Fatigue Failure Mechanism Of Steel-High Performance Concrete Composite Bridge Deck With Large-Size U-Ribs

Fatigue cracking at welded joints and pavement damage have been posing formidable challenges to the development and application of orthotropic steel decks(OSDs)for a long time.One of the most effective solutions to address the presented issues at structure system level is to enhance the local stiffness of the traditional OSDs by using an innovative orthotropic steel-high performance concrete composite deck.The composite deck system consists of OSDs with large-size U-ribs and a high performance concrete layer.The composite action between these two components is achieved by shear connector group.Two types of high performance concrete are used as layer materials: ultra-high performance concrete(UHPC)and engineered cementitious composites(ECC).The foundation and key to investigate the fatigue failure mechanism of the innovative composite deck are: identify the fatigue failure modes and fatigue-prone details of the composite deck system;clarify the deterioration process of the mechanical characteristics of the composite deck system;reveal the relationship between the mechanical degradation and the fatigue performance of the composite deck system;explain the fatigue damage evolution process of the composite deck system.The fatigue failure mechanism of the composite deck system is investigated by model tests and theoretical analysis.The following works are performed and completed:(1)Shear behavior of short studs embedded in UHPC and ECC layerEight static push-out tests and nonlinear finite element numerical simulations are conducted to investigate the shear behavior of short studs embedded in UHPC and ECC layer.Based on the theory of beam on elastic foundation and considering the reaction coefficient of the high performance concrete,an equation with clear physical meaning is proposed to predict the load-slip behavior of short studs embedded in ECC and UHPC layer.A nonlinear finite element parametric study is carried out to investigate the effects of height-to-diameter ratio of studs,compressive strength of ECC and weld collar geometry on the shear strength of short studs.Based on the results of the finite element parametric analysis,an equation for calculating the shear strength of short studs embedded in ECC is derived.This equation can describe the influence of height-to-diameter ratio on the shear strength of short studs.(2)Fatigue behavior of short studs embedded in ECC layerSix fatigue push-out tests are conducted based on the static ones.Based on the fatigue tests results,an S-N curve related to a survival probability of 95% is derived to facilitate design and evaluation of the short studs embedded in ECC.Based on the evolution of the plastic slip and elastic stiffness with the number of cycles,empirical formulae(fatigue degradation model)are proposed for predicting the load-slip behavior of the short studs under fatigue loading.The fatigue crack growth characteristic of short studs embedded in ECC is studied by performing crack propagation analysis based on linear elastic fracture mechanics.(3)Fatigue degradation mechanism of the innovative orthotropic steel-high performance concrete composite deck with large-size U-ribFour large-scale composite deck specimens,divided into two groups,are tested to investigate the transverse and longitudinal fatigue failure mechanism of the composite deck system.According to the fatigue test results,the fatigue failure modes,key mechanical components and fatigue-prone details are identified;the influence of mechanical degradation of key components on the local mechanical behavior of the composite deck system is investigated.Experimental and numerical methods are combined to quantify the influence of mechanical degradation of key components on the damage evolution process of the fatigue-prone details.Based on the above investigations,the fatigue failure mechanism of the composite deck system is revealed.(4)Time-dependent fatigue analysis method for the composite deck based on composite action degradationCombining the fatigue degradation model of short studs with nonlinear finite element analysis methods,a time-dependent fatigue analysis method that can consider the degradation of composite action is established.The feasibility and accuracy of established time-dependent fatigue analysis method are verified by a comparison of the analytical results with experimental results.Based on the established time-dependent fatigue analysis method,the fatigue performance of the composite deck is studied,and the influence of degradation in composite action on the fatigue behavior of the composite deck is quantified.Finally,a simplified fatigue evaluation method is proposed by introducing a damage amplification factor.