Research On Dynamic Behavior And Damage Assessment Of Cable-Stayed Bridges To Conventional Weapons Effects

Bridges are the hinges and lifelines of the traffic systems, especially in wartimes. It is very meaningful but new in technicality to discuss such topics as the dynamic responses, the destructive characteristics and degrees, the damage assessment methods and rush-repair techniques of bridges in wartime at the attack of a conventional weapon. And the technical policies, criterions and transport schemes of the important bridge constructions in traffic systems can be built for the relevant departments, to meet the national defense needs and adapt to the requirements of safety guarantee in wartime. According to "task book of the feasibility research on emergency repair technique of long-span bridge and fly-over bridge ( [2002] Num.43 )", which is jointly issued by Country traffic combat readiness office and military affairs traffic office of the general logistics of PLA of China, the dynamic behaviors of cable-stayed bridges are theoretically analyzed and numerically simulated with the finite element method. The results are given as follows:Firstly, the local destructions of thin-walled members subjected to the impaction or explosion loads are discussed. Cratering, scabbing and perforating effects are three typical local damages of the RC structures in the early time responses of structures under impaction or explosion. To cable-stayed bridges, which are consisted mainly of the thin-walled members, perforation is the major factor that influences the resistance of structures to the local damages. Based on the failure modes in perforation experiments and fracture mechanisms, two engineering analytical models for resistance to perforation have been developed for the thin-walled members under impaction and contact detonation respectively. In the models, such factors as cratering resistant force in the front face, punching shear resistant force in the concrete slabs and the resistant forces of rebars with double layer arrangements are sufficiently taken into account. And the effective evaluation methods for the resistant abilities of thin-walled members to perforation under impact and explosive load are developed by means of the given models and analysis formulas.Secondly, in spite of the pre-stress methods have been widely applied to thin-walled structures, the investigative experiences of dynamic behaviors of pre-stressed members under impact are deficient now. Simulations for the dynamic responses of pre-stressed concrete slab to impact loading were performed by the nonlinear finite element method. The results show that the pre-compression can effectively mitigate the scabbing effect produced by the reflected tensile waves and restrain the cracks surrounding the local fracture zones, as well as improve anti-cracking performances of structures in the whole vibration and deformation processes.Thirdly, it is essential to extend the dynamic analyzing from basic members into the whole structures, to analyze the mechanical behaviors of basic members in the whole structure surroundings, and the influences of local damages on the deformations of the whole structures. With the nonlinear finite element method, numerical simulations for the dynamic responses of a cable-stayed bridge with twin towers and twin-cable planes, of which the important locations are impacted by the high technological weapon, have been done to probe into the damage characteristics and degrees of the whole structure. Considering the simulated damage results, the variations of eigen frequencies and vibration modes caused by the damage states are analyzed, to research the influences of the local damages on the vibration characteristics of the whole structure. Some valuable conclusions are summed out to assist protection designs and rushing repairs of the cable-stayed bridges in the wartime.Fourthly, With the nonlinear finite element method, simulations for the dynamic responses of impacted cable-stayed bridges with different structural systems, such as the suspension system, the rigid-frame system and the support system, are proceeded to compare the effects of structural systems on the dynamic behaviors. The results show that the local plastic deformation of the suspension system is the least in three systems. To a rigid-frame system, the largest dynamic plastic deformations are distributed in the intermediate column of pylon, so the bridge pylon is easiest to fracture and the vibration responses of the main longitudinal girder are also most obvious.Fifthly, the damage assessment methods of cable-stayed bridges are researched. The simplified theoretical criterion, at which condition the bridge members will loss their load bearing capacities, is established, and provides the foundation for a practical damage criterion to grade damage degrees, which is proposed here. In relation to the damage positions and the destruction effects, the damage assessment methods are presented, in which assessing the damaged members by the simplified theoretical criterion mentioned above and estimating the dangerous degrees of the internal force states of the whole structure, which are caused by the internal force redistribution due to local impacted damages, are adopted.Sixthly, based on the damage states with different characteristics, the primary and feasible rush-repair techniques and schemes for cable-stayed bridges are put forward.