| Streamlined steel box girder has several advantages,such as light weight,high bearing capacity,and strong integrity.Therefore,it has been widely used as the girder in long-span bridges.The girders usually serve under severe loading and working conditions,which inevitably causes different fatigue problems.As the fatigue problems getting worse,the safety of bridges can be significantly impaired,and thus reinforcement need to be conducted accordingly.Compared with traditional reinforcement measures,such as replacing materials and re-welding operations,Fiber Reinforced Polymer(FRP)exhibits better performance in terms of light weight,high strength,ease of construction,and excellent durability,etc.Studies have shown that FRP reinforcement can effectively improve the fatigue performance of steel structures.However,for long-span steel bridges,the stress state and working environment of the steel box girder are more complicated than laboratorial conditions.Hence,the combined influences of stress,temperature and corrosion on the fatigue performance in reinforced components need to be studied.Therefore,the fatigue behaviors of FRP reinforce steel bridge box girder were investigated with comprehensive methods(laboratory tests,numerical analysis,real bridge reinforcement and monitoring)in this study.The bullet points of this study and relevant results are summarized as follows:(1)Effect of FRP angle reinforcement on out-of-plane fatigue behaviors of welds in steel box girders.An FRP angle was designed to strengthening the out-of-plane fatigue performance of welds in steel box girder,and fatigue strengthening tests were carried out.The results of fatigue tests show that debonding is one of the main failure modes of this reinforcement structure.The FRP angle can significantly reduce the stress concentration near the weld details,thereby increases the fatigue life of the welds accordingly.Furthermore,numerical analysis shows that the stress intensity factor at the crack tip can be reduced significantly by bonding FRP angle.(2)Field fatigue inspection of GFRP-reinforced deck of steel box girder.FRP angles were used for reinforcing the weld details of the deck on real steel bridge.Its performance was examined via field measurements.The field-testing results show that the effective stress amplitude of the FRP-reinforced weld area has been significantly reduced up to 66.9%.In addition,even under the most severe load condition,the Von Mises stress and the stress intensity factor at the crack tip of the reinforced welds also decreased obviously,with reductions of 44.8%and 18.0%,respectively.(3)Static behaviors and fatigue behaviors of FRP-steel specimens under different curing conditions.Double-shear joint GFRP-steel bonded specimens were tested to study their mechanical properties and bonding strength under different curing conditions.The effects of curing method,curing temperature and curing cycle were investigated.The results indicate that the corrosive environment can change the failure mode of the specimens.The corroded specimens exhibited notable degradation of tensile capacities.Accordingly,the interfacial fracture energy and fatigue strength were also reduced.By studying the energy dissipation rate of the specimens under a single cycle,it was found that the energy dissipation rate E_dof all specimens increases continuously as the cycle number increases,and the E_dof the harsh cured specimen grew faster than that of the reference specimen.(4)Interfacial fatigue performance of FRP-steel specimens under different temperatures and multi-step loadingThe fatigue performance of GFRP-steel bonded specimens under two-step and four-step variable amplitude loading in different temperatures was studied.Studies show that the fatigue life decreases with increasing temperature.Based on tensile tests of the specimens which were pre-loaded under fatigue loading,it is found that the bonding strength of the GFRP-steel interface degenerated continuously during the fatigue loading process.Based on the results of the multi-step test and nonlinear strength degradation model,a modified model for GFRP-steel bonded specimens was adopted.By using the modified degradation model and the results of the two-step loading test,the fatigue life of the four-step loading was evaluated.By comparing the evaluation value and the test value,it is proved that using the modified nonlinear strength degradation model can accurately assess the fatigue life of the GFRP-steel bonded specimen under variable amplitude loading. |
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