Research On Shear Performance Of New Joints Of Precast Segmental Bridges

With the promotion and application of accelerated bridge construction,precast concrete segmental bridges(PCSBs)have been adopted both domestically and internationally in an extensive manner,owing to the standardization and rapid manufacturing thereof.Although PCSBs have obvious construction advantages and there are numerous engineering examples,the structural weakness thereof is the complete disconnection of longitudinal reinforcement at the joints.The mechanical properties of PCSBs have not been fully understood either domestically or internationally,and thus,are not reflected in the international mainstream specifications.The expression ‘beyond the specifications’ means that the technology is more dependent on the user’s understanding of its nature,and thus,the risks are prone to unpredictable problems and even errors,which will also affect the further promotion and development of PCSBs.The focus of the present study was on the shear mechanism of joints,including direct shear and bending shear.In terms of direct shear research,to simplify the joint form,improve the joint force transmission,and improve the joint local bearing capacity and ductility,a steel shear key was designed for precast concrete segmental beams.In terms of bending shear research,a new precast segmental beam with continuous longitudinal reinforcement at the joints was proposed based on the theory of grid shear reinforcement.Part one: Design theory of steel shear keyed joint.To fully explore the shear behavior of steel shear keyed joints under different load conditions,shear key types,joint types,and steel shear key geometric effects,10 specimen tests were conducted.Said tests were combined with theoretical analyses and finite element numerical simulations,so as to facilitate further investigation into the crack development,failure mode,shear slip,cracking load,ultimate bearing capacity and residual bearing capacity of steel shear keyed joints.The results reveal that the joints relied on the mechanical occlusion between the steel keys and concrete to transmit the shear forces.The joints had high shear bearing capacity and good ductility.When the load-displacement curve entered the horizontal stages,the joints could still bear large relative deformation while the bearing capacity did not decrease.The stiffness and bearing capacity of the steel shear keyed joints was higher than the concrete key joints,and the structural system was more stable than the concrete key joints at the moment of cracking.Both the concrete key and steel shear key epoxied joint suffered direct shear failure,but the steel key epoxied joints demonstrated a large residual capacity.After cracking,the bearing capacity of the square key was more stable than that of the circular key specimen.By adding 1 or 2steel shear keys to a flat joint,the ultimate bearing capacity could be increased by3.81 and 6.06 times for dry joints,and 43% and 41.58% for epoxied joints,respectively.The ultimate bearing capacity of the epoxied joints with 1 and 2 steel keys were respectively 151.39% and 69.68% higher than dry joints.Meanwhile,the epoxied joints were found to become brittle and fail in a relatively sudden manner.The failure modes of the dry joint were determined by the cracking strength of concrete and the shear strength of the steel key,while direct shear failure occurred in the epoxied joint.In the construction stage,the inelastic deformation of crossing-joint teeth should be used to control the temporary load design.In the service stage,the direct shear strength of the joints should be controlled by the shear strength of steel key’s material.The direct shear resistance of steel keyed joints could be calculated according to the shear formula.The material of the keys and the diameter of the crossing-joint teeth were identified as the key factors affecting the bearing capacity of the joints.Based on the aforementioned research results of the mechanical properties of the steel shear keyed joints,and combined with the CEB-FIP MC90 local compression model,a mechanical model suitable for steel shear keyed joints was established.The force transmission mechanism and theoretical failure modes of steel shear keyed joints were revealed,including shear failure of steel key,partial compression failure of concrete(contact surface crushing,contact surface expansion,splitting failure)and concrete tearing failure.The results of the present study suggest that the shear failure of steel shear keyed joint should be used as the basis of joint resistance design,but the strength of concrete should be checked.Predicated on the design calculation method and the support project,a design principle for steel shear keyed joints was established,and a whole set of design theories of steel shear keyed joints was formed.In terms of construction methods,in accordance with the traditional construction method of segmental beams,the construction of steel shear keyed joints using the short-line method,the long-line method and the modular method was introduced.As verified in the study,the steel shear keyed joint was suitable for industrial production according to the construction experiment of two kinds of segmental T-beams.Finally,three experimental beams were designed to explore the bending-shear behavior of steel shear keyed joints.The experimental results show that although the local shear mechanical properties of steel key joints were significantly improved compared with those of concrete key joints,the bending-shear properties of steel key joints were basically the same as those of concrete key joints.Part two: Research on shear mechanism of precast segmental concrete girders with longitudinal steel reinforcement across joints.Three test specimens were designed,and the beam body deformation,reinforcement strain,crack development,cracking load and failure load were taken as research objects.Moreover,the shear mechanism and mechanical properties of precast segmental concrete girders with longitudinal steel reinforcement across joints were experimentally investigated.The experimental results show that the crack location,length,width and cracking load of the monolithic beam,the traditional segmental beam and the longitudinal reinforcement continuous segmental beam were basically the same at the initial stage of crack development.In the failure stage,the main cracks of traditional segmental beams were concentrated at the joint position,and there were obvious signs before the failure of the beam body.The beam body on both sides of the joint rotated plastically around the joint,and an inverted V-shaped opening was formed on the joint.Due to continuous longitudinal horizontal web reinforcement in the joint position,the longitudinal reinforcement continuous segmental beam could better inhibited crack open at joint position,and limited the main fracture width,thus effectively improved the stiffness of segmental beam.The load-displacement curve was similar to the development of the monolithic beam,and the specimen stiffness and bearing capacity significantly greater than the traditional segmental beam.The reinforcement strain development of the longitudinal reinforcement continuous segmental beam was similar to the monolithic beam: When the specimens fail,the stirrups near the joints of the longitudinal reinforcement continuous segmental beams can effectively bear the vertical component of the main tensile stress,and give full play to the role of shear reinforcement.And most of the web longitudinal reinforcement reached or were very close to yield strains.The web longitudinal reinforcement bore the axial tension caused by shear force and played the same shear resistance role as the stirrup.However,the joint position of traditional segmental beam lacked longitudinal reinforcement.With the opening of the joint,the structural stress on both sides of the joint was fully released,and the longitudinal reinforcements and stirrups near the joint did not reach the yield strain and failed to exert their shear resistance.The shear stress could only be transmitted from the shear compression zone concrete and prestressed reinforcement,and the shear effect of the stirrup and longitudinal reinforcement near the joint was weakened.Further,the stress state of the longitudinal reinforcement continuous segmental beams met the plane cross-section assumption.The mechanical behaviors of the shear capacity,stiffness,crack development mode and failure mode of the specimens were basically consistent with the monolithic beam.The experimental results fully verify the necessity of continuous longitudinal reinforcement at the joints and provide an experimental basis for the design theory of continuous longitudinal reinforcement segmental beams.Finally,a discussion on the direction of future work is provided.The research results of the present study are original and universal,and will contribute to the development of segmental bridge construction in the field of civil engineering.