In 1960, a 2-lane, 2.4-km (1.5-mile) -long, concrete floating bridge was designed and built by the Washington State Highway Department to extend Highway 104 across Hood Canal, a fjord-like arm of Puget Sound west of Seattle, Washington.

A fixed bridge design was ruled out because the canal is more than 92 m (300 feet) deep and has a tidal variation of more than 5 m(16 feet). The floating bridge included a 183-m (600-foot) -long draw span to allow passage of vessels through a navigation channel to Bangor Naval Base.

In February 1979, a storm destroyed the western half of the Hood Canal Bridge-the principal highway link from Seattle to the Olympic Peninsula. The loss of this critical bridge link resulted in a 160-km (100-mile) detour and activation of limited and costly emergency ferry service. The state determined that the structure should be replaced as a floating bridge but that the replacement should be designed according to storm design criteria more stringent than those used for the original bridge. The Washington State Department of Transportation (WSDOT) engaged PB as the lead firm in a joint venture to design the replacement structure.

In 1999, PB was retained to update the 1979 plans and replace the eastern half of the structure.

Traffic has increased so much in the intervening 20 years, that we now have to widen the western half "under traffic"-no easy feat, particularly on a long floating bridge in a near ocean-like environment.

1979 Staged Replacement Program

View from water level of bridge across Hood Canal showing fixed truss section.

The 1979 design included a staged replacement program carried out under tight scheduling. This program involved reconstruction of the western portion of the bridge and design for the subsequent replacement of the eastern portion, which survived the storm with only minor damage.

To speed the restoration of highway service across the canal, WSDOT obtained Coast Guard approval to restrict the navigation channel during the first stage of construction to the 82-m (300-foot) width provided by the remaining half of the original 183-m (600-foot) -wide twin draw spans.

In the next stage, a new draw span on the western half replaced a portion of the stage 1 construction, restoring the navigation opening to its full width. The final stage, which is planned to be advertised next year, is to replace the eastern portion of the bridge, some 20+ years later.

West Half Floating Bridge Structures. We designed the replacement bridge to withstand sustained waves generated by 110-kph (83-mph) winds and wind pressure from 176-kph (110-mph) gusts. Adding to design complexity, all construction materials were required to withstand a highly corrosive marine environment.

Concrete Pontoons. The replacement floating bridge structures consist of continuously linked longitudinal concrete pontoons held in place by 0.8-km (0.5-mile) -long anchor cables attached to concrete anchors weighing 1350 metric tons (1,500 tons) each. The prestressed concrete pontoons, anchors and anchor cables are 2.5 times stronger than those on the original bridge.

Each pontoon weighs 7470 metric tons (8,300 tons) and contains 36 watertight cells. This compartmentalized design keeps water from migrating in the event of cell flooding and improves safety should a ship strike the pontoons. Special submarine-type, screw-down hatch covers provide access to each compartment to facilitate inspections. The pontoons are post tensioned vertically, longitudinally and transversely. Precast segments were used to speed construction and improve quality.

Roadway. The pontoons-3 m (10 feet) wider and 1.2 m (4 feet) deeper than the originals-support a 2-lane roadway of 18-m (60-foot)-long, precast, prestressed concrete spans. The roadway is supported on columns above the pontoons to keep it above storm waves and spray.

Lift-draw Spans. The lift-draw section combines a 92-m (300-foot) -long steel deck and a floating draw span. In addition to being more economical, the lift-draw design allows a safer and more efficient traffic flow than was possible on the original bridge, which required a sharply curved, split roadway to leave room for draw-span retraction. To open a span, the deck is lifted hydraulically to allow the draw span to retract into an open well beneath the deck. When the draw span is extended, the deck is lowered hydraulically to roadway level. The local population of seals seems to like the well and can be found in it from time to time.

The draw span is operated by a rack-and-pinion mechanism with twin 132-m (432-foot) -long racks. Both spans can be controlled from either of two control towers. This combination of hydraulic and mechanical drives with a floating, hollow, prestressed concrete structure involved a rare combination of civil, marine, mechanical, and electrical engineering.

Hinged Pontoon Joint. The bridge construction also included a special hinged pontoon joint and flexible deck section. When dynamic analysis simulating storm forces showed high torsional moments about the pontoon joint at the draw span, the answer was a structural hinge-a 2.4-m (8-foot) -diameter, steel-lined, concrete cylinder sliding on teflon-coated neoprene bearings within a steel-lined can-a "wrist" held together by cable. Across this joint, a flexible superstructure span of steel stringers with partially filled grating deck can twist with the pontoons yet maintain a smooth roadway.

Superstructures. The control tower and storage building superstructures used fiberglass-reinforced concrete panels to provide an attractive, lightweight, and durable surface. The building panels and concrete surfaces were coated to provide a uniform color, as well as improved durability.

The bridge was reopened to traffic in October 1982, 44 months after the February 1979, sinking, and the project won a number of awards, as shown in box above.

2000 Update is in Progress

WSDOT recently retained PB to update the eastern half replacement plans and specifications so that construction on the replacement can be completed.

The update will include incorporating current standards, lessons learned since the replacement of the western half, and revisions due to changes in available equipment. Because of traffic growth, it was also decided to widen both the existing western half and new eastern half under the same contract. This widening will be particularly challenging because it will have to be done on the western half while maintaining traffic.

A recently passed referendum in Washington has resulted in greatly reduced funds available for any public project. This project will move forward, however, because the eastern half of the bridge is nearing the end of its life. To get the job done and keep within funding limits, we are now working closely with the state to maximize its share of the work. We are sharing in the design effort, almost as a joint venture and the state is undertaking much of the drafting. In effect, we have PB working for WSDOT, which is working for PB.

Another consideration is that any shut down will have major repercussions, so we are working with the local contracting community to develop a scheme to change out the eastern half in a short time period by floating out the old eastern half then floating in the new eastern half, while at the same time replacing the fixed approach spans within the shortest possible time period.

We were also retained to provide maintenance inspection of the bridge's mechanical and electrical elements and to prepare plans for maintenance repairs to these items. [Ed. Note: See "Hood Canal Bridge: A Study of the West Structure Control System" by Mark VanDeRee for information on that work.]