Investigation On The Mechanical Properties Of Steel-high Performance Concrete Composite Girders In The Negative Moment Region

The steel-concrete composite girder not only has the characteristics of lightweight,strong spanning ability and excellent seismic performance,but also has good construction operability and simple and beautiful structural form.However,the steel-concrete composite girder in the negative moment region avoids the advantages of the two materials.It is in an unfavorable state: concrete in tension and steel girder in compression.Under the influence of vehicle load and complex environment,the cracking of Normal Concrete(NC)bridge deck is severe,leading to decreased structural stiffness,bearing capacity,and structural durability.Effectively improving the cracking of the bridge deck in the negative moment region has been one of the issues of concern in the research,design,and construction of steel-concrete composite girders.High-performance concrete(Engineered Cementitious Composite(ECC),Ultra-high Performance Concrete(UHPC),etc.)has excellent tensile strength,toughness,and durability.It can be used in the negative moment region of steel-concrete composite bridges to improve the cracking of bridge decks and the performance of steel-concrete composite bridges.It has good application prospects.This paper studies the bending behavior of steel-concrete composite girders under the negative moment by model test,simulation,and theoretical analysis.The main work and conclusions are as follows:(1)The static load tests of two steel-UHPC composite girders(reinforced and unreinforced),one steel-ECC composite girder,and one steel-NC composite girder were carried out under negative moment.The failure modes,stiffness,and bearing capacity changes of composite girders and the development and distribution of strain and crack were studied.The results show that the cracks develop fully when the reinforced composite girder is damaged and the steel girder is locally buckled under compression.The crack development of the unreinforced composite girder needs to be improved,the main crack is formed,and the local buckling of the steel girder is insignificant.Compared with the steel-NC composite girder,the stiffness and bearing capacity of the reinforced steel-ECC/UHPC composite girder are significantly improved.The cracks show small and dense distribution characteristics,effectively improving the bridge deck’s cracking problem.The cracks of the unreinforced composite girder are rare,and there are wide through the main cracks,indicating that the steel bar has a significant effect on the cracks of the concrete slab;the strain distribution of the composite girder section conforms to the plane section assumption.(2)A nonlinear simulation analysis model of a steel-concrete composite girder is established based on ABAQUS software.It is verified that the finite element results agree with the experimental results.Based on this,the finite element parametric analysis was carried out.The results show that the strain deformation of the concrete slab and the buckling of the steel girder are mainly distributed at the intermediate support,and the main strain of the reinforced ECC slab and UHPC slab is lower than the ultimate strain of the material.The stiffness and bearing capacity of the composite girder are sensitive to the width-thickness ratio of the steel girder flange and the height-thickness ratio of the web.Increasing steel strength can effectively improve composite girders’ bearing capacity.The influence of reinforcement ratio on the bearing capacity of composite girders is divided into two stages: the bearing capacity of the first stage increases rapidly,and the bearing capacity of the second stage increases slowly.The reinforcement ratio should be reasonably selected in design;other parameters in this paper have little effect on composite girders’ stiffness and bearing capacity.(3)Using the generalized strain energy density as the characteristic parameter of the structural stress state analysis method,combined with the energy increment curve,the characteristic failure load of the steel-concrete composite girder is identified,and the stress state of the steel-concrete composite girder is analyzed.The structural failure load is studied in the displacement and strain response of the steel-concrete composite girder.The structural failure load conforms to the variation law of the displacement and strain response of the structure,and the rationality of the structural failure load is verified.(4)The stiffness degradation law of steel-high performance concrete composite girders is explored based on the experimental results and finite element results.The fiber contained in the matrix can slow down the stiffness degradation of the structure.The stiffness calculation formula of steel-UHPC composite girders in the negative moment region is proposed by regression analysis.The calculation results are in good agreement with the measured values.It can be seen that the participation of the ECC slab and UHPC slab can be maintained until the structural failure.The mechanical mechanism of steel-high performance concrete composite girder is analyzed,and the ultimate bearing capacity calculation method of the steel-ECC/UHPC composite girder is derived considering the action of ECC and UHPC.It is verified that the calculation method has high accuracy.