Non-ductile Reinforced Concrete Research Articles

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.

Nonlinear Frame Element with Shear\u2013Flexure Interaction for Seismic Analysis of Non-Ductile Reinforced Concrete Columns

This paper presents and emphasizes the essence of inclusion of shear response and shear–flexural interaction in the investigation of reinforced concrete (RC) columns characterized by light and inadequately (substandard) detailed transverse reinforcement. This column type commonly exists in old-constructed RC frame buildings before the regulation of modern seismic codes. A stiffness-based RC frame element with shear–flexure interaction is formulated within the framework of Timoshenko beam kinematics assumption. Linked displacement interpolation functions are employed to remedy the problematic shear-locking phenomenon. The axial and flexural actions are interacted via the fiber-section model while shear-strength deterioration with inelastic flexural deformations is accounted for within the framework of the UCSD shear-strength model. The numerical procedure for shear–flexure interaction is modified from the Mergos–Kappos procedure. The proposed element is simple, computationally efficient and able to describe several salient features of RC columns with substandard detailed transverse reinforcement, including gradual spread inelasticity, shear–flexure coupling effects, and shear-strength deterioration with increasing curvature ductility. Three correlation studies are conducted to examine the model accuracy and its capability to predict the rather complex responses of non-ductile RC columns. Comparison with conventional flexural frame element is also presented to emphasize the essence of inclusion of shear response and shear–flexure interaction.

Seismic performance of non-ductile RC frames strengthened with CFRP

To study the seismic performance of non-ductile reinforced concrete (RC) frames strengthened with carbon fiber-reinforced polymer (CFRP), two 1/2-scale, 2-bay, 2-story RC frames were prepared and tested under combined axial load and cyclic lateral load. One of the RC frames served as a control specimen (bare specimen) and the other was strengthened with CFRP sheets (strengthened specimen) at its joint regions. Based on the hysteretic curves, the envelop curves, and several important indexes (e.g., strength, stiffness, ductility, energy dissipation capacity) of the two specimens, the performance of the employed seismic strengthening technique in improving the seismic performance of RC frames was discussed in detail. The experimental results indicated that the externally bonded CFRP reinforcement had little influence on the lateral stiffness of the RC frame. While the lateral strength of the strengthened RC frame increased by about 20% compared to the control RC frame. Furthermore, it is worth noting that the energy dissipation capacity of the strengthened RC frame is as much as 3.01 times that of the control RC frame. The test results of the current study indicate that the employed strengthening method can substantially improve the seismic performance of the strengthened RC frames.

Financial risk evaluation of non-ductile reinforced concrete buildings in eastern and western Canada

Financial risks due to catastrophic earthquakes can be severe. This is particularly applicable to existing buildings that were constructed prior to modern seismic hazard maps and seismic design provisions. Pre-1970 non-ductile reinforced concrete (RC) frame buildings in Canada are of typical examples, where design base shear coefficients were essentially the same across the country, despite very diverse seismic hazards in the east and the west. This study implements a comprehensive performance-based earthquake engineering methodology to evaluate the financial risks of non-ductile RC frame buildings with different numbers of stories for seismicity in Montreal and Victoria. In addition to seismic loss curves that consider a comprehensive event set of regional seismicity, conditional loss distributions for critical scenarios are developed. The results of the financial risk evaluations for the buildings strongly indicate the effects of regional seismicity are significant and overall seismic risks for buildings in Victoria are greater than those in Montreal. Nevertheless, seismic vulnerability of these buildings remains to be high for both locations, meaning that should a major earthquake strike near the location, the buildings studied will suffer major damage. The results from the detailed risk assessments are also useful for developing effective financial risk mitigation strategies through earthquake insurance.

Seismic vulnerability of non-ductile RC buildings in Pakistan through stochastic analysis of design/construction deficiencies

Reinforced concrete frame structures constitute a major portion of the building stock of Pakistan. In 2005, Kashmir earthquake exposed the current deficient local design and construction practices in Pakistan. It is hypothesized that inhomogeneity of concrete in general and faulty construction practices in Pakistan cause huge uncertainties in parameters affecting strength, serviceability and seismic assessment results for structures in this country. The aim of this study is to assess the vulnerability of existing reinforced concrete (RC) building stock of Pakistan with respect to a stochastic analysis of the parameters affecting the built infrastructure. For this purpose, typical building characteristics with uncertainties are identified. The probability density functions of material and geometric parameters are defined for these building characteristics. These functions are then used to account for the uncertainty in modelling of structural configurations. Structural response is evaluated through static pushover analysis and vulnerability curves for different structural configurations are derived using the framework developed for deficient RC buildings. The performance of buildings is evaluated in different seismic zones as classified in the new Building Code of Pakistan (BCP-SP 2007). Selected buildings performed satisfactorily in Zones 1, 2A and 2B. Buildings in Zones 3 and 4 are found to be more vulnerable.

Experimental behavior of strengthening of masonry infilled reinforced concrete frames by adding rebar‐reinforced stucco

Strengthening of structures may become a necessity to overcome the mistakes made during the design and construction phases of existing reinforced concrete (RC) buildings. In recent years, researches have been intensified on the economical and easy application of the strengthening of masonry infill walls in RC buildings. Strengthening of masonry infill wall with steel reinforcing mesh and additional mortar layer, fiber reinforced polymers and prefabricated concrete panels have become in the forefront of these techniques. In this study, the RC frame infilled with masonry walls and strengthened with steel reinforcing mesh and mortar layer was examined experimentally. One of the most important reasons for choosing this method is that the strengthening could be achieved with less cost, workmanship and without the need to prepare molds for reinforcement. The main variables that have been examined in the current study are: the anchorage length, spacing, and configuration which were applied to the frame and infill wall and the added layer are on the inside or outside wall. A total of 11 specimen, 1/3 scale, one‐bay, one‐story nonductile RC frames with masonry infill wall which representing the deficiency in the existence building by using low‐compressive concrete, insufficient stirrups, and strong beam in comparison to column were constructed and tested under the effect of repeated seismic load. Load–displacement behaviors, initial stiffness, displacement ductility rates, and energy consumption capacities are interpreted.

Probabilistic Assessment and Cost-Benefit Analysis of Nonductile Reinforced Concrete Buildings Retrofitted with Base Isolation: Considering Mainshock–Aftershock Hazards

AbstractNonductile reinforced concrete (RC) frame buildings were found to be vulnerable to earthquakes. Thus retrofitting of these buildings is necessary, as addressed in the recent retrofit progra...

Impact of Earthquake Types and Aftershocks on Loss Assessment of Non-Code-Conforming Buildings: Case Study with Victoria, British Columbia

This paper presents a study on the impact of earthquake types (shallow crustal, deep inslab, and megathrust Cascadia interface earthquakes) and aftershocks on loss assessment of non-code-conforming reinforced concrete (RC) buildings. The loss assessment is formulated within the performance-based earthquake engineering framework. The dependency between the maximum and residual inter-story drift ratios are captured using copulas. Finite-element models that take into account key hysteretic characteristics of non-ductile RC frames were adopted and incremental dynamic analysis is utilized to compute collapse risk. The proposed procedure is applied to a set of 2-, 4-, 8-, and 12-story non-ductile reinforced concrete frames located in Victoria, British Columbia, Canada. From the results, the aftershock showed marked difference for the 2-story building. At annual probability of 10−2–10−3, crustal and inslab events with Mw6.5 to Mw7.5 contributed the most to the loss as these events occur more frequently. At rarer annual probability of 10−3–10−4, the Cascadia event having Mw8.5 to Mw9.0 is predominant and contributed the most to the loss.

Influence of concurrent horizontal and vertical ground excitations on the collapse margins of non-ductile RC frame buildings

Recent earthquakes worldwide show that a significant portion of the earthquake shaking happens in the vertical direction. This phenomenon has raised significant interests to consider the vertical ground motion during the seismic design and assessment of the structures. Strong vertical ground motions can alter the axial forces in the columns, which might affect the shear capacity of reinforced concrete (RC) members. This is particularly important for non-ductile RC frames, which are very vulnerable to earthquake-induced collapse. This paper presents the detailed nonlinear dynamic analysis to quantify the collapse risk of non-ductile RC frame structures with varying heights. An array of non-ductile RC frame architype buildings located in Los Angeles, California were designed according to the 1967 uniform building code. The seismic responses of the architype buildings subjected to concurrent horizontal and vertical ground motions were analyzed. A comprehensive array of ground motions was selected from the PEER NGA-WEST2 and Iran Strong Motions Network database. Detailed nonlinear dynamic analyses were performed to quantify the collapse fragility curves and collapse margin ratios (CMRs) of the architype buildings. The results show that the vertical ground motions have significant impact on both the local and global responses of non-ductile RC moment frames. Hence, it is crucial to include the combined vertical and horizontal shaking during the seismic design and assessment of non-ductile RC moment frames.

In-Plane Seismic Strengthening of Nonductile Reinforced Concrete Shear Walls Using Externally Bonded CFRP Sheets

AbstractThis study evaluated the effectiveness of using externally bonded carbon fiber–reinforced polymer (CFRP) sheets to enhance the seismic performance of nonductile reinforced concrete shear wall specimens representative of low aspect ratio walls. The specimens were designed to replicate structural deficiencies typically found in older concrete structures constructed during the 1960s and 1970s. The nonductile reinforcement details included insufficient shear reinforcement, lack of boundary elements, and low concrete compressive strength. The specimens were repaired or strengthened using vertical and horizontal CFRP sheets and then cyclically tested to simulate seismic loading. A tube anchor system was used to transfer the tensile force carried by the CFRP sheet to the foundation of the specimens. Experimental results showed that the CFRP retrofitting system was able to restore the original strength and initial stiffness of severely damaged specimens and significantly increase the flexural strength, du...

Seismic Performance of Nonductile Reinforced Concrete Frames with Masonry Infill Walls—I: Development of a Strut Model Enhanced by Finite Element Models

Reinforced concrete (RC) frames with masonry infill walls are prevalent in high-seismicity areas worldwide and have experienced significant damage in earthquakes. This paper proposes a finite element–enhanced strut model to simulate the in-plane seismic response of masonry-infilled RC frames through time-history analysis. The strut backbone defining the behavior of the wall is developed from the response extracted from the finite element (FE) model(s) for the infill and frame configuration of interest. These struts are combined with models capturing flexural and shear failures of beam-columns to simulate building response. The strut model takes advantage of the accuracy of the FE modeling results, yet is computationally efficient for use in nonlinear dynamic analysis. The robustness of the proposed strut model is examined through comparison with experimental results for frames with different failure modes. This modeling approach is used in the companion paper to simulate the collapse response of 1920s-era California frames.

Seismic Performance of Nonductile Reinforced Concrete Frames with Masonry Infill Walls—II: Collapse Assessment

This paper quantifies the collapse performance of a set of masonry-infilled reinforced concrete (RC) frame buildings that are representative of 1920s-era construction in Los Angeles, California. These buildings have solid clay-brick infill walls and vary in height (2–8 stories), wall configuration (bare, partially, and fully infilled frames), and wall thickness (1–3 wythes). The buildings’ collapse behavior is assessed through dynamic analysis of nonlinear models. These models represent the walls by diagonal struts whose properties are developed from finite-element (FE) analyses, as described in the companion paper, and represent beam-columns with lumped-plasticity models. The results indicate that the presence of infill walls can increase the risk of collapse. The most collapse prone of the buildings considered are those with strong, heavy infill walls, which induce large force demands in the frame elements. The partially infilled frames, which have a soft and weak first story, also perform poorly.

ECC 날개벽 요소로 보강된 비내진상세를 갖는 철근콘크리트 골조의 내진성능

본 논문에서는 고인성 시멘트 복합체(ECC)가 적용된 날개벽 요소의 면 접합방식에 따른 평가를 실시하였다. 또한 비내진상세를 갖는 RC 골조에 ECC날개벽 요소 보강하여 보강 유무에 따른 내진성능평가를 실시하였다. 면 접합 방법에 따른 거동특성을 비교하기 위하여 2면 접합은 상·하부 보에 3면 접합은 상·하부 보 및 기둥에 접합하여 실험을 실시 하였다. 또한 비내진상세를 갖는 기존 구조체와의 일체로 거동하는 합성거동을 위해 3면 접합 방식으로 ECC날개벽 요소 보강을 실시하였다. ECC날개벽 요소 실험과 골조 실험은 점증되는 층간변위에 따라 2회씩 반복가력하여 실험을 진행하였다. 실험 결과 ECC 날개벽 요소 실험체의 경우 3면 접합이 2면 접합보다 우수한 내진성능 나타내었다. 각각의 실험체는 우수한 재료 특성으로 인하여 미세한 다수의 균열이 ECC날개벽 요소 전면에 폭넓게 분포하였다. 또한 보통 콘크리트와 달리 최대강도 이후 연성적인 거동을 나타내었으며, 이에 우수한 에너지소산능력을 나타내었다. ECC날개벽 요소를 보강한 실험체와 기존 골조에서는 ECC날개벽 요소 보강에 따른 최대강도 이후 연성적인 거동을 나타내었다. 이에 따라 에너지소산능력이 증가하였으며, 강성저하 또한 완만한 곡선을 나타내며 기존 골조보다 우수한 내진특성을 나타내었다. 이에 ECC 날개벽 요소의 보강이 비내진상세를 갖는 구조체에 우수한 내진특성을 부여하는 판단된다.

Seismic Performance of Nonductile Reinforced Concrete Frames with Masonry Infill Walls: I. Development of a Strut Model Enhanced by Finite Element Models

Reinforced concrete (RC) frames with masonry infill walls are prevalent in high-seismicity areas worldwide and have experienced significant damage in earthquakes. This paper proposes a finite element–enhanced strut model to simulate the in-plane seismic response of masonry-infilled RC frames through time-history analysis. The strut backbone defining the behavior of the wall is developed from the response extracted from the finite element (FE) model(s) for the infill and frame configuration of interest. These struts are combined with models capturing flexural and shear failures of beam-columns to simulate building response. The strut model takes advantage of the accuracy of the FE modeling results, yet is computationally efficient for use in nonlinear dynamic analysis. The robustness of the proposed strut model is examined through comparison with experimental results for frames with different failure modes. This modeling approach is used in the companion paper to simulate the collapse response of 1920s-era California frames.

Seismic vulnerability assessment of high-rise non-ductile RC buildings considering soil–structure interaction effects

Traditionally fragility curves of reinforced concrete (RC) buildings are estimated with the assumption of fixed base structures. The objective of the present research is to study whether soil-structure interaction (SSI) and site effects may affect the seismic performance and vulnerability of reinforced concrete moment resisting frame (MRF) buildings and consequently modify the fragility curves. SSI is modeled applying the direct one-step approach considering either linear elastic or nonlinear soil behavior while site effects are inherently accounted for. To further examine the contribution of site and SSI effects, a two-step uncoupled approach is also applied, which takes into account site effects on the response of the fixed base structure, but neglects SSI effects. Additional analyses are performed investigating the influence of the soil depth and stratigraphy under nonlinear soil behavior on the seismic response and fragility of RC buildings. A 9-story RC MRF designed with low seismic code provisions is adopted as a reference structure. A comparative dynamic analysis is performed highlighting various trends in the seismic response of the considered SSI and fixed base systems. Fragility curves are derived as a function of rock outcropping peak ground acceleration for the immediate occupancy and collapse prevention limit states for the fixed base and SSI models based on the statistical exploitation of the results of incremental dynamic analysis (IDA) of the given structural systems. Results show the significant role of SSI and site effects under linear or nonlinear soil behavior in altering the expected structural performance and fragility of high-rise fixed base structures.

Numerical investigation of the affecting parameters on the shear failure of Nonductile RC exterior joints

In this paper, a parametric study has been carried out on reinforced concrete (RC) exterior beam-column joints without transverse reinforcement in the joint region. This type of joints belongs to a group of RC structures classified as non-ductile RC buildings. In the previous investigations, special attention is paid to the joint aspect ratio and beam longitudinal reinforcement ratio. These parameters have been shown by other researchers to govern the joint shear behavior, although the degree of influence was not scrutinized to the degree required. The results of the current study confirmed once again that these parameters have a significant influence on the shear failure in RC joints. Apart from that, the degree of influence has been quantified in great detail. The findings suggest that the correct selection of these parameters is extremely important in the design of RC buildings, and a wrong choice may adversely affect the behavior.

Structural Behaviors of Non-Ductile Reinforced Concrete Frames with Engineered Cement Composite (ECC) Wing Wall Elements

In this paper, an experimental investigation was carried out to evaluate the application of engineered cement composite (ECC) wing wall elements for seismic strengthening of reinforced concrete (RC) buildings with non-ductile reinforcement details. The ECC is mixed with cement, silica fume, fly ash and polyvinyl alcohol (PVA) fiber. The ECC wing wall elements were fitted on three side of RC frame; upper beam, lower beam and column. Two specimens, non-ductile RC bare frame and RC frame strengthened with ECC wing wall, were made for this study. These specimens were made as a third scale for this study. The specimens were loaded through 1,000 kN actuator that consists of two cycles at a displacement level. The test results showed that the seismic strengthening method of ECC wing wall elements had a significant effect to improve seismic performance of non-ductile RC bare frame. As a result, the application of ECC wing wall elements in non-ductile RC bare frame may be effective alternative to seismic retrofit of non-ductile buildings.

Cyclic Response of Non-ductile RC Frame with Steel Fibers at Beam-Column Joints and Plastic Hinge Regions

An experimental study has been conducted on a reduced-scale gravity-load designed test frame to investigate its overall performance due to the addition of steel fiber-reinforced concrete (SFRC) at the critical regions. Two geometrically similar specimens, namely, reinforced concrete (RC) and SFRC, are tested under slow-cyclic lateral loading. End-hooked steel fibers (aspect ratio = 80) of 1.0% volume fraction were used in the SFRC mix for a distance of one-and-half times the member size near the joint regions. The addition of steel fibers improved the damage tolerance, lateral load resisting capacity, lateral stiffness, ductility, and energy dissipation of the frame.

Cyclic Response of Nonductile Reinforced Concrete Frames with Unreinforced Masonry Infills Retrofitted with Engineered Cementitious Composites

AbstractA new seismic retrofit technique for unreinforced masonry infills in nonductile reinforced concrete frames is presented. The objective was to develop a light and easy to apply retrofit technique for the infills that both delays shear failure at the columns of the nonductile bounding frame when subjected to an earthquake by preventing severe deterioration of the infill wall, and enhances the ductility of the system. The retrofit technique uses sprayable engineered cementitious composites (ECCs), a micromechanically designed cement-based material reinforced with short, randomly distributed polymeric fibers. Four small-scale unreinforced masonry infilled nonductile reinforced concrete frames were tested under in-plane quasi-static cyclic lateral loading. One unretrofitted wall and three different retrofit schemes were evaluated. Continuous steel reinforcement was embedded in the ECC layer and a bonding agent was applied between the masonry wall and the ECC layer. In two retrofits the impact of two di...

Simulation of Seismic Collapse in Nonductile Reinforced Concrete Frame Buildings with Masonry Infills

AbstractImproved analysis methods and guidelines are presented to simulate the seismic collapse of nonductile concrete frame buildings with masonry infills. The analysis tools include an inelastic dual-strut model that captures the post-peak behavior of the masonry infill and its interaction with the surrounding frame. The dual compression struts capture the column-infill interaction that can cause shear failure of the columns and loss of their vertical load carrying capacity. A rigid softening shear degradation model is implemented in the beam-column elements to capture the shear failure of nonductile RC columns. Guidelines are presented to determine the strut model parameters based on data from 14 experimental tests on infill frames. The models are applied in three-dimensional nonlinear dynamic analyses of a three-story nonductile concrete frame prototype building with infills. The incremental dynamic analyses technique is utilized to understand the effect of the infill-column interaction and the rockin...