Pfeiffer Canyon Bridge

Record rainfall in the winter of 2016/2017 effectively made an island out of a portion of Big Sur in Monterey County, Calif.

A landslide undermined a support for the Pfeiffer Canyon Bridge on scenic coastal State Route 1, causing severe damage that was beyond repair. The bridge was closed to traffic on February 15, 2017, and its loss devastated the people stuck between the closed bridge to the north and a large landslide to the south. Groceries and fuel had to be helicoptered into the area. Children were no longer able to attend school, which was located on the other side of the deep canyon. The community lost its tourism-based revenue source, with State Route 1 closed on either side.

Caltrans immediately contracted with Golden State Bridge to demolish and construct a new bridge, designed by Caltrans, under an emergency force account. The team quickly determined that a temporary bridge was not feasible at this narrow mountainous site; there was no room for both a temporary and permanent bridge as well as the required equipment and staging areas. The design and construction of the new bridge became even more urgent.

The team quickly determined that a single 310-ft-long composite welded-steel-plate-girder bridge would be the best replacement for the existing three-span concrete box-girder bridge. Golden State Bridge received plans for the steel plate girders less than two weeks after the damaged bridge was closed to traffic. The plans included two options for the girders: 1) hybrid girders consisting of Grade 50 steel for the top flanges and webs, and Grade 70 steel for the bottom flanges and 2) all Grade 50 steel girders. The second option won out because it had the quickest delivery of all evaluated bid packages.

The girders were designed to have unstiffened webs to simplify and speed up their fabrication. The webs were 1¼-in. thick to meet this criterion. The thicker unstiffened webs were also a benefit for launching because the shear resistance of the webs would be constant and not dependent on locations of the transverse stiffeners.

The new bridge width is 40 ft, incorporating three girder lines, and the total structure depth is 14 ft (the steel girders alone are just under 13 ft deep). Each girder line was fabricated in five segments for transport to the site and required four bolted field splices. The girders were shipped to the site lying on their sides and required special Highway Patrol escort due to the width of the load on the narrow two-lane highway leading to the site. The largest transported segment was 63 ft long and weighed 56.6 tons.

Early on, Golden State Bridge decided it wanted to launch the girders across the canyon; the girders could not be delivered to the south side of the canyon and erecting all girders from the north side would require a temporary trestle halfway across the deep canyon with an active landslide. Some of the temporary erection towers at the girder field splices would actually have to be located on the landslide.

The girder plans incorporated several details to accommodate the launching. To keep the bottom surface of the bottom flange level and flush for the rollers, the web plate height varied depending on the flange plate thickness (instead of constant web plate height). Also, the lower field splice plates were redesigned to be three separate plates instead of a single plate so that the middle plate could be left off during launching, allowing the rollers to pass though the splice. The existing, damaged bridge was on a horizontal curve. The team straightened the highway alignment for the new bridge to simplify the girder details to save design and fabrication time and allow for the girder launching.

To facilitate the launch, temporary pipe supports were constructed on each abutment extending from the seat to just above the back walls, and a central temporary tower was also constructed in the canyon at mid-span. This temporary tower consisted of multiple WACO shoring towers founded on a temporary concrete footing supported by cast-in-drilled hole piles. The approximately 75-ft-tall towers were also guyed at the top. A jacking frame was constructed on the south bank to pull the girders across the canyon using prestressing strands and two 235-kip hydraulic jacks.

All the girders were assembled on the north side of the canyon with a launching nose. Timber soffit formwork for the concrete deck and overhangs was added to the girders during assembly on the launching bed; the catwalks were also installed while the girders were on the launching bed.

The launching plan involved a 14-stage process that included vertical alignment changes to raise the nose up and over the central tower and south abutment supports. The launch took three days following the very controlled and methodical launch plans. The girder assembly was pulled in 12-in. to 18-in. increments as each hydraulic strand jack piston cycled. After each pull, the team took measurements to check for deflection and alignment to ensure the process was proceeding correctly. This process was repeated again and again until the assembled girders reached the south abutment—and marked the state’s first bridge launch.

After the launch was completed, the top portion of the central temporary tower was removed along with the supporting rollers and guides. The girders were then lowered approximately 14 ft onto the abutment seats. The concrete deck was poured and then the see-through bridge railing was constructed. The new bridge opened to traffic on October 13, 2017, just eight months after the existing bridge was closed, reestablishing this vital link to Big Sur and the surrounding communities.

Project Team

• Steel fabricator: XKT Engineering Inc., Vallejo, Calif. *AISC Certified*

• Steel erector and general contractor: Golden State Bridge, Benicia, Calif. *AISC Member*

• Engineer: Caltrans Structure Design, Sacramento, Calif.

• Owner: Caltrans District 5, San Luis Obispo, Calif.