Basem H. Alsamman and Mark A. Darnall, California Department of Transportation
The California Department of Transportation (Caltrans) is currently building a new fourspan HPC precast, prestressed, post-tensioned bridge to carry Interstate 5 across the Sacramento River in Northern California. This bridge with a length of 614 ft (187 m), uses spliced Caltrans bulbtee girders with a cast-in-place reinforced concrete deck. It is the first bridge of this type to be built in California.
The bridge was designed subject to several stringent constraints that included (1) minimizing the foundation footprint of the bridge in the Sacramento River, (2) reducing substructure influence to the river flow and minimizing channel disruption during construction, and (3) only working within the waterway from May 1 to October 15 of each year with all falsework removed from the river by October 15.
Due to these constraints, the minimum acceptable span length was 148 ft (45 m), and the maximum permissible structural depth was 78 in. (1.98 m) to provide the required freeboard. Therefore, Caltrans engineers opted for continuous high performance concrete (HPC) precast, prestressed, posttensioned, spliced bulb-tee girders.
Actual span lengths were 153 ft (46.6 m) for the end spans and 154.2 ft (47.0 m) for the center spans. Each span incorporated thirteen 70-in. (1.77-m) deep girders at 10.25-ft (3.13-m) centers. Web thickness of the girders was 8 in. (200 mm). The spliced girder in each span consisted of three prestressed concrete segments approximately 49 ft (15 m) long, erected on temporary supports and spliced together with a 2-ft (610-mm) long HPC closure pour. The reinforcement between girder segments was made continuous and the segments were post-tensioned longitudinally for the complete length of the bridge. The first-stage longitudinal post-tensioning and transverse post-tensioning at pier diaphragms made the girders self-supporting. This enabled the removal of falsework and access pad from the river. The second stage post-tensioning was performed after the deck was cast.
The specified concrete compressive strength for the girders, girder closure pours, intermediate diaphragms, and pier cap diaphragms was 8700 psi (60 MPa). An additional 7 days beyond the normal 28 days was allowed in the special provisions for strength development. However, due to time constraints, it was necessary to achieve the specified strength for the cast-in-place concrete within 10 days. Consequently, a concrete mix with a watercementitious materials ratio of 0.33 and a highrange water reducing admixture was used. The average 10- and 35-day strengths were approximately 10,000 psi (69 MPa) and 11,000 psi (75 MPa) respectively. At the time of its design, this was one of the highest concrete strengths used by Caltrans.
Phased construction of this bridge started in April 2001 and is expected to be completed in August 2004. It has a projected total cost of $16.1 million. In the meantime, thousands of motorists who traverse this route daily will hardly have noticed the work in progress. Clearly, the design and construction incorporating HPC precast, prestressed, spliced bulb-tee girders for the superstructure have provided an outstanding solution to a complex engineering problem and have addressed the needs and concerns of the traveling public and other partners.
