Pontoons support a two-lane roadway of 60-foot, precast, prestressed concrete spans. The roadway is supported on columns above the pontoons to keep it above storm waves and spray.

Projects to rebuild America's roads and bridges have many goals – improving safety, traffic flow, reliability, and maintainability, and getting the job done with minimal impact on existing traffic. A recent project to replace the east half of Washington's Hood Canal Bridge and widen the entire structure shared all of these goals and heightened the challenges for the Washington State Department of Transportation (WSDOT) and Parsons Brinckerhoff.

At a total length of 8,000 feet with a floating portion 6,530 feet long, the Hood Canal Bridge is one of the world's longest floating bridges. Composed of east and west approach spans, transition spans, and floating pontoon spans, the bridge also has a 600-foot center floating draw span, which provides a navigation channel for U.S. Navy ships passing to and from Bangor Naval Base.

As the only crossing of the Hood Canal, the bridge is a vital component of the highway infrastructure for residential, commuter, and commercial traffic in the South Puget Sound region, with an average daily usage of 15,000 to 20,000 vehicles.

Moreover, any detour during bridge construction would require costly emergency ferry service.

Replacing the east-half floating section involved a complex marine operation that required closing the bridge, yet this process was accomplished in just 34 days, enabling the bridge to reopen to traffic eight days ahead of schedule. Prior to that, only two weekend bridge closures were required for replacement of the west and east approach spans. The existing west half of the floating section was widened using staged construction to maintain traffic over the bridge.

The east-half replacement was designed by WSDOT in partnership with Parsons Brinckerhoff.

Built in 1960, half destroyed in 1979
In 1960, the existing two-lane, 1.5-mile concrete floating bridge was designed and built to extend Highway 104 between Kitsap and Jefferson counties over Hood Canal, a 340-foot-deep fjord-like arm of Puget Sound. A floating bridge design was selected because the canal is more than 300 feet deep and has a tidal variation of more than 16 feet, ruling out a fixed bridge design.

The Hood Canal is subject to frequent storms off the Pacific Ocean that generate large waves and fierce winds. In February 1979, a storm destroyed the west half of the Hood Canal Bridge. Although the east half of the bridge survived the storm with minor damage, the loss of this critical link from Seattle to the Olympic Peninsula resulted in a 100-mile detour and activation of costly emergency ferry service.

Parsons Brinckerhoff was the lead firm in a joint venture for the reconstruction of the west half of the bridge. In 1982, the program was completed with a new floating draw span replacing the floating portion of the west half of the bridge.

WSDOT had the joint venture complete plans the following year for future replacement of the east half. However, because of funding limitations and a predicted remaining life of approximately 20 years, the east-half replacement was not undertaken at that time.

Fast forward: Upgrading the entire bridge
In 1998, WSDOT retained Parsons Brinckerhoff to update the existing plans and specifications for the east-half replacement based on lessons learned from the 1979 bridge failure. It was determined that the bridge roadway should be widened from 30 feet to 40 feet to accommodate two 12-foot lanes and two 8-foot shoulders to improve traffic flow and safety for motorists and bicyclists. The wider roadway was needed to accommodate traffic growth in the region.

Widening a fixed bridge is a familiar challenge to bridge engineers. However, widening a floating bridge to accommodate a wider superstructure with a higher dead and live load is a much more significant challenge because the floating bridge's capacity is limited by its buoyancy.

The project team developed the bridge-widening design concept that was able to accommodate the wider superstructure while accommodating the increased displacement requirements for the pontoons that support the substructure. WSDOT completed the final design plans for the widening of the existing fixed-approach spans, existing west-half superstructure above the pontoons, and the new east-half superstructure above the pontoons; as well as the new parallel chord pipe transition span trusses, which will minimize ongoing maintenance.

To minimize the added weight of the superstructure, lightweight concrete was used in the traffic barrier, diaphragms, and the roadway shoulder for widening on the west half. For the east half, a more efficient girder was used together with normal-weight concrete.

Parsons Brinckerhoff, in association with Streeter Associates, designed a new east control tower and storage building and new east and west generator buildings. The control tower building was designed with fiberglass-reinforced concrete panels to provide an attractive, lightweight, durable surface, and building panels and concrete surfaces are coated for uniform color and improved durability. Plans were also prepared for new electrical, control, mechanical, and hydraulic systems for both the east and west halves of the bridge. These new systems improve the reliability of bridge openings.

Design challenge:
Increase load and hydrodynamic resistance
Adding weight to a floating structure is a challenge in and of itself. But in this case, the design of the east half needed to accommodate an increased dead and live load while resisting a complex set of hydrodynamic forces associated with major storms – forces that had caused the earlier bridge to fail.

The replacement bridge was designed to withstand sustained waves generated by 83-mph winds and wind pressure from 110-mph gusts. The design team used computer modeling for static analysis and for structural dynamic analysis of wave forces. Adding to design complexity, all construction materials were required to withstand a highly corrosive marine environment.

The replacement floating bridge structures consist of continuously linked longitudinal concrete pontoons held in place by half-mile-long anchor cables attached to concrete anchors weighing 1,500 tons each. The prestressed concrete pontoons, anchors, and anchor cables are 2.5 times stronger than in the original 1960s design.

The rebuilt 8,000-foot-long Hood Canal Bridge in Washington has a 6,530-foot-long floating portion and a 600-foot center floating draw span.

Each pontoon weighs 8,300 tons and contains 36 watertight cells. The compartmentalized design of the east half, which is an improvement over the original 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 – 10 feet wider and 4 feet deeper than the originals – support a two-lane roadway of 60-foot, precast, prestressed concrete spans. The roadway is supported on columns above the pontoons to keep it above storm waves and spray. The pontoons are post-tensioned vertically, longitudinally, and transversely. Precast segments were used in many of the west-half pontoons to speed construction while the east-half pontoons were almost entirely fabricated using cast-in-place construction.

Each lift-draw span combines a 300-foot-long steel deck and a floating draw span. To open a span, the deck is lifted hydraulically to create an open well into which the draw span is retracted beneath the deck. 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.

The U-shaped pontoon structure for the draw span provides an open well into which the draw span can be retracted beneath the raised steel deck. When the draw span is extended, the deck is hydraulically lowered to roadway level. The draw span is operated by a rack and pinion mechanism with twin 432-foot-long rack gears. Both spans are electronically controlled from a single control house. 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.

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: an 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.

The east half of the floating section was replaced in a complex marine operation that required closing the bridge. Of a total of 17 pontoons, all but three were fabricated off site in a graving dock; three pontoons that had been saved from replacement of the west half of the bridge in 1982 were retrofitted and reused. Ranging in length from 60 feet to 360 feet, the pontoons were outfitted with a new superstructure and assembled in units so that they could be floated into place.

At the same time, 26 new anchors were installed; and the existing east half of the floating section, including the east transition span, was removed. Finally, the replacement units were towed to the bridge site and anchored.

Since its reopening on June 3, 2009, the Hood Canal Bridge has provided motorists and ship traffic with improved safety, traffic flow, and reliability. The east-half replacement project earned a 2011 Engineering Excellence Award from the American Council of Engineering Companies of New York; it also was named Number 1 of the Top 10 Bridges of 2010 by Roads & Bridges magazine.

Michael Abrahams, P.E., is technical director of structures in the New York office of Parsons Brinckerhoff. During a 46-year career he has had significant roles in the planning, design, and construction of all types of bridges – especially major long-span and movable bridges including lift, sliding, floating, bascule, and swing. He is the recipient of the 2011 John A. Roebling Medal from the International Bridge Conference.