Research On Seismic Behavior And Shear Capacity Theory Of High-Ductility Fiber-Reinforced Cementitious Composite Coupling Beams With Small Span-to-Depth Ratio And Diagonal Reinforcement

Coupling beams connect wall piers in coupled shear walls and are located in the plane of shear walls.Coupling beams are the first seismic fortification line of shear wall system,similar to “a fuse”.Due to the functional requirements of buildings,coupling beams are characterized by small span-to-depth ratio and large shear-to-bending ratio and are easy to experience britttle shear failure under horizontal loads.In the paper,ultra high toughness cementitious composites(UHTCC)which present good strain hardening characteristic and diagonal reinforcement are applied in the coupling beams to save reinforcement,simplify construction and attain good shear capacity,stiffness,ductility and energy dissipation.The specific research contents and conclusions are as follows.1)Four coupling beams with diagonal reinforcements and span-to-depth ratio of 1.0 and 1.5 has been tested and seismic performance of RUHTCC coupling beams and RC beams has been compared.Results show that X-type shear failure occurs in all coupling beams.The number of cracks on the surface of RC coupling beams is limited,while the cracks in the bending-shear zone and diagonal tension-compression zone of RUHTCC coupling beams are well developed,and the micro-cracks are distributed throughout the specimen,which show obvious multi-cracking characteristics.There are no large area of UHTCC peeling and exposure of stirrups and diagonal bars in the RUHTCC coupling beams.Their damage is significantly reduced.Compared with RC coupling beams,RUHTCC coupling beams show higher seismic performance even at low diagonal reinforcement ratio.However,for RC or RUHTCC coupling beams,when the span-to-depth ratio is not more than 1.5,the internal force is mainly transmitted through the diagonal strut-and-tie mechanism and truss mechanism during the whole loading process.The proportion of the two mechanisms is different with the loading time.At the time of peak load,diagonal strut-and-tie mechanism is the dominant mode of force transmission.2)Six coupling beams with different span-to-depth ratio,stirrup ratio and diagonal reinforcement ratio has been tested uhder cyclic loading,and the influence of various factors on the seismic performance of the copling beams has been analyzed.Results show that the stirrup ratio has little effect on shear capacity of the coupling beams with span-to-depth ratio less than 1.5,but the increase of stirrup ratio can significantly improve ductility and energy dissipation.With the increase of the amount of diagonal reinforcement,the shear capacity will increase and the energy dissipation will improve,but the ductility will reduce.The span-todepth ratio has great impact on the seismic performance of RUHTCC coupling beams.The smaller the span-to-depth ratio,the larger the shear capacity,the weaker the energy dissipation,but the ductility increase because of no shear-tension failure.3)In the paper,for coupling beams with small span-to-depth ratio and diagonal reinforcements,test results of 85 coupling beams has been collected and analyzed to discuss the rationality of the formulas for calculating the shear capacity of the Chinese and American codes and the existing formulas for calculating the shear capacity.Results show that the ACI code only consider the contribution of diagonal reinforcemnts,neglects the contribution of concrete or UHTCC,and the calculation results are too conservative.Code for design of concrete structures(GB50010-2010)is good enough to predict RC coupling beams,but conservative to RUHTCC coupling beams.The empirical formula of Che Jialing is also conservative for RUHTCC coupling beams.When the formula of Park completely ignores the role of stirrups,the calculation results are conservative,but the shear capacity is overestimated when considering effect of all the stirrups in the midspan of coupling beams.4)Based on the Mohr-Columb theory and the softened diagonal strut-and-tie model,the formulas for calculating the shear capacity of coupling beams with small span-to-depth ratio and diagonal bars are derived and the rationality of the formulas is verified by comparing with the collected experimental results.The shear capacity formula based on the two theories can take into the tensile contribution of UHTCC on diagonal crack surface and the compression softening account and the mechanical concept is clear.