A floating bridge is one which crosses a river, estuary or lake and is supported by pontoons, instead of fixed permanent supports. The first historical records of such bridges date back to the Persians and Greek where the deck was supported on pigskins or boats. These bridges were built for military purposes. One such bridge was built across the Bosporus spanning 1km by Darius in 493BC. The Persians constructed a similar bridge over the River Euphrates in 536BC.
This type of bridge became very popular down through the ages, particularly where installing fixed foundations was difficult. In addition, the technology evolved because of the benefits to military operations. Such bridges could be assembled on a shoreline and moved over into position with the minimum of labour and machinery in a short period of time. When the bridge is in position the pontoons are anchored to the river bed to ensure stability. The top of the pontoons may act as the road carriageway or a truss, which is continuous across the pontoons, may also be used.
On waterways, where there are large volumes of shipping, an opening span can be incorporated in the centre to allow passage upstream and downstream. In early times the pontoons were actually boats made of timber. This practice evolved into steel pontoons but nowadays concrete is also used where practicable.
The main disadvantage of this type of bridge is the high cost of maintenance and its susceptibility to damage from weather events and shipping. In addition the anchoring has to cater for the rise and fall of the water levels. As a result very few such bridges have been constructed in recent years. The exception is Lake Washington adjacent to Seattle in Washington State, USA, where a number of such bridges in excess of 2kms in length have been built in recent years. The pontoons are fabricated from pre-stressed concrete in a dry dock, they are then floated into position and fixed to the previous one with bolted or pre-stressed connections.
These bridges, very often incorporate an opening span. This can be achieved by moving one of the pontoons out of position. In the Washington State bridges it is achieved in two ways, namely by creating two lines of pontoons so that the carriageways in each direction diverge. The pontoon, where the carriageways merge together can then be floated into the space between the parallel pontoons. The alternative is that one pontoon is lifted vertically and the next one floated in beneath it thus giving passage for ships.