Fatigue Performance Study Of Transverse Rib WEB In The Composite Deck System Composed Of Orthotropic Steel Deck With Large Longitudinal Ribs And UHPC Layer

As for the problem of weld failure in orthotropic Steel Bridge Deck,a new structure of the orthotropic steel deck(OSD)with large longitudinal ribs and ultra high performance concrete(UHPC)layer was proposed.The length of welding line of the stucture can be decreased and the performance of fatigue will be improved effectively.The rib-to-crossbeam welded joint is a typical fatigue detail in orthotropic Steel Bridge Deck.Because service bridges are subjected to frequent vehicle wheel loads,and due to the manual welding at this place,welding initial defects can easily occur.What's more,the structure of this site is complex and the stress concentration is serious.Therefore,the details of the structure at the weld toe are prone to fatigue cracks.This problem still exists in the OSD with large longitudinal ribs and UHPC layer.In order to improve the fatigue resistance of this detail and to study the details of the fatigue crack growth law and predict the residual fatigue life after cracking,it is of great significance to the use and promotion of this structure.Therefore,the numerical simulation study of this paper is carried out.The main research work was carried out and the main conclusions obtained are listed as follows:(1)The difference of fatigue performance between traditional OSD and the OSD with large longitudinal ribs and UHPC layer were researched.The results shows that the new type of composite bridge deck can improve the fatigue performances effectively in the position of rib-to-crossbeam welded joint.According to criterion and the researches on traditional OSD,four typical form of cutouts in the OSD with large longitudinal ribs and UHPC layer were designed and analyzed using ANSYS.Based on the hot spot stress S-N curves and the linear cumulative damage theory,the hot spot stress influence lines can be got through loading the standard fatigue car.The hot spot stress influence lines are superimposed by the 1.2m wheelbase,and the hot-spot stress processes were carried out.The hot stress amplitude was obtained by using the rain-flow counting method.And then,the structural detail fatigue life was predicted.The thesis aims to obtain the influence of form of cutouts on the OSD with large longitudinal ribs and UHPC layer.(2)To improve the fatigue performances,a form of cutout on diaphragm plate for the OSD with large longitudinal ribs and UHPC layer was proposed.The finite element analysis shows that the structure can achieve infinite fatigue life in the absence of residual stress.(3)Based on the linear elastic fracture mechanics and the Paris formula,a new method for simulating semicircular fatigue crack propagation in the position of diaphragm plate was proposed.The sub-model technology can be applied in numerical simulation research which is similar to programming language.The 1/4-node singular element in finite element models and interaction integration method were used to calculate the stress intensity factor of the crack front.And then,the crack propagation law and fatigue life was calculated by the formula.(4)The study of crack propagation law shows that the stress intensity factor amplitude begins to decrease when the crack grows to about 75% of the thickness in the thickness direction of the longitudinal rib.And the crack growth rate also shows a decreasing trend,while the surface crack increases with the crack extension length and the stress factor amplitude increases to the peak value.In a steady state,the crack growth rate tends to be stable.The stress intensity factor is slightly lower than the peak when the crack is in a steady state.One of the reasons is that the stress at the joint between the longitudinal ribs and the transverse ribs is complex and the stress concentration phenomenon is obvious.When the crack propagation reaches a certain length and exceeds the more complex stress region,the crack stress intensity factor tends to be stable.The other reason is that the restraining effect of the longitudinal ribs and the transverse ribs at the arc cuts gradually weakens by the increase of the crack propagation length,and stress redistribution occurs in the area where the cracks in the slab extend.(5)Compared with the traditional OSD,the stress intensity factor and the expansion rate of crack in the OSD with large longitudinal ribs and UHPC layer decreased obviously.The research indicates that the OSD with large longitudinal ribs and UHPC layer will improve the fatigue performance significantly.