Balanced Cantilever Segmental Bridge System Test
By Kelly Burnell(1), José Restrepo(2), and Frieder Seible(3)
Preliminary Results Summary (June 10th and 11th, 2004)
This test examined the seismic behavior of a precast post-tensioned segmental bridge with cast-in-place column and internal bonded tendons under longitudinal seismic motion.
The column was prismatic and had octagonal boundary elements on each of the corners. The longitudinal reinforcement in the column remained unchanged along the column height. Nonetheless, the spacing of the transverse reinforcement and corner spiral spacing was increased in the region away from the plastic hinge.
The primary objectives of the test were to investigate the response of the column-superstructure interaction, possible opening of the superstructure CIP joints, plastic hinge formation in the column, and the overall system failure mechanism.
Test Specimen The specimen was built at half-scale of the prototype. To indicate general dimensions of the specimen, an elevation drawing is shown in Fig. 27, and a photo of the specimen is presented in Fig. 28.
The force-displacement diagram for the bridge system is shown in Fig. 29. The graph shows the lateral force versus the displacement. The overall performance of the column is typical for well-confined, reinforced concrete showing very high-energy dissipation capacity, stable hysteretic response and high displacement ductility. No strength loss was noted as the ductility levels increased.
No signs of opening of the joints was noticeable during the test. The joints most susceptible to opening are those closest to the column. These joints were carefully monitored during the testing both by visual inspections at all stopping points as well as with numerous instruments. The initial examination of the instrument data does not show any cracking in the joint region.
One pair of tendons in the top of the superstructure was not grouted so they could be removed in the later stage of testing. Removing these strands reduces the amount of prestressing in the top of the superstructure by approximately 25%. This significantly reduces the opening capacity of the joints in the top. The level of prestressing remaining corresponds approximately to the design level of not allowing any cracking of the joints under service loads, using a cracking stress of 6√f'c.
As well as loosening up the superstructure, the loading on the bridge will be intensified by the addition of a simulated 0.75g vertical acceleration. This will be accomplished by adding additional dead load to the specimen.
1 Graduate Student Researcher, University of California, San Diego
2 Associate Professor, University of California, San Diego
3 Dean, Jacobs School of Engineering, University of California, San Diego