Theoretical And Experimental Research On Shear Capacity Of High Strength Concrete Beams With High Strength Stirrups

As one of the greatest resource-consuming industries and to realize sustained development, construction industry must adjust the structure of material consumption and popularize high-strength steel and high-strength concrete to increase material using ratio and to realize economized development. At present,high-strength concrete such as C60 and C80 has began to be used in high-rise buildings and large-span structures, and HRB400 steel has been recommended according to Design Code of Concrete Structure(GB50010-2002). HRB500 steel has been in the stage of studying before being used in general.Research on shear strength of reinforced concrete beams is one classical issue in basic theory of concrete structure, and is focused by many scholars due to its complexity of failure mechanism and various influential variables. In the past century, a variety of theoretical formulas have been established by scholars from different points of view. However, due to the complexity of these theoretical formulas, it is difficult to be unified and put into practice. Therefore, formula with certain reliability serving as design principle have been proposed in terms of statistical regression analysis based on experimental data by most countries.With the popularization and application of high strength steel and high strength concrete, it is necessary for the formula in Code to have some adjustment correspondingly and the shear failure mechanism of reinforced concrete beams also should have further improvement. In this background, the following works are completed in this paper: 1. Based on the theory of energy-absorbing, a stiff testing facility is contrived aimed at obtaining stable and controllable shear failure of reinforced concrete beams. A total of 20 reinforced concrete beams with shear span-to-depth ratio equal to 3 are tested under this facility, and complete load-deflection curves including the post-peak portion are obtained. Variables such as concrete strength grade (from C30 to C80), stirrup strength grade (HPB235,HRB400,HRB500), inclined stirrup angle (60°,90°), stirrup ratio (0,0.20%,0.25%), longitudinal reinforcement ratio (0.65%,1.31%,2.03%) and the amount of longitudinally distributed reinforcement are considered in this experiment. Failure modes of tested beams mainly include diagonal-tension failure, shear-compression failure and flexure-shear failure. Herein, for beams with shear-compression failure, the stirrups yielded and the longitudinal reinforcement did not yield. For beams with flexure-shear failure both the stirrups and the longitudinal reinforcement yielded.2. Experimental results reveal that shear strength of beams increase with the increase of concrete strength and stirrup strength. With the same stirrup ratio, shear strength of high-strength concrete beams with high-strength stirrups are higher markedly than that of normal strength concrete beams with normal strength stirrups. Inclined stirrups can effectively restrain the propagation of diagonal cracks, and shear strength of beams increased to about 13%. With an increase of longitudinal reinforcement ratio, the shear strength of beams increased correspondingly. When the longitudinal reinforcement ratio increased from 1.31% to 2.03%, shear strength of beams increased about 30%. A 10% increase of shear strength can be made for beams with longitudinally-distributed web reinforcement. Design of beams with high-strength concrete and high-strength stirrups according to China Code (GB50010-2002) is safe, however, it is unsafe to follow the code for designs of high-strength concrete beams without stirrups, beams with little amount of longitudinal reinforcement and normal-strength concrete beams with high-strength stirrups.3. Experimental results show that as for beams with shear span-to-depth equal to 3 and appropriate web reinforcement, the failure mode is characteristic of certain ductility. Although beams with shear-compression failure have a better ductile performance than beams with diagonal-tension failure, both of them displayed brittle failure mode. The deflection of beams increased with yielding of longitudinal reinforcement, while shear force decrease the ductility of beams. With increase of concrete strength and stirrup strength, shear ductility coefficient increase. Longitudinally distributed reinforcement to some extent improve shear ductility, and the bigger the longitudinal reinforcement ratio, the smaller the shear ductility. Diagonal stirrups increase shear ductility performance of beams substantially and prevent the propagation and widening of diagonal cracks. As for beams with high-strength concrete and high-strength stirrups, the shear ductility coefficient is twice higher than that with normal strength concrete and stirrups. It is shown that combination of high-strength concrete beams with high-strength stirrups take full advantage of these two material properties and high-strength stirrups are suitable for high-strength concrete beams.4. Based on the section mechanical analysis of tested beams, a model is developed to predict the shear strength of reinforced concrete beams with and without web reinforcement. It is assumed that the shear strength of concrete beams is primarily provided by concrete in the compressive zone on the top of critical section of beam. The shear strength of concrete in the compressive zone is determined by the tensile stress and shear stress generated from section flexural moment. The failure mode of slender beam in the compressive zone is diagonal tension failure, and the failure mechanism of compressive concrete and the failure in shear compressive multiplex condition were in accordance with Ottosen's material failure criteria. This proposed model can accurately predict the shear strength not only for beams in this testing but also for 391 beams with shear span-to-depth ratio bigger than 2.5. In order to be generally applied in design practice, a simplified theoretical formula is developed based on analysis of a majority of experimental results. A comparison between this simplified design formula and China Design Code method shows that this formula has a better prediction for the shear strength of concrete beams than method in China Code and is little scatter.5. In order to consider other effect of variables on shear strength of beams without this paper, experimental data of 682 beams with shear failure was collected. The testing results are compared with failure shear strength calculated by GB50010-2002, ACI318-05, EC2-02, and Zsutty equation, not only the advantages and disadvantages of these methods but also the feasibility of these considerations on influential variables of shear strength are analyzed. Based on non-linear statistical regression analysis of these 682 beams, a formula used to calculate the shear strength of beam is proposed. This formula takes into account concrete strength, span-to-depth ratio, longitudinally reinforcement ratio and size effect. Besides, distributed longitudinally reinforcement, arch action of deep beams and beam action of slender beams are also considered. In comparison with other methods, statistical regression formula has the best prediction for shear strength of concrete beams and the least Demerit Point.