Research On Mechanical Performance Of Steel-UHPC Composite Beams In Negative Bending Moment Area

The concrete is tensioned in the negative bending moment area of the steel-concrete composite beam bridges,and the bridge deck is prone to cracking.Using ultra-high performance concrete(UHPC)is a solution to improve the performance.Through experimental research,numerical simulation and theoretical analysis,the shear strength and slip characteristics of the steel-UHPC push-out specimen were firstly studied.Secondly,based on the research of the push-out test,the mechanical performance of the steel-UHPC composite beam under the action of negative bending moment was studied.The main research contents and conclusions are as follows:(1)The steel-UHPC push-out test was carried out,and four specimens were designed and manufactured to study the shear bearing capacity and load-slip relationship of the stud connector between the steel and UHPC.The test results show that the test specimens were damaged by studs shearing,and UHPC crushed on the compressed side of the bolt root,the degree and scope of damage were small.(2)The shear bearing capacity and the load-slip relationship of the push-out specimen were calculated,and the proposed modification formula of the theoretical formula suitable for UHPC were given.(3)Two composite beams with UHPC-NC laminated slab,which separately connected by uniform stud arrangement and cluster stud arrangement,were designed and manufactured,and the negative loading test was carried out,putting the laminated plate in the tensile zone.The bearing capacity,load-slip relationship,displacement and section strain of composite beams were tested.The development of cracks were observed.The test results show that the steel beam yielded and the concrete slab breaked during failure,and the failure modes were bending failure;During the test,cracks first appeared on normal concrete,and then developed to UHPC slabs.The load capacity of the two composite beams was not much different,and the relative slip was small.(4)A refined finite element model of the composite beam was established,considering the slippage of the stud connector,and based on the test results,the spring unit was used to simulate the sliding of the steel-concrete interface,and the whole process of the composite beam failure was simulated.The analysis results show that the greater the shear strength of the stud connector,the smaller the amount of slip.Before the load reached 0.9 times of the limit load,the relative slip of each section of the composite beam was basically the same.The minimum value of slip occurs at the mid-span and the maximum value was between the L / 4 section and the mid-support section.(5)Based on the analysis results of the test and finite element calculation,the cracking moment of the composite beam was calculated.The cracking moment formula of the UHPC composite beam was modified by the steel fiber crack resistance coefficient.The formula of ultimate flexural bearing capacity of composite beams considering the UHPC tensile effect was proposed,which supplemented the standard formula,and the calculation results were in good agreement.(6)A trial design was carried out for the actual steel plate concrete composite beams,and a refined finite element model was established to analyze the stress state of UHPC concrete slabs under the lane loading.The results show that when UHPC or UHPC-NC composite slabs were used in the negative bending moment area under the lane loading,the stress was greater than that of normal concrete slabs.UHPC's stress reserve was higher than normal concrete.UHPC limited the development of displacement and slip.(7)When UHPC was used as the negative bending moment area concrete slab,the negative bending moment area length was about 0.26 L,and the normal concrete slab was about 0.3L.The longer the UHPC length,the smaller the tensile stress area near the midsupport section.