. Furthermore, the building response during the earthquake was reconstructed from the acceleration records obtained from instruments installed throughout the building. The site seismic demands were also obtained from a local free-field instrument. The data collected from this damage investigation provide a high-quality dataset that is valuable to researchers investigating different parameters related to the seismic performance of reinforced concrete moment frame buildings. The dataset is published and publicly available on DesignSafe-CI (project PRJ-3444); https://www.designsafe-ci.org/data/browser/public/designsafe.storage.published/PRJ-3444v2. The data collection methodology is described, and a roadmap for navigating the dataset is presented to support future use of the archived dataset. [ABSTRACT FROM AUTHOR]

Recent earthquakes have demonstrated that code-conforming modern (i.e. post-1970s) reinforced concrete (RC) buildings can satisfy life safety performance objectives. However, the accumulated earthquake damage in these modern buildings raised concerns about their performance in future events, contributing to widespread demolition and long-term closure of damaged buildings. The economic and environmental impacts associated with the demolition and long-term closure of modern buildings led to societal demands for improved design procedures to limit damage and shorten recovery time after earthquakes. To address societal demands, this study proposes a damage-control-oriented seismic design approach that targets functional recovery by ensuring structural component demands do not exceed the damage-control limit state (DLS) under design-level events. Herein, DLS is defined as the post-earthquake state beyond which the strength and deformation capacity of a structural component is compromised, and its performance in a future event cannot be relied upon without safety-critical repair. This study proposes a methodology to determine component deformation limits for the design of structures for damage control. Using the developed methodology, we propose component rotation limits for RC beams, columns, and walls. The seismic performance and capability of buildings designed using the proposed design approach to satisfy recovery-based performance objectives is demonstrated through nonlinear response history and recovery analyses (using the ATC-138 methodology) of four archetype frame buildings, designed per New Zealand standards to different beam deformation limits. The analyses show that building codes can achieve functional recovery using the proposed component deformation limits without the need for sophisticated recovery analyses. [ABSTRACT FROM AUTHOR]

Hollow-core flooring systems were damaged in Wellington buildings during the 2016 Kaikoura earthquake (7.8 Mw) and have been shown to be susceptible to undesirable failure mechanisms (loss of seating, negative moment, and positive moment failure modes) at low drift demands. These undesirable damage mechanisms have also been observed in sub-assembly and super-assembly laboratory testing of hollow-core flooring systems and the test data obtained has enhanced the state-of-the-art knowledge of the probable seismic behaviour of hollow-core floor units. In this study, using currently available sub-assembly test data, fragility functions are defined for hollow-core flooring systems. Furthermore, the proposed fragility functions are combined with fragility information derived from nonlinear dynamic analyses for two eight-storey bare-frame reinforced concrete (RC) buildings designed based on New Zealand standards. This study shows that, in comparison with RC buildings with flooring systems that are not susceptible to gravity load failures, RC buildings with vulnerable hollow-core floors have a significantly higher likelihood of exceeding the collapse prevention limit state, as defined in this study. [ABSTRACT FROM AUTHOR]

One of the key damage observations in modern reinforced concrete (RC) frame buildings, damaged following the 2010/2011 Canterbury and 2016 Kaikoura earthquakes, was localised cracking at the beam‐column interface of capacity‐designed beams. The localised cracking in the beams was due to curtailed longitudinal bars at the beam‐column interface. Following these observations, without experimental data to justify desirable seismic performance, modern beams controlled by localised cracking were assumed to be potentially earthquake‐vulnerable. To address this, an experimental program was carried out on six RC beam specimens susceptible to single‐crack plastic hinge behaviour due to curtailed longitudinal bars. The experimental data show that RC beams with single‐crack plastic hinge behaviour can undergo significant inelastic drift demands without loss of lateral resistance. However, contrary to conventional beams with distributed cracking, the response of RC beams with single‐crack plastic hinge behaviour due to curtailed longitudinal bars is mainly dominated by hinge rotation (via bond‐slip) and shear sliding at the column face. The current paper studies the interdependence of axial elongation, bond‐slip and shear sliding deformation of RC beams with single‐crack plastic hinge behaviour under cyclic demands. A procedure for seismic assessment of RC beams with single‐crack plastic hinge behaviour due to curtailed longitudinal bars is proposed. The proposed formulations can be adopted to develop adequate numerical models for simulating the response of RC frames with beams susceptible to single‐crack plastic hinge behaviour due to curtailed longitudinal bars. [ABSTRACT FROM AUTHOR]

Following the 2010/2011 Canterbury and 2016 Kaikoura earthquakes, a number of reinforced concrete (RC) beams in high-rise structures developed a single primary crack at the beam-column interface without the formation of distributed secondary cracks along the beam length. Detailed assessments showed that these beams have conforming longitudinal steel ratios and the single-crack mechanism may be due to design and/or construction practices for beam-column joints in the 1980s. In order to investigate the seismic behavior of reinforced concrete beams with detailing that inhibited the spread of flexural yielding, an experimental program was carried out on RC beam specimens, having similar reinforcement detailing to that of beams that developed a single crack at their ends during the Kaikoura earthquake to understand their seismic behavior, postearthquake repairability, and residual low-cycle fatigue life. Experimental results showed that the beams were able to undergo significant inelastic drift demands without loss of lateral resistance and have sufficient residual drift capacity following moderate and large earthquake demands. The response of the beam specimens was dominated by hinge rotation via the bond-slip mechanism. Comparisons showed that the measured drift capacities of the beams exceeded the predicted drift capacities computed using state-of-the-practice procedures. [ABSTRACT FROM AUTHOR]

Uncertainty about when post‐earthquake repairs are needed delays decision making, and can lead either to unnecessary demolition or repair, or to inadequate repair actions. This study proposes a framework for assessing the effect of earthquake damage on the future seismic performance of a building. In this framework, we first assess drift demands during a maximum considered earthquake (MCER) level ground motion in an undamaged building, and then compare the drift demands in the same building during the same motion when it has been damaged by a prior shaking. We assess each building for a range of prior shaking events and 15 MCER‐level motions. From these analyses, we determine the relationship between damage in the first motion—quantified as peak story drift demand—and the MCER‐level motion drift demands—quantified as the ratio of peak drift demands in the damaged building to those in the undamaged building. This trend is used to indicate the level of damage at which performance is impaired, and repair is needed due to structural safety concerns. We apply this framework to a set of single‐degree‐of‐freedom structures and reinforced concrete moment frame buildings representative of construction in the United States. The results show that code‐conforming structures do not see significant drift amplifications in the damaged building when story drifts in the damaging motion do not exceed 2%, indicating they would not require major repairs for structural safety. Structures with less deformation capacity saw larger amplifications, and were affected when story drift demands in the damaging motion were lower. We also found that stiffer structures tend to see larger amplifications of drift demands. [ABSTRACT FROM AUTHOR]

Several studies have looked at the development of hinge models to simulate the hysteretic response of flexure‐ and shear‐critical reinforced concrete (RC) beam‐column components from damage initiation to onset of gravity collapse. However, few studies have been conducted to develop similar models for older type bond‐critical beam‐column components. First, using existing experimental data, this paper describes the calibration of simple and efficient hinge models to simulate the inelastic hysteretic response of as‐built and retrofitted splice‐deficient columns. Subsequently, to demonstrate the applicability of the hinge models, the seismic performance of as‐built and fibre‐reinforced polymer (FRP) ‐retrofitted two‐ and four‐story nonductile RC frame buildings with splice‐deficient columns are assessed using nonlinear dynamic analysis procedures. The results show that buildings with bond‐critical columns may have a lower collapse potential than buildings with shear‐critical columns, implying that in certain cases a longer splice length may actually worsen performance. According to the analyses, local retrofitting of the columns can significantly improve the seismic performance of the buildings. Contrary to results presented in this study, a significant U.S. evaluation methodology for RC frame buildings, FEMA P‐2018, indicates that buildings with bond‐critical columns have similar collapse potential as buildings with shear‐critical columns. Modifications are proposed to improve the FEMA P‐2018 provisions. The modelling approach presented in this paper are recommended for incorporation into ASCE/SEI 41, engineering practice and future research. [ABSTRACT FROM AUTHOR]

Existing reinforced concrete (RC) columns with short splices in older-type frame structures are prone to either a shear or bond mechanism. Experimental results have shown that the force–displacement response of columns exhibiting these failure modes are different from flexure-critical columns and typically have lower deformation capacity. This article presents a failure mode-based approach for seismic assessment of RC columns with short splices. In this approach, first, the probable failure mode of the component is evaluated. Subsequently, based on the failure mode, the force–displacement response of the component can be predicted. In this article, recommendations are proposed for evaluating the probable failure mode, elastic rotation, drift at lateral failure, and drift at axial failure for columns with short splices experiencing shear, flexure, or bond failures. [ABSTRACT FROM AUTHOR]

Despite the awareness of the adverse effects of biaxial seismic loading on the response of reinforced concrete (RC) components, the current knowledge of seismic performance of RC columns under biaxial loading is limited in comparison to that of columns under uniaxial lateral loading. Several questions on the effect of biaxial lateral displacement history still remain unanswered. Without proper progress on this topic, the codification of seismic assessment of RC columns under the influence of biaxial lateral loading will remain elusive. This study appraised test results from various experimental programs and identified the characteristics and effects of biaxial lateral displacement paths on seismic response of RC columns. Using the available dataset, recommendations are provided on how the influence of biaxial lateral load on lateral strength, deformation capacity, and stiffness degradation can be accounted. [ABSTRACT FROM AUTHOR]

Recent research studies have shown that provisions of current seismic assessment and design standards overestimate the effective stiffness of reinforced concrete (RC) components. The influence of slip and shear deformations on the elastic response of RC components is not adequately addressed by these provisions. This paper presents a methodology for estimating the yield rotation and effective stiffness of RC components. Based on first principles, a simple analytical approach accounting for the effect of bar slip and shear deformations is presented. Using a dataset of past experiments on RC columns, the adequacy of the simplified model is demonstrated. The approach is extended to RC components with high-strength steel. Furthermore, recommendations on effective stiffness of RC coupling beams are provided. [ABSTRACT FROM AUTHOR]

A wide range of reinforced concrete (RC) wall performance was observed following the 2010/2011 Canterbury earthquakes, with most walls performing as expected, but some exhibiting undesirable and unexpected damage and failure characteristics. A comprehensive research programme, funded by the Building Performance Branch of the New Zealand Ministry of Business, Innovation and Employment, and involving both numerical and experimental studies, was developed to investigate the unexpected damage observed in the earthquakes and provide recommendations for the design and assessment procedures for RC walls. In particular, the studies focused on the performance of lightly reinforced walls; precast walls and connections; ductile walls; walls subjected to bi-directional loading; and walls prone to out-of-plane instability. This paper summarises each research programme and provides practical recommendations for the design and assessment of RC walls based on key findings, including recommended changes to NZS 3101 and the NZ Seismic Assessment Guidelines. [ABSTRACT FROM AUTHOR]

Modern reinforced concrete structures are typically designed to form plastic hinges during strong earthquakes. In post-earthquake situations, repair of moderate plastic hinging damage can be undertaken by filling the crack system with epoxy resin and reconstituting spalled cover concrete. This study uses available experimental test data, including three large-scale ductile beams tested by the authors, to investigate the effects of epoxy repair on the structural behaviour of plastic hinges, with a focus on beam elements. Factors that have been neglected in past studies, including the effects of residual deformations at the time of repair, are given special attention. It is found that epoxy-repaired plastic hinges can exhibit different behaviour from identical undamaged components in terms of stiffness, strength, deformation capacity, and axial elongation. Potential explanations for the observed differences in behaviour are given, and recommendations are made for how these differences can be quantified in order to relate the expected response of an epoxy-repaired plastic hinge to the response that would be calculated for an identical undamaged component. [ABSTRACT FROM AUTHOR]

Many pre-1970s reinforced concrete (RC) buildings were constructed with smooth longitudinal reinforcement, particularly in southern Europe and New Zealand. It is conventionally accepted that the cyclic behaviour of components with smooth longitudinal reinforcement differ from those with deformed bars. An important factor that influences the cyclic behaviour of RC components is bond properties. Under cyclic loading in the nonlinear range, the bond between concrete and smooth longitudinal reinforcement is lost due to Poisson effect, resulting to lack of stress transfer between the concrete and the reinforcing bar thereby inhibiting the development of secondary cracks. Hence, the post-yield behaviour of columns with smooth reinforcement is similar to that of a rocking rigid body with all the deformation and damage concentrated at the column-footing or column–joint interface. This rocking mechanism enhances the deformation capacity of columns with smooth reinforcement, leading to a 'pseudo-ductile' behaviour. Using a database of 65 experimental tests on reinforced concrete columns with smooth reinforcement, a simple rocking model has been developed and proposed for assessment of RC columns with smooth reinforcement. The proposed model is compared with provisions of international seismic assessment standards. [ABSTRACT FROM AUTHOR]

Poor seismic performance of older reinforced concrete buildings in past seismic events has frequently been attributed to failure of non-ductile columns not detailed for seismic demands. The Seismic Assessment of Existing Buildings Guidelines developed in New Zealand (NZ Guideline) provides a performance-based engineering framework for assessment of existing buildings, with concrete buildings covered in section C5. This study compares the probable failure mode and deformation capacity assessed based on NZ Guideline, ASCE/SEI 41-13, and ASCE/SEI 41-17 with the results from quasi-static cyclic tests conducted on 52 rectangular and 13 circular reinforced concrete columns with reinforcement details similar to those of nonductile columns. Results indicate that the general curvature-based method of the NZ Guideline was not able to identify the observed failure mode but generally provides a conservative estimate of deformation capacity in comparison with ASCE/SEI 41-17. Based on the results of this study, a direct rotation-based acceptance criteria is proposed for NZ Guidelines. Also, slight modifications, to reduce conservatism, have been proposed for the curvature-based method. [ABSTRACT FROM AUTHOR]

Summary: Understanding the impact of prior earthquake damage on residual capacity is important for postearthquake damage assessment of buildings; however, interpretation of such impact is challenging when based on tests using traditional reversed‐cyclic loading protocols. A new loading protocol, consisting of a dynamic earthquake displacement history followed by quasi‐static reversed‐cyclic loading to failure, is presented as an alternative to traditional simulated seismic loading protocols. Data are analyzed from a set of 12 nominally identical ductile reinforced concrete beams that were tested by using variations of this protocol and traditional reversed‐cyclic and monotonic protocols. Differences in the cycle content of the earthquake displacement histories applied to the test specimens allowed for the effects of load history variation below 2.2% drift to be isolated. It is found that such variation had no effect on the beam deformation capacities. The effects of dynamic loading rates are also analyzed and compared against control quasi‐static specimens. Relative strength increases due to dynamic loading are found to be more significant at yield than at ultimate. Dynamic loading rates led to modest reductions in the beam deformation capacities, but the presence of causality between these variables remains uncertain. [ABSTRACT FROM AUTHOR]

Recent earthquakes have highlighted discrepancies between the intended and observed performance of RC walls and significant research is in progress to improve the seismic performance of RC wall buildings. An international group of researchers and practitioners developed a research framework in order to conduct a project mapping and prioritisation exercise for RC wall research. The process by which this research framework and mapping exercise were conducted is described. The framework was used to identify research priorities that would provide a basis for the direction of future research. High priority topics included, shear demands and capacities, effect of load-rate and loading history, seismic assessment of older walls, residual capacity and repairability, non-rectangular and core walls, and whole of building response. [ABSTRACT FROM AUTHOR]

Following the devastating 1931 Hawke's Bay earthquake, buildings in Napier and surrounding areas in the Hawke's Bay region were rebuilt in a comparatively homogenous structural and architectural style comprising the region's famous Art Deco stock. These interwar buildings are most often composed of reinforced concrete two-way space frames, and although they have comparatively ductile detailing for their date of construction, are often expected to be brittle, earthquake-prone buildings in preliminary seismic assessments. Furthermore, the likelihood of global collapse of an RC building during a design-level earthquake became an issue warranting particular attention following the collapse of multiple RC buildings in the February 22, 2011 Christchurch earthquake. Those who value the architectural heritage and future use of these iconic Art Deco buildings--including building owners, tenants, and city officials, among others--must consider how they can be best preserved and utilized functionally given the especially pressing implications of relevant safety, regulatory, and economic factors. This study was intended to provide information on the seismic hazard, geometric weaknesses, collapse hazards, material properties, structural detailing, empirically based vulnerability, and recommended analysis approaches particular to Art Deco buildings in Hawke's Bay as a resource for professional structural engineers tasked with seismic assessments and retrofit designs for these buildings. The observed satisfactory performance of similar low-rise, ostensibly brittle RC buildings in other earthquakes and the examination of the structural redundancy and expected column drift capacities in these buildings, led to the conclusion that the seismic capacity of these buildings is generally underrated in simple, force-based assessments. [ABSTRACT FROM AUTHOR]

Existing and proposed models of the effective stiffness of reinforced concrete columns subjected to lateral loads are assessed using the experimental response of 329 concrete columns. Existing models appropriate for design applications tend to overestimate the measured effective stiffness and are unacceptably inaccurate, because they generally neglect the influence of anchorage slip on the effective stiffness of the column. A three-component model that explicitly accounts for deformations due to flexure, shear, and anchorage-slip is shown to provide a more accurate estimate of the measured effective stiffness for the database columns. This model is simplified by neglecting small terms and approximating the results of moment-curvature analysis to obtain an accurate and rational effective stiffness model appropriate for design applications. For this model, the ratio of the measured stiffness to the calculated stiffness had a mean and coefficient of variation of 1.02 and 22% for circular columns and 0.95 and 25% for rectangular columns. [ABSTRACT FROM AUTHOR]

A large number of reinforced concrete buildings constructed prior to the mid-1970s in western North America rely on lightly reinforced, perforated, perimeter shear walls to resist earthquake-induced lateral forces. Although a substantial number of piers may exist, deformation demands can significantly exceed acceptable levels for plastic deformations published in common acceptance criteria such as FEMA 356. A shear-friction model is used to establish the ability of wall piers to support vertical loads after substantial loss of pier lateral-load capacity. The model results indicate that typical wall piers are capable of sustaining relatively large lateral drift ratios prior to loss of vertical load-carrying capacity, which is consistent with postearthquake observations. However, preliminary test results indicate that the drift capacity may be substantially less for poorly detailed walls, where axial failure occurred at a lateral drift ratio of approximately one percent. [ABSTRACT FROM AUTHOR]

Reinforced concrete columns with light transverse reinforcement may be susceptible to shear and subsequent axial-load failures during strong earthquake ground shaking. An experimental program examined the behavior of two half-scale, one-story frames with axial loads representative of those expected for the lower story of a multistory building. The frames were subjected to unidirectional simulated earthquake motion applied at the base. Shear failures of an interior column led to axial-load failure and redistribution of internal forces to adjacent framing components. Recorded data illustrated the details of the shear and axial failures and the redistribution to adjacent components. This paper discusses the global response of the frames and the shear and axial-load response of the shear-critical interior column. [ABSTRACT FROM AUTHOR]

A proposed supplement to ASCE/SEI 41 Seismic Rehabilitation of Existing Buildings has been developed for the purpose of updating provisions related to existing reinforced concrete buildings. Based on experimental evidence and empirical models, the proposed supplement includes revisions to modeling parameters and acceptance criteria for reinforced concrete beams, columns, structural walls, beam-column joints, and slab-column frames. The revisions are expected to result in substantially more accurate and, in most cases, more liberal assessments of the structural capacity of concrete components in seismic retrofit projects. [ABSTRACT FROM AUTHOR]

Experimental research and post-earthquake reconnaissance have demonstrated that reinforced concrete columns with light or widely spaced transverse reinforcement are vulnerable to shear failure, and in turn, axial failure during earthquakes. Based on experimental data, failure surfaces have been used to define the onset of shear and axial failure for such columns. After the response of the column intersects the failure surface, the shear or axial strength of the column begins to degrade. This paper introduces a uniaxial material model that incorporates the failure surfaces and the subsequent strength degradation. When used in series with a beam-column element, the uniaxial material model can adequately capture the response of reinforced concrete columns during shear and axial load failure. The performance of the analytical model is compared with results from shake table tests. [ABSTRACT FROM AUTHOR]

• Accounting for all sources of uncertainties in probabilistic seismic assessment. • Failure mode variability in reinforced concrete components. • Concept of competing failure mode analysis in seismic assessment. • Correlation between mechanics-based and statistical learning-based models in development of fragility curves. This paper makes a case for the propagation of relevant uncertainties in prediction of the seismic response of reinforced concrete components. Conventional probabilistic seismic assessment studies only consider parameter uncertainty and model error as the dominant forms of uncertainty in a component's capacity. In this paper, firstly, experimental evidences of failure mode variability in nominally identical specimens are presented. Furthermore, a simple approach to account for aleatory uncertainty due to variability in failure mode is presented. Although this paper is focused on reinforced concrete components, the concept of failure mode variability can be applied to components of other materials as well. The paper suggests that failure mode uncertainty needs to be adequately treated in component fragility functions as well as in probabilistic assessment of global collapse capacity. [ABSTRACT FROM AUTHOR]

• The benchmark wall formed multiple flexural cracks for even curvature distribution. • Debonding of reinforcement helped to distribute strain and delay bar buckling. • An improved debonding detail is suggested for future evaluation. • Walls with advanced cementitious materials had increased crack propagation initially. • Walls with advanced cementitious materials fell short of expectations at large drifts. As a consequence of recent earthquakes in New Zealand, many concrete buildings have been demolished due to structural damage. Observations of damage to concrete walls led to substantial research and revisions to design standards to ensure that a satisfactory ductile response was achieved. However, even when the current performance objectives of the design standards are met, reinforced concrete walls may still require extensive or costly post-earthquake repairs. The objective of this project was to evaluate simple modifications to conventional reinforced concrete walls to increase their repairability. Four modified walls were constructed and subjected to cyclic lateral in-plane loading until failure, and compared to a previously tested conventional ductile reinforced concrete benchmark wall that failed at 2.5% drift. The modifications considered included debonding of reinforcement at the wall base, substituting fiber-reinforced concrete (FRC) for conventional concrete, and substituting engineered cementitious composite (ECC) for conventional concrete in the ends of the plastic hinge region (applied in two walls). Debonding delayed vertical reinforcement buckling, but failure occurred shortly thereafter (2.5% drift) due to constricted movement of the buckled bars within the debonding sleeves. The FRC and both ECC walls had increased crack propagation up to a drift demand of 0.5%, but then the cracks localized to a single dominant crack and the walls failed at drifts lower than the benchmark wall (about 1.5%). Modifications of the tested details are recommended for future test programs that investigate the repairability of concrete walls. [ABSTRACT FROM AUTHOR]