Light Transverse Reinforcement Research Articles

Drift-based fragility assessment of nonductile reinforced concrete columns failing in shear under cyclic loading

The paper presents drift-based fragility functions for nonductile reinforced concrete (RC) columns having light transverse reinforcement steel ratio which predominantly fail in shear. These fragilities were developed for four key damage states (i.e., light cracking, severe cracking, shear failure, and loss of axial load carrying capacity) associated with their structural performance observed in laboratory tests and repairability. For this purpose, experimental results of 71 column specimens tested under cyclic lateral loading were collected from 22 research programs; particularly, drift ratios associated to the column specimens tested to reach axial collapse. It is shown that the shear span-to-depth ratio and level of axial load ratio have influence on the drift-based fragilities associated to the damage states considered in this investigation. Additionally, several sources of epistemic uncertainty due to specimen-to-specimen, finite-sample, column shear span-to-depth aspect ratio, and level of axial compression load were quantified from the experimental database. The introduced drift-based fragilities are a very useful tool to assess the expected damage of existing buildings, such as school buildings that might exhibit the short captive column effect.

Shear strength model of large-scale reinforced concrete rectangular columns with light transverse reinforcement

Columns with low transverse reinforcement due to poor detail, small rebar size, and wide spacing were commonly used in old buildings. These types of columns often fail in shear with the brittle mode that could lead to a sudden collapse of the entire building. Thus, the accurate determination of the shear strength is significant for estimating the response of reinforced concrete (RC) columns under earthquake events. Numerous experiments were conducted to investigate the shear strength, however, there is a lack of tests on large-scale lightly RC columns under high axial compression load. Therefore, this study tested four large-scale RC columns with dimensions of 800 mm × 800 mm × 3200 mm. The paper highlights the main factors that influence the shear strength of the RC columns and based on the test results, an equation with high predictive power is established. In addition to the experimental results, the analytical/empirical models’ predictions from design codes or other researchers were evaluated. Furthermore, the interaction diagram between bending moment (M)—shear force (V)—axial compression force (N) was conducted to estimate the strength ability of RC columns. It was found that the simple processes to build the backbone curves and the hysteresis loops that were recommended had good agreement with the tested results and experimental database.

Seismic behaviour of full-scale lightly reinforced concrete columns under high axial loads

Failure of lightly reinforced concrete (RC) columns has been the primary cause of the seismic collapse of aged or under-designed structures. Moreover, the columns in medium to high-rise buildings are often subject to high axial compression, further increasing their seismic vulnerability. Many past studies experimentally investigated the seismic behavior of lightly RC columns based on scaled specimens with cross-sections less than 500 mm and subject to a low to moderate axial load ratio (ALR) of less than 0.3. However, the size effects are recognized to influence the RC column behavior significantly. This study aimed to provide new insights into the influence of critical conditions, including high ALR and light transverse reinforcement, on the seismic behavior of full-scale RC columns. Based on the test results of a series of full-scale columns under displacement reversals, the increase of ALR led to brittle failure modes with decreased displacement ductility, but the shear strength, stiffness, energy dissipation, and plastic hinge length could be enhanced. While the ALR adversely impacted the drift capacity of flexure-dominant columns, it had an insignificant influence on the drift capacity of flexural-shear- and shear-dominant columns. Furthermore, the ratios of the displacement components were slightly affected by the ALR but were dependent on the transverse reinforcement ratio. In addition to the experimental results, predictions by analytical/empirical models from design codes and the literature were presented and discussed. It was found that the ACI shear model became less conservative when the ALR of the column was increased from 0.1 to 0.58. Moreover, the influence of the displacement ductility and aspect ratio on the ratio of the experimental and predicted shear strengths of the tested full-scale columns generally followed the trend of the data of the thirty-two shear-critical columns collected from previous studies.

Seismic behavior of low-corroded reinforced concrete short columns in an over 20-year building structure

In reinforced concrete (RC) structures, reinforcement corrosion, concrete carbonization, and chloride ion erosion occur during service life, which deteriorate the seismic structural capacity of the structures. Most researchers have focused on the effect of the reinforcement corrosion on the seismic behavior of RC columns showing flexural behavior. On the other hand, research on the influence of the combined reinforcement corrosion and concrete carbonization on the seismic behavior of in-service RC columns showing shear-flexural behavior has not been performed. The present study evaluates the seismic performance of low-corroded in-service RC columns showing shear failure under cyclic load. Four RC columns with shear-span ratio 2.3 and light transverse reinforcement were obtained from an in-service RC moment frame building constructed in 1987, and quasi-static cyclic tests were conducted to investigate the seismic behavior of these columns. For the test parameters, axial compression force ratios and longitudinal bars arrangements were considered. The test results showed that as the compression force ratio increased, the failure modes of the in-service RC columns shifted from shear-bond failure to shear-compression failure, and the ductility and energy dissipation capacity decreased. The shear strength of the column specimens was evaluated by five existing shear strength models, considering the effect of reinforcement corrosion. Finally, seismic performance of RC short columns with corroded bars was discussed.

Refined dynamic progressive collapse analysis of RC structures

Post-earthquake field investigation has revealed that if reinforced concrete (RC) structures in seismic regions were not designed to the level required by modern seismic design codes, they are vulnerable to severe damage or even progressive collapse when subjected to strong earthquakes. To gain insight into the mechanism and prevention of progressive collapse, it is imperative to develop an effective and efficient approach to capture the physical collapse of RC structures by numerical simulation. Finite element method (FEM)-based numerical simulation is one of the most widely used approaches for progressive collapse analysis, but some uncertainties still exist in its element removal algorithms. This paper refines FEM-based dynamic progressive collapse simulation for RC structures by introducing the concept of degree-of-freedom (DOF) release. With the concept, a RC beam-column element removal is a natural consequence of release of all DOFs of the element. This concept is implemented in an open-source finite element code of OpenSees. The test results of a RC beam with two ends fixed under a static load and a RC column with light transverse reinforcement under lateral pseudo-static load are first used to assess and demonstrate the advantages of the DOF release over the beam element removal. The refined progressive analysis method is then applied to a RC frame structure to demonstrate its dynamic progressive collapse with catenary effect included. Finally, a two-span continuous RC bridge with a two-column pier is taken as an example to demonstrate its flexure-shear-axial failure predicted by the proposed method.

Seismic performance of lightly shear reinforced RC columns

Previous experimental studies on reinforced concrete columns subjected to cyclic loading simulating earthquake loading have shown that the columns with non-seismic details of transverse reinforcement are vulnerable to the cyclic loading. In this study, to investigate the behavioral characteristics of concrete columns with light transverse reinforcement, ten half-scale specimens were fabricated and tested under repeated cyclic loading together with axial loading. The main test parameters included axial load level, hook angle, longitudinal reinforcement ratio, transverse reinforcement ratio, and shear span to depth ratio. The performances and characteristics of the test specimens were analyzed in terms of load-drift relationship, dissipated energy, damping ratio, strain profile, drift capacity, effective stiffness, and ductility. The test results showed that the axial load level is the major parameter affecting seismic performance including the drift capacity. Based on the test results, the nonlinear modeling parameters for concrete columns presented in ASCE 41-13 were investigated using fragility analysis.

Seismic Performance of Full-Scale FRP Retrofitted Substandard RC Columns Loaded in the Weak Direction

FRP confinement of sub-standard columns with low quality concrete, light transverse reinforcement and improper reinforcement detailing is widely accepted as an efficient retrofitting strategy. This paper introduces an improved method using carbon fiber reinforced polymers (CFRP) and external steel ties for seismic retrofitting of full-scale rectangular reinforced concrete columns loaded in their weak directions. Three cantilever columns with a cross-sectional aspect ratio of two (600 mm x 300 mm) are tested under constant axial load and reversed cyclic lateral loads. The columns are representative of existing substandard members with characteristics such as low concrete quality, low transverse reinforcement ratio, plain bars and high axial load level. The test results indicate that columns retrofitted with FRP jacketing and external steel ties significantly benefit from the applied retrofit scheme particularly in terms of ductility and energy dissipation. Additionally, the experimental results are compared with the performance predictions of seismic assessment and design documents.

Experimental Studies on the Backbone Curves of Reinforced Concrete Columns with Light Transverse Reinforcement

This paper presents the investigations carried out on the backbone curves of reinforced concrete (RC) columns with light transverse reinforcement. An experimental program consisting of five half-scale RC columns with light transverse reinforcement was carried out. The specimens are tested to the point of axial failure to obtain the backbone curves of such columns. Quasi-static cyclic loading simulating earthquake actions is applied. The backbone curves obtained from the tested specimens are then compared with the existing seismic assessment guidelines. The test results indicate that the initial column stiffness and ultimate displacements (displacements at axial failure) are overestimated and underestimated by some guidelines and provisions. The existing initial stiffness and ultimate displacement models are briefly reviewed and compared with the experimental results. The results show that the existing initial stiffness and ultimate displacement models produced better results than the existing seismic assessment guidelines.

CONTRIBUTION OF CONCRETE TO SHEAR STRENGTH OF RC BEAMS FAILING IN SHEAR

Reinforced concrete (RC) beams with light transverse reinforcement are vulnerable to shear failure during seismic response. In order to prevent brittle shear failures at beam plastic hinge regions of earthquake-resistant structures, the Turkish Earthquake Code and ACI318 require the use of sufficient transverse reinforcement to resist the total expected shear demand. These codes tend to be excessively conservative and, in some cases, the contribution of the concrete to the shear strength is neglected. The aim of this study is to investigate the contribution of concrete to shear strength of RC beams failing in shear experimentally. The beams were tested under monotonically increasing reversed cyclic loading to determine the concrete contribution to shear strength. It is observed that the concrete contribution to the shear strength at ultimate state ranges from 18% to 69% of the ultimate strength.

Seismic Behavior of Reinforced Concrete Columns with Light Transverse Reinforcement under Different Lateral Loading Directions

Considerable progress has been made on the strength and ductility research of reinforced concrete (RC) columns over the last two decades. However, the seismic performance of RC columns under lateral loading in different directions remains limited. This paper presents an experimental and numerical investigation carried out on RC columns with light transverse reinforcement with an emphasis on how varying the directions of seismic loading influences the seismic failure mechanisms of the columns. Seven half-scale RC columns were tested with the columns subjected to axial load and cyclic forces under reversed double-curvature bending. The parameters varied in the test program included the axial load ratio and loading directions. The overall performance of each specimen is examined in terms of cracking patterns, hysteretic response, initial stiffness, shear strength, drift ratio at axial failure, and energy dissipation capacity. A three-dimensional (3-D) finite element (FE) model is developed to supplement the experimental results. The direction of seismic loads is found to have a significant effect on the drift capacities and failure modes of both rectangular and square columns. The extent of change in the shear resistance capacity resulting from the different angles of seismic load could be predicted. A simple upper limit for shear strength contributed by concrete is proposed.

Ultimate Displacement of Reinforced Concrete Columns with Light Transverse Reinforcement

This article presents the analytical and experimental investigations carried out on reinforced concrete (RC) columns with light transverse reinforcement. A semi-empirical model is developed to estimate the ultimate displacement (displacement at axial failure) of RC columns with light transverse reinforcement subjected to simulated seismic loading. The developed model is calibrated using the collected data of RC columns tested to the point of axial failure. A series of experiments is conducted on five RC columns with light transverse reinforcement to validate the applicability and accuracy of the developed model. It is concluded from the study that the mean ratios of the experimental to predicted ultimate displacement and its coefficient of variation are 1.077 and 0.194, respectively, showing a good correlation between the developed model and the experimental data.

Dynamic collapse test on eccentric reinforced concrete structures with and without seismic retrofit

Reconnaissance reports on structures damaged or collapsed by severe earthquakes have revealed several common characteristics in their structural members and systems, such as insufficient reinforcement details in beam-column joints and transverse confinements, low aspect ratios, soft and or weak stories, and eccentric plans. Dynamic tests were carried out to investigate the collapse process of reinforced concrete structures that had seismically deficient reinforcement details (light transverse reinforcement) and seismic systems (soft/weak stories and eccentric plans). A comparison of collapse behaviors with and without seismic retrofits also verified the effectiveness of the SRF (super reinforced with flexibility) strengthening method, which was developed to prevent the loss of axial load carrying capacity even at excessive lateral deformation. The columns of one specimen were strengthened with polyester fiber belts and its shear walls with polyester fiber sheets, while the members of the other were not. Each specimen was designed following old (1970s) reinforcement detail practice in Japan, and is a one-third-scale reinforced concrete structure with considerable stiffness and strength eccentricity in the first story. The specimens were composed of independent column frame and shear wall frame. Torsional response resulting from the eccentricity in the 1st story induced a displacement concentration on the weak frame, and eventually the independent columns of the RC specimen failed in shear and lost their axial load carrying capacity. On the other hand, the SRF specimen survived not only an identical earthquake load to the one that caused the RC specimen to fail, but also three additional earthquake loads, although significant strength deterioration and considerable lateral and vertical deformation were generated at the end of the test. The following conclusions were drawn from the comparison of the two specimens’ responses: axial column collapse cannot be predicted from vertical responses since the vertical behavior of bare RC columns was not discernibly different from that of SRF columns until axial collapse was initiated, and the SRF strengthening method is effective in confining the column and preventing the cracking progress, thus modifying the failure mode of the RC columns from brittle shear failure after flexural yield to flexural dominant behavior.

Obtaining ductile performance from precast, prestressed concrete piles

This paper reports the findings of a parametric study of the inelastic seismic response of precast, prestressed concrete piles conducted to determine whether piles with only light transverse reinforcement could act as ductile structural elements. A nonlinear, inelastic finite element program written specifically for this project was used to validate results for both laboratory and in-place testing. The study examined single piles using several types of pile cap connections, the addition of mild-steel reinforcement, varying levels of axial load, and a range of soil stiffnesses. The piles were modeled with 1% transverse reinforcement, less than one-third of that required by ACI 318. The results indicate that modest levels of transverse reinforcement will allow for ductile response. Assuming that pile-cap connection is detailed to allow and support the formation of a plastic hinge, with subsequent redistribution of moment down the shaft to form a secondary subgrade hinge in the pile shaft, the pile configurations analyzed provided a minimum displacement ductility of 2. The addition of mild-steel longitudinal reinforcement did not enhance ductility, though it did increase flexural strength. The optimum pile-cap connection to maximize ductility is embedment of the pile head into the cap. Rotation capacity is maximized by embedment of prestressing tendons and any mild-steel longitudinal reinforcement present into the pile cap.

Dynamic Shear and Axial-Load Failure of Reinforced Concrete Columns

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.

Seismic Tests of Concrete Columns with Light Transverse Reinforcement

Earthquakes and laboratory experience show that columns with inadequate transverse reinforcement are vulnerable to damage including shear and axial load failure. To study this behavior, four full-scale columns with light transverse reinforcement were tested quasistatically under unidirectional lateral load with either constant or varying axial loads. Test results show that responses of columns with nominally identical properties vary considerably with magnitude and history of axial and lateral loads. Observed behavior is compared with expected behavior based on available analytical models. The FEMA 356 assessment model predicted the column strengths well, but underestimated the displacements.

Drift Capacity of Reinforced Concrete Columns with Light Transverse Reinforcement

Existing reinforced concrete columns with light transverse reinforcement are vulnerable to shear failure during seismic response. Shear strength models, modeling the degradation of shear strength with increasing displacement ductility demand, have been widely used to evaluate the interstory drift capacity of such columns. The application of a shear strength model to determine the drift capacities for a database of 50 shear-critical columns demonstrates significant inaccuracies with such a method. An empirical drift capacity model based on the shear-critical column database provides a better estimate of the interstory drift at shear failure. The new drift capacity model identifies the most critical parameters affecting the drift capacity of shear-critical columns, namely, transverse reinforcement ratio, shear stress demand, and axial load ratio.

Shear Strength Model for Lightly Reinforced Concrete Columns

Parameters affecting the shear strength of reinforced concrete columns having a rectangular cross section and light transverse reinforcement are investigated using data from numerous column tests. A new model is proposed to predict the column shear strength based on theoretical formulations and experimental evidence. The proposed shear strength equation includes contributions from the concrete and transverse reinforcement. Primary parameters in the shear strength model are the column cross-sectional dimensions, concrete compressive strength, column aspect ratio, axial load, and displacement ductility demand. The proposed model is compared with other shear strength models using the available column test data and is shown to result in improved accuracy. A conservative shear strength model for use in design and assessment is proposed based on statistical evaluation of computed and actual shear strengths.

Strength of prestressed concrete beams in torsion

An analytical model with tension softening for the prediction of the capacity of prestressed concrete beams under pure torsion and under torsion combined with shear and flexure is introduced. The proposed approach employs bilinear stress-strain relationship with post cracking tension softening branch for the concrete in tension and special failure criteria for biaxial stress states. Further, for the solution of the governing equations a special numerical scheme is adopted which can be applied to elements with practically any cross-section since it utilizes a numerical mapping. The proposed method is mainly applied to plain prestressed concrete elements, but is also applicable to prestressed concrete beams with light transverse reinforcement. The aim of the present work is twofold; first, the validation of the approach by comparison between experimental results and analytical predictions and second, a parametrical study of the influence of concentric and eccentric prestressing on the torsional capacity of concrete elements and the interaction between torsion and shear for various levels of prestressing. The results of this investigation presented in the form of interaction curves, are compared to experimental results and code provisions.