Float-in of complete arches and spans

How bridges get built · Cable-supported and arch bridges · Float-in of complete arches and spans

Also called: float-in erection, barge-assembled arch, whole-span float-in

Float-in of complete arches and spans — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Assemble the span on a quay, slipway or barges near the site, completing as much as the barge capacity allows (bracing, deck plate, sometimes the deck slab).
Prepare the permanent piers with landing bearings or temporary seats, guides and jacking points, and survey them against the as-built span length at a reference temperature.
Load out onto barges by SPMT, skidding or floating off; check barge stability, ballast plan, moorings and the span's stresses on its temporary supports.
Tow at slack water within the forecast window, using tugs and winches to warp the barges between the piers, with abort points along the route.
Position over the piers and set down by ballasting the barges, lowering with jacks on the barges, or lifting with strand jacks on the piers where the span must rise above barge level.
Transfer the load to the bearings, release and remove the barges before the tide or current changes, then connect joints and approaches.

Key numbers

Spans moved wholeabout 80–300 m tied arches and trusses
Weightsabout 1,000–10,000 t (Fremont centre span about 5,400 t)
Weather windowweather-restricted operations planned within a reliable forecast of up to about 72 hours (DNV)
Bargesabout 5,000–20,000 t displacement
Marine operationUSD 1–5 million (indicative)

Where it fits

Tied arches and trusses of 100-300 m over navigable water, harbour crossings, replacement spans.

Where it does not

Rough water, sites without an assembly area, spans too heavy for available barges.

Choosing it

Choose float-in over piece-by-piece erection when navigation must stay open except for one short closure and an assembly site is within towing distance.
Prefer SPMT or a lateral slide when the crossing is over land or the water is too shallow for loaded barges.
Prefer a floating crane when the span is within crane capacity: less ballast engineering and fewer marine days.
Avoid it where currents are strong or the weather window is unreliable, unless the operation can be held safely partway.

Plant, pace and money

PlantBarges 5,000-20,000 t, tugs, ballast systems, winches, weather forecasting.

ProductivityMonths of assembly, one day of float-in.

CostAssembly on the bank is cheaper than in the air; the marine operation USD 1-5 million.

Risks and controls

What goes wrong

Weather during the tow, barge stability, misalignment on landing.

Quality assurance

Naval architecture check of the tow, float-in procedure with abort criteria, survey on landing.

Origins

Robert Stephenson floated the wrought-iron tubes of the Britannia Bridge (1850) into position on pontoons and raised them with hydraulic presses; floating complete spans became routine for river trusses and arches in the 20th century, the Fremont Bridge lift (1973) being a landmark.

Examples

Fremont Bridge (Portland) centre span, several Dutch and German river arches, Chinese Yangtze tied arches.

Case studies

Fremont Bridge, PortlandUSA · 1973The 275 m centre section of the tied arch, about 5,400 t, was assembled on Swan Island, floated up the Willamette and jacked up about 52 m into place, a record lift at the time.

Providence River BridgeUSA · 2007Network tied arch of about 120 m assembled away from the site and floated up Narragansett Bay on barges to its piers.

Lake Champlain BridgeUSA · 2011Modified network tied arch centre span of about 120 m assembled on shore, floated on barges and raised onto its piers by strand jacks.

In the bridge game

What Bridge Master applies when you pick it

Deck types: Steel arch, Steel truss, Steel box girder, Network tied arch. Spans 100-250 m, up to 300 m.

+30: Tied arch over a channel: float it in.
-45: No draft for barges.
-20: Fast current: positioning a floated arch is a day-long gamble.
-25: A one-off operation for one span: on a multi-span viaduct it is the exception for the main span, not the method for the deck.

Try it in the game →