Drift Demands Research Articles

Seismic Performance of Coupled Shear Walls Designed according to Old and New Codes and Retrofitted with Eb-CFRP Composites

ABSTRACT Coupled shear walls (CSW) are an efficient lateral load-resisting system. Their seismic performance depends primarily on their coupling beams (CBs) ability to provide adequate stiffness and strength. However, many existing buildings with CSW resisting system were designed and constructed according to old codes and standards with insufficient requirements for seismic design. These systems feature unsatisfactory behaviour and prematurely collapse in case of major earthquake events. Therefore, their seismic retrofit is urgently needed. The main objectives of this study are, i) evaluate and compare the seismic performance of CSWs designed to old and new codes; ii) highlight the deficiencies of CSWs designed to old codes; iii) investigate a retrofit method using externally bonded (EB) carbon fibre reinforced polymer (CFRP) composites to enhance the seismic response of deficient CSWs. To that end, a 20-story CSW is considered with different design details including one according to old National Building Code of Canada (NBCC) before the 1970s and the other one designed consistent with the new NBCC 2015 and the Canadian Standard Association, CSA A23.3–14, for Eastern seismic Canadian zone. The nonlinear analyses of CSWs under earthquake records are conducted in two steps. At first, the nonlinear time-history analyses of two types of CSWs are carried out using RUAUMOKO program and the deficiencies of old-designed CSW are identified through the comparison of obtained results. In the second step, the old design CSW is retrofitted using CFRP sheets and reanalysed to investigate the benefits of the retrofit technique in mitigating the shortcomings. The results indicate that CFRP retrofitting is an efficient method to enhance the seismic performance of deficient old CSWs in terms of story displacement, inter-story drift, CBs rotation, and ductility demand.

Shear behavior of geopolymer concrete panels under diagonal tensile stresses

The use of geopolymers has spread over the last 60 years as an alternative to traditional cementitious systems, due to their environmental benefits and mechanical properties for construction. Nowadays, the state-of-the-art on reinforced geopolymer concrete has focused on the micro-scale analysis of the materials, but it has vaguely dabbled into the macro-scale context. This research experimentally assessed the shear behavior of geopolymer concrete panels that may be promising for construction of low-rise and low-cost housing. The parametric testing program included diagonal tension tests of 27 panels built with three different concrete types and three shear reinforcement ratios. The measured response of the specimens is compared in terms of cracking patterns, shear-capacity and drift demands. Overall results indicated that the shear capacity of panels mainly depends on the concrete type, but it is slightly governed by the reinforcement ratios. Furthermore, the program contributes towards developing new provisions for code-based structural design of geopolymer concrete walls as an environmentally friendly construction option.

Effect of earthquake statistically correlated vertical component on inelastic demand to regular reinforced-concrete frames

Post-earthquake survey and structural analysis prove that the vertical seismic action can damage buildings and bridges, even those with regular configuration. Moreover, there is evidence that one principal axis of ground motion may be not vertical. The inelastic demand to regular building structures is assessed here considering both uncorrelated and cross-correlated random seismic components, concurrent to deterministic weight. The method is conventional statistical equivalent linearization. Planar reinforced-concrete frames are analyzed parametrically varying the number of stories, the design ductility class, or the foundation ground type, as well as variance and cross-covariance of the vertical seismic component. Following a lumped plasticity approach, the bending moment-rotation relationship is a Bouc-Wen equation extended for asymmetry and interaction with axial force. Based on mean values, variances, and percentiles of response, the effect of uncorrelated vertical excitation is appreciable only on stress resultants of little practical consequence. Much more important is the effect of cross-correlated vertical excitation. As extreme results, the interstory drift demand increases up to four times. The rotation ductility demand to columns, less than 2 under uncorrelated excitations, rises to 5, meaning very likely yielding of columns designed as strong. The probability of axial force in the columns being tensile grows from negligible value to 8%. The beams may yield also around mid-span. The frame plastic mechanism worsens towards soft story, the peak deformation ductility being demanded more than the cyclic ductility. Not only variance of the vertical ground motion, but also cross-covariance with the horizontal one should be considered in the near field.

Optimum Strength Distribution for Structures with Metallic Dampers Subjected to Seismic Loading

A key aspect of the seismic design of structures is the distribution of the lateral strength, because it governs the distribution of the cumulative plastic strain energy (i.e., the damage) among the stories. The lateral shear strength of a story i is commonly normalized by the upward weight of the building and expressed by a shear force coefficient αi. The cumulative plastic strain energy in a given story i can be normalized by the product of its lateral strength and yield displacement, and expressed by a plastic deformation ratio ηi. The distribution αi/α1 that makes ηi equal in all stories is called the optimum yield-shear force distribution. It constitutes a major aim of design; a second aim is to achieve similar ductility demand in all stories. This paper proposes a new approach for deriving the optimum yield-shear force coefficient distribution of structures without underground stories and equipped with metallic dampers. It is shown, both numerically and experimentally, that structures designed with the proposed distribution fulfil the expected response in terms of both damage distribution and inter-story drift demand. Moreover, a comparison with other distributions described in the literature serves to underscore the advantages of the proposed approach.

An energy dissipating hold down device for cold-formed steel structures

A two-phase experimental investigation has been conducted for the development and evaluation of a hold down device that has energy dissipating feature. Focus of the first phase was the development of the device itself, while the second phase concentrated on the investigation of the performance of this device when used in a CFS framed sheathed wall panel that is subjected to reversed cyclic lateral loading. Hold down subassembly test results indicate that the load capacity of the hold downs is mostly limited by the shearing capacity of connection screws. Hold down devices themselves were observed to remain virtually undamaged under tensile loading, indicating a superior mechanical behavior in terms of strength and stiffness. As part of the second phase of the study, cyclic lateral load tests were conducted on seven CFS framed sheathed wall panels in order to study the energy dissipation ability of the proposed hold down device when used in CFS wall systems. Aligned with this focus, the anchor rods used to connect the hold downs to the foundation system in wall panel specimens were altered mainly by reducing the rod diameter within the middle portion over a certain length to provide controlled rod yielding under tensile forces. Controlled yielding of anchor rods was observed to reduce the drift demands imposed on wall panels due to seismic effects, resulting in a minimal damage on wall panels themselves.

Evaluation of design modifications for enhanced repairability of reinforced concrete walls

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.

Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands

This paper presents results from an experimental evaluation on the pre- and post-buckling behavior of 12 steel wide-flange cantilever columns under axial load and lateral drift demands. The influence of several loading and geometric parameters, including the cross-sectional local web and flange slenderness ratios, applied axial load, and lateral and axial loading history on the performance of these columns is thoroughly examined. The test data indicate that cross-sectional local buckling is highly asymmetric in steel columns under variable axial load. A relatively high compressive axial load can significantly compromise the steel column seismic stability and ductility, but this also depends on the imposed lateral loading history. The AISC axial load–bending moment interaction equation provides accurate estimates of a steel column’s yield resistance. However, the same equation underestimates by at least 30% the column’s peak resistance, regardless of the loading scenario. Measurements of column flange deformation, axial shortening, flexural resistance, and lateral drift are combined in a single graphical format aiding the process of assessing steel column repairability after earthquakes. The test data suggest that current practice-oriented nonlinear component modeling guidelines may not provide sufficient accuracy in establishing both the monotonic and first-cycle envelope curves of steel columns. It is also shown that high-fidelity continuum finite-element models should consider geometric imperfections of proper magnitude, in addition to the steel material inelasticity, to properly simulate the inelastic buckling of wide-flange steel columns and generalize the findings of physical tests. Issues arising due to similitude are also discussed to properly limit steel column instability modes in future studies.

Effects of early-arriving pulse-like ground motions on seismic demands in RC frame structures

Forward-directivity effects cause most of the seismic energy from the rupture to arrive at the beginning of the motion as a large pulse. This characteristic of ‘early-arriving’ pulses is often adopted to identify pulse-like motions caused specifically by directivity effects. However, not all early-arriving pulses are caused by the forward-directivity effects. Also, current criteria to select near-source directivity pulses mainly include the presence of large pulses in the velocity time series, whether the velocity pulse is early-arriving or located within 20 or 30 km closest distance to the fault. This study is intended to provide important insights into the differences in the response attributes of reinforced concrete (RC) frame structures subjected to early-arriving pulse-like ground motions caused by different physical processes, i.e., forward-directivity versus non-directivity effects. Nonlinear time-history analyses of three generic RC frame structures to two extensive suites of unscaled and scaled ground motions were performed to examine the distinct effects of these two types of ground motions. Results indicate that for the short period (2-story) frame, the mean drift demands by directivity motions are equal to or smaller than that caused by non-directivity motions. For the medium (6-story) and long period (20-story) frames, the directivity-induced motions result in larger demands in the lower half of the structures compared to non-directivity motions, especially for high-intensity ground motions. Spectral analyses of the ground motions provide key information as to why the non-directivity records cause larger demands in the 2-story frame. Simulations using pulse models that represent pulse-like ground motions are carried out to gain additional understanding on the primary findings for the 6-story and 20-story structures.

Development of collapse‐consistent loading protocols for experimental testing of steel columns

SummaryIn order to effectively utilize results from quasi‐static cyclic testing on structural components for the earthquake‐induced collapse risk quantification of structures, the need exists to establish collapse‐consistent loading protocols representing the asymmetric lateral drift demands of structures under low‐probability of occurrence earthquakes. This paper summarizes the development of such protocols for experimental testing of steel columns prone to inelastic local buckling. The protocols are fully defined with a deformation‐ and a force‐controlled parameter. They are generally applicable to quantify the capacity and demands of steel columns experiencing constant and variable axial load coupled with lateral drift demands. Through rigorous nonlinear earthquake collapse simulations, it is found that the building height, the column's local slenderness ratio, and ground motion type have the largest influence on the dual‐parameter loading protocol indexes. Comprehensive comparisons with measured data from full‐scale shake table collapse tests suggest that unlike routinely used symmetric cyclic loading histories, the proposed loading protocol provides sufficient information for modeling strength and stiffness deterioration in steel columns at large inelastic deformations.

Seismic Fragility of Tall Concrete Wall Structures in Malaysia under Far-Field Earthquakes

Introduction: In recent years, the seismic vulnerability of structures in Malaysia has attracted the attention of researchers mainly because the majority of existing structures have not been designed for seismic actions. In this study, seismic vulnerability of tall concrete wall buildings has been investigated through the development of seismic fragility curves. Methods: Two 25-story tall buildings with similar plans but with the different number of parking levels were analyzed through the incremental dynamic analysis. The tall buildings were excited by 15 far-field earthquakes, and their inter-story drift demands and capacities were estimated. Nonlinear response of beams and columns was simulated through the lumped plasticity model. The inelastic response of concrete walls was taken into account through the use of distributed plasticity fibre-based elements. Results and Conclusion: The obtained results indicated that the probability of exceeding minor damage to the tall concrete wall buildings located in the Kuala Lumpur city was around 55%. However, the probability of collapse of these structures in the same city was less than 15%.

Influence of shape memory alloy brace design parameters on seismic performance of self‐centering steel frame buildings

This paper investigates the seismic and collapse performance of shape memory alloy (SMA) braced steel frame structures considering the effects of various brace design parameters and ultimate state of SMAs. An SMA-braced steel frame building is designed to have comparable strength and stiffness with a steel-moment resisting frame selected as case study building. Then, the stiffness and ultimate deformation capacity of the SMA braces in the initially designed reference SMA-braced frame are systematically varied. First, the static pushover analysis and incremental dynamic analysis are employed to illustrate the significance of SMA brace failure consideration in seismic performance assessment of steel frames with SMA elements. Then, the influence of SMA brace initial stiffness and ultimate deformation capacity on the seismic and collapse performance of SMA braced frames are studied through pushover analyses, nonlinear response history analyses, and incremental dynamic analysis. The results show that the SMA brace initial stiffness does not affect the interstory drift and floor absolute acceleration response at design and maximum considered earthquake level seismic hazard or collapse capacity of the frame. However, it has considerable influence on post-event functionality of the frame. It is also found that the SMA brace ultimate deformation capacity should be at least 80% of maximum interstory drift demand at maximum considered earthquake level for satisfactory seismic performance, whereas larger values provide higher collapse capacity for the SMA-braced frame.

F(1H‐Pyrazol‐4‐yl)methylene‐Hydrazide derivatives: Synthesis and antimicrobial activity

AbstractThis paper investigates the seismic and collapse performance of shape memory alloy (SMA) braced steel frame structures considering the effects of various brace design parameters and ultimate state of SMAs. An SMA braced steel frame building is designed to have comparable strength and stiffness with a steel‐moment resisting frame selected as case study building. Then, the stiffness and ultimate deformation capacity of the SMA braces in the initially designed reference SMA braced frame are systematically varied. First, the static pushover analysis and incremental dynamic analysis (IDA) are employed to illustrate the significance of SMA brace failure consideration in seismic performance assessment of steel frames with SMA elements. Then, the influence of SMA brace initial stiffness and ultimate deformation capacity on the seismic and collapse performance of SMA braced frames are studied through pushover analyses, nonlinear response history analyses, and IDA. The results show that the SMA brace initial stiffness does not affect the interstory drift and floor absolute acceleration response at design and maximum considered earthquake (MCE) level seismic hazard or collapse capacity of the frame. However, it has considerable influence on post‐event functionality of the frame. It is also found that the SMA brace ultimate deformation capacity should be at least 80% of maximum inter‐story drift demand at MCE level for satisfactory seismic performance, while larger values provide higher collapse capacity for the SMA braced frame.

An Optimal Seismic Force Pattern for Uniform Drift Distribution

The force distribution proposed by codes, which in many cases is framed in the equivalent static force procedure, likely leads to design structures with non-uniform drift distribution in terms of inter-storey drift and ductility demands. This can lead to an unbalanced drift demand at certain storeys. This phenomenon may also amass cyclic damage to the dissipative elements at this very storey, therefore increasing the probability of premature failure for low-cycle fatigue. This work proposes a new force design distribution that accounts for higher mode effects and limits the displacement concentration at any storey thus improving the dissipative capacity of the whole structures. The main advantage of the proposed method stands in its formulation, which allows to spare any previous set up with structural analyses. The proposed force distribution has been applied to multi-degree-of-freedom systems to check its effectiveness, and the results have been compared with other proposals. In addition, in order to obtain a further validation of the proposed force distribution, the results obtained by using a genetic algorithm have been evaluated and compared. Additionally, the results provided in this work validate the proposed procedure to develop a more efficient lateral load pattern.

Design criterion for the self-centering capacity of precast segmental UHPC bridge columns with unbonded post-tensioning tendons

This paper aims to provide a novel quantitative design criterion for the self-centering (SC) capacity of precast segmental ultra-high performance concrete (UHPC) bridge columns based on the concept of the SC factor, which is deemed capable of depicting the formation mechanism of residual drift. A refined numerical model was developed to simulate the seismic performance and was verified by the results of the cyclic loading test. Parametric analysis was conducted to research the influence of seven common design parameters on the SC factor based on the proposed numerical model. According to the results of the parametric analysis, a simplified formula was established to assess the residual drift when the SC factor and the maximum drift were known. Nonlinear dynamic analysis was conducted to validate the simplified formula for precast segmental UHPC bridge columns subjected to far-field, no-pulse near-field, and pulse near-field earthquake motions. A design criterion for the SC capacity was proposed based on the SC factor limits determined by the simplified formula. Research results show that the SC factor corresponding to the ultimate drift is generally conservative for assessing the SC capacity of precast segmental UHPC bridge columns. The residual drift is dependent on the SC factor and the maximum drift. The residual drift decreases with the increasing SC factor or decreasing maximum drift. The proposed formula is verified by the results of nonlinear dynamic analysis to be conservative to calculate the residual drift. According to the proposed design criterion, the SC factor limits are increased with the drift demand up at the same earthquake design level. The adverse effects of pulse near-field earthquake motions on the earthquake resilience of precast segmental UHPC bridge columns should be taken seriously.

Rapid assessment of peak storey drift demands on reinforced concrete frame buildings

The peak storey drift demands that an earthquake imposes on a building can be assessed through a detailed engineering seismic assessment or recorded if a building is instrumented. However, for the rapid seismic assessment of a large number of buildings, it is desirable to have a simplified means of estimating storey drift demands. Consequently, this paper proposes a simplified means of quickly estimating storey drift demands on reinforced concrete (RC) frame buildings. Expressions for peak storey drift demand as a function of ground motion intensity are developed by utilising concepts and simplifications available from displacement-based seismic design and assessment methods. The performance of the approach is gauged by comparing predicted storey drift demands with those obtained from rigorous non-linear time-history analyses for a number of case study buildings. The promising results suggest that the approach proposed will be useful for rapidly assessing the likelihood of damage to a range of drift-sensitive elements in modern RC frame buildings.

Evaluation of Building Collapse Risk and Drift Demands by Nonlinear Structural Analyses Using Conventional Hazard Analysis versus Direct Simulation with CyberShake Seismograms

AbstractLimited data on strong earthquakes and their effect on structures pose challenges of making reliable risk assessments of tall buildings. For instance, although the collapse safety of tall buildings is likely controlled by large‐magnitude earthquakes with long durations and high low‐frequency content, there are few available recorded ground motions to evaluate these issues. The influence of geologic basins on amplifying ground‐motion effects raises additional questions. Absent recorded motions from past large magnitude earthquakes, physics‐based ground‐motion simulations provide a viable alternative. This article examines collapse risk and drift demands of a 20‐story archetype tall building using ground motions at four sites in the Los Angeles (LA) basin. Seismic demands of the building are calculated form nonlinear structural analyses using large datasets (∼500,000 ground motions per site) of unscaled, site‐specific simulated seismograms. Seismic hazard and building performance from direct analysis of Southern California Earthquake Center CyberShake motions are contrasted with values obtained based on conventional approaches that rely on recorded motions coupled with probabilistic seismic hazard assessments. At the LA downtown site, the two approaches yield similar estimates of mean annual frequency of collapse (λc), whereas nonlinear drift demands estimated with direct analysis are slightly larger primarily because of differences in hazard curves. Conversely, at the deep basin site, the CyberShake‐based analysis yields around seven times larger λc than the conventional approach, and both hazard and spectral shapes of the motions drive the differences. Deaggregation of collapse risk is used to identify the relative contributions of causal earthquakes, linking building responses with specific seismograms and contrasting collapse risk with hazard. A strong discriminative power of average spectral acceleration and significant duration for predicting collapse is observed.

Hybrid approach for simulating shear–flexure interaction in RC walls with nonlinear truss and fiber models

An alternative approach to the nonlinear truss model (NLT) is proposed to simulate the seismic behavior of reinforced concrete (RC) walls with various aspect ratios (ratio between the height and the length of the wall, hw/lw). The alternative consists of a hybrid model comprising an NLT panel connected in series with a forced-based fiber-beam–column model through a rigid elastic beam element (HyNLT-F model). The HyNLT-F approach saves computational time as compared to the default NLT model while keeping the capabilities of modeling inelastic shear response and shear–flexure interaction under static cyclic or dynamic loading. These capabilities are first validated for the default NLT model using the experimental test data of two RC-wall panels subjected to reversed cyclic loading, one whose response is flexure-dominated and another with a shear–predominant behavior. The numerically computed lateral force–displacement relationships, lateral displacement contributions, shear–flexure interaction, and vertical strain distribution at different levels of drift demand show a good agreement with the experimentally recorded responses. The hybrid alternative is then validated using three RC-wall panel experiments with high (hw/lw = 3.1), moderate (hw/lw = 2.3) and low aspect ratio (hw/lw = 1.5). The capabilities of the hybridization are evaluated at the global and local level of response, as well as in terms of computational time needed to run the model in comparison with the default NLT model. As an application, a HyNLT-F model is implemented in a multistory non-ductile RC frame-wall structure, to evaluate the impact of the nonlinear shear response at the critical section in the global structural behavior under static and dynamic loading. The results show that the HyNLT-F can model features of the wall response such as inelastic shear and shear–flexure interaction at the critical section, which of interest for certain structural typologies.

Influence of shear‐axial force interaction on the seismic performance of a piloti building subjected to the 2017 earthquake in Pohang Korea

AbstractThis study aims to investigate the influence of shear‐axial force interaction on the seismic performance of a piloti‐type building subjected to the Pohang earthquake. For this purpose, a four‐story piloti‐type building in Pohang, Korea whose columns were brutally damaged by the 2017 Pohang earthquake, one of this country's strongest earthquakes in modern history, was chosen as the object of the study. Columns of this piloti‐type building were designed with insufficient transverse reinforcement, resulting in an increased potential for shear‐axial failure. The piloti‐type building was simulated using Zeus‐NL, an efficient analysis and simulation platform developed for earthquake engineering applications to perform nonlinear time history analysis. A hysteretic shear model in Zeus‐NL capable of accounting for the variation in column axial force was adopted to capture the effect of the shear‐axial force interaction, caused by a combination of horizontal and vertical ground motions. The shear model was validated by comparing the simulated and experimental results. The seismic performance of the piloti‐type building under the effect of the shear‐axial force interaction was compared with that under the shear effect only. Results have indicated that shear effect and shear‐axial force interaction effect significantly reduce column maximum shear force and increase column drift demand. The influence is more intense considering shear‐axial interaction. The seismic response in terms of hysteretic behavior, energy dissipation, and stiffness degradation is presented and discussed.

Effects of pulse-like nature of forward directivity ground motions on the seismic behavior of steel moment frames

In the structures with high level of ductility, the earthquake energy dissipation in structural components is an important factor that describes their seismic behavior. Since the connection details play a major role in the ductile behavior of structure, in this paper, the seismic response of 3-, 5- and 8-story steel special moment frames (SMFs) is investigated by considering the effects of panel zone modeling and the influence of forward-directivity near-field ground motions. To provide a reasonable comparison, selected records of both near and far-field are used in the nonlinear time-history analysis of models. The results of the comparison of the median maximum inter-story drift under excitation by near-field (NF) records and the far-field (FF) ground motions show that the inter-story drift demands can be obtained 3.47, 4.86 and 5.92 times in 3-, 5- and 8-story structures, respectively, undergoing near-field earthquakes.

Multi-material core as self-centering mechanism for buildings incorporating BRBs

Conventional buckling restrained braces used in concentrically braced frames are expected to yield in both tension and compression without major degradation of capacity under severe seismic ground motions. One of the weakness points of a standard buckling restrained braced frame is the low post-yield stiffness and thus large residual deformation under moderate to severe ground motions. This phenomenon can be attributed to low post-yield stiffness of core member in a BRB. This paper introduces a multi-core buckling restrained brace. The multi-core term arises from the use of more than one core component with different steel materials, including high-performance steel (HPS-70W) and stainless steel (304L) with high strain hardening properties. Nonlinear dynamic time history analyses were conducted on variety of diagonally braced frames with different heights, in order to compare the seismic performance of regular and multi-core buckling restrained braced frames. The results exhibited that the proposed multi-core buckling restrained braces reduce inter-story and especially residual drift demands in BRBFs. In addition, the results of seismic fragility analysis designated that the probability of exceedance of residual drifts in multi-core buckling restrained braced frames is significantly lower in comparison to standard BRBFs.