Dynamic Response Of Cable-Stayed Bridge Under Blast Load

Blast loads have been recognized as one of the extreme events that must be considered in the design of important structures such as cable-stayed bridges.Currently the blast loading has not been considered as an extreme limit state in bridge design codes both in China and abroad.However,the vulnerability of the bridges that is exhibited in car bomb and terrorist attacks has attracted more and more attentions of the bridge engineers and transportation authorities in recent years.The damages of the key bridge components or even collapse of the bridges caused by blast loadings will lead to serious traffic congestion and tremendous personnel and property losses.Therefore,it is necessary to analyze the dynamic response of the blast loaded bridges to characterize the structural performance globally and locally.The conclusions can be used to evaluate the performance of existing bridges and provide the effective methods for new structural design.Based on LS-DYNA,this paper establishes the global model of a cable-stayed bridge,the local model of the stay cable,and the segmental model of the steel box girder.For each model the detail dynamic analyses that consider different blast load cases,are conducted,and the conclusions are made as follows:Firstly,in order to investigate the dynamic response of the blast loaded cable-stayed bridge,based on load blast enhanced(LBE)method,the finite element model of a cable-stayed bridge was established,and the blast resistance analyses for different detonation locations on the bridge deck were performed.It is found that the damages of the bridge exhibit the localized property and the risk of the progressive collapse is low.The vibration of the superstructure is governed by the first mode shape in the direction of the explosion shock wave propagation.The pylon bends toward to the detonation center.When the explosive is detonated in the middle of main span,the tower deflection is greater than those the explosive detonated in the middle side spans.The results show that the standoff has significant effect on the blast resistant performance of the stay cables.The cables closed to the detonation center and tower show more vulnerable than those that have larger standoff distance.It is suggested the more attentions should be paid to those cables separated with superstructure due to the eccentric blast loading.Secondly,in order to investigate blast resistance of stay cables,and near field explosion effects on the stay cable dynamic behavior,based on arbitrary Lagrange-Euler method,the finite element model of stay cable was established and the accuracy of the numerical method was verified according to the steel plate explosion experiment,the stresses and the deflections of cables were analyzed,and the parametric study was carried out for the different proportional distances.The results show that for a near filed event,the cable's failure mode is governed by yield and no brittle fracture was found in this case.The most vulnerable position of the cable is the anchorage zone.The stress of the cable of in area was larger than that of the cable near the detonation center due to the reflection of the explosion shock wave.The minimum safe proportional distance is 0.287 kg/m~3,which is the critical survival proportional distance in a near filed event.Finally,in order to investigate the blast resistance of steel box girder,based on arbitrary Lagrange-Euler method,the finite element model of steel box girder was established,the steel box girder damage and pressure were analyzed,and then parametric analysis was carried out for different charge sizes,different plate thickness and different stiffener arrangements.The results show that there are two main failure modes of steel box girders:plastic deformation and local rupture.In the narrow space of the wing plate,the explosion shock wave will be reflected multiple times,and the sealing effect is significant.The peak value of the maximum overpressure inside the box girder occurs at the stiffening ribs of the top plate where is closed to the explosion point.Taking into account of the blast resistance,economy,and deck pavement,an optimal thickness,2cm,of steel box girders is recommended.Stiffeners and top plates can be designed as a fuse system,and the damage zone can be limited between two stiffeners by using the stiffness difference between the stiffeners and the top plate.