Floating (pontoon) bridge construction and towing

How bridges get built · Heavy lift, moving and accelerated construction · Floating (pontoon) bridge construction and towing

Also called: pontoon bridge, floating bridge, concrete pontoon bridge, float-out and tow of pontoons

How it is done

Design pontoons, moorings and transitions together: pontoon size and freeboard, compartments arranged so that flooding of one cell cannot sink the bridge, anchor type (gravity, fluke or pile) and spacing, and transition spans that absorb water-level changes at each shore.
Cast the concrete pontoons in batches in a graving dock or casting basin (or fabricate steel pontoons in a yard), post-tension them where specified and pressure-test every compartment.
Install anchors and anchor cables in the water beforehand, with buoys marking each cable end, and prove anchor capacity by test pulls.
Flood the basin, float out the pontoons and tow them to site within an agreed weather window along sheltered routes or through locks; hold them at a mooring area until needed.
Join the pontoons afloat into the bridge string, aligning them with winches and tugs and connecting them by post-tensioning or bolted or welded joints; attach and pretension the anchor cables.
Build the superstructure on the pontoons, adjusting ballast so the string floats level, then connect the transition spans and any navigation opening.
Commission monitoring (leak detection, bilge pumps, anchor tension, motion sensors) and inspect every compartment before opening.

Key numbers

Floating lengthabout 300–2,400 m (Evergreen Point / SR 520, about 2,350 m)
Pontoonsconcrete pontoons up to about 110 m × 23 m × 8.5 m and 10,000 t (SR 520 longitudinal pontoons)
Water depthtypically 30–500 m where piers would be uneconomic
Design lifetypically 75–100 years for permanent pontoon bridges
CostUSD 8,000–15,000 per m² of deck (indicative)

Where it fits

Deep lakes, fjords and sheltered sounds of about 30–500 m depth or with very soft beds; moderate waves and currents; temporary and military crossings; shores where transition spans can absorb water-level change.

Where it does not

Open sea with large waves, strong tidal currents, heavy ice, busy navigation without a drawspan or high transition, water bodies with large level variation and no room for long transition spans.

Choosing it

Choose a floating bridge over a fixed bridge or tunnel when the water is very deep or soft-bottomed, waves are moderate, and navigation can be served by a drawspan, a high transition or a gap.
Prefer a fixed bridge on piers where foundations are affordable; prefer an immersed or bored tunnel where navigation must be unobstructed or waves and ice are severe.
Use continuous pontoons with side anchors for lakes of moderate fetch (Washington State); discrete pontoons under a curved girder anchored only at its ends for deep fjords where side anchors are impractical (Bergsøysund, Nordhordland).
Budget for maintenance and storm management: hatches, bilge systems and watertight compartments are life-safety items.

Plant, pace and money

PlantGraving dock or casting basin, concrete and post-tensioning plant, tugs and towing gear, anchor-handling vessels and anchors (gravity blocks, fluke or pile anchors), winches, ballast and bilge systems, survey.

ProductivityPontoon casting in batches over one to three years; towing and joining a few pontoons per week in good weather; superstructure on the floating string at conventional deck pace.

CostIndicative: USD 8,000–15,000 per m² of deck for permanent concrete pontoon bridges including moorings; a dedicated casting facility can be a major item.

Risks and controls

What goes wrong

Flooding of pontoons through open hatches or damaged compartments (Hood Canal 1979, Lacey V. Murrow 1990), anchor cable failure, storms beyond the construction-stage design, leaks from cracked concrete, ship collision, fatigue at pontoon joints and transitions.

Quality assurance

Watertightness tests of every compartment, crack control and post-tensioning records, anchor test pulls and cable tension monitoring, tow and mooring analysis to marine operations standards, storm procedures with hatches closed.

Origins

Bridges of boats are ancient (Xerxes crossed the Hellespont on one in 480 BC) and military pontoon bridging was refined through the 20th century; the first permanent concrete floating bridge was the Lacey V. Murrow Bridge on Lake Washington (1940), and Washington State still has most of the world's longest floating bridges.

Examples

Lacey V. Murrow (1940), Hood Canal (1961, west half rebuilt 1982, east half replaced 2009), Evergreen Point / SR 520 (2016, about 2,350 m), Bergsøysund (1992) and Nordhordland (1994) in Norway, and the Yumemai floating swing bridge in Osaka (2001).

Case studies

Lacey V. Murrow Memorial BridgeUSA · 1940First concrete pontoon bridge, on Lake Washington; it sank in a storm in November 1990 during renovation, after water entered pontoons through openings made for the work.

Hood Canal BridgeUSA · 1961The western half sank in a storm in February 1979 and was rebuilt by 1982; the eastern half was replaced in 2009 with new pontoons towed to site.

Evergreen Point Floating Bridge (SR 520)USA · 2016About 2,350 m floating length on 77 concrete pontoons, the longest floating bridge; the largest pontoons, about 110 m long and 10,000 t, were cast in a basin at Aberdeen and towed to Lake Washington.

Nordhordland BridgeNorway · 1994About 1,250 m floating section on discrete concrete pontoons carrying a curved steel girder anchored only at its ends.