Experiment And Mechanism Research Of Steel Box Girder Under The Action Of Near Explosion Load

As a direct bearing member of bridge structure under vehicle load,the dynamic characteristics and damage mode of steel box girder structure under explosion impact are an important part of the research on anti-explosion performance of long-span Bridges.Existing researches on anti-explosion of Bridges were mainly based on numerical simulation,and mainly focus on the influence of explosion effect(including explosion yield and explosion location)on its failure mode.With the method of test,theoretical analysis and numerical simulation,the failure mode,failure mechanism and influencing factors of the steel box girder section scaling model under the action of short-range explosion were studied in detail,and this paper mainly carried out the following research work.(1)Taking the section of steel box girder of a suspension bridge as the object of study(Scale of 1:10),the short distance explosion impact test of steel box girder structure was carried out.The steel box girder section was selected with the commonly used single-box,double-room and single-box,three-room structures in engineering,and the test program was determined from the aspects of material selection,scale model design,selection of explosives and test instruments,test device and charging position.By changing the structural parameters of the steel box girder,a total of 14 working conditions(GL-series)of the steel box girder scale model explosion tests were carried out.The results show that,under the action of near explosion,the shock wave caused stress concentration only to the partition area nearest to the explosion point.Large plastic deformation or fracture occurred in this area.With the increase of the dosage,the fracture would exceed the stiffener and diaphragm,and the stiffened rib would fail in buckling.The damage state and damage degree were a comprehensive result influenced by multiple parameters.According to the test results,parameters such as thickness of roof,U rib and spacing of diaphragm could be effectively designed to improve the anti-explosion ability of steel box girder roof.In particular,the restraint effect of U-rib was particularly significant(the damage degree can be reduced by more than 80%).(2)By changing the conditions of steel box girder roof pavement,the model of 8working conditions(PZL-series)was studied experimentally.According to the damage condition and strain condition of steel box girder roof and the damage condition of pavement layer itself,the anti-explosion effect of concrete pavement layer with steel wire mesh and concrete pavement layer with kevlar layer was compared,and the anti-explosion performance of pavement layer was analyzed.The results show that the anti-explosion performance of concrete pavement could be effectively improved by adding steel mesh inter layer and fiber-like energy absorbing materials in the pavement layer(the damage of steel box girder could be reduced by more than 50%).The anti-explosion ability of kevlar concrete pavement layer was better than that of pure steel wire mesh concrete pavement layer.The anti-explosion performance of concrete pavement with double steel mesh was better than that with single steel mesh.After setting the pavement layer,the peak value of strain time-history curve at the same position of steel box girder roof under the action of explosion load was significantly reduced and hysteresis.For concrete pavement,steel wire mesh had obvious effect on limiting concrete cracking.The anti-explosion performance of concrete pavement layer could be improved by putting fiber energy absorbing materials(such as kevlar)into concrete,which was manifested by the increase of spalling area and fragment volume of concrete and the reduction of plastic deformation of steel box girder roof.When the thickness of fiber energy absorbing material reached a certain thickness,it would aggravate the damage of pavement layer,so it needs reasonable design and optimization.(3)Study on surface pressure test method of steel box girder and reflection coefficient of box girder roof under near-field explosion was carried out.In this test,according to the commonly used test method for explosive power of small-dose charge warhead,the over pressure test method that deviates from the detonation point of the explosive was adopted.Based on the classical formula,the reflection coefficient of shock wave considering the deformation and failure of steel box girder was obtained.The results show that the peak value of over pressure curve was inversely proportional to the proportional distance from the sensor to the explosive.The roof fracture would absorb shock wave energy,which was shown in some working conditions,although the proportional distance was the same,but the over pressure value would be greatly different due to different structures or materials that lead to different roof damage degree.The over pressure time history curves of all working conditions have reflected signals near 3500 us and 4000 us.If the box girder roof plastic deformation or aslight crack,the curve between the two reflected signals in the curve was relatively smooth.If serious cracking and buckling of stiffener occured on the box girder roof,serious interference signals would occur between the two reflection points in the curve.Considering the deformation of the box girder roof and other factors,when the reflection coefficient of the shock wave was1.63,the error analysis showed that the calculated value corresponding to the reflection coefficient was in good agreement with the measured value,and the standard deviation was the smallest.(4)Theoretical analysis and research on the failure mode of steel box girder roof was carried out.By means of analytical method and energy method,the lattice failure mode of steel box girder roof was theoretically analyzed and calculated.It could be seen from the test results that the deformation was mainly concentrated in the nearest lattice to the explosive,so the deformation of the adjacent lattice could be ignored and the damage caused by the blast wave was all concentrated in this lattice.The failure state and the maximum deflection deformation value of the failure state of the rectangular plate with fixed four edges were analyzed and calculated approximately according to the analysis of the rectangular plate with fixed four edges,and compared with the measured value of the failure state in the test.At the same time,the minimum explosive quantity Q* of crack critical state of steel box girder roof was proposed.The failure state and extent of the box girder roof can be predicted by comparing the experimental explosive amount with the minimum explosive quantity Q*.The greater the gap,the greater the degree of damage.In addition,from the derivation of the formula Q*,it could be seen that the damage of the structure was related to the proportional distance,the ultimate strength of the material and the thickness of the roof.When the proportional distance was relatively close,the ultimate strength of the material and the thickness of the roof and the size of the grid are the main factors affecting the damage of the structure.(5)Numerical simulation of dynamic response of steel box girder under the action of near explosion was carried out.The beam section model for the analysis of local explosion impact response of steel box girder was established.By changing the thickness of steel box girder roof and stiffening rib,the influence on the lattice failure of steel box girder roof was compared and analyzed.When the thickness ratio between the top plate and the stiffener was certain,the longitudinal cracking size of the top plate caused by decreasing the thickness ofthe stiffener was larger than that caused by changing the thickness of the top plate,and the difference was up to 17%.The effect of increasing the thickness of the roof was better than that of decreasing the thickness of the stiffener.When the thickness ratio between the top plate and the stiffener was certain,the depth of the concave hole caused by decreasing the thickness of the stiffener was greater than that caused by changing the thickness of the top plate,and the difference was up to 92%.Increasing the thickness of the plate had a significant effect on reducing the depth of the roof cavity and the approximate plastic failure volume(the maximum could be about 90% smaller).It could also effectively reduce the longitudinal crack size.