Research On Bending Performance Of Ultra-thin Steel-UHPC Composite Bridge Deck In Negative Moment Area

The wide application of orthotropic steel box girder bridges benefits from the excellent mechanical properties of orthotropic steel box girders,but the bridge deck has always suffered from fatigue cracking problems.in order to meet the needs of the renovation and reinforcement of the bridge deck,a steel ultra-thin UHPC composite bridge deck,referred to as the ultra-thin system,is proposed by Shao Xudong team of Hunan University.The ultra-thin system,with 15 mm protective layer thickness,is formed by connecting steel plate and thin-layer UHPC through ultra-short shear studs,and its engineering feasibility has been proved by relevant studies.Compared with the traditional steel-concrete composite system,the overall mechanical performance of the ultra-thin system is better,which can significantly reduce the tensile stress amplitude and effectively solve the fatigue cracking problem of concrete in the negative bending moment area.It is the preferred solution for bridge renovation and reinforcement.Obviously,when the ultra-thin system is used for bridge renovation,the bending test in the negative bending moment area of the local model should be carried out in advance.In order to clarify the optimal form of the ultra-thin system,the transverse strips in the negative bending moment region were selected for local model bending tests.The influence of key design parameters such as steel bar diameter,reinforcement ratio,thickness of longitudinal reinforcement protective layer,and number of reinforcements on the flexural performance of ultra-thin systems was studied.The nominal cracking load of composite bridge decks was calculated by using the section stress method based on the ideal elastic-plastic model.The real cracking process of the composite bridge deck is simulated by ANSYS finite element.The research results show that:(1)The load-deflection curve of the composite deck can be divided into three stages:elasticity,crack propagation and yielding.The larger the steel bar diameter,the smaller the reinforcement spacing,the higher the reinforcement ratio,and the thicker the protective layer,the higher the bearing capacity of the composite deck.Among them,the steel bar diameter and reinforcement ratio are the most important factors affecting ultimate elastic load,nominal cracking load and ultimate failure load of composite deck.When the diameter of steel bar increases from 6mm to 8mm,the increase of ultimate failure load can reach 60.47%.When the number of steel bar increases from 6 to 8,the load increase can reach 23.32%.When the reinforcement ratio increases from 2.98% to 3.97%,the load increase can reach 31.29%.When the thickness of the protective layer increases from 15 mm to 25 mm,the load increase is only 11.34%.(2)Increasing the diameter of the steel bar can not only significantly increase elastic limit load,nominal cracking load,and ultimate failure load,but also significantly reduce the corresponding deflection.The denser the rebar arrangement,the more numerous and dens the cracks are at the time of failure.Although increasing the thickness of the protective layer can increase the loads,it will also increase the corresponding deflection,and will significantly increase the final crack width of the composite deck.Comprehensive analysis shows that,for the ultra-thin system,the optimal choice for the diameter of the steel bar is 8mm,the optimal number of steel bars is 6,the optimal thickness of the protective layer is 15 mm,and the optimal solution for the reinforcement ratio is 3.97%.(3)The load-strain curve of the mid-span section reflects the change law of load and strain during the bending process of the composite deck,which points out:(1)The relative slip between steel deck and UHPC layer is very small,which indicates that the ultra-thin system has very good integrity.(2)The elastic limit load given by the load-strain curve in the elastic stage is very close to the load value given by the load-deflection curve,and the maximum error is only 6.67%,which indicates that the elastic limit load given by the load-deflection curve has high accuracy.(4)Compared the measured value with the theoretically calculated value of the nominal cracking load,it can be seen that the guaranteed rate of the obtained results is between0.92～0.96,which shows that: for the ultra-thin system,it is reasonable to take the hardening height as the thickness of the protective layer when estimating the nominal cracking load using the section stress method based on ideal elastic-plastic model.(5)Numerical analysis shows that:(i)The main influencing parameters of the real cracking load are the thickness of longitudinal reinforcement protective layer and the total thickness of UHPC.(ii)The measured value of the real cracking load is greater than the finite element iteration value,indicating that the UHPC has entered the plastic stage before cracking.(iii)the nominal cracking load is 2.08～2.71 times the numerical cracking load,which indicates that the composite bridge deck has a strong ability to work with cracks.(iv)dense reinforcement is beneficial to inhibit the plastic deformation of UHPC.(v)combined effect can improve the tensile strength of UHPC.when the uniaxial tensile strength is used as the design standard value,the stress is conservative,and the structure is biased towards safety.