Segmental Seismic Tests at the University of California, San Diego

The segmental seismic research project consists of three phases of which Phase I is currently in progress. The objective of Phase I of the project is to study the seismic performance of segment-to-segment joints in precast segmental bridges with different ratios of internal and external post-tensioning. This research program was initiated by the American Segmental Bridge Institute (ASBI), funded by Caltrans and conducted at the University of California San Diego (UCSD) under the direction of Prof. Frieder Seible. This first phase of the research program includes testing of four large scale units with different ratios of internal bonded and external post-tensioning. Table 1 provides the test matrix. Test Units 1 and 2, which use 100 percent internal post-tensioning (bonded tendons), have recently been tested at UCSD. Test Unit 2 had a continuous cast-in-place deck at the location of segment-to-segment joints. The reinforced cast-in-place deck detail across each joint is similar to the detail proposed for the new East Span Skyway of the San Francisco-Oakland Bay Bridge. The remaining portion of the joints, along the web and bottom flange, were epoxy bonded. Test Unit 1 was similar to Unit 2 except that the deck was not continuous at the locations of joints. It means that the entire surfaces of the precast segments were joined together by epoxy. Figure 28 shows one of the segments of Unit 1 during application of the epoxy to the joint surface. The test units were designed at 2/3 scale in relation to a prototype box girder superstructure. However, only half of a single cell box girder in the shape of an equivalent I-section (Figure 28) was modeled in the experiments to simplify the test setup. Figure 29 shows the load frame and a typical test unit simply supported at both ends. The test units were loaded by four vertical actuators as shown in Figure 29. Each test unit was tested as follows:
• Testing Stage I (Fatigue Test): The test unit was loaded with 100,000 cycles at prescribed load levels which correspond to maximum service loads. Test Units 1 and 2 did not experience any cracking during this testing stage.
• Testing Stage II (Seismic Test): The test unit was subjected to fully reversed cyclic displacements with increasing amplitude up to failure. The forces in all the four actuators were maintained equal at all times. The loading during this stage was controlled by mid-span vertical displacement.

In Test Unit 1, under downward loading, the first crack occurred at the mid-span joint followed by cracking at the two joints adjacent to the mid-span joint. The cracks actually occurred in the concrete layer adjacent to the epoxy. In the upward direction, cracking occurred only at mid- span joint. Unit 1 failed when the prestressing strands fractured at the mid-span joint at a downward displacement of 5 inches and a total applied load of 490 kips. Figure 30 shows the mid-span joint after failure of Test Unit 1. Figure 31 shows a closeup view of the fractured prestressing strands.

Test Unit 2 also experienced cracking at the mid-span and the two adjacent joints under downward loading. However under upward loading, Unit 2 had several closely spaced cracks inside the segments because of the continuity of the deck at joint locations. Failure of Test Unit 2 initiated by buckling of the cast-in-place deck reinforcing bars which was followed, after repeated displacement cycles, by compression failure of the deck at mid-span. Test Unit 2 failed at a downward mid-span displacement of about 6 inches and a total load of 480 kips. Figure 32 shows the mid-span joint of Test Unit 2 after failure.

Figure 33 shows the history of total applied load versus vertical deflection measured at 6 inches from mid-span for Test Units 1 and 2. Sign convention in Figure 30 is positive for downward loading and displacement. The performance of Units 1 and 2 was similar in the downward loading direction. However, the performance of the two test units was substantially different under upward loading. Unit 1 did not have any continuous deck reinforcement crossing the joints. Thus, the total upward load dropped after cracking of the mid-span joint. The joint opening widened whereas the total load dropped with increasing the applied displacement. The maximum upward loading reached for Unit 1 was about 93 kips which is the cracking load. Because Unit 2 had a continuous deck with reinforcing bars at the locations of joints, multiple closely spaced and relatively narrow cracks occurred under upward loading, rather than one wide crack at mid-span joint for Unit 1. The strength of the deck reinforcing bars could be developed resulting in a maximum total upward load of 327 kips, rather than 93 kips for Unit 1. Figure 33 also shows the enhancement in energy dissipation with provision of the cast-in-place deck joints in Unit 2.