Study On Transverse Seismic Performance Of Cable-stayed Bridge Based On A Novel Type Of Oil Damper

Cable-stayed bridges have been widely built as river-crossing and sea-crossing bridges because of their elegant shape,economic construction and excellent seismic behavior.Cable-stayed bridges built at the southeast coast of China are threatened by earthquakes and wind simultaneously,and the demand to restrain the deck to resist the wind load and to isolate the deck under earthquake should be considered simultaneously.Therefore,the conventional dampers are not applicable since they are not capable to resist the wind load.Wind resistance bearings are introduced at the deck-tower and deck-pier connection at the transverse direction in the construction of cable-stayed bridges,and the deck is restrained.To solve the problem,a novel type of oil damper with variable stiffness was developed.A seismic reduction system for cable-stayed bridge in transverse direction was proposed,which consistd of the novel type of oil dampers and shear links.A simplified mechanical model of cable-stayed bridge in the transverse direction which considering the stiffness of damper was developed and a simplified design method to calculate the stiffness and damping ratio of the oil damper was proposed.The main research work and outcomes are as follows:(1)A novel type of oil damper with variable stiffness was developed,which exhibited high stiffness to resist the wind load in service condition,and exhibited low stiffness to isolate the deck under earthquake.The mechanism of the damper was explained in detail,and the stiffness and damping characteristics of the damper were analyzed.A restoring force model was developed accordingly.The mechanical property and energy dissipation capacity of the novel oil damper were studied through an axial mechanical property test.The results show that,the initial stiffness of the novel type of oil damper is high in the initial state,and reduce to zero after the sliding of the piston,which exhibits the mechanical behavior of the ideal elastic-plastic material.The relief force of the oil damper is determined by the working pressure of overflow valves and the inner diameter of oil damper,and the damping coefficient is determined by the dimension of the oil tubes and overflow valves.The relief force and damping coefficient are not related,and can be designed separately.The axial mechanical property test of the oil damper shows that in quasi-static loading conditions,the shapes of the hysteretic curves are almost perfect rectangle,and with the increase of loading velocity,the shapes of hysteretic curves convert from rectangle to ellipse due to the increase of damping force.The average deviation between the stiffness,damping coefficient and the energy dissipation of the oil damper calculate by the experimental curves and curves calculated with the restoring force model are 5.7%,4.9% and 5.3% respectively.(2)The “low stiffness high damping” design principle was proposed.The influence of the damper stiffness to the transverse seismic response of the cable-stayed bridges was studied,which proved that a lower stiffness of damper was beneficial.The relief force of the novel oil damper was recommended to be designed by the wind load to reduce the effective stiffness of damper,and the damping coefficient was recommended to be designed high enough to guarantee that the damper is capable to mitigate the seismic response of cable-stayed bridges.Thus,the novel oil damper was recommended to be designed as “low stiffness high damping”damper.The seismic mitigation effect of the novel oil damper was validated and compared with that of metallic dampers.The influence of spectral characteristics of earthquake records,bridge parameters and damper parameters to the seismic mitigation effect of the novel oil damper were studied.The results show that,with the increase of damper stiffness,the transverse displacement at the end of deck increase226～642%.The stiffness of the novel oil damper designed with the “low stiffness high damping” principle is only 0.037～0.063 of that of metallic dampers.The transverse displacement at the end of deck of cable-stayed bridges with various main span length reduces 50.3～53.8% with the oil damper,which performance is better than the metallic dampers.The seismic mitigation effect of the novel oil damper is satisfactory with various spectral characteristics of earthquake records and bridge parameters.(3)A transverse hybrid seismic reduction system consisted of the novel type of oil damper and shear links was proposed.The contribution of the vibration modes of the cable-stayed bridge to the transverse seismic response at the bottom of tower was studied.The influence of the novel type of oil damper and shear links to the contribution of the vibration modes were studied.A transverse seismic reduction system consisted of the new types of oil damper installed at the deck-tower and deckpier connection and shear links installed at the crossbeam of the tower was proposed.The results show that,for cable-stayed bridges with H-shape tower and main span length larger than 700 m,the model contribution coefficient of the first order transverse bending vibration mode of tower is larger than 0.83.However,the influence of the novel oil damper to the vibration period of the the first order transverse bending vibration mode of tower is less than 8%.Therefore,the oil damper cannot reduce the bending moment at the bottom of tower effectively,and the tower cannot remain elastic under strong earthquakes.The shear links cannot prevent the coupling of the tower and deck vibration mode,and the tower cannot be protected with the shear links alone.The tower can be protected the tower form yielding under strong earthquake with the proposed hybrid seismic reduction system.(4)A simplified mechanical model of cable-stayed bridge in the transverse direction considering the stiffness of damper,which can be used to design the damper rapidly was established.A simplified mechanism model considering the stiffness of damper,cable system and tower in the transverse direction was developed,which was based on the transverse floating vibration mode of the deck.A simplified mechanical model of cable-stayed bridge in the transverse direction considering the vibration of tower and deck was established based on the first order transverse bending vibration mode of tower.The energy method was applied to derive the simplified formula to calculate the period of the two vibration modes,and the response spectrum method was applied to derive the simplified formula to calculate the transverse displacement at the end of deck and shear at the bottom of tower.A simplified method to design the parameters of the oil damper and various types of metallic dampers was established based on the simplified mechanism model.The results show that,the accuracy of the simplified formula is high,and the deviation to calculate the period of the transverse floating vibration mode of the deck and the first order transverse bending vibration mode of tower with the simplified mechanical model are 2.8% and 1.1%,respectively,compared with the results calculated with finite element model.The deviation to calculate the transverse displacement at the end of deck and shear at the bottom of tower with the simplified mechanical model are 7.5% and 6.9%,respectively,compared with the results calculated with finite element model.Based on the simplified mechanical model,the influence of the stiffness and damping ratio of the damper on transverse seismic response of cable-stayed bridge can be calculated rapidly,and the stiffness and damping ratio of damper can be designed rapidly.