The word ‘pontoon’ is derived from the Latin word for a ferryboat (‘ponto’) and the word for bridge (‘pons’). Pontoon bridges, also known as floating bridges, consist of interconnected floating pontoons or shallow-draft vessels, over which is laid a deck or track for the easy transportation of vehicles and/or pedestrians. These bridges have been constructed since ancient times, and their earliest recorded use was in Ancient China in the 11th century BC.
Different Types of Pontoon Bridges
Although pontoon bridges can be built as permanent structures, they are more often used in times of civil emergency or wartime. These temporary structures are made of more lightweight materials such as rubber, wood or inflatable metal tubes. Nowadays they often come pre-fabricated in sections which then have to be connected on site, but this makes them compact and easy to transport. The main disadvantage of their lightweight construction is that they are easily dislodged or damaged in bad weather or if their load is exceeded.
Permanent pontoon bridges might be constructed in locations where the soil at the bottom of the waterway is unsuitable for supporting the piers on which traditional bridges are built. They are also preferable for sheltered crossings where the construction of a bridge with anchored piers is not economically viable as the expanse of water is too deep and/or wide. Permanent floating bridges are built of more robust materials such as concrete and asphalt over a metal framework. Concrete makes the pontoon bridge more resistant to damage caused by saltwater corrosion and can also absorb the vibrations from traffic crossing the bridge or the impact of waves much better than temporary structures.
Both permanent and temporary bridges are usually built with an anti-skid surface. This makes their use possible even in adverse weather conditions and for heavy-duty traffic.
The Design of Pontoon Bridges
Before starting construction, a civil engineer has to consider the topography and purpose of the pontoon bridge. Locations with strong winds and/or waves are not suitable for these bridges. The connection of the bridge to the shoreline should also be taken into account. The approach should not be too steep while the shoreline should be protected from soil erosion. The bridge should be given freedom to move slightly especially during changes in water level due to the ebb and flow of the tide.
The natural buoyancy of the water supports a pontoon bridge. However, the size of the load it can support is limited to the amount of water which is displaced in its construction, and the mass of the bridge itself must be included in the calculation of the load it can bear. If the load is exceeded, the bridge will sway and some pontoons might even separate and become submerged. Pontoon bridges can be continuous (with pontoons along their whole length) or separate (when the pontoons are placed at regular intervals.) The pontoons themselves are kept in place by both heavyweight anchors and mooring lines.
To avoid disruption to waterborne traffic, a pontoon bridge can be constructed to allow vessels to pass underneath it. Alternatively, it can have a drawbridge function when overhead traffic is temporarily stopped while a section of the bridge is elevated or even removed.