Seismic performance of glass fiber reinforced polymer tube confined prefabricated concrete pier

The application of prefabricated assembly pier systems in medium-high intensity seismic regions is often restricted by their seismic performance. Fiber reinforced polymer (FRP) has been widely applied in the field of bridge seismic resistance. However, up to now, the seismic performance of the "equivalent cast-in-place" fabricated piers with GFRP tube-confined and grouting sleeve connections has not been studied. A simple and effective new structural form is proposed to meet the higher seismic fortification requirements by combining the FRP confined concrete system with the prefabricated pier system. The research results can provide reference for the seismic design and practical engineering application of GFRP tube confined prefabricated concrete pier.

Two GFRP tube confined concrete specimens and one unconfined concrete comparison specimen were designed and fabricated. The seismic performance of pier was studied based on pseudo-static test, including hysteretic properties, deformation and load bearing properties, stiffness, energy dissipation capacity and residual displacement. To measure the deformation development of pier specimens and the restraining effect of GFRP tube during the loading test, the vertical strain gauges of connecting steel bars, the circumferential and vertical strain gauges of concrete surface, the circumferential strain gauges of GFRP tube surface and the linear displacement sensors distributed along the pier height were laid. Based on the Park-Ang two-parameter seismic failure model and the improved Park-Ang model, the damage index of the specimen was calculated, and the drift rate was directly used to correspond to the damage state of the pier, and the suggested damage quantification interval and qualitative description were given.

The failure height at the top of the pier column in the SPC loading direction was less than 250mm, and the failure height at the bottom was less than 175mm, and the degree was lighter than that at the top. The damage of GFRP tubes for SPCG1 and SPCG2 was not serious, and only the local fiber cracking, peeling and whitening phenomena were observed on the surface of GFRP tubes within the height range of 100mm at the bottom. The maximum circumferential strain of SPCG1 and SPCG2 at 100mm and 175mm height was 0.96% and 0.35%, 0.23% and 0.2%, respectively, and the strain at other heights was at a low level. The curvature was basically symmetric along the mid-span section of the column, and the curvature of SPCG1 and SPCG2 was smaller than that of SPC at most loading displacement levels. Compared with SPC, the ultimate displacement of SPCG1 and SPCG2 was increased by 23.2% and 30.9%, the ductility coefficient was increased by 16.7% and 54.6%, and the residual bearing capacity of SPCG1 and SPCG2 was increased by 103.3% and 90.4% respectively at 7%. When the drift rate was 0~7%, the cumulative hysteretic energy consumption of the specimen was SPC>SPCG2>SPCG1. When the drift rate was 7%, the residual displacement of SPC, SPCG1 and SPCG2 was 106.0mm, 83.3mm and 72.1mm, respectively. The results of damage index calculated by Park-Ang model and improved Park-Ang model were not much different in most cases. Compared with SPCG1 and SPCG2, the difference of calculation results of SPC using different models was greater. In the whole process of displacement loading, the damage index of SPC was obviously higher than that of SPCG1 and SPCG2.Conclusions: Both the cast-in-place column top and the prefabricated column bottom of SPC form sufficient plastic hinge area, which belongs to ductile bending failure. The prefabricated structure at the bottom has the same seismic performance as the cast-in-place structure at the top. The failure of concrete and the fracture of steel bar at the joint of the prefabricated pier are the main reasons limiting its seismic performance. The measured average curvature distribution satisfies the deformation mode of the double-curvature column, in which the GFRP tube constrained specimen has lower curvature and better bending resistance. GFRP tube effectively limits the development of shear oblique cracks and lateral expansion of concrete, prevents the concrete protective layer from falling off, and delays the yield and fracture of steel bars. Constrained by GFRP tubes, the ultimate deformation, ductility and bearing capacity under large displacement of prefabricated concrete piers are improved, and the residual displacement is reduced. Therefore, the specimens have stronger lateral resistance, greater safety reserve and certain self-resetting ability. The damage quantization interval and qualitative description of the prefabricated pier are given based on the drift rate of the pier top directly.