Study On Dynamic Response And Fatigue Damage Of Sea-crossing Bridges Under Wave Loads

With the rapid development of the bridge technology,sea-crossing bridges are gradually developing from offshore to deep sea.The influence of ocean waves on sea-crossing bridges in deep water cannot be ignored.Therefore,it is necessary to study the effect of ocean waves on sea-crossing bridges in deep water.In this paper,the five-span continuous beam bridge in the Qinhuangdao to Dalian Changxingdao sea-crossing channel scheme was used as a prototype,and the hydrodynamic model of the pile group foundation was established according to the geometric scale of 1:40.At the same time,four kinds of irregular waves were designed based on different wave energies.By considering different water levels and different direction angles of incident waves to carry out model tests,the wave forces of the pile group foundation under the action of random waves were obtained.Meanwhile,the MIDAS finite element software was used to model the continuous beam bridge,and the converted measured wave forces were input into the bridge numerical model to study the dynamic response of continuous beam bridge under the wave actions.On this basis,the stress time history data of the bridge structure was extracted,and the fatigue damage of the bridge structure under the action of wave loads was further discussed.The main works and conclusions of this paper are as follows:(1)Through physical model tests,the effect of different water depths,incident wave directions,and random wave energies on the wave forces of the pile group foundation were discussed.The results shown that the greater the wave energy was,the greater the wave force on the pile foundation was.With the water level rises,the wave forces on the pile foundation also shown an increasing trend.When the pile cap participated in the action between waves and the pile foundation,different wave incidence angles had an obvious influence on the pile foundation wave forces,and the wave forces of the pile foundation caused by the high-order velocity potential generated by the incident waves and the diffracted waves could not be ignored.(2)Through finite element analysis and comparing the displacement and acceleration at the top and bottom of the bridge pier,it could be found that the greater the wave energy was,the more severe the vibration at the top and bottom of the bridge pier was.Bridge vibrations consisted of two parts,which caused by wave actions and bridge natural vibrations.For the displacement response,the bridge vibration induced by the wave action dominated,while for the acceleration response,the natural vibration characteristics of bridges dominated.The loworder mode of the bridge had a greater impact on acceleration than the high-order mode.(3)By changing the parameter of pier heights,water depths and foundation types,the influence of different factors on the dynamic response of the bridge was studied.The results shown that large wave incidence angles,the uniform excitation,the high pier,the deep water,soft foundations,and other conditions increased the dynamic response of the bridge.Therefore,these factors should be taken into account when studying the effect of waves on the sea-crossing bridges and designing the sea-crossing bridges,the vibration response of bridges under wave load could be then predicted accurately.(4)The cumulative fatigue damage was analysed by using the rain flow counting method with MATLAB software,and fatigue damage values caused by different stresses were calculated.It could be found that the problem of fatigue damage for sea-crossing bridges caused by wave actions could not be ignored,and with the increase of wave energies,the fluctuation range of the stress response amplitude of the bridge gradually increased,resulting in the gradual increase of the fatigue damage of bridge structures.No matter what wave conditions,foundation conditions and water depths,the fatigue damage of steel pipe piles at top section was higher than that of steel pipe piles at sea bed section,which indicated that the fatigue damage at the joint between the steel pipe pile and the pile cap should be considered.With the decrease of soil strengths and the increase of water depths,the fatigue damage of steel pipe piles at the top section and the seabed section increased,which will reduce the fatigue life of bridge structures.Therefore,in order to more accurately predict the fatigue damage of bridge structure,the soil strength and water depth should be fully considered,and the fatigue damage of bridge structures under wave actions should be reasonably evaluated.