Mechanical Behavior And Design Method Of Precast Segmental Bridge Columns Confined By Grouted Steel Tubes

The adoption of assembly technology and consideration of earthquake resilient performance will become a trend in the development of modern bridge engineering.The precast bridge columns using rapid assembly technology such as components factory production and on-site assembly construction provide an effective way for the high-quality,high-efficiency and ecological construction of bridges.However,the precast bridge columns using ’equivalent cast-in-place bridge column’ and based on ductility design still have the possibility of large residual deformation after a severe earthquake.The precast segmental bridge columns(PSBCs)assembled with unbonded post-tensioned prestressing tendons(UPPTs)have good self-centering capability,but when the mild steel bars used to increase the energy dissipation capacity only cross the bottom joint,concrete crushing or obvious torsion may occur near the joints above the bottom segment.Considering the new demand of earthquake resilient performance and the composite action between the steel tube and the concrete,a novel type of precast segmental bridge column confined by a grouted steel tube(TPSBC)was proposed.The grouted steel tube was employed to improve the integrity of the precast segments and mitigate the seismic damage of the concrete at column end.UPPTs were applied to assemble precast segments and reduce the residual deformation of the TPSBC.Experimental work,numerical simulation,and theoretical analysis were conducted to systematically study the axial compressive behavior and seismic behavior of TPSBCs.The damage process and working mechanism of the TPSBCs under axial loading and lateral cyclic loading were revealed.The effects of the main parameters on the mechanical behavior of TPSBCs were evaluated.Based on the completed research,a seismic design method for TPSBCs that utilizes residual deformation was proposed.The following are the main contents and conclusions of the research:(1)Experimental tests were carried out on two PSBCs and twelve TPSBCs to investigate their axial compressive behavior.The effects of six parameters on the axial compressive behavior of TPSBCs were evaluated,including prestress,loading forms,diameter-thickness ratios of steel tubes,the number of precast segments,reinforcement arrangements in precast segments,and sandwich concrete strengths.The test results showed that prestress and the number of precast segments had little effect on the maximum capacity and ductility of the TPSBCs.It was observed that the maximum capacity of the TPSBCs decreased with the decrease of compression area.However,the ductility factors of the TPSBCs under local compression were more than 30% higher than those under full compression.It was also found that the ductility factors of the TPSBCs were more than 1.6 times those of the PSBCs.In addition,it was found that an increase in the diameter-thickness ratio of the outer steel tube had a negative impact on the maximum capacity and ductility of the TPSBCs under local compression.It was observed that the arrangement of both inner and outer reinforcement layers in the precast segments improved the ductility of the TPSBCs,while the use of steel tube and outer spiral stirrups played a superimposed role in enhancing their capacity.With consideration of the combined confinement effects of the steel tube and the outer spiral stirrups,analytical models were developed to predict the ultimate capacity of the TPSBCs under different loading forms,and it was found that the calculated results were in good agreement with the experimental peak loads.(2)A total of nine large-scale bridge columns,including two conventional PSBC specimens and seven TPSBC specimens,were designed and tested under lateral cyclic loading.The effects of parameters such as the steel tube thicknesses,energy dissipation steel bar amounts,initial prestress,and sandwich concrete strengths on the seismic behavior of TPSBCs were evaluated.Experimental results showed that the initial stiffness,lateral peak load,ductility,and energy dissipation capacity of the TPSBC increased after the outer steel tube was arranged,while the residual deformation remained small.In addition,residual deformation decreased with the increase of the steel tube thickness.With the increase of energy dissipation steel bars amounts,initial stiffness,lateral peak load,and energy dissipation capacity of the TPSBC increased,but residual deformation also significantly increased.The residual deformations of the TPSBCs assembled with UPPTs decreased significantly;however,the effects of initial prestress levels on residual deformation was not obvious.Lateral peak loads and initial stiffness of the TPSBCs increased with initial prestress increases.When the area of sandwich concrete was less than 20% of the gross cross-sectional area of internal concrete,small fluctuations in sandwich concrete strength had little effect on seismic performance of the TPSBCs.Lateral displacements of the TPSBCs were mainly caused by rotations near bottom joints after lateral drift ratios exceeded 3%.(3)A detailed three-dimensional finite element model was developed,which can well simulate the joint opening and closing of the TPSBCs and the stress-strain development of the unbonded regions of energy dissipation steel bars under cyclic loading.Based on the verified modeling method,the effects of six parameters on the seismic behavior of the TPSBCs were systematically studied,including axial compression ratios,initial prestress levels,steel tube thicknesses,energy dissipation steel bar amounts,internal concrete strengths,and sandwich concrete strengths.The results of parametric studies showed that the lateral peak loads of TPSBCs increased with the increase of the above parameters.However,the lateral peak loads may occur before drift ratios of 5% with the increase of axial compression ratio,initial prestress level,and internal concrete strength.The residual drift ratios and equivalent viscous damping coefficients of the TPSBCs increased with the increase of energy dissipation steel bar amount and the decrease of internal concrete strength.However,with the increase of axial compression ratio,initial prestress level,and steel tube thickness,the residual drift ratios and equivalent viscous damping coefficients decreased first and then increased.In the preliminary design of TPSBCs,it is recommended that the reinforcement ratio of outer energy dissipation steel bars should be no more than 1.0%,initial prestress level should be no less than 0.05,and the sum of axial compression ratio and initial prestress level should be between 0.15 and 0.30.(4)A section analysis method was proposed to calculate the flexural bearing capacity of TPSBCs,which can be used to address the strain incompatibility of the critical section caused by the presence of UPPTs and energy dissipation steel bars.Additionally,a simplified lateral load-displacement analysis model was provided for TPSBCs.By utilizing appropriate hysteresis rules,the load-displacement restoring force model of the TPSBCs was developed.The validity of the suggested analysis models was verified by the satisfactory agreements between analytical and experimental results.Based on the above research and current seismic design specifications of bridges in China,modified seismic fortification objectives were proposed to meet the repairability requirement of bridges after earthquakes.A seismic design method for TPSBCs in regular bridges was also developed based on the dual control of maximum lateral displacement and residual drift ratio.There are 112 figures,9 tables,and 274 references in this dissertation.