To reinforce the abutments supporting the bridge deck at either end, engineers put in place a floating slab, continuous with the deck, keyed into a massive pile cap with six 72-inch (1,800 mm) diameter cast-in-drilled-hole (CIDH) piles behind each abutment. To support the towers, engineers designed a full height structural wall that was integrated within each of the two existing towers. During the retrofit, they removed the top portion of the towers, including the roadway, and replaced them with a prestressed diaphragm that anchors the full height of the vertical tower. The diaphragm simultaneously distributes the vertical prestressing forces uniformly to the new concrete structural wall and the existing tower's concrete. The deck, which curves from one end to the other, was reinforced by adding heavily confined edge beams encasing high strength steel along the inside face of the exterior longitudinal girders underneath. These rods extended from one end of the roadway to the other. The reinforced edge beams ensure continuity across the many expansion joints and help distribute the bending strains due to lateral flexure. In addition to the reinforced edge beam, four large prestressing tendons were installed the length of the bridge along the underside of the deck slab. These tendons are stressed to pre-compress the concrete deck to approximately 800 psi and also serve as flexural reinforcement along with the high strength rods. Finally, engineers found a way to reinforce the bent columns attached to the arch, which possess complex and varying geometric challenges. They encased the bent columns with thin, lightweight, composite carbon fiber jackets that provide the necessary degree of confinement to ensure ductile response and also mimic the original design. In addition to the analyses performed by Buckland and Taylor, Caltrans commissioned Lawrence Livermore National Laboratory to perform an independent study of the structure both with and without the proposed retrofit measures in place. The final report, which was published in June 1999, concludes that the retrofit appears to be appropriate even for earthquake ground motions including near-field displacement pulses, which were not considered in the original analyses. As a result of the retrofit, the continuous, stiffened deck has four lateral reaction points: two new massive abutments anchored by large-diameter, cast-in-drilled-hole piles. The two towers are strengthened and anchored to rock with tie-down anchors within the towers. The arch ribs are laterally supported at their crowns by new shear keys that link them to the reinforced deck.