Study On Seismic Performance Of Reinforced Concrete Piers With Rectangular Hollow Cross-sections

Reinforced concrete pier with rectangular hollow cross-section is one main type of piers always used in long span bridges. However, several problems in seismic properties of the piers need more researches, such as bi-axial seismic load, the height of piers exceeds which allowed in Guidelines for Seismic Design of Highway Bridges in China. Therefore, in this paper study on seismic performance of reinforced concrete piers with rectangular hollow cross-sections were carried out by bi-axial qusia-static test and earthquake simulation three shaking table test of a continuous rigid frame bridge model. The main works and extracted conclusions are listed as follows:(1) The effect of different loading laws on hysteretic characteristics of the 4.8 meters high test piers was studied by bi-axial quasi-static test with four different bi-directional load patters including diagonal loading patter, square loading patter, diamond loading patter as well as circle loading patter. It is concluded that the diagonal loading rule has several advantages, such as simple loading process, easy implementation, short loading path and time, and also, the bi-directional hysteretic properties and failure modes are well revealed according to this type of pier, so this loading rule is available and recommended in bi-axial quasi-static test for the piers.(2) Bi-axial quasi-static test on 14 reinforced concrete piers with rectangular hollow cross-sections were carried out, the hysteretic characteristics were discussed in consideration of the influence of slenderness ration, axial-load ratio and ratio of reinforcements. Results show:1) Flexural failure is the main failure mode of the piers, the damage area of short piers focuses on the bottom area while the damage area of high piers moves upward obviously.2) The piers with bigger slenderness ratio and smaller axial-load ratio have more full hysteresis curves, bigger deformation capacity, show more excellent seismic performance.3) As the slenderness ratio ranges from 6.9 to 13.1, displacement ductility of the pier decreases as slenderness ratio increases, but the displacement ductility of the pier with slenderness ratio of 16.3 is obviously bigger.4) Because the influence of second order effect, the tall piers with larger slenderness ratio has bigger plastic damage area while the ultimate curvature declines obviously.(3) The process of bi-axial quasi-static test of the piers were analyzed by nonlinear finite element analysis method using solid element model and nonlinear fiber beam element model, both models predict well for the nonliear characteristics of the piers. The analysis based fiber beam element has high calculation efficiency, while the calculation results are smaller than the test ones. The analysis based solid element model has a good simulation both for damage process and test results.(4) Combined with moment curvature analysis of reinforced concrete rectangle hollow cross-section in action of bi-axial moment action, the seismic properties of the piers subjected to bidirectional horizontal load were studied. The safety factors for ultimate plastic rotation capacity were analyzed by comparison between the testing curvatures and the calculating ones, the equivalent plastic hinge length calculation formula was present and the ductility seismic performance analysis method also was established.(5) Study on load displacement restoring force model was carried out. Combined the test results, the relationship among displacement in ultimate load and ultimate displacement with slenderness ration, axial-load ratio and ratio of reinforcements was constructed by data-fitting, the calculating method for field load, ultimate load as well as unloading stiffness were present, and the restoring force model of the piers subjected to bidirectional horizontal load was established. Results show that calculating hysteretic curves agree well the testing ones, so the proposed restoring force model can be referenced during seismic analysis of the piers.(6) Performance index of bridges with reinforced concrete rectangular hollow cross-section piers was discussed. It is concluded that Park-Ang index based bi-axial quasi-static test can reflect the damage condition in each performance level, and the performance based seismic design process for girder bridges with rectangular hollow cross-section piers was preliminarily put forward.(7) An earthquake simulation three shaking table test of a continuous rigid frame bridge mode with 3.2 meters high piers and two 6 meters long spans, as well as the seismic evaluation were carried out. The results show:1) the main damage feature is the crack in piers in the test, the strain or the piers in action of bidirectional earthquake is obviously bigger than that in action of a single directional action.2) The dynamic analysis model of the test model constructed by the fiber beam element can reflect test dynamic response in a certain degree.3) The Park-Ang index considering bi-directional load action can reflect failure characeteristics and is available in seismic evaluation of the bridges with rectangular hollow cross-section piers.