Research On Shear Theory And Application Of Reinforced Concrete Beam Based On SMCFT

The shear performance of reinforced concrete beams is a very important content in the theoretical system and structural design of reinforced concrete structures.However,due to many factors affecting the shear capacity,it is still a very challenging problem to reasonably explain the shear mechanism and give the accurate shear capacity prediction.For more than 100 years,many scholars from different countries have put forward a series of shear analysis theories,such as classical truss model,variable angle truss model,strut-and-tie model（STM）,compression field theory（CFT）,modified compression field theory（MCFT）,simplified modified compression field theory（SMCFT）,etc.Among them,the MCFT theory is a more accurate and effective shear analysis model that has been tested and considered by many countries and scholars.The proposed SMCFT theory is a reasonable simplification of the MCFT theory,which reduces the original complex calculation steps to 6 steps,providing great convenience for engineering practice.However,scholars at home and abroad lack of research results on the accuracy and applicability of the SMCFT theory,especially in the case of small shear span-to-effective depth ratio.Based on the SMCFT theory,this paper explores the accuracy of the theory and the scope of application of shear span ratio,and presents a modified model suitable for a wide range of shear span-to-effective depth ratio combined with combination theory.At the same time,the SMCFT theory is applied to FRP shear strengthened beams.The main research contents are as follows:（1）The SMCFT theory and the STM model are used to deal with the experimental data of 160 groups of normal RC beams in databaseⅠ.Compared with the STM model,it is found that the SMCFT theory is more suitable for the case of larger shear span-to-effective depth ratio（a/d>1.8）.（2）In order to modify the problem that the SMCFT theory is not applicable to the small shear span-to-effective depth ratio,RC beams are divided into the shear compression zone,the diagonal tensile zone and the tension zone.Among them,the shear capacity contribution of the concrete in the shear compression zone is calculated according to the failure criterion of main compressive stress,and then the shear capacity contribution of the reinforced concrete in the area where main shear crack penetrates（the diagonal tensile zone and the tension zone）is calculated by using the SMCFT theory,so as to establish advanced SMCFT model（A-SMCFT model）.According to the calculation results of the test beams in databaseⅠusing the A-SMCFT theory,it is shown that the A-SMCFT theory can more effectively reflect the shear mechanism of ordinary RC beams with web reinforcement,and the modified theory is more suitable for its shear capacity analysis.（3）According to the two main failure modes of FRP strengthened beams（FRP rupture and FRP debonding）,this paper applies the SMCFT theory for the externally bonded FRP strengthened beams.Then,according to the layered theory of the beam and the A-SMCFT theory,the FRP strengthened beams are also divided into the shear compression zone,the diagonal tensile zone and the tensile zone.The A-SMCFT（Rupture）theory,which is suitable for the main failure mode of FRP rupture,and the A-SMCFT（Debonding）theory,which is suitable for the main failure mode of FRP debonding,are established respectively.According to the results of calculating the test beams in databaseⅡand databaseⅢ,the data points of the ratio of experimental shear capacity(V_exp)to the shear prediction(V_pre)in this paper fluctuate on the straight line of V_exp/V_pre=1 in a small range,and the average ratios are 1.142 and 1.140 respectively.At the same time,for single-layer or multi-layer FRP,vertical paste or inclined paste,using A-SMCFT（Rupture）theory and A-SMCFT（Debonding）theory can give more accurate prediction results of shear capacity.