To: The Members

Pontoon Bridges

There are an estimated 12 floating, large-scale pontoon bridges in operation world-wide, with the majority located in the Pacific Northwestern United States. These unusual types of bridges are composed of a series of watertight concrete boxes (similar to a barge) that are bolted together to form a continuous structure and held by cables anchored in concrete at the bottom of a river or lake bed. The top surface forms the roadway for vehicular traffic. Typically these bridges are found in locations where there are low fluctuations in water levels, and where soft soil conditions and significant water depths make the sinking of pilings cost-prohibitive.

Two such large-scale pontoon bridges in Washington have experienced failures stemming from a combination of the under-design of the anchoring system, obsolescence of the design criteria and lack of engineering experience with pontoon bridge design. Poor maintenance may also have played a part in the losses. Despite these two pontoon bridge failures the Washington Department of Transportation (WDOT), and bridge designers in general, remain committed to the design and have worked to refine it over many decades. This may signal new applications.

While the average underwriter may not frequently come across large-scale pontoon bridges, smaller scale pontoon bridges can be found throughout the country, including those that are used to cross shallow, marshy waterways or as temporary crossings while permanent stationary bridges are under construction or rehabilitation. With temporary pontoon bridge applications, economics rather than geography is often the primary factor in their usage. In addition, the privatization of such bridges is an emerging trend that may also result in new underwriting opportunities.

This bulletin will examine two infamous Washington pontoon bridge losses, and also discuss underwriting issues related to completed pontoon bridges as well as builders' risk involving rehabbing and replacement construction of these bridges.

Hood Canal Bridge -- 1979
The Hood Canal Floating Bridge, which connected Kitsap Peninsula with the Olympic Peninsula west of Seattle, was built in 1961 at a cost of $24.6 million. The bridge was 6,520 feet long and consisted of 50 floating concrete pontoons, each 360 feet long and weighing 500 tons. At the time the bridge was built, two contracting firms issued independent statements questioning its design, and noting that maximum wind, wave and current conditions at the site could produce forces in excess of the design specifications.

The firms' fears proved to be well-founded when a vicious winter storm hit the area in February, 1979. With winds registering a sustained 87 miles per hour (mph) and gusting to 100 mph, the waves and currents produced forces that resulted in destructive stresses. The storm's intensity was the highest ever recorded at the bridge site, and caused damage to half of the pontoons. Water rushed into hatches and bolt holes, and 13 pontoons sunk to the bottom of the 300 foot deep canal. The amount of excess water in the pontoons was too much stress for the design.

Repairs were under way by 1982. The WDOT decided that the depth of the canal bottom would make rebuilding as a conventional bridge both costly and difficult. The western section of the bridge (3,800 feet long) had to be completely replaced, while the eastern section required repairs of only some of the existing pontoons.

The new bridge length would total 7,131 feet. The rebuilt bridge would be one of only five floating bridges in the world at that time, and the only one to be located over tidal waters. The new bridge section would be 10 feet wider, four feet deeper and 50% stronger than its predecessor. Anchors and pontoons would be heavier, and
the steel cables would be thicker to resist the high winds, strong currents and heavy seas which devastated the original bridge. The replacement of the western section would be the main focus of reconstruction, and the contract was bid for $60 million.

One of the most intriguing construction features of the replacement bridge project was the construction of a barge system that resembled a catamaran. The barge system was designed to provide stability for lowering the bridge anchors in place. The crane and winching systems were also custom built. The pontoons were built 600 miles away and were towed to the construction site.

Lacey V. Murrow Bridge -- 1990
The Lacey V. Murrow Floating Bridge, the first major floating concrete bridge in the world, was built in 1940 and crossed Lake Washington -- connecting Seattle and Bellevue on Mercer Island. The floating portion of the bridge consisted of 25 pontoon sections anchored in 200 feet of water by steel cables. Each pontoon was a hollow, airtight steel box encased in concrete, measuring approximately 350 feet in length, 60 feet wide and 15 feet deep, and weighing approximately 4,900 tons. The floating pontoons were bolted together to form one continuous structure, with the top surface utilized as a roadway for commercial and passenger traffic.

At the time of loss, the Murrow Bridge had been undergoing extensive rehabilitation (estimated contract cost of $35 million) and had been closed to traffic for approximately 17 months. The sinking began the morning of November 25, 1990, with pontoon "A5," centrally located in the bridge. As the pontoon settled into the water, additional water was taken on through open maintenance hatches on the north side of the bridge. Pontoon A5 sank, dragging additional sections of the bridge underwater. Within 8 hours, eight pontoons were at the bottom of Lake Washington. The remaining pontoons were towed to temporary storage sites along the lake in an effort to mitigate any additional damage.

Considerable controversy arose over the cause of loss -- with the contractor and the WDOT each pointing fingers at the other. After extensive investigations spearheaded by both sides, the dispute was ultimately resolved through a mediated settlement.

The WDOT findings indicated that the primary cause of sinking was water accumulating in the bridge pontoons during the renovation work. The sources of the water, in order of volume, were from the hydro demolition machines used to remove concrete from the pontoons, rainwater that fell on the bridge, and wave splash water. Environmental restrictions mandated that water used in the demolition process be contained in the pontoons and carted off-site for disposal, rather than discharged into the Lake. Although the plan was to limit the accumulation of water stored in the bridge at all times, significant hydro demolition work was conducted during this period. On November 22-23, a severe wind storm occurred, and on November 23-34 heavy rains (4-5 inches) fell. These circumstances, together with the age and condition of the bridge, were held by the WDOT to contribute to the failure.

The contractor's investigation revealed that the probable cause of failure was a progressive bond slip at lapped splices in the bottom slab of pontoon A5, due to fatigue in bond. The failure resulted in the widening of existing cracks, allowing water to flow into the pontoon. The deterioration was the result of cumulative damage caused by wave-induced stress reversals experienced over the 50 year life of the bridge.

While the original Murrow bridge had been designed to carry about 2,500 vehicles per day, it supported 105,000 vehicles per day just prior to its close for rehabilitation. While the bridge design remains practical its structural integrity was diminished. The replacement bridge, which cost an estimated $80 million, utilized the same design concept while incorporating the following engineering improvements:

  • Prestressed high performance silica fume concrete for pontoon fabrication
  • Larger, heavier and thicker walled pontoons with heavier rebar enforcement
  • Post tensioning of pontoons
  • "Egg carton" pontoon design for airtight interior compartments

Additional measures taken to prevent water accumulation in the pontoons included patrolling by maintenance boats outfitted with pumps for water removal, utilization of scuppers along the bridge to drain off water, and installation of high water level sensors and automatic bilge pumps on each pontoon.

Underwriters need to know what wind and flood loads a pontoon bridge design can handle. As a minimum standard, the design should meet at least a 100-year event (e.g., 100-year storm, 100-year flood).

There are other important considerations for underwriters, including:

  • How have the values been established? The potential repair costs of older pontoon bridges are often several times the original cost of the structure. How are the values updated? Are appraisals available?
  • The business interruption (or loss of use) from the loss of a bridge can be substantial. Are values reported accurately? Are there alternatives routes that could mitigate a business interruption loss?
  • Does the insured have a comprehensive maintenance program in place? Are records available for review? Are inspections conducted annually? What is the frequency of underwater inspection of the cables/anchors?
  • For new or rehabilitation of existing bridges, underwriters need to know the qualifications of the bridge design team. Do they have experience with this type of bridge? What are the qualifications of the General Contractor and the key subcontractors?

Note: For underwriting concerns relevant to other bridge types, see the 1990 IMUA paper entitled "Bridges, What Can We Learn?".

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