End Patch Loading Behavior And Strengthening Of Locally Corroded Steel I-beams

The steel girder bridge is a widely used bridge form in the world,many of them are seriously corroded after natural exposure in air for long years,which brings great hidden dangers to the bridge safety.Due to the leakage of deck and poor ventilation,many cases of corrosion were found at the end of steel I-beam,and the corrosion tend to be localized.The section of the beam end will be weakened by local corrosion,leading to a reduction in the end patch loading resistance of the steel beam.However,few study has been reported on the end patch loading behavior of locally corroded steel I-beam.Therefore,the end patch loading behavior and reinforcement of locally corroded steel I-beams are reported herein.The main work and conclusions are as follows:1.A total of 15 specimens were tested to reveal the influence of corrosion pattern,web corrosion thickness and length,bearing length and bearing type on the end patch loading behavior of steel I-beam.Tentatively,the test results are compared with the predicted ultimate loads by the existing formulas using the measured remaining thickness of the corroded section,and the failure mechanisms of the steel beams with different bearing types under end patch loading were explored.It can be found that:(1)the web corrosion has a more negative effect on the end patch loading resistance of the steel I-beams,as compared with the flange corrosion,the ultimate load of the steel beam decreases significantly with the increase of the web corroded thickness and length,but the influence becomes very limited after the corroded length exceed a certain value;(2)the influence of the bearing type on the end patch loading resistance of the corroded beam is more obvious than that of the intact beam,compared with those of the corresponding ones with curved plate bearings,the ultimate loads of specimens with flat plate bearings increase by 8.8% and 25.7% for the intact and the damaged beams,respectively;(3)for the specimens with flat plate bearings,the combined effect of web and flange corrosion on the end patch loading resistance is less than the sum of the effects of each alone;(4)the fluctuation of the margin of safety for the predicted results by the existing formulas is too much.2.By the use of ABAQUS,a finite element model of steel I-beam with local corrosion was established,and the model was validated by the tested results of the above-mentioned 15 specimens,and on this basis the mechanism of the end patch loading behavior of the steel I-beam was further explored,moreover,the influence of initial imperfection,boundary conditions,shear span-to-depth ratio and the fillet of the transition region on the end patch loading behavior of the steel I-beam.It can be found that:(1)the torsion restraint of the upper flange has little influence on the end patch loading resistance of the steel beam;(2)the shape of the imperfection has little influence of on the load versus the vertical displacement at the end,but the amplitude of the imperfection has a relatively large influence on the end patch loading resistance,and mostly on the load undertaken by the web right above the bearing;(3)when the shear span-to-depth ratio is larger than 0.6,the influence of shear span-to-depth ratio on the end patch loading resistance of steel I-beam is very small;when the shear span-to-depth ratio is smaller than 0.6,the end patch loading resistance of steel I-beam deceases with the decrease of the shear span-to-depth ratio,and the reduction increases;hence the influence of shear span-to-depth ratio should be accounted for in the design formulas;(2)the influence of the fillet on the end patch loading resistance of steel I-beam is overestimated by the web local yielding limit.3.Based on the results of finite element analysis,the three existing design formulas for the end patch loading resistance of steel I-beam were evaluated,and the direct strength method was improved.A method was proposed to simplify the irregular and non-uniform local corrosion.The relations between the reduction coefficient of the end patch loading resistance for the corroded steel I-beam and major factors are examined,and a practical calculation method was proposed.It can be found that:(1)the discreteness of the predicted results with direct strength method is smallest among the three existing formulas,but the predicted results are too conserved on the whole,and can not take the influence of bearing type into account;(2)the improved direct strength method can predict the end patch loading resistance of the steel I-beam with flat plate bearing and curved plate bearing;(3)the reduction coefficient of the end patch loading resistance for the steel I-beam with the web corroded has little influence with the beam section,but decreases almost linearly with the web residual thickness,and first decreases obviously and then tend to be stable with the increase of the web corroded length;(4)a practical calculation method was proposed for the reduction coefficient of the end patch loading resistance for the steel I-beams with local corrosion,and the predicted results are satisfying.4.For the steel I-beams with severe local corrosion,a combined strengthening method with stiffeners and high strength grout.Three damaged specimens were strengthened to illustrate the applicability and effectiveness of the proposed method.It can be found that:(1)the end patch loading resistance of the corroded steel beams can be easily restored by welding a pair of stiffeners,and can be further improved by the encasement of high-strength grout;(2)the specimen strengthened with stiffeners failed due to local buckling of the damaged web and the stiffeners at the bottom,while the specimen strengthened with stiffeners and high-strength grout failed by shear buckling of the web panel.