I-5 Antlers Bridge Replacement – Shasta County, California

I-5 Antlers Bridge Replacement – Shasta County California

The existing steel truss Antlers Bridge had reached the end of its life and needed to be replaced.

The new bridge is a five-span continuous, post-tensioned, cast-in-place, segmental concrete box-girder bridge, 104-ft. wide and 1,942-ft. long, with a 591-ft. main span, all founded on 12-ft.-diameter drilled shafts.

Structural components include: 212 box-girder segments, 13 – 15 ft. long, 12 – 30 ft. deep; 63 x 104 x 36 ft. deep pier tables supported by 12-ft.-diameter drilled shafts; and a transversely post-tensioned, cast-in-place concrete deck.

Innovation of Design and/or Construction

New Design – Designers located the piers to miss the greatest depth of the lake and a recreational hot spot fronting the Antler’s Shasta Lake public boat ramp. The result was a bridge with five continuous spans and a 591-ft.-long main span. Symmetry in the layout and the box girder dimensions helped to economize on equipment and formwork costs.

Design Challenges – Extra pier-table reinforcement was needed to meet Caltrans’s Seismic Design Criteria.

Because of the statewide drought conditions, the designers sought a foundation type that was both constructible and visually appealing in either high or low water conditions. Potential high-water conditions precluded traditional cofferdams, and low water would make an eyesore and boating hazard out of any suspended pile caps. The design team used 12-ft.-diameter drilled shafts extending between 95 and 140 ft. to the superstructure. The shafts were a versatile solution, but constructing them required much time and money.

Durability for the Future – Transverse post-tensioning, epoxy-coated reinforcement, 2.5-in. concrete cover, and a polyester concrete overlay contribute to the durability of the new deck.

Rapid Construction

Since delivery and access to materials was not accessible nearby, the contractor built their own concrete plant on site.

The end spans were cast-in-place on steel falsework, and were founded on concrete shafts, drilled into the rock on the slopes.

To accelerate construction the bridges were cast-in-place with 4 form travelers in balanced cantilever method.

Heavy rains flooded the site and a combination of barges and even loftier trestles— up to 95 ft. tall— needed to be built.

After excavating within the construction casing, the contractor used reverse circulation drilling to advance the shaft up to 70 ft. into more competent rock. To reduce the risk of caving in the open-hole, several operations were conducted in quick succession after the drill string was removed. A sonic caliper assessed construction casings and access trestles were built when the lake was full but exposed when the lake is low. The hole for plumbness, and a miniature drilled shaft inspection device verified the bottom cleanliness. Prefabricated, 80-ft.-long reinforcing cages were then lowered into the shafts, followed by tremied concrete with a 6-ksi strength requirement. Caltrans tested the hardened concrete for homogeneity using gamma-gamma density logging. Of the 12 shafts, six contained minor anomalies that were repaired by grouting.

Pier Tables – After the foundations and columns were completed 63 x 104 x 36-ft.-deep pier tables were constructed. Just one of these massive elements would consume 4,350 cu. yds. of concrete from an on-site batch plant.

Architectural details required complicated, curving formwork. Brackets embedded in the bridge columns supported an impressive falsework system above the water line. Seismic design required nearly one million pounds of reinforcing steel in each of the main pier tables.

Segments – Once a pier table was finished, the contractor erected four 1,200-kip form travelers to support the cast-in-place concrete segmental construction. The 212 superstructure segments were 13 – 15 ft. long and weighed up to 400 kip.

Cost Competitiveness

The cost ranges from $650/sf. – $1,350/sf. cost per square foot.
Segmental bridge cost: $131 million.
Total deck area: 203,000 sf.

Aesthetics and/or Harmony with the Environment

Located in the Shasta-Trinity National Forest, the bridge incorporated several aesthetical and environmental details. The structure includes 16 bat houses under the pier table diaphragms.

With such a large structure, Caltrans thought it would be poor stewardship not to consider incorporating aesthetics into the design. The two large-mouth bass motifs on the webs of the pier table are part of the structure. These motifs were colored with a stain which is expected to require lower maintenance costs than painting.

Now, boaters, swimmers and water skiers who frequent the area can look forward to open water while motorists will enjoy an open road. Caltrans expects the new Antlers Bridge to last more than 100 years.

Minimization of Construction Impact on the Traveling Public

To minimize impacts to the traveling public and trucking routes, the existing bridge remained opened during construction of the new bridge. To improve safety, a 0.4-mile long section of highway south of the bridge was realigned.

Jury Comments

This segmental structure combines the clean and simple elegance that balances with its natural environment, as well as the structural robustness needed to withstand an earthquake event in this seismically active region. This is a beautiful curved segmental bridge, with deliberate focus on aesthetics, fitting both the environment and the culture of the area. The bridge has a visually interesting pier table with aesthetics that deliver form and function.