Seismic Retrofit of Bridge Joints in Central U.S. with Carbon Fiber-Reinforced Polymer Composites

ACI Structural Journal, Mar/Apr 2007 by Silva, Pedro F, Ereckson, Nicholas J, Chen, Genda D

Strengthening of bent cap/joint region

Retrofit of RC columns and beams using CFRP composites has received wide attention, but significantly less attention has been devoted to a problem of equal importance, which deals with the seismic rehabilitation of RC joints. A few research projects have been conducted in this area, however, which were used in this research to assist in the design of the retrofit.

In a research program conducted by Gergely et al. (2000), a series of 14 1/3-scale exterior T-joints were tested under simulated seismic loads. In this program, shear strengthening of the T-joints was achieved by bonding CFRP sheets to the concrete surface and the variables considered were the composite system, the fiber orientation, and the surface preparation. One of their main conclusions was that, by providing only a few layers of CFRP, the shear capacity of RC joints was considerably increased in comparison to the unstrengthened cases (Gergely et al. 2000). Of equal relevance to the research described in this paper was that in the direction of the diagonal compression struts running through the joints, the strains registered in the composite layers were at a minimum, indicating that in the design of the composite layout only the layers crossing the diagonal strut or subjected to tensile strains should be included in the design or analysis.

Another research program that has dealt with the retrofit of bridge joints using CFRP composites was led by Pantelides et al. (1999, 2004), which successfully retrofitted the State Street Bridge, in Salt Lake City, Utah. In this research led by Pantelides, the main retrofit scheme consisted of implementing an ankle X-wrap for improving the joint shear strength and U-wraps for improving the anchorage of the column bars. Because in this research program the prototype bent cap had reserved capacity against shear failure, it was decided to implement a similar detail for the joint region using an ankle X-wrap, but U-wraps were implemented only in the vicinity of the joint region rather than along the entire length of the bent cap. The complete retrofit scheme for Units 1 and 2 is shown in Fig. 7(a), respectively, which consisted of diagonally and vertically placed CFRP sheets. Design of the retrofit scheme is discussed in the following.

In this research program, the design of the retrofit scheme was based on the strut-and-tie model depicted in Fig. 8. Based on recommendations by Priestley (1993), only 50% of the column tension force must be clamped by the joint shear mechanism, because the other 50% are properly clamped by the diagonal compression strut near the column compression zone. Based on the proposed mechanism, it was reasonable to assume that from this 0.50T C , 50% are clamped inside the joint by the struts DI , and the remaining 50% are clamped outside the joint by the strut DO.

Interior joint shear retrofit

Inside the joint the column tension force is clamped by the strut DI and the force present is

... (11)

where l is the length of each of these struts DI (refer to Fig. 8). Based on this strut force and the length of the tie FI , which is denoted as w in Fig. 8, it is possible to establish a relation between FI and DI in terms of