Drift Demands Research Articles

Seismic-Resistant Design Regulations for Structures, Earthquake and Experimental Results Lessons

The Mexico City earthquakes of September 19, 1985, and September 19, 2017, showed that the seismic requirements for these earthquakes exceeded those specified in the Mexico City building code. This study illustrates limitations in current procedures for the seismic analysis of structures. The Uncertainties in the assessment of seismic design requirements and limitations in the seismic analysis of structural systems indicate that the lateral strength specified in building codes is not sufficient and that structures must have lateral deformation capacity. It follows that structural systems with low or intermediate ductility, such as those allowed in the Mexico City Building Code (MCBC, 2017), would not be desirable for resisting strong earthquakes. The last part of this work shows computed story drifts in buildings with different structural systems, either based on frames or using structural walls. This study compares story drifts in these structural systems for both using the seismic demands specified by MCBC 2017 and for the seismic demands corresponding to the September 19, 2017, earthquake in Mexico City. The results for these story drifts show the importance of rigid structural systems for reducing story drift demands, not only for avoiding collapse in strong earthquakes but also for significantly reducing building damage.

New Behavioural Trends of Plan Asymmetric Limited Ductile Buildings Revealed by Incremental Dynamic Analysis

ABSTRACT The drift demand of plan-asymmetric buildings in low seismicity regions was studied by employing Incremental Dynamic Analysis to track the influence of torsion. A stepped increase in the (3D/2D) drift demand ratio was observed as the limit of yield was surpassed. This amplification phenomenon, which is typically muted with high, or moderate, ductility demand has not been reported in research literature on building responses in high seismic conditions. However, the undesirable torsion-induced amplification cannot be ignored as limited ductile buildings are particularly vulnerable depending on the disposition, number, size and orientation of the structural walls.

Maximum Drift Demands of Earthquake Damaged Reinforced Concrete Columns Based on Residual Flexure Cracks

Assessment of existing reinforced concrete (RC) structures after an earthquake is a challenging task that must somehow relate qualitative and quantitative observations in the plastic hinge regions and the associated residual deformation capacity of damaged structures. Having an estimate available for the remaining drift capacity will result in more economical and informed decisions regarding demolition or strengthening options. This study aims to develop a practical methodology to estimate the maximum drift demand of an RC column based on the residual crack width. For this purpose, fiber-based frame elements are used to model the RC column considering appropriately concrete behavior in compression and tension stiffening effects. Afterwards, the accuracy and reliability of the proposed methodology are demonstrated by validating the computational approach with two cyclic experimental results from literature and new test data for a one-bay one-story RC frame conducted within the course of this study. A comprehensive parametric study is performed for RC columns with different axial loads, longitudinal and transverse reinforcement ratios, and ground motions to exhibit the stochastic behavior. The study identifies the axial load ratio as the predominant parameter. Key findings include strong correlations between maximum drift ratios and total residual crack widths, as well as maximum compressive strains, with regression analysis yielding equations for accurate drift ratio estimation. Simple predictive models are proposed to estimate the maximum deformation demands based on observed residual crack widths. Residual cracking exceeding 5 mm poses significant risk for the columns with axial load ratios above 0.4, with 90% probability of exceedance 2% drift ratio.

Effects of axial compression ratio on seismic behavior of shallow‐buried rectangular stations: Hybrid simulation and quasi‐static test

AbstractIn the absence of experimental investigations on column members of underground structures, full‐scale column specimens were tested to explore the seismic behavior of shallow‐buried subway stations at various depths. The axial compression ratios of internal column specimens were set as 0.16, 0.33, and 0.40. Both hybrid simulations and quasi‐static tests were performed on the station columns. The hybrid simulations illustrated the drift demands of internal columns, while the load‐carrying capacity and deformation capacity were obtained from the quasi‐static tests. Hybrid simulations at low, moderate, and high‐intensity levels were conducted to study the seismic responses of shallow‐buried rectangular stations. The hybrid simulations suggest that the most severe damage occurred in the station when the axial compression ratio of the tested column reached 0.40. Central columns suffered severe stiffness deterioration under high‐level earthquake excitation, especially in stations at greater depths. Meanwhile, the quasi‐static test results indicate that the ultimate load of the central columns increases with increasing axial compression, but this leads to a significant decrease in the ductility of columns. Besides, the sectional analysis results show that the central columns are prone to tension‐controlled failure, and the safety margin for flexural response deteriorates with an increasing axial compression ratio. The test results indicate that shallow‐buried rectangular stations are susceptible to flexure‐controlled structural failure when their central columns possess a relatively low axial compression ratio and a high shear span‐to‐depth ratio. The failure mechanism of station columns is revealed by both the hybrid simulations and quasi‐static tests, and the findings from the full‐scale tests are beneficial for the practical design of shallow‐buried rectangular stations.

A study of the response of reinforced concrete frames with and without masonry infill walls to earthquake motions

In developing countries prone to frequent earthquakes, it is common practice to use masonry infill walls as partitions in reinforced concrete frame buildings. Frames with infill have smaller drift demands and smaller drift capacities relative to similar frames without infill. Whether the relative reduction in drift demand offsets the plausible reduction in drift capacity remains open to discussion. This investigation provides experimental evidence on drift demands and drift capacities for both frames with and frames without infill. To make fair comparisons, only experiments where sets of nominally identical reinforced concrete frames, at least one frame with no infill and at least one frame with infill, are considered. Because many static experiments provide evidence for the reduction in drift capacity caused by infill, the objective of the dynamic experiments described in this study is to quantify the reduction in drift demand attributed to infill. Results of dynamic tests show that frames with infill drifted no more than one-third the amount similar frames without infill drifted in simulated base motions of the same intensity. Results of static tests show that the smallest drift capacities of frames with infill were approximately one-half of the drift capacities of similar frames without infill. These two experimental observations indicate that infill reduces drift demand more than it reduces drift capacity suggesting an overall beneficial effect of the infill. Field data were used to test this idea. Based on surveys of low-rise school buildings, RC frames with more full-height infill walls tend to have less earthquake damage than frames with less infill. A convenient method to quantify the amount of infill walls in buildings is computing wall ratios defined as the ratio of cross-sectional area of masonry infill walls on ground floor to total floor area above ground floor. Projections of the field data suggest that infill wall ratios of at least 0.5% in both directions of a building reduce the likelihood of severe structural damage by 300% compared with smaller ratios. The use of infill presents an inexpensive yet effective retrofit technique, even if only for temporary use, for strengthening buildings vulnerable to earthquake damage.

Variability of coal mine overburden incorporated concrete and its effect on the seismic safety of reinforced concrete building

AbstractImproved utilization of coal mine overburden material (CMOB)‐incorporated concrete in the construction sector can contribute to developing a more sustainable built environment if the results of proven research on the behavior of structures constructed of CMOB‐incorporated concrete are applied. Even though there have only been a few studies on CMOB‐incorporated concrete, no research has been done on how this type of concrete can be used in structures. In particular, researchers have not paid much attention to the fact that the strength properties of CMOB‐incorporated concrete tend to be very variable, even though this variability is needed for a wide range of structural applications (such as limit state design formulation, reliability‐based structural analysis, etc.). For the first time, this research provides quantitative estimates of the variability in compressive‐, flexural‐, and splitting tensile strengths of cured concrete using varying amounts of a CMOB sample as fine aggregate. Taking into account the suggested distribution model, the research also provides a reliability‐based seismic risk assessment of a standard CMOB‐incorporated framed building. The failure probability and dispersion in the drift demand were found to be the same for the frames modeled with CMOB and standard concrete (natural sand); the results demonstrate that CMOB samples can be safely used for earthquake‐resistant constructions.

The role of masonry infills on the interstory drift demand of reinforced concrete frames

Most frame buildings, especially those with shear-type moment resistance frames, are affected by masonry infill panels, which change the mechanical properties of whole systems in a big way. In turn, seismic performance varies depending on the infill panel and frame interactions. In conventional structural design practice, such interaction has been overlooked. This study looks at the range of local displacement demands in shear-type frames with and without infill panels. Generic frames are developed by tuning the story stiffness and mass to produce a reasonable period range between 0.2 and 2.0 s. The masonry infill panels are simulated through equivalent diagonal struts. A Bouc–Wen-based hysteretic model is applied to incorporate the post-yielding hysteresis degradations of both columns and masonry panels. The hysteresis loop control parameter values are also given for incorporating masonry infill properties. The correlation analysis between the strength and stiffness of RC frames and masonry infills is supplied as an instrument for calibrating the hysteretic model. In the collection of records, there are a lot of near-fault ground motions, which puts a lot of seismic demands on the buildings. The modification factors via regression analysis are proposed using over 1254 nonlinear response history analyses. This modification factor is figured out by looking at the difference between the mean drift spectrum for a set of generic frames that are both bare and filled. The nonlinear analysis shows that residual drift demands can be reduced in the case of panel effects that exist for masonry-infilled mid-rise RC shear-type frames.

The importance of hazard-consistency when estimating seismic residual drifts in steel moment frames

Residual drifts are routinely used to assess the post-earthquake safety of buildings. Despite their importance, studies on the probabilistic assessment of residual drifts in multi-storey buildings with Steel Moment Resisting Frames (SMRF) are far less common than those dealing with their transient peak drift counterpart. More seriously, although residual drift prediction models have been developed with a particular broad ground-motion type or soil condition in mind, all models proposed to date remain oblivious to the central issue of hazard consistency. Hence, missing the causal connection between the seismic hazard level and the ground-motion suite used and potentially compromising the meaningfulness of their results. This oversight is expected to introduce a bias in the estimation of residual drifts but its magnitude has not yet been properly evaluated. In this paper, we evaluate and quantify the significance of these effects. To this end, we use the Conditional Scenario Spectra framework, which provides a set of realistic earthquake spectra with assigned rates of occurrences based on their spectral shape and intensity, thus preserving the critical relationship of hazard consistency. Nonlinear Response History Analyses (NRHA) of 24 deteriorating SMRFs under 816 ground-motion records are performed to construct a database of residual drift demands. These NRHA results are used to examine the residual drift trends and to construct benchmark residual drift hazard curves. An extensive feature selection process employing several Machine Learning (ML) algorithms precedes the development of hybrid data-driven predictive models. Finally, we compare our hazard-consistent predictions with currently available models and quantify the massive under- and over-estimations associated with previously proposed non-hazard-consistent assumptions.

Seismic Performance of Steel Buildings with Eccentrically Braced Frame Systems with Different Configurations

Although eccentrically braced frames (EBFs) can be used with different configurations according to architectural requirements, it has not yet been indicated which configuration has a better seismic performance; therefore, this paper presents an analytical study focused on evaluating the seismic behavior of various steel buildings with EBF systems, factoring in different configurations. Furthermore, the objective is to compare the performances of EBF systems with one another, to learn more about their structural efficiency. The results obtained indicate that seismic response, in terms of peak interstory drifts, depends on the structural period and hysteretic behavior of the links, because high levels of plastic rotation increase lateral displacement. In addition, it was observed that maximum drift demands are concentrated in the lower floors where the links exhibit inelastic behavior, while the level of interstory drift decreases as height increases.

Seismic fragility of reinforced concrete buildings with hollow-core flooring systems

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.

Shaking table tests and numerical simulations on RC frames retrofitted with friction-damped brace

In this study, the dynamic response of double-story reinforced concrete (RC) space frames retrofitted with diagonally arranged friction damped-brace system was evaluated through shaking table tests. A simplified friction damper was proposed and tested under cyclic loading, later used to retrofit an RC frame. The splice of a conventional steel brace was converted into a friction damper, which requires limited technical expertise on site. Commercially available brake-pads were used as a friction material between the splice plates and bracing elements. An artificial ground motion characterized by a short period, similar to the Korean earthquake histories, was generated and used for the shaking table tests. The tested RC frame half-scaled of existing old structures in Korea. The shaking table tests results demonstrated that the dynamic response of the retrofitted RC frame was improved with reduced drift demands. Additionally, a numerical modelling method for friction damped-brace system was developed, and the dynamic responses of retrofitted RC frame with the proposed friction damper was numerically investigated using OpenSees. The numerical simulation results agreed well with the experimental observations.

Dynamic response of RC frames retrofitted with friction-damped brace system through shaking table tests

This study presents the shaking table test results of two-dimensional reinforced concrete (RC) frames retrofitted with a simplified friction damped brace. An easy-to-manufacture and install friction damper with economical design, was proposed and tested under reversed cyclic loading and used to retrofit RC frames. The splice of a conventional steel brace was converted into a friction damper, which not only dissipated seismic energy but also helped avoid the buckling of the steel brace. The test scheme consisted of four RC frames, and the behaviors of the three retrofitted frames were compared with that of a non-retrofitted (bare) frame. The shaking table tests were conducted using an artificial seismic wave that reflected the short-period response characteristics of the Korean earthquake. The test results of the bare and retrofitted frames were analyzed by considering several important performance indicators, such as the cracking and fracture mechanism in the RC frames, base share, and story drift ratios. The results of the shaking table tests demonstrated that the proposed damped brace effectively reduced earthquake-induced drift demands and successfully shifted the fracture and damage concentration from the columns to the beams.

Out-of-Plane behaviour of dowel type precast Panel-to-Foundation connections

Precast concrete wall panels are a common structural system, particularly in low-rise industrial and commercial buildings. In past earthquakes the connections between precast concrete panels and other structural members have been found to perform poorly. Fourteen panel-to-foundation assemblages were tested to investigate the out-of-plane performance of common connection detailing. These connection details included both dowel starter bars formed from conventional reinforcement, such as hooked bars, as well as starter bars connected to the panel via the use of cast-in threaded inserts. Variations in panel strength and connection strength were incorporated across the range of tested assemblages, and both cyclic and monotonic loading conditions were considered within the test program. It was found that conventional starter bars performed well because the presence of the hook returns of the starter bars within the joint region elevated the joint strength. Conversely it was found that panels having threaded inserts performed unsatisfactorily due to flexural cracking that propagated vertically behind the insert heads, leading to separation of the panel from the starter bars. It was determined that because the panels were close to the minimum reinforcement ratio, that the drift demands were very sensitive to the assumed cracking moment of the panel. Due to the brittle behaviour of the panel-to-foundation connection, it is advised that existing panels utilizing this connection be limited to less than 2% drift in the out-of-plane direction in order to avoid joint failure.

Earthquake response of reinforced concrete frames with infill and active external confinement: Tests and dataset

One option to retrofit reinforced concrete (RC) frames is the construction of infill walls. Infill increases lateral strength and stiffness but tends to reduce drift capacity relative to bare frames. Few studies have quantified the reductions in drift demand attributable to infills prior to failure. This report summarizes data from experiments designed to compare drift demands of frames with and without infill. Included data come from two experimental programs completed at Purdue University which focused on the in-plane dynamic response of reduced-scale, non-ductile RC frames to uniaxial simulated earthquake ground motions. 254 dynamic tests were conducted on bare frames, frames with masonry infill walls, and frames with timber infill walls. In 11 of 14 test series, active confinement was applied to columns using external post-tensioned reinforcement. The complete dataset is open-access online on Zenodo, DOI: 10.5281/zenodo.6954967.

Numerical and analytical studies for a novel steel brace retrofit system

To upgrade the existing RC frame structure with insufficient seismic design, a novel and advanced gapped eccentric steel brace (GESB) system was proposed. The finite element modeling methods based on fiber cross-section and macroscopic models were established and calibrated through the test results. The impact of three key design parameters, including axial load ratio (ALR) isolation thickness (IT) and Clamping Joint Spacing (CJS), was investigated. Based on the results of the numerical analysis, it is recommended that the drift demand controlled by IT should be between 0.6% and 1%, while the CJS should be 1/4 to 1/3 in response to the ductility and strength requirements. And the effect of ALR on the retrofitted frame can be ignored. An analytical method was also presented, which proved to be accurate for estimating the mechanical response of the retrofitted frame by comparison with the experimental and numerical response.

Characteristics and selection of near-fault simulated earthquake ground motions for nonlinear analysis of buildings

Earthquake-induced ground shaking near rupturing faults is highly sensitive to the rupture characteristics, seismic wave propagation patterns and site conditions, and field recordings of near-fault shaking are relatively sparse. These challenges complicate the assessment of the seismic performance of near-fault structures. A common approach to representing near-fault ground motion in engineering analysis is to explicitly consider and select records with strong directivity pulses (pulse records). We use three-dimensional high-resolution physics-based earthquake simulations to test this approach in the context of scenario-based ground motion record selection, and to study the important characteristics of near-fault ground shaking. We highlight the deficiencies associated with classifying near-fault simulated records as “pulse” or “non-pulse,” based on the presence of a single dominating pulse in the velocity time history. We show that this approach is inadequate for characterizing near-fault shaking on soft soils which can be dominated by both forward rupture directivity and basin amplification effects. We conduct ground motion selection experiments for the analysis of near-fault structures with and without explicit classification of the pulse features in the records, and evaluate the bias in the predicted structural demands. We find that the maximum interstory drift demands on building structures imposed by unscaled site-specific simulated ground motion records selected based on relevant spectral shape features are not sensitive to the classification of records as pulse/non-pulse. Therefore, with regard to predicting the maximum interstory drifts in near-fault buildings, we do not find justification for the binary pulse classification of near-fault records.

Assessing the feasibility of achieving functional recovery goals through seismic retrofit of existing reinforced concrete buildings

Damage from past earthquakes has significantly hampered post-earthquake building function, threatening community resilience, and motivating consideration of functional recovery in building design and assessment. This study examines whether it is feasible to achieve functional recovery in retrofit of existing reinforced concrete buildings, focusing on seven buildings retrofit with various motivations and strategies. The seismic response of these buildings was nonlinearly simulated, and functional recovery was probabilistically assessed. The results show that retrofits targeting life safety may or may not achieve functional recovery goals. Achieving functional recovery depends especially on the reduction of drift demands and collapse probability. However, the acceleration increase associated with many retrofits can increase function loss due to the criticality of acceleration-sensitive nonstructural components if such components are not retrofitted. We also examine other performance metrics, that is, economic losses and immediate occupancy limits of ASCE/SEI 41, showing that these provide imprecise, and in the case of the immediate occupancy conservative, proxies for functional recovery.

Hysteretic behaviour of welded connections with highly inelastic panel zones

AbstractThe current practice in capacity‐designed steel moment resisting frames (MRFs) worldwide allows for limited shear yielding in the column web panel zone. As such, inelastic deformations concentrate near the beam ends, thereby leading to flexural strength deterioration due to local buckling often at modest lateral drift demands. Experiments on post‐Northridge welded connections suggest that prior to a story drift angle of 5% rad, panel zone kinking does not induce fracture for panel zone shear distortions of up to 10γy. Within such a context, this paper presents an alternative design of a welded moment connection with highly inelastic panel zones. The connection features simplified weld details including a customised beveled backing bar that can be kept in place after the completion of the complete joint penetration welds at the beam flange‐to‐column flange joint. Full scale experiments suggest that the hysteretic response of the connection remains stable up until a story drift angle of 8% rad. The specimen did not exhibit visible structural damage prior to story drift angles of 4% rad, which are characteristic to a maximum considered earthquake. Modest local buckling occurred near the beam ends only after a story drift angle of 5% rad, which is a typical drift threshold in current collapse assessment methodologies. This paper summarises the primary findings of the experimental program for one of the test specimens.

Geometric tolerances of welded connections with inelastic panel zones

AbstractPrequalified beam‐to‐column connections in steel moment resisting frames usually concentrate inelastic deformations in the steel beam ends. As such, nonlinear geometric instabilities (i.e., local buckling) may occur in these regions at modest lateral drift demands, leading to potentially high residual deformations. Previous research has shown that welded connections featuring inelastic panel zones exhibit a stable hysteretic behaviour. In this case, residual deformations in steel beams are often not visually detectable after inelastic cyclic straining. This paper contrasts the residual deformations of steel beams as part of welded connections with elastic and inelastic panel zones through continuum finite element analyses. The employed finite element model is thoroughly validated with available test data. Besides the actual hysteretic response of the connections, the primary focus of the present work is on a number of geometric tolerance indicators, such as the web concavity, flange out‐of‐square and beam axial shortening. These are strongly correlated to the potential for reuse of structural steel members. The results suggest that connection designs featuring inelastic panel zones delay beam local buckling relative to their strong panel zone counterparts, achieving acceptable residual deformations even at lateral drift demands higher than 4% when considering the geometric tolerances of international standards.

Out-of-plane behavior of innovative infill walls made of autoclaved aereated concrete (AAC) subjected to shake table tests

Introduction: Seismic damage affecting the non-structural components of buildings (such as infills) can significantly impact the expected economic losses over a structure’s service life. Recently, autoclaved aerated concrete (AAC) infills have gained attention in the construction industry due to their light weight, energy efficiency, and durability.Methods: This study investigates the out-of-plane behavior of an innovative AAC infill wall system by means of shake table tests. The wall system was made of low-density AAC units with innovative joints made from polyurethane resin and steel plates for connection to a structural frame. Shake table tests were carried out using artificial accelerograms based on ICBO-AC156, which can reproduce a wide range of interstory drift demands. The study is articulated in three stages: the mechanical characterization of the AAC infill wall, shake table tests, and seismic behavior analysis of the wall based on current code requirements.Results and Discussions: The results demonstrate the effectiveness of the innovative AAC infill wall system in resisting out-of-plane seismic loads, also considering different levels of seismic action, with base accelerations reaching up to 0.80 g.