Diagonal Shear Cracks Research Articles

Effect of existing cracks on shear failure behaviour of reinforced concrete members

The effect of existing cracks on shear failure behaviour of reinforced concrete (RC) members was investigated in this paper. The authors conducted the experiments on shear response of pre-cracked RC members with various quantities of stirrups. The objective of the study is to identify the effect of pre-cracks on concrete that resists shear through diagonal tension, as in beams without stirrups, and through compressive struts, as in beams with large amounts of stirrups. In the former case, it is found that pre-cracks delay diagonal tension failure, thus elevating the shear capacity. The apparent increase in concrete tension capacity arises from the crack arrest mechanism. The propagation of a diagonal shear crack is arrested by the pre-crack. A considerable increase in shear capacity is obtained, and the failure mode is changed from brittle shear to flexural yielding. On the other hand, in the case of a beam containing large amounts of stirrups, pre-cracks decrease the web-crushing capacity. Pre-cracks are supposed apparently to weaken the concrete struts by two mechanisms: a reduction in the effective contact area of crack surface and a reduction in the compressive strength of concrete near pre-crack owing to micro-fracturing damages. In the case of beams with a fair amount of stirrups, the failure is governed by yielding of stirrups rather than crushing of web concrete, hence the web-crushing mode is not relevant. The increase in diagonal tension capacity of concrete therefore leads to the increase in the overall shear capacity.

Strut-tie model evaluation of behavior and strength of pre-tensioned concrete deep beams

To date, many studies have been conducted for the analysis and design of reinforced concrete members with disturbed regions. However, prestressed concrete deep beams have not been the subject of many investigations. This paper presents an evaluation of the behavior and strength of three pre-tensioned concrete deep beams failed by shear and bond slip of prestressing strands using a nonlinear strut-tie model approach. In this approach, effective prestressing forces represented by equivalent external loads are gradually introduced along strand's transfer length in the nearest strut-tie model joints, the friction at the interface of main diagonal shear cracks is modeled by the aggregate interlock struts along the direction of the cracks in strut-tie model, and an algorithm considering the effect of bond slip of prestressing strands in the strut-tie model analysis and design of pre-tensioned concrete members is implemented. Through the strut-tie model analysis of pre-tensioned concrete deep beams, the nonlinear strut-tie model approach proved to present effective solutions for predicting the essential aspects of the behavior and strength of pre-tensioned concrete deep beams. The nonlinear strut-tie model approach is capable of predicting the strength and failure modes of pre-tensioned concrete deep beams including the anchorage failure of prestressing strands and, accordingly, can be employed in the practical and precise design of pre-tensioned concrete deep beams.

Shear strength of concrete beams reinforced with steel bars milled from scrap metals

The paper reports a study on the shear resistance of concrete beams reinforced with mild steel bars that are milled from scrap metal such as old vehicle parts and obsolete machinery. It has been previously reported that because the chemical compositions of carbon, sulphur and phosphorus in these reinforcing steel bars exceed the maximum allowable limits, the characteristic tensile strengths are too high and ductility too low for standard mild steel. Concrete beams reinforced with such bars to resist flexural tensile and shear stresses were tested under a two-point loading system to provide a central constant moment region and outer shear spans. Tested beams exhibited little deflection and very low ductility prior to collapse. Experimental failure loads for the beams averaged 123% of the theoretical failure load, which was generally governed by either shear or yielding of the tension steel. Shear failure was mostly initiated by diagonal tension cracks, followed by either crushing of the concrete, or splitting of the concrete over the longitudinal tensile bars near the supports. Failure of the beams was brittle and the post-cracking strain energy absorption averaged 357.9 Nm. At failure the maximum crack width in the beams ranged from 1.12 to 5.0 mm, the largest sizes forming in the diagonal shear cracks.

Nonlinear Strut-Tie Model Analysis of Pre-Tensioned Concrete Deep Beams

To date, many studies have been conducted for the analysis and design of disturbed regions. However, prestressed concrete deep beams have not been the subject of much dedicated effort. This paper presents an evaluation of the behavior and strength of two pre-tensioned concrete deep beams tested to shear failure using a nonlinear strut-tie model approach. In this approach, the effective prestressing forces represented by equivalent external loads are gradually introduced along tendon's transfer length in the nearest strut-tie model joints, the friction at the interface of main diagonal shear cracks is modeled by diagonal struts along the direction of the cracks in strut-tie model, and additional positioning of concrete ties at the place of steel ties is incorporated. Through strut-tie model analysis of pre-tensioned concrete deep beams, the nonlinear strut-tie model approach proved to present effective solutions for predicting the essential aspects of the behavior and strength of pre-tensioned concrete deep beams.

Interaction of Pre-Induced Damages along Main Reinforcement and Diagonal Shear in RC Members

The mechanical interaction of pre-induced damage and diagonal shear was investigated with regard to corrosion cracks in concrete and rupture of the web reinforcement along the main reinforcement. First, localized corrosion cracking was produced in concrete around the mid-span of beams in both compression and tension fibers, and a mechanical interaction was no longer identified. Next, cracking occurred near the beam supports inside a shear span and around the anchorage zones of longitudinal steel. A diagonal shear crack was observed to meet the pre-existing cracking around the anchorage zones of longitudinal steel. Crack propagation into the anchorage zones resulted in significant decline in the overall capacity. On the other hand, the overall capacity could be maintained or even increased by keeping anchorage failure from occurring. Finally, RC beams with artificial defects in both concrete and web reinforcement over the whole shear span were tested. Damage around anchorage zones was concluded to play a critical role in triggering shear sliding failure in pre-damaged reinforced concrete.

Fibre reinforced polymer shear reinforcement for concrete members: behaviour and design guidelines

This paper describes an experimental program conducted to examine the structural performance of fibre reinforced polymer (FRP) stirrups as shear reinforcement for concrete structures. A total of ten large-scale reinforced concrete beams were tested to investigate the contribution of the FRP stirrups in a beam mechanism. The ten beams included four beams reinforced with carbon fibre reinforced polymer (CFRP) stirrups, four beams reinforced with glass fibre reinforced polymer (GFRP) stirrups, one beam reinforced with steel stirrups, and one control beam without shear reinforcement. The variables were the material type of stirrups, the material type of the flexural reinforcement, and the stirrup spacing. Due to the unidirectional characteristics of FRP, significant reduction in the strength of the stirrup relative to the tensile strength parallel to the fibres is introduced by bending FRP bars into a stirrup configuration and by the kinking action due to inclination of the diagonal shear crack with respect to the direction of the stirrups. A total of 52 specially designed panel specimens were tested to investigate the bend and kinking effect on the capacity of FRP stirrups, along with two control specimens reinforced with steel stirrups. The variables considered in the panel specimens are the material type of the stirrups, the bar diameter, the bend radius, the configuration of the stirrup anchorage, the tail length beyond the bend portion, and the angle of the stirrups. Based on the findings of this investigation, shear design equations for concrete beams reinforced with FRP, appropriate for the Canadian Standards Association (CSA) code, are proposed. The reliability of the proposed equations is evaluated using test results of 118 beams tested by others.Key words: shear, fibre-reinforced polymers, CFRP, cracks, GFRP, kink, stirrups, bend capacity.

Shear failure and ductility of RC columns after yielding of main reinforcement

During the Hanshin-Awaji Earthquake, catastrophic shear failure accompanying plenty of diagonal shear cracks was experienced in RC structures and bridge piers serving transportation facilities. In this paper, shear capacity and ductility of RC columns in the post-yield range of longitudinal reinforcement are discussed for seismic-resistant-design. First, the shear failure of large-scale RC columns is computationally simulated for an understanding of the mechanism of shear collapse of bridge piers. FEM computational results on ductility are compared with the experiments and a parametric study is conducted concerning factors that affect the post-yield deformability of RC columns. Finally, a simple formula for ductility is presented for summarizing the sensitivity of influential factors.

Simplified Method for Shear Design of Prestressed Concrete Girders

The new AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications incorporate the modified compression field theory for shear design of prestressed and non-prestressed concrete members. This method is based on the variable-angle truss analogy model and takes into account the inclination of diagonal shear cracks, the strain in longitudinal steel, and the applied shear stress on a section. This is a realistic approach to predict the shear resistance of any section of a concrete member. However, for prestressed concrete sections, an iterative process is required in conjunction with charts and tables to determine the shear resistance of the sections. The process is cumbersome and time consuming. In an effort to simplify the process, Washington State Department of Transportation has completed a parametric study that results in a simplified method of using fixed shear design parameters. The simplified method saves significant time in shear design of prestressed girders, especially when it is used to standardize the shear reinforcement in a series of standard prestressed concrete girders.

SHEAR RESISTANCE AND DUCTILITY OF RC COLUMNS AFTER YIELD OF MAIN REINFORCEMENT

During Hanshin-Awaji Earthquake, catastrophic shear failure accompanying plenty of diagonal shear cracks were experienced in RC structures and bridge piers serving transportation facilities. In this paper, shear capacity and ductility of RC columns in the post-yield range of longitudinal reinforcement are discussed for seismic resistant design. First, the shear failure of large scale RC column is computationally simulated for understanding the mechanism of shear collapse of bridge piers. FEM computational results on ductility are compared with the experiments and parametric study is conducted concerning factors that affect the post-yield deformability of RC columns. Finally, a simple formula for ductility is presented for summarizing the sensitivity of influential factors.

Evaluation and Rehabilitation of Concrete Pier

This paper presents a case study of repair and strengthening work carried out on a defective reinforced concrete pier. The pier manifested severe cracking and spalling at the joint between its walls and platform. The distress was due to twisting of the platform as a result of torsional stress, which had also caused diagonal shear cracks to appear on the walls. A comprehensive investigation was carried out in order to determine the causes and extent of the problem and to define the remedial measures needed to rectify the problem and prevent future deterioration. The investigation included three phases: field investigation, which included soil borings, platform temperature measurement, and measurement of twist and tilt of the platform; laboratory investigation, which included soil testing and concrete core testing; and analysis of the structure. The repair work consisted mainly of providing a silica-fume-blended cement shotcrete jacketing system around the structure. The repair work was successful, economical, technically feasible, and not very labor-intensive.

NUMERICAL SIMULATION OF SIZE EFFECT IN SHEAR STRENGTH OF RC BEAMS

本研究は, 主鉄筋降伏以前に発生するRCはりのせん断破壊と強度に現れる寸法効果について有限要素解析による解析を行い, 使用した構成モデルの妥当性の検証と機構に関する検討を報告するものである. 鉄筋コンクリート領域を, 鉄筋との付着作用によってひび割れ以後も十分な引張応力の伝達が期待できる領域と, 急激な引っ張り軟化を呈する無筋領域とに分割し, 個々に引張およびせん断伝達に硬化及び軟化モデルを適用することを提案している. 釣り合い解を探索する繰り返し計算中に進展するひび割れ領域の観察から, せん断破壊モードを数値的に判定する方法を提示するとともに, はりのせん断破壊に現れる寸法効果を解析的に示した.

Collapse Load Test of 1:3 Shell Bridge Model

Previous analytical and experimental work by the authors has determined that integrated shell-deck segments can be used to construct short- or medium-span bridges. This system consisted of integrated shell-deck concrete segments that are post-tensioned together to form a simply supported bridge. A 1:3-scale segmental concrete shell bridge model was tested to failure. The bridge model was subjected to increasing load until failure occurred. Before failure, excessive deflection, cracking, and buckling of some of the steel elements connecting the deck to the shell edge beams were observed. Failure of the model resulted from the formation of excessive diagonal shear cracks in the shell portion of one of the segments. The fabrication and the behavior of the bridge model indicated that shell bridges can be used to construct short- and medium-span segmental bridges.

Planning and scheduling concrete repair work — A case study

In this case study the authors demonstrate the importance of recording and studying the details of concrete repair work to enable future projects to be carried out more efficiently with less slippage in the time schedule and therefore less financial damage over the time plants, such as kilns, are shutdown. In this case study, the distress in the concrete pier was apparently due to the twisting of the platform as a result of tortional stress, as well as to diagonal shear cracks in the walls.

鉄筋コンクリート部材のせん断力伝達機構に関する実験研究

The seismic properties of reinforced concrete members have been increasing elucidated while the shear transfer mechanism has not yet been grasped. The shear failure of reinforced concrete members is a complex phenomenon influenced by a diversity of factors, and this is one of the reasons that a number of experiments carried out to date were unable to fully grasp the phenomenon of the shear failure of the reinforced concrete members. As it is impossible to assess all phenomena of shear failure only by experiments, the results of an experiment must be recorded accurately and carefully to create an adequate model for analyses. This paper discusses in detail the results of experiments on reinforced concrete members subjected to shear force which monotonously increases under a constant axial load. The shear reinforcement ratio and the degree of compressive axial force are used as parameters. The paper concludes as described in the subsequent paragraphs. 1) With a diagonal shear crack occurring in a reinforced concrete member, the concrete element is almost unable to transfer the shear force, which shear reinforcement is able to. 2) The magnitude of shear force transferred by the shear reinforcement is affected by that of the axial force, and the ultimate shear strength is largely affected by the axial force. 3) Photographing facilitates the measurement of two-dimensional deformation as well as that of the deformation in the vicinity of a crack which cannot be predicted before experiment.

鉄筋コンクリート部材に関する従来型曲げせん断変形分解法の力学的意味と適用限界

Researchers engaged in structural testings often decompose the deflection of a member into flexural and shearing components. The physical meaning of this decomposition, however, has not fully been discussed yet. Moreover, there are some disagreements among researchers as to the definition of flexural and shearing deformation. This paper firstly presents a mathematial and rigorous expression for the conventional definition of flexural and shearing deformation. The shearing component of deflection is defined as the factored average of the shearing strain within the member. The flexural component of deflection is defined as the factored integration of the curvature calculated from the axial strain of the edges of the member. In case of a framed shear wall where the peripheral beams and columns carry only the axial stresses and the web concrete and shear reinforcement in the wall carry only the shear stress, the shear-flexure decomposition technique has physical meanings as follow. (a) The hysteresis loop of load vs. shearing deformation relation represents the energy absorbed within the web concrete and shear reinforcement. (b) The hysteresis loop of load vs. flexural deformation relation represents the energy absorbed by the peripheral frame. In case of a beam or a column where the longitudinal reinforcement carry only the axial stresses and the web concrete and shear reinforcement carry only the shear stress, the shear-flexure decomposition technique has physical meanings as follow. (a) The hysteresis loop of load vs. shearing deformation relation represents the energy absorbed within the web concrete and shear reinforcement. (b) The hysteresis loop of load vs. flexural deformation relation represents the energy absorbed by the longitudinal reinforcement. Stress flow in truss action resembles the stress states stated above. Therefore, the hysteresis loops of load vs. flexural and shearing components of a truss-action-dominated member tend to have such physical meanings. In case of a member where an arch action dominates, however, these loops do not have such physical meanings. In this case, even a sliding deformation along a diagonal shear crack may be partly considered as a flexural deformation.