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1.4. DESIGN

When designing a pontoon bridge, the civil engineer must take into consideration

the Archimedes' principle. Each pontoon can support a load equal to the mass of the water
that it displaces. This load includes the mass of the bridge and the pontoon itself. If the

maximum load of a bridge section is exceeded, one or more pontoons become submerged.

Flexible connections have to allow for one section of the bridge to be weighted down more
heavily than the other parts. The roadway across the pontoons should be relatively light, so

as not to limit the carrying capacity of the pontoons.

The connection of the bridge to shore requires the design of approaches that are not too

steep, protect the bank from erosion and provide for movements of the bridge during (tidal)
changes of the water level.

Floating bridges were historically constructed using wood. Pontoons were formed by
simply lashing several barrels together, by rafts of timbers, or by using boats. Each bridge

section consisted of one or more pontoons, which were maneuvered into position and then
anchored underwater or on land. The pontoons were linked together using

wooden stringers called balks. The balks were covered by a series of cross planks

called chesses to form the road surface, and the chesses were secured with side guard rails.

A floating bridge can be built in a series of sections, starting from an anchored point on the

shore. Modern pontoon bridges usually use pre-fabricated floating structures.

Most pontoon bridges are designed for temporary use, but bridges across water bodies with

a constant water level can remain in place much longer. Hobart Bridge, a long pontoon
bridge built 1943 in Hobart, was only replaced after 21 years. The fourth Galata Bridge that

spans the Golden Horn in Istanbul, Turkey was built in 1912 and operated for 80 years.

Provisional and lightweight pontoon bridge are easily damaged. The bridge can be
dislodged or inundated when the load limit of the bridge is exceeded. The bridge can be

induced to sway or oscillate in a hazardous manner from the swell, from a storm, a flood

or a fast moving load. Ice or floating objects (flotsam) can accumulate on the pontoons,
increasing the drag from river current and potentially damaging the bridge. See below for

floating pontoon failures and disasters.

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