Study On Seismic Safety Behavior Of Biaxial-loaded Bridge Piers

At present,simplified planar structures is widely used to calculate seismic capacity of bridges.In fact,the bridge piers are loaded in three directions during the earthquake.This traditional method of calculation ignores the coupling effects.The widely use of grouting sleeves has proved that building industrialization has become the mainstream of development.But the research on the seismic capacity of the assembled bridge piers is not perfected.In order to clarify the difference between the biaxial and uniaxial forces,major factors in the seismic capacity of assembled piers are considered.Based on piers of one subway line,models of six spring pile-group foundations were established by OpenSees.From the perspective of capability-demand,the static and dynamic calculation methods have been used.By parametric study,the seismic safety capabilities of monolithic piers,grouting coupled piers and posttensioned piers(with grouting sleeves)have been compared,aiming to propose reasonable suggestion formulas for the design of bridge piers.The main contents of the dissertation are described as follows:(1)In the part of biaxial cyclic-loaded analysis,the elastoplastic monolithic models were subjected to biaxial and uniaxial cyclic loading respectively.The relationship between biaxial and uniaxial forced states were analyzed in cyclic load-displacement curve,skeleton curve,equivalent stiffness,equivalent viscous damping and other parameters.And both biaxial and uniaxial amendatory restoring models were summarized.The result of this part showed that the capacity of column was significantly reduced under the biaxial coupling effect.Biaxial peak lateral force along bridge(weak axis)was decreased by 19.6%.And biaxial peak lateral force cross bridge(strong axis)was decreased by 16.7%.That means the hysteretic behaviors are significant different in two lateral directions,if the pier`s reinforcement ratios cannot be equivalent in two directions.(2)In the part of biaxial Pushover analysis,the elastoplastic monolithic models were subjected to biaxial and uniaxial Pushover loading respectively.Triangle and uniform loads were used to obtain the load-displacement curves of the piers.Referring to ATC40,the capacitydemand spectrums were calculated.Then the target displacements of monolithic piers would be obtained.All target displacements were compared with dynamic responses to verify whether biaxial-loaded piers can meet the seismic requirements.The results of the Pushover analysis showed that when comparing biaxial-loaded piers with uniaxial-loaded piers,the peak lateral loads of weak and strong axis were respectively decreased by 14.4% and 48.3% under uniform loading,while what were decreased by 13.4% and 48.5% under triangular load.Combining Pushover analysis with dynamic analysis,if we simply take target displacements as ultimate displacements,the bridge piers cannot accord the completely elastic requirement.In fact,the plastic deformation has occurred,only the elastoplastic evaluation can meet the seismic requirements.(3)Through a general chamfered section,the bearing capacity formulas were deduced.And the influence of ignoring chamfers on calculation was analyzed.It can be simulated monolithic piers,grouting coupled piers and prestressed piers(with grouting sleeves)by the general section.Materials of longitudinal steels,axial compression ratios,longitudinal steel ratios,post-tensioned steel ratios and other parameters were analyzed to find the possible factors on the bearing capacity.Dimensionless formulas were fitted,and they were applied to the actual project in Shanghai.To verify whether those formulas were reliable for seismic design,we compared the results with the three-axis dynamic response.When the chamfered rectangle section was simplified to the rectangular,according to theoretical formulas,only if the chamfered side length was not greater than 0.075 times cross-section height,the reliability of calculation can be more than 95%.From section analysis reports,we conclude that the longitudinal steel ratio can not change the overall shape of the failure surface.The plump area of failure surface always comes with the axial compression ratio fixed in the range of 45%-55%.When the longitudinal steel ratio is increased,the bearing capacity and ultimate bending capacity will increase at the same time.Post-tensioned steels have little influence on the bending capacity of the piers with normal longitudinal steel ratio.When post-tensioned steel ratio ranges from 0.14% to 0.27% and longitudinal steel ratio is about 1%,the bending capacity of bridge piers will be best.By checking proposed dimensionless formulas,the scope of use is equivalent rectangular sections with internal rectangular constraints.(4)The biaxial-loaded analysis is the main method for assembling piers.Three types of results were compared including cross-section calculation,cyclic-loaded calculation and dynamic calculation.The formula in current specifications would be modified to make the design of grouting sleeve assembling piers better.We analyzed the influences of post-tensioned steels to seismic capacities of grouting coupled piers,and fitted recommended formulas of design for the best range of post-tensioned steel ratios.Under cyclic loading,the peak lateral force of post-tensioned piers(with grouting sleeves)in the weak axis and the strong axis were 6.8% and 5.3%,respectively,which were lower than those of uniaxial monolithic piers,but 15.9% and 13.6% higher than those of biaxial monolithic piers.Under dynamic analysis,the weak axis and strong axis displacement of grouting coupled piers were 8% and 6% larger than those of monolithic piers,respectively,However,comparing post-tensioned piers with monolithic piers.,it was reduced by 12.6% and 4.3%,respectively.Although capacities of grouting coupled piers were less than those of monolithic piers,post-tensioned steel bars combining with grouting sleeves can equal or better than monolithic piers.When deformability was calculated,the plastic hinge length should be shifted upward by one sleeve height.And requisite displacements of equivalent monolithic piers should be multiplied by 1.1.Altogether,along and cross bridge,the actual safety factors for design should be 1.3 and 1.2,respectively.