Through arch

Through arch bridge

Sydney Harbour Bridge, a bridge of this type.
Ancestor	arch bridge
Carries	Pedestrians, vehicles, light rail, sometimes heavy rail
Span range	Medium
Material	Steel
Movable	No
Design effort	High
Falsework required	Seldom

A through arch bridge, also known as a half-through arch bridge and through-type arch bridge, is a bridge made from materials such as steel or reinforced concrete in which the base of an arch structure is below the deck, but the top rises above it, so the deck passes through the arch. Cables or beams in tension suspend the central part of the deck from the arch.

For a specific construction method, especially for masonry arches, the proportions of the arch remain similar no matter what the size: wider arches are thus required to be taller arches. For a semi-circular arch, the height is half of the span. Bridges across deep, narrow gorges can have their arch placed entirely beneath a flat roadway, but bridges in flatter country rise above their road approaches. A wide bridge may require an arch so tall as to become a significant obstacle and incline for the roadway. Small bridges can be hump-backed, but larger bridges such as the Old Bridge, Pontypridd may become so steep as to require steps, making their use for wheeled traffic difficult. Railways also find arched bridges difficult as they are even less tolerant of inclines. Where simple arched bridges are used for railways on flat terrain the cost of building long approach embankments may be considerable.

Further issues are the foundations for the bridge. Arch bridges generate large side thrusts on their footings and so may require a solid bedrock foundation. Flattening the arch shape to avoid the humpback problem, such as for Brunel's Maidenhead bridge, increases this side thrust. It is often impossible to achieve a flat enough arch, simply owing to the limitations of the foundations - particularly in flat country. Historically, such bridges often became viaducts of multiple small arches.

With the availability of iron or concrete as structural materials, it became possible to construct a through arch bridge: a bridge where the deck does not have to be carried over the top of the arch. This requires a structure that can both support the deck from the arch by tension rods, chains or cables and allow a gap in the arch, so the deck can pass through it. The first of these in particular cannot be achieved with masonry construction and requires wrought iron or steel.