Arch Bridge Seismic Performance Analysis

Bridge is the pivotal project of transport and is a crucial part of the anti-earthquake disaster prevention system. The damage and collapse of the bridge facilities in the earthquake costs huge directly economy loss, and it usually cuts off the local transportation route, and the secondary disaster is aggravated by the huge difficulties in the rescue after the damage of lifeline engineering. Therefore raising the anti-vibration ability of the bridge is one of the basic measures that eases the earthquake loss and strengthens the regional security. The half-through and bottom-through arch bridge has the outstanding esthetics effect, so it is extensively applied in our country bridge engineering. Because the half-through and bottom-through arch bridge is a new type of bridge that developed in the recent years, they didn't come through the strong earthquake examination. It puzzles the designers and managers of the bridge that how is the the anti-vibration ability of the new bridge. This dissertation carries on the research of the seismic response of the half-through and bottom-through arch bridge, and the conclusion has the important practical value to find out the anti-vibration ability of the half-through and bottom-through arch bridge.In this dissertation, 3 kinds of reinforced concrete arched bridges-- one arch bridge with parallel arch ribs, one X-shaped arch bridge and one Double X-shaped arch bridge-- were taken as the solid example of the engineering. Finite element software MIDAS/Civil special for bridges' structure analysis was applied to the emulate calculation, with the spatial beam element simulating arch ribs, the longitudinal tied beams and the like, and with the plate element simulating the bridge deck, and with the truss element simulating the suspenders. Three half-through and bottom-through arch bridge's spatial mechanical computational finite element model are established totally—one arch bridge with parallel arch ribs, one X-shaped arch bridge and one Double X-shaped arch bridge. It carries on the research of the dynamic property and seismic response behavior of three different kinds of arch bridges, and similarities and differences of their natural frequency are also analyzed. The maximum of seismic displacement response and seismic inner force response of the 3 kinds of arch bridges were calculated out, and the conclusion can afford a reference to the aseismic design and maintenance of this kind of bridges. The main work of this dissertation and the conclusions that obtains are as follows:1. The results of dynamic property of half-through and bottom-through concrete arch bridge:(1) This kind of arch bridge's modes of vibration mainly involves three types: the arch ribs' transverse vibrations out of plane, the vertical vibrations of the whole bridge and torsional vibrations, and the transverse vibrations of the bridge deck was appeared in the higher sets of modes of vibration.(2) The vibration modes' range of double X-arch bridges is more dense than the other two kinds of arch bridges', and the stiffness of bridge deck in plane and the whole vertical stiffness and the torsional stiffness are also biger than the other two kinds of arch bridges'.(3) Three kinds of arch bridge' first vibration was the arch ribs' transvers vibration out of plane, and the arch ribs' transvers vibration out of plane play a dominant role in the lower sets of modes of vibration. The arch ribs' transverse natural frequency is less than vertical natural frequency, and the arch ribs' transvers stiffness out of plane is obviously less than vertical stiffness.(4) The Double X-shaped arch bridge' transverse natural basic frequency is lager than that of X-shaped arch bridge and the arch bridge with parallel arch ribs, and its basic natural vibration period is the least, and the first natural vibration period is 1.616s, which indicates that this kind of arch bridge belongs to the more flexible construction, and when analysing the bridge's earthquake response, it should take more higher sets of modes of vibration in count.(5) The vertical vibration of three kinds of arch bridges all express the whole vertical vibration of the bridge, the reason is that the ribs and the longitudinal tied beams were connected with many suspenders, which makes the ribs and bridge deck appear the synchronous vibration characteristic in vertical plane.(6) Three kinds of arch bridges' torsional vibration all appear in the higher sets of modes of vibration, the reason is that the arch bridge' ribs and the longitudinal tied beams were connected with many suspenders, and the longitudinal tied beams and the end floor beams constitute the frame system in bridge deck plane, which makes the torsional stiffness of this kind of bridge larger.2. The results of seismic analysis of half-through and bottom-through concrete arch bridge with time-history method:(1) Under the longitudinal seismic load, 3 kinds of arch bridges' longitudinal displacement and vertical displacement are larger than the transverse displacement; however, the displacement of every direction is small generally. The maximum of longitudinal displacement and vertical displacement appear at 1/4 of the bridge span or nearby. The difference of corresponding station's longitudinal, transverse and vertical displacement among 3 kinds arch bridges is feeble.(2) Under the transverse seismic load, transverse displacement is main displacement response of every kind of arch bridges, and the transverse displacement is obviously larger than the other two directions' displacement. The maximum of transverse displacement appears at middle span of the bridge or nearby. For the rigidity out of plane is small, the arch ribs' transverse displacement is quite remarkable under the transverse earthquake load which is a kind of function out of plane. The results conform to physical concept.(3) Under the longitudinal earthquake load, 3 kinds of arch bridges' inner force have some characteristics as follows: Axial force is larger; Shear force is smaller, and the vertical shear force is larger than the transverse shear force; Each kind of inner force at the arch feet of ribs is larger; At the middle span, the vertical shear force has a remarkable value, while the other forces are small. At the station where the two arch ribs are connected, arch ribs vertical and transverse shear force both have a notable value. At the middle span, the longitudinal earthquake load is a kind of horizontal anti-symmetrical load for the longitudinal direction of bridge, so symmetrical inner force response is smaller, and anti-symmetrical inner force response is the larger. The results conform to physical concept. The most unfavorable mechanical position is arch feet and bracings connection at the top of arch.(4) By comparison of every kind of arch bridge's inner force under the transverse earthquake load and longitudinal earthquake load, we can get the following conclusion: transverse shear force and moment out of plane increase obviously, and axial force and moment in plane have little change. Each kind of inner force is still larger at the arch feet of ribs and increases at bracings connection on different degree, disclosing that the arch feet of ribs and the bracings connection whose stress state is complex are the most unfavorable mechanical position(5) Each kind of arch bridges' seismic response has some change under the function of Elcentro wave and Taft wave. Seismic response under the former is larger than under the later, which indicates that the frequency and spectrum characteristics of the earthquake wave has some influence to the seismic reaction of the bridge structure.(6) The absolute value of inner force or stress of the 3 kinds of bridges at the arch feet of main arch and bracings at the top of arch is larger, so these section should be control section of seismic design. Stress under transverse earthquake load is larger than under longitudinal earthquake load at the two positions, which indicates that transverse earthquake load is the controlling factor. In synthesized consideration of longitudinal displacement, transverse displacement and vertical displacement, Double X-shaped arch bridge's displacement response is smaller, and its seismic characteristic is better.