Preliminary Study On Pier-bottom Isolated System Of Continuous Rigid Frame Bridge With Tall Pier And Large Span

Within the range of 150～300m,the continuous rigid frame bridge is the most reasonable,simplest and adaptable type of bridge,and also has significant economic advantages compared with arch bridges and cable-stayed bridges under the same span.Currently,seismic measures are used to reduce earthquake response by increasing the resistance of the bridge pier section,as isolation bearings cannot be installed due to the fixed connection of pier and beam.However,as the span and pier height increase,the seismic response of continuous rigid frame bridges with tall pier and large span becomes complicated in high intensity areas.If seismic measures are continued to be used,it is necessary to increase the section to resist the excessive bending moment at the bottom of pier,which has reached an unsustainable stage and makes seismic design increasingly difficult.In order to better adapt to the development trend of bridge construction in China and ensure the safety of bridges in complex environments,this paper conducts a preliminary study on the isolation system at the bottom of the pier for tall pier and large span continuous rigid frame bridge in high intensity areas where seismic design is difficult to achieve.The rationality,effectiveness,and feasibility of the isolation system at the bottom of the pier were analyzed through a combination of theoretical analysis and numerical simulation.The main research work and conclusions of the paper are as follows:(1)In terms of rationality,the mechanism of the isolation system at the bottom of the pier for tall pier and large span continuous rigid frame bridge was analyzed by referring to the mechanism and realization of building structure base-isolation.Several possible ways were proposed.The results showed that by using isolation devices to extend the natural vibration period of the continuous rigid frame bridge,it can be separated from the excellent period of the site,thus isolating the seismic action and greatly improving the seismic performance of the structure.Among them,the friction pendulum bearing,which is a relatively feasible way to achieve isolation of the bottom of tall pier and large span continuous rigid frame bridge.(2)In terms of effectiveness,the seismic models and isolation models of typical three-span continuous rigid frame bridges were established respectively using Midas/Civil 2021.Five earthquake waves were selected for dynamic time-history analysis,and the seismic response results of the two models were compared.The results showed that compared with the seismic system,the seismic response of the central pier was greatly reduced when the isolation system was used at the bottom of the pier for the continuous rigid frame bridge,while the seismic response of the side piers increased slightly,and the seismic response of the beam increased significantly but did not exceed its resistance.Obviously,it is very advantageous because in the traditional seismic system,the load-bearing capacity of the beam is basically not utilized,and the piers bear most of the earthquake internal forces.The isolation system at the bottom of the pier fully utilizes the unused bending resistance of the main beam to resist seismic energy,thereby effectively reducing the excessive bending moment at the bottom of the central pier.It should be noted that after using the isolation system at the bottom of the pier,the deformation at the end of the beam increased,and the transverse deformation at the end of the beam was amplified to 0.30-0.40 m.However,with the use of dampers,it can be effectively controlled.(3)In terms of feasibility,a friction pendulum bearing with horizontal rotational function was proposed,and its performance was verified during the construction phase and bridge completion phase.The results showed that the isolation device is safe under unbalanced moments and transverse static wind loads,with sufficient safety margin.Moreover,under nonseismic loads,the isolation device remains locked,and the system maintains a continuous rigid frame bridge,meeting the requirements of normal use.(4)In the case analysis,the seismic isolation model of the pier bottom of the expansive street crossing bridge was established based on Midas/Civil 2021,viscous dampers were added between the side piers and the beam,and two 8-degree seismic waves were selected for dynamic time analysis.The research results show that: in terms of seismic performance,after adopting the pier bottom isolation system,the seismic moment damping rate of the pier bottom of the middle pier was 75%～94%,and the shear damping rate was 50%～75%,with significant seismic isolation effect.After setting a viscous damper of 2700 k N,the beam end deformation was controlled and the maximum beam end displacement in the cross-bridge direction was reduced to 0.287 m.Under the same pier cross-sectional size,both seismic system and seismic isolation system can reach seismic protection 8 degree,but the side piers and main girders of pier-bottom isolation system share part of the seismic internal force,effectively using the beam bending bearing capacity,the seismic response sharing mechanism of middle pier,side piers and main girders is more uniform and reasonable,in terms of continuous rigid fram bridge with tall pier and large span,the safety of the structure is greatly improved and the seismic performance is better.