| Since the 911 incident in the United States,terrorist activities have become one of the main threats to the security of the world.Bridges,as the vital link in transportation lifelines,are one of the main targets of terrorist bombing attacks.Additionally,accidental explosions of oil,gas,fireworks,etc.in the transportation process cannot be ignored,either.Presently,the total number of highway bridges in China has exceeded 850,000,of which more than 90% are concrete girder bridges.None of these bridges have been designed for explosion resistance and protection.Whether in wartime or in peacetime,the safety and smoothness of key bridges in important transportation lifelines should be guaranteed.Therefore,research on the explosion resistance and protection of bridge structures has important theoretical significance and practical value for improving the existing design concept and enhancing the safety and disaster prevention ability of traffic infrastructure.This dissertation,backgrounded on prestressed concrete continuous girder bridges,aimed to study the distribution law of pressure field,dynamic response and failure modes caused by explosion under the bridge.The main research work included:(1)A model with a scale of 1:5 of a two-span prestressed concrete continuous girder bridge was designed and built,and explosion experiments under the bridge with varying explosive locations and equivalents were conducted.Seven working conditions were included in the tests,which were divided into three stages.(2)The overpressure in the free field,caused by the reflection from the columns,and induced by the reflection from the bottom of the T-shaped girders during the explosions was measured and studied.Based on the explicit dynamic analysis software LS-DYNA,the interaction between the explosion wave and the model bridge during the explosions was simulated.(3)The dynamic response and failure modes of the piers of the model bridge under near-field explosions were tested and studied,and then a threedimensional local model of the pier of the model bridge was established in LS-DYNA to simulate the test process.The local failure mechanism of the pier under the near-field explosion load was studied,and the influence of the axial compression ratio and boundary conditions on the local failure of the pier were studied.(4)The collapse of the prestressed concrete continuous girder bridge under contact explosion was experimented and studied.A refined three-dimensional finite element model of the two-span prestressed concrete continuous girder bridge model was established,and the local failure and collapse process of the model bridge under contact explosion were simulated via the stage method;meanwhile the local failure mechanism of the piers under contact explosion load and the collapse caused by gravity after the explosion were studied.The main conclusions are as follows:(1)There were multiple peaks in the measured time history curves of the overpressure in the free field,on the column and on the bottom of the T-shaped girders during the explosion below the bridge deck.The ground cratering would cause the arrival of the wave front to delay and the peak values of the overpressure to decrease significantly.The measured freefield overpressure peak values were larger than the values in the chart of UFC 3-340-02,and the time when the measured free-field overpressure was positive was shorter than the value in the chart of UFC 3-340-02.(2)Established in LS-DYNA,the three-dimensional model including the ground,the air,the explosive and the prestressed concrete continuous girder bridge could accurately predict the distribution of the pressure field in the test process.The numerical simulation showed that reflection and diffraction occurred when the shock wave encountered the webs of the Tshaped girders,diaphragms and piers,and that the blast wave reflected repeatedly and was restrained in the confined space between the webs;the peak values of the overpressure tended to decrease with the increase of the pier height,but at the middle and the top of the column,the peak values would rebound,of which the rebound was more significant on the surface facing the blast.(3)The near-field explosion test of the pier of prestressed concrete continuous girder bridge shows that,several transverse cracks were detected on the front surface,but the concrete did not spall;the concrete cover of the side surfaces spalled and the rebars inside were exposed;the concrete on the back surface was not damaged much.The local numerical model can well simulate the local failure phenomenon and dynamic response characteristics of the pier under the action of near-field explosion.Simulation results show that,the expansion caused by poisson effect will result in the spalling of concrete protective layer on the side under the action of near-field explosion.Because of the boundary effect of pier crosssection,the stress wave propagation in the column is non-one-dimensional.There is a large impulse in the area where the foundation and column meet,the diffraction effect of blast wave on column surface is quite limited.Parameter analysis shows that,the axial compression ratio has a certain influence on the local failure of piers under the action of nearfiled explosion,but the restraint effect between the bridge superstructure and the pier has little influence on it.(4)Under the action of contact explosion,the main failure mode at the bottom of the pier was local punching shear failure,and the magnitude of the axial force had a significant impact on the dynamic response of the pier under contact explosion.After the bottom of the double-column pier was destroyed in the contact explosion,most of its bearing capacity was lost.Consequently,the model bridge would collapse under the action of its own weight without any external forces.(5)Under the action of the contact explosion at the bottom of the pier,the variation trend of the time history curve of the acceleration at the measurement point on the bridge deck above the fixed bearing of the continuous girder bridge was different from that at other measurement points.The maximal value of the peak acceleration of the span near the back surface was much smaller than that of the span near the front surface;the response at the measurement points of the prestressed tendons in the negative moment section started much earlier than that in the positive moment section,and the variation range at the measurement points in the negative moment section was larger than that in the positive moment section.(6)The established numerical model could simulate the punching shear failure at the bottom of the pier and the crack distribution on the pier and reproduce the whole process of the transmission of the explosion stress wave and its interaction with the structure well.By the multi-stage simulation method,the collapse process of the model bridge under its weight could be simulated,in which tensile cracks occurred on the concrete in the T-shaped girders on the both sides of the cast-in-place section at the top of the middle pier.For the prestressed tendons in the negative moment section,an obvious downward trend of the element tension at the top of the middle pier was observed,while the tension of the elements at other locations did not change significantly.For the prestressed tendons in the positive moment region,the element tension near the midspan showed an upward trend,while the element tension near the anchorage zone showed a downward trend. |
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