Arch

The arch was the first bridge structure understood by man. The strength and suitability of the arch was initially developed in Mesopotamia and then transferred around the world. Its strength as a resistant structure was quickly recognised as the Indian proverb explains; “the arch never sleeps”. Arch bridges work by directly transferring the weight of the bridge and its loads into a thrust restrained by abutments at either side. The earliest bridges of this type date from 1300 BC with the Mycenaean Arkadiko bridge in Greece being an early example.

Stone, brick and other such materials are strong in compression and somewhat so in shear, but cannot resist much force in tension. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary timber frame, known as a centring. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together and the mortar is allowed to set before the falsework is removed.

The shape of the arch has also evolved over many years from semi-circular to elliptical with a low rise/span ratio. In addition the single arch bridge evolved to multiple arches. When using multiple arches the location and the gradient of approach roads is critical. Increasing the number of arches also creates construction complexity with temporary supports in waterways whilst erecting them. In the long term a large number of support piers restricts the width of the river and increases the flow through the bridge giving rise to the potential of erosion around the bridge foundations. This method has fallen from favour in recent times.

When the arch is used as a pedestrian bridge it is possible to use the actual arch for walking on although steps may be needed if too steep. Where the arch has to carry a roadway then a secondary structure has to be supported by the arch. This additional structure is the road deck and is designed to cater for the restricted gradients imposed by the traffic or trains using the bridge. The space between the road deck and the arch particularly at the abutment was initially in-filled with material such as stone or rubble. The introduction of steel and concrete saw this material replaced by vertical walls. This latter construction gives the appearance of lightness and visibility through the bridge.

In more recent times the advantages of arch construction has been expanded. This is best illustrated in a situation where it is not possible to position the road on top of the arch so the roadway is placed at the base of the arch. The arch in these situations is actually two arches usually constructed along either side of the roadway. The road deck is then either suspended or supported by the arch depending on whether it is below or above the arch. James Joyce Bridge at Blackhall Place in Dublin is built in this manner.

One of the longest steel arch bridges in the world is the New River Gorge Bridge in the USA with a span of 518 metres and 267 metres above the valley bottom.