Longitudinal Reinforcement Bars Research Articles

Experimental investigation of stress transients of blasted RC columns in the blasting demolition of buildings

Blasting demolition is the preferred method for safely and efficiently demolishing high-rise buildings, and numerical simulation is very helpful for the design of complicated blasting schemes. In simulations of demolition projects, the explosion processes and blast loadings are usually ignored to save computer time. However, the stress transients of blasted columns at the bottom of buildings are important boundary conditions, which may affect the accuracy of simulation results. Yet, few works have investigated it and provided a detailed model. To investigate the features of stress transients of reinforced concrete (RC) columns, an experiment was conducted in the present study. In a 56-m-high frame structure demolished by controlled blasting technology, a 1000 mm × 700 mm × 4100 mm column was blasted. The collected data include strain-time histories and failure mode of the column. The study shows that explosion-induced stress-time history curves far from the blastholes are jagged-triangular, which may have multiple peaks because of the delay time errors of detonators. After detonation of explosives, the bearing capacity of the column gradually decreases, which takes several milliseconds. When the bearing capacity reduces to less than the initial axial compressive stress in the column, the initial stress releases linearly, which also takes several milliseconds. The movements of the concrete fragments bent the longitudinal reinforcement bars, which induces a strong tensile stress in the residual column; their stress-time history curves are triangular. Based on the experiment results, a stress transient concept model is proposed to detailly describe the stress state of blasted RC columns in demolished high-rising buildings, which can be applied in the numerical simulation of demolition blasting.

Experimental study on seismic behavior of exterior composite beam-to-column joints with large size stiffened angles

The top-and-seat angles with double web angles are commonly used in the design of beam-to-column joints in Asian and North American countries. The seismic behavior analysis of these joints with large cross-section size of beam and column (often connected by four or more bolts) is a challenge due to the effects from the relatively larger size of stiffened angles and the composite action from the adjacent concrete slab. This paper presents an experimental investigation on the seismic performance of exterior composite beam-to-column joints with stiffened angles under cyclic loading. Four full-scale composite joints with different configuration (only one specimen contain top angle in concrete slab) were designed and tested. The joint specimens were designed by considering the effects of top angles, longitudinal reinforcement bars and arrangement of bolts. The behavior of the joints was carefully investigated, in terms of the failure modes, slippage, backbone curves, strength degradation, and energy dissipation abilities. It was found that the slippage between top-and-seat angles and beam flange, web angle and beam web led to a notable pinching effect, in addition, the ability of the energy dissipation was significantly reduced. The effect of anchored beams on the behavior of the joints was limited due to premature failure in concrete, the concrete slab that closes to the column flange and upper flange of beam plays an significant role when the joint subjected to the sagging moment. It is demonstrated that the ductility of the joints was significantly improved by the staggered bolts and welded longitudinal reinforcement bars.

Comparison between cyclic response of RC columns transversely reinforced with FRP strips and carbon steel

This study investigates the effects of different confinement methods for longitudinal reinforcement bars in reinforced concrete, RC columns. Comparison is made between three RC columns; two were transversely reinforced with carbon steel in single tie and continuous spiral form, and the third sample were transversely reinforced with Fibre Reinforced Polymer, FRP strips in continuous spiral form. Numerical simulation is conducted using validated numerical models. All three RC column models were subjected to a constant axial load and cyclic loading. Hysteresis curves and backbone curves were used to analyse cyclic behaviour of each samples. Comparisons between the RC columns were made based on the ultimate load capacity and energy dissipation capacity ratio. Results show that, RC column transversely reinforced with FRP strips exhibits the highest ultimate load and energy dissipation capacity. The ultimate load capacity is 15% and 10% higher than the RC column transversely reinforced with carbon steel in single tie and continuous spiral form respectively. The FRP strips also increases the energy dissipation capacity up to 35% and 22% compared to the one transversely reinforced with carbon steel in single tie and spiral form respectively. It can be concluded that the usage of FRP strips as transverse reinforcement in RC column is able to improve the cyclic responses of the conventional RC columns.

Experimental investigations of failure modes of reinforced concrete beams without web reinforcement

This paper reported an experimental investigation into the stochastic nonlinearities inherent in reinforced concrete (RC) beam without web reinforcement. A series of specimen tests were conducted with a static monotonic four-point bending regime. Three kinds of RC beams (total of 27 specimens) without web reinforcement of three shear span-depth ratios of 5.1, 4.6 and 4.0 were designed. Each kind of beams had 9 samples. All beams were constructed in the same condition and at the same time. Specifically, the materials (both concrete and reinforcement bars) used in the beams were derived from the same batch, and the same testing program was performed during the experiments. Statistical nonlinearity responses to beams with three shear span-depth ratios, including the strain-deflection curves of longitudinal reinforcement bars, crack patterns, failure modes, and load-deflection curves were obtained from the tests. Different failure modes occurred in the beams with the same shear span-depth ratio. The probabilities of different failure modes of RC beams without shear reinforcement were statistically calculated based on the test results. It is found that the randomness of the mechanical properties of the materials results in the stochastic initial damage distribution and the subsequent stochastic nonlinear evolution of the beams. In addition, the shear span-depth ratio is an important parameter, which significantly affects the stochastic failure mode in the investigated members.

Performance of lightweight granulated glass concrete beams reinforced with basalt FRP bars

This paper presents an investigation into the flexural behaviour of basalt FRP reinforced concrete beams through experimental and analytical methods. To achieve the research objectives, four concrete beams reinforced with steel and four identical concrete beams reinforced with BFRP bars were tested under four-point bending. The main parameters examined under the tests are the type of concrete (lightweight foam glass concrete and normal concrete) and the type of longitudinal reinforcement bars (BFRP and steel). Test results are presented in terms of failure modes; deformation crack pattern and the ultimate moment of resistance are presented. The experimental results are analysed and compared to predictive models proposed by ACI 440.1R, 2006 and BS EN 1992, Eurocode 2, for deformations and ultimate flexural capacities of the steel and BFRP reinforced concrete beams. The experimental results indicated that the flexural capacity decreased for the beams reinforced with BFRP bars compared to that of a corresponding beam reinforced with steel bars. Both types of beams failed in the modes predicted. The prediction models underestimated the flexural capacity of BFRP reinforced concrete beams. The increase in foam glass aggregate content was observed to reduce the cracking load by almost 10-40% and 25-50% for steel and BFRP reinforced concrete beams, respectively. The flexural capacities of BFRP reinforced beams were underestimated by using equations stipulated in ACI 440.1R and Eurocode 2 codes of practice. © 2019 Growing Science Ltd. All rights reserved.

Experimental assessment and theoretical evaluation of axial behavior of short and slender CFFT columns reinforced with steel and CFRP bars

This paper presents the test results of an experimental program to investigate the structural performance of short and slender concrete-filled fiber-reinforced polymer (FRP)-tubes (CFFTs) columns under pure axial compression load. The test parameters include: transverse reinforcement type (steel spirals versus glass-FRP (GFRP-tube), type of longitudinal reinforcement (steel and carbon-FRP (CFRP) bars), GFRP tube thickness, and slenderness ratio.Comparisons between the experimental test results and the theoretical design equations are performed in terms of ultimate load carrying capacity. The design equation is modified to accurately predict the ultimate load capacities of CFFT columns reinforced with FRP bars. Furthermore, the experimental results showed that the axial compressive strength of CFRP-reinforced CFFT columns was reduced by 22% with increasing the slenderness ratio from 8 to 20. In general, the use of the FRP tube induces a confinement effect for concrete columns which enhances the strength and ductility of such column’s section. This enhancement resulted in a slender section which increased the possibility of buckling instability of the CFFT columns. Thus, a theoretical model was conducted to develop a simplified formula for critical slenderness limit of FRP-reinforced CFFT columns to control the buckling instability mode of failure. Results indicated that the slenderness limit of 14 was suggested as a safe value for the design purposes. A parametric study was conducted which showed the significant effect of the concrete compressive strength and hoop stiffness of FRP tube on the critical slenderness ratio of FRP-reinforced CFFT columns.

SEISMIC PERFORMANCE OF BEAM-COLUMN JOINTS IN REINFORCED CONCRETE BUILDINGS SUBJECTED TO REVERSIBLE VERTICAL CYCLIC LOADING

Three identical half-scale of exterior beam-column joints of RC buildings with different arrangement of connections detailing are designed, constructed and tested under reversible vertical cyclic loading. Each set consists of reinforced concrete beams, column and foundation beam which represents the exterior joints of the ground floor of RC buildings. The objectives of this study are to examine seismic performance, to observe visual damages and to classify different types of damages which occur in beam-column joints. The first set is designed beam-column joint in accordance to BS 8110 where no anchorages of bars are tied between beam and column. Second set comprises of three anchorage bars from column are placed and tied with top of reinforcement bars in the beam. Third set is designed with cross-bracing of reinforcement bars at the intersection of beam-column joint. Results showed that the beam-column connections which designed according to BS 8110 without anchorage bars suffer the most severe damages as compared to connections with anchorage bars and cross-bracing bars. Beam-column joint with cross-bracing bars experienced less damage as compared to two types of joints. Visual structural damages of beam-column joints include cracks prorogation, the spalling of concrete shell, buckling of longitudinal reinforcement bars and fractured of shear reinforcement bars. Therefore, it is an appropriate time to revise and replace the current code of practice to seismic code so that reinforced concrete buildings which are constructed in Malaysia are save under long-distant earthquake excitation and local earthquake.

Confinement Effect of Geo-grid and Conventional Shear Reinforcement Bars Subjected to Corrosion

An experimental study was performed to examine the effects of polyester geo-grid as a shear reinforcement bar for corroded and uncorroded reinforced concrete beams. The main aim of this study was to investigate the effect of geo-grids on flexural strength, moment-curvature, bond–slip and crack patterns for uncorroded and corroded reinforced concrete beams. For each parameter, a geo-grid confinement technique was compared with conventional reinforced concrete beams as a function of corrosion level. The experimental program consisted of testing ten reinforced concrete beams for two different configurations of shear reinforcement bars at six different corrosion levels (0% and approximately 2, 3, 5, 6, and 9%). Test specimens were subjected to an accelerated corrosion method to corrode the reinforcement bars using a full-scale corrosion pool. Actual corrosion levels were obtained from fully extracted reinforcement bars by breaking the concrete. At the same overall corrosion levels which were differed at longitudinal and transverse reinforcement bars at the same applied corrosion current time for different RC beams. Obtained actual corrosion levels with the aid of fully extracted reinforcement bars from concrete provided to examine the effect of corrosion levels by considering the type of reinforcement bars. The results indicated that the performance of the polyester geo-grid was very poor compared to conventional steel stirrups, even at this stage of corrosion. It was found that the utility of a geo-grid as shear reinforcement bars in reinforced concrete beams is not proper.

Seismic Behaviors of Steel Bar Reinforced Joints of Concrete Filled Steel Tubular Laminated Columns

This paper presents the numerical studies on seismic behaviors of steel bar reinforced joints of Concrete Filled Steel Tubular Laminated Columns (CFSTLC). First, the material constitutive relations and modeling details are introduced. Then, the failure mode, bearing capacity and energy dissipation of CFSTLC joints under cyclic loading are studied, in which the effects of concrete grade, steel grade and reinforcement arrangement are discussed. Finally, the restoring force model is proposed to describe the seismic behaviors of bar reinforced joints. Results show that the bar reinforced joints perform good seismic performance. There are three main failure modes, including flexural failure of beam, shear failure of column and mixed failure. Increasing concrete grade and reinforcement ratio of column can increase the ultimate bearing capacity and energy dissipation capacity. Using bundled longitudinal reinforcement bars in beams can slightly reduce the ultimate bearing capacity, but can improve the energy dissipation capacity of specimens with flexural failure modes. Increasing the reinforcement ratio and steel grade can decrease the degradation rate of structural stiffness. The tri-linear restoring force model can make a good prediction of hysteretic behaviors of CFSTLC joints.

Flexural and shear behaviours of plain and reinforced polyvinyl alcohol-engineered cementitious composite beams

In this paper, the results of an experimental investigation on the flexural and shear behaviours of plain and reinforced polyvinyl alcohol-engineered cementitious composite (PVA-ECC) beams are presented. The PVA-ECC employed in this study, which has a tensile strain capability up to 1%, has been recently developed by the authors using local sand to reduce cost. The aim of this study is to investigate the effects of PVA-ECC matrix, transverse reinforcement (stirrups) and longitudinal reinforcement bars on the flexural and shear structural performances of reinforced PVA-ECC beams. Four-point bending tests were conducted and normal steel reinforced concrete beams were tested for comparison. Digital Image Correlation technique was used to monitor single crack development and was validated by LVDT measurements. Experimental results show that PVA-ECC beams present significantly improved flexural behaviour compared with normal concrete beams. Furthermore, PVA-ECC beams without stirrups eventually fail in flexure rather than in shear, and exhibit similar load-deflection, moment-curvature relationships and crack development history when compared with PVA-ECC beams with stirrups.

10.01: Analytical methods for the prediction of fire resistance of “reinforced” slim floor beams

ABSTRACTThe present paper describes a new analytical method for the prediction of temperature in longitudinal reinforcement bars of slim floor beams subjected to fire. This method is based on the isotherms of EN 1992‐1‐2 Annex A developed for concrete beams and slabs and is consequently applicable only to slim floor beams subjected to standard ISO fire curve. In addition to vertical heat fluxes, this method also considers 2D temperature field induced by the hot part of the beam web and by the lateral heating in case of long span composite flooring systems. An adaptation is proposed for reinforcement bars when situated close to the beam web.This new method leads to very good temperature predictions for R60, R90 and R120 fire resistance requirements. For less severe requirements, the reinforcement usually remains below 400°C and the resistance of hot‐rolled reinforcement bars is not reduced for this range of temperatures. The better predictions of the sagging bending moment of the section under ISO fire by use of this new method have also been demonstrated within the present paper.

Experimental and numerical study on the behavior of circular RC columns under impact loading

Columns of overpasses are highly exposed to the potential hazard of vehicular collisions. This type of collisions occurred in the past, sometimes with catastrophic consequences. Previous research reported in the literature has explored the dynamic response of RC column under impact loading using both numerical and experimental methods. The majority of the available tests were performed on horizontal elements featuring beam characteristics. In this context, a new facility for the testing of vertical elements featuring the real characteristics of RC columns was realized in the Collision Laboratory of Nanjing Tech University.Four impact tests on cantilever circular RC column were conducted using this facility. The diameter of the column is 330 mm and the height is 1700 mm. The column is a one-third scaled model of a 5.2 m tall RC bridge pier. All the columns have the same configuration of the longitudinal reinforcement bars and two different hoop spacings both representing the case of a low shear reinforced structural element. Impact force, lateral displacements and accelerations were measured. A high-speed camera was adopted to record the evolution of the damage of the RC column during the impact. A brittle shear-type damage was observed characterized by one main diagonal crack located from the rear base of the column to the base of the impact point. Finally, a numerical study was also conducted using LS-DYNA showing a good agreement with the experimental results, especially in the low-velocity impacts. Results highlighted the vulnerability of these structural elements under impact loading.

An effective model for analysis of reinforced concrete members and structures under blast loading

The traditional fiber beam model has been widely used in the seismic analysis of reinforced concrete members and structures. However, the inability to capture shear failure restricts its application to blast loadings. In this article, a numerical model that considers both rate-dependent shear behavior and damage effect is proposed based on the traditional fiber beam element. This is achieved using the modified compression-field theory with a concrete damage model and bilinear steel model in the principal directions. Meanwhile, a condensed three-dimensional stress–strain relation from the isotropic hardening plasticity model is implemented to simulate longitudinal reinforcement bars, as large shear strain would be produced under severe blast loads. The proposed model is validated by comparing the numerical and test results. The high-fidelity physics-based finite element model, validated by the same experiment, is also used in the study to prove the efficiency of the proposed model. Case studies of a reinforced concrete beam and a six-story reinforced concrete frame structure subjected to blast loads are then carried out. The results indicate that the proposed model is reliable compared with the high-fidelity physics-based model. In addition to the accuracy, comparisons of the computational time show an excellent performance with respect to the efficiency of the proposed model.

Behaviour of composite frames with castellated steel beams at elevated temperatures

The behaviour of composite frames with castellated steel beams at elevated temperatures is investigated, and the results obtained were compared with the companion results of simply supported composite castellated steel beams. Overall, it is aimed to investigate the effect of axial and rotational restraining of the steel beam via rigid connections with protected steel columns at elevated temperatures. A previously reported finite element model for the analysis of composite beams in fire was extended to perform the nonlinear analyses of composite frames at elevated temperatures. The finite element model has accounted for the frame geometries and boundary conditions, nonlinear material properties of steel, concrete, profiled steel sheeting, longitudinal and lateral reinforcement bars as well as shear connection behaviour at ambient and elevated temperatures. The thermal properties at the steel beam top flange/profiled steel sheeting and profiled steel sheeting/concrete elements interfaces were considered in the thermal heat transfer analysis that allowed the temperatures to be accurately predicted in the composite slab during fire exposure. The comparison of composite frames and composite beams behaviour has shown that if the columns were sufficiently protected and the connection maintained its rigidity during fire exposure, such that it can restrain the composite beam thermal expansions, this would result in considerable reduction in fire resistances of the composite frames compared with that of simply supported composite beams. This is attributed to the premature failure at elevated temperatures due to local buckling at the bottom flange of the steel beams in the frame connections. Furthermore, the variables that influence the fire resistance and behaviour of the composite frames comprising different load ratios during fire, different fire curves, presence of web openings and different steel grades were investigated by parametric studies. It is shown that the strength of steel and the fire scenarios have a considerable effect on the time–displacement behaviour and fire resistances of the composite frames.

Performance of RC columns with debonded bars subjected to cyclic lateral loading

SummaryConcrete columns with light confinement reinforcement subjected to cyclic lateral load exhibit brittle shear failure along with flexural yielding of longitudinal steel reinforcement bars. The present study investigates the effect of debonding of longitudinal bars on the performance of columns under lateral loading. Column specimens were tested under quasi‐static reverse cyclic lateral loading. A comparative study has been carried out on the performance of columns with and without debonding using circular steel tubes of varying lengths. Steel tubes were provided to the longitudinal reinforcement in the potential plastic hinge zone. It is observed that the specimens with debonded reinforcement have significant enhancement in deformation capacity, ductility and energy dissipation capacity than the specimens without it. The congestion of reinforcement at the joints can also be eliminated by reducing the confining reinforcement for these debonded specimens. Copyright © 2015 John Wiley & Sons, Ltd.

Nonlinear analysis of composite castellated beams with profiled steel sheeting exposed to different fire conditions

This paper discusses the nonlinear analysis and design of unprotected composite castellated and noncastellated steel beams with profiled steel sheeting at elevated temperatures. The nonlinear material properties of steel, concrete, profiled steel sheeting, longitudinal and lateral reinforcement bars as well as shear connection behaviour at ambient and elevated temperatures were considered in the finite element models. The thermal properties at the steel section/profiled steel sheeting and profiled steel sheeting/concrete element interfaces were considered in the thermal heat transfer analysis that allowed the temperatures to be accurately predicted in the composite slab during fire exposure. It has been shown that the finite element models can efficiently predict the thermal and thermal–structural behaviour of the composite beams at elevated temperatures during the heating and cooling phases of fire. Furthermore, the variables that influence the fire resistance and behaviour of the composite beams comprising different load ratios during fire, different fire curves, presence of web openings and different steel grades were investigated in parametric studies. It is shown that the fire resistances of the unprotected simply supported composite castellated and noncastellated steel beams could be below 30min when heated using the standard fire curve. It is also shown that the strength of steel beam has a considerable effect on the behaviour and failure modes of the composite beams, which could change the failure mode of the composite beams in fire. The fire resistances of the castellated and noncastellated composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 is conservative for most of the unprotected composite castellated and noncastellated steel beams, except for some composite beams heated using the standard fire curve under load ratios of 0.4 and 0.5.

Domino-type progressive collapse analysis of a multi-span simply-supported bridge: A case study

Hongqi Viaduct, a multi-span simply-supported bridge in Zhuzhou city, Hunan Province, China, collapsed progressively during the mechanical demolishing of the bridge on May 17, 2009. Totally nine spans collapsed in the accident and it is a typical domino-type progressive collapse. The accident resulted in the loss of 9 lives and 16 injuries. Investigations were conducted after the accident to determine the cause of the unexpected progressive collapse. This paper is aimed at presenting a summary of the bridge before and after the incident, the demolishing plans and field investigations after the accident. To better understand the cause and mechanism of the progressive collapse, a numerical simulation is carried out. A detail 3D finite element (FE) model is developed by using the explicit FE code LS-DYNA. The bridge components including the bridge slabs, wall-type piers, longitudinal and transverse reinforcement bars are included in the model. The non-linear material behaviour including the strain rate effects of the concrete and steel rebar are considered. The model is used to simulate the bridge collapse induced by demolishing, and the domino-type progressive collapse of the bridge is clearly captured. Based on the numerical results, the reason for the failure is discussed and better understood. Finally, the possible mitigation methods of such progressive collapses of multi-span viaducts are suggested.

Reinforcement to Induce Ductile Behavior in an Ultra High Performance Fiber Reinforced Concrete Beam

The purpose of this study was to reduce production costs for UHPFRC members by using a bundle of longitudinal reinforcement bars as a substitute for steel fiber. Experiments on the ductile behavior of Ultra High Performance Concrete (UHPC) rectangular beams with a combination of steel fiber and longitudinal reinforcement bars were performed.The volume fractions of steel fiber were 0%, 0.7%, 1%, 1.5%, 2%, and the reinforcement ratios of longitudinal reinforcement bars that promoted ductile behavior were 0.0036, 0.016, 0.029 and 0.036.Fifteen UHPC beams were made with the combination of these test factors. Both the steel fiber and the longitudinal reinforcement bars had the effect of induciing ductile behavior for UHPC structural members. The load-deflection relationship, the concrete stress variation and the crack pattern indicated the usefulness of the bundle of longitudinal bars that have a small diameter with close arrangement.

Plastic Hinge Length of FRP-Confined Square RC Columns

The confinement effect of fiber-reinforced polymer (FRP) jacket on the plastic hinge length of square RC columns is studied through experimental testing and analytical study. Seven half-scale RC square columns with different confinement ratios were tested. The strain of longitudinal reinforcement bars and the extent of yielding were measured by strain gauges mounted inside the reinforcement bars. The variation of the strain field of the external column face was recorded continuously by digital image correlation (DIC) during a test. More rational approaches of data analysis are used for identification of plastic hinge length from measured strain fields. The obtained results show that compared with unconfined RC columns, FRP jacketing increases the plastic hinge length when the confinement level is low, but reduces it when the confinement level is high. The analytical study shows that all existing models of plastic hinge length are inadequate in one way or another because no model includes all important factors. The proposed model can correctly capture the trends but it needs to be further improved for accuracy when more data are available.

Strain Distribution on Reinforcement of Concrete Beams Reinforced with Glass Fiber Reinforced Polymer (GFRP) Bars

This paper presents a part of the results from an experimental study of strain distribution on reinforcement of concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars. Under static loading conditions, eight concrete beams reinforced with GFRP bars were tested and as comparison eight beams with steel reinforcement were also tested. All of the beams were prepared with varying ratios of longitudinal reinforcement bars and stirrups. The effect of shear span-effective depth ratio on strain distribution of longitudinal reinforcement was also observed. Furthermore, the behavior of strain on stirrups due to different materials of longitudinal reinforcement was also discussed in this report. The test results show that the ratio of longitudinal reinforcement significantly influence the strain distributions on reinforcement where the beams with higher ratio exhibit higher strain. Moreover, it was also obtained that the different types of longitudinal reinforcement considerably influences the strain behavior on stirrups as higher strain was observed in beams reinforced with GFRP bars.