| The composite box girder with corrugated steel webs is a new type of steelconcrete composite structure.This structure makes full use of the advantages of good compressive performance of concrete,high shear strength and lightweight of corrugated steel webs.Moreover,the simple and short period of construction process can reduce the cost and improve economic efficiency.Therefore,it has been rapidly developed and applied in bridge engineering abroad.Over the past decade,corrugated steel web composite girder bridges have been widely used in urban and rural bridge construction projects in China,and more than 60 such bridges have been built so far.More than 70% of the failure of engineering structures in practice is caused by fatigue failure,which is often sudden and catastrophic.The bridge structure will be subjected to dynamic loads such as vehicle operating for a long time during its service.The deterioration of its working state and the reduction of its safety are mainly caused by the local fatigue failure of key components.Fatigue design and fatigue life prediction have become the vital technologies of composite structures in bridge design and safety assessment.Therefore,to ensure the safety,reliability and fatigue durability of corrugated steel webs composite box girder bridge,it is of great academic significance and engineering application value to study the mechanical properties and anti-fatigue characteristics of corrugated steel webs and concrete composite box girder.In this paper,based on the cause and development of failure of composite box girder with corrugated steel webs(hereinafter referred to as composite box girder)under fatigue load,combined with experimental analysis,theoretical calculation and finite element simulation,the mechanical behavior and fatigue characteristics of key materials,components and structures of composite box girder were systematically studies.The main contents and conclusions are summarized as follows:(1)The basic mechanical behavior of Q345 b structural steel was studied,and the effect of pre-torsional strain on low cycle fatigue behavior of the steel was analyzed.The reasonable parameters describing the Coffin-Manson relationship of low cycle fatigue of Q345 b structural steel were determined,the strain amplitude-fatigue life relationship was established,and the energy absorption rate of Q345 b was obtained.The pre-torsional strain had clear adverse effects on the low cycle fatigue and seismic properties of Q345 b structural steel,the material shows cyclic softening phenomenon,and the plasticity decreases.The lower of the fatigue performance,the lower of the energy absorption of the material after pre-torsion angle 90 and 180 with reductions to the values of 69.8% and 91.2% compared with the original base material,respectively.(2)The static load test and fatigue test of corrugated steel web beams were carried out,and the effects of over-welded holes,welding mode and inclined web angle were analyzed.The stress model of corrugated steel webs showed that the upper and lower flanges were mainly subjected to bending moment,while the web was mainly subjected to shear force.Corrugated steel webs had good durability,and fatigue cracks originated at weld toes and develop along the longitudinal direction of beams,resulting the fatigue failure.Reserving welding holes or increasing the angle between straight and inclined webs would lead to the reduction of stiffness of beams.The stiffness of beams could be increased by overlapping and butt welding.(3)Taking PBL(Perfobond-Leiste Strip)shear connector as the research object,the failure mode and ultimate bearing capacity of PBL shear connector were studied by static push-out test method,and the effects of hole diameter and thickness of steel plate on ultimate bearing capacity were explored.The main failure mode of PBL shear connector was the shear failure of concrete pins in PBL shear connector under shear stress.Increasing the aperture size and steel plate thickness of PBL shear connector could effectively improve the shear strength and ultimate bearing capacity of PBL shear connector.(4)Aiming at corrugated steel webs-fiber reinforced concrete composite box girder,the failure law of corrugated steel webs-fiber reinforced concrete composite box girder under static and cyclic loads were studied experimentally,and the influence of shear span ratio was analyzed.The fatigue crack behaviors of composite box girder could be divided into three stages: surface crack initiation,steady growth of main crack width and sudden change of crack.Firstly,crack initiated on the side or bottom of the bottom plate corresponding to the loading point(about 5% of the total fatigue life).Later,new and old cracks coalesced with the main crack,and the main crack width increased steadily as the increasing of cyclic loading.When the width of the main crack reached 1 mm,cracks occured at the weld toe of the steel webs,and the composite box girder was at the critical point of fatigue failure.With the increase of shear span ratio,the resistance of composite beams to deformation decreases,and the earlier the fatigue cracks occurred.The more of the number and the more complex of the distribution of the fatigue cracks,the lower fatigue life of CFRP strengthened floor.It could effectively improve the flexural rigidity and fatigue resistance of tested beams.The finite element analysis results showed that,the fatigue life of composite box girder was decreased exponentially with the increase of shear span ratio,increased exponentially with the increase of flange thickness or width,increased linearly with the increase of web thickness,and decreased first and then increased linearly with the increase of Web wave height.With the increase of web angle,the fatigue life was decreased linearly,but the stress concentration was also increased,so it was suggested to take 25 ° to 45 °.(5)CFRP(Carbon Fibre Reinforced Plastics)was used to reinforce and repair damaged fiber reinforced concrete-corrugated steel webs composite box girders,to study the fatigue resistance and residual bearing capacity of the damaged composite box girders in service.The fatigue cracks of CFRP strengthened composite box girders generally originated at the weld toes of steel flange and web in the mid-span pure bending section of corrugated steel web bottom plate.CFRP could restrain the longitudinal slip between concrete bottom plate and web plate,and the original crack propagation was slow,hence it could greatly improve the fatigue life of composite beams and achieve the purpose of repairing and continuing use.The smaller of the shear-span ratio,the greater improvement of fatigue life by CFRP reinforcement.CFRP reinforcement was also conducive to improving the residual bearing capacity of composite box girder after fatigue and increasing structural ductility.Before the overall instability failure of composite box girder after CFRP reinforcement occurs,there were obvious signs: Y-type cracks appeared in the bottom plate of the end support of the test beam and interconnection could be used for damage warning. |
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