A through arch bridge, also known as a through-type arch bridge, is a

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually someth ...

that is 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. It can either be lower bearing or mid-bearing. Thus, the deck is within the arch, and cables or beams that are in tension suspend the central part of the deck from the arch.

Function

For a specific construction method, especially for

masonry Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...

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 The Old Bridge ( cy, Yr Hen Bont), which is now also known as the William Edwards Bridge or Pontypridd Bridge, was originally known as the New Bridge or Newbridge, is an arched single-span footbridge that spans the River Taff at Pontypridd in ...

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

viaduct A viaduct is a specific type of bridge that consists of a series of arches, piers or columns supporting a long elevated railway or road. Typically a viaduct connects two points of roughly equal elevation, allowing direct overpass across a wide v ...

s 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 Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...

or steel. The use of a through arch does not change the proportions or size of the arch: a large span will still require a tall arch, although this can now reach any height above the deck without obstructing traffic. The arch may also reach downwards at its sides, to either reach strong foundations or to place the roadway at a convenient height for spanning a deep valley from a plateau above. The

Tyne Bridge The Tyne Bridge is a through arch bridge over the River Tyne in North East England, linking Newcastle upon Tyne and Gateshead. The bridge was designed by the engineering firm Mott, Hay and Anderson, who later designed the Forth Road Bridge, ...

demonstrates both of these advantages.

Notable examples

A well-known example of this type is the

Sydney Harbour Bridge The Sydney Harbour Bridge is a steel through arch bridge in Sydney, spanning Port Jackson, Sydney Harbour from the Sydney central business district, central business district (CBD) to the North Shore (Sydney), North Shore. The view of the bridg ...

in Australia, which is based on the

Hell Gate Bridge The Hell Gate Bridge, originally the New York Connecting Railroad Bridge or the East River Arch Bridge, is a steel through arch railroad bridge in New York City. Originally built for four tracks, the bridge currently carries two tracks of Amtr ...

New York City New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...

. Other bridges include the Chaotianmen Bridge in China, the world's longest through arch bridge;

Newcastle upon Tyne Newcastle upon Tyne ( RP: , ), or simply Newcastle, is a city and metropolitan borough in Tyne and Wear, England. The city is located on the River Tyne's northern bank and forms the largest part of the Tyneside built-up area. Newcastle is ...

; the

Bayonne Bridge Bayonne Bridge is an arch bridge spanning the Kill Van Kull and connecting Bayonne, New Jersey with Staten Island in New York City. It carries New York State Route 440 (NY 440) and New Jersey Route 440. It is the sixth-longes ...

that connects New York City to

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delawa ...

, which is longer than the Sydney Harbour Bridge; the Ahwaz White Bridge; the

Bourne Bridge The Bourne Bridge in Bourne, Massachusetts carries Route 28 across the Cape Cod Canal, connecting Cape Cod with the rest of Massachusetts. It won the American Institute of Steel Construction's Class "A" Award of Merit as the "Most Beautiful S ...

and

Sagamore Bridge The Sagamore Bridge in Sagamore, Massachusetts carries Route 6 and the Claire Saltonstall Bikeway across the Cape Cod Canal, connecting Cape Cod with the mainland of Massachusetts. It is the more northeastern of two automobile canal crossings, ...

, smaller, near-twin bridges over the

Cape Cod Canal The Cape Cod Canal is an artificial waterway in the U.S. state of Massachusetts connecting Cape Cod Bay in the north to Buzzards Bay in the south, and is part of the Atlantic Intracoastal Waterway. The approximately canal traverses the neck ...

; the Pennybacker Bridge in

Austin Austin is the capital city of the U.S. state of Texas, as well as the seat and largest city of Travis County, with portions extending into Hays and Williamson counties. Incorporated on December 27, 1839, it is the 11th-most-populous city ...

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

and as the Hernando de Soto Bridge in

Memphis, Tennessee Memphis is a city in the U.S. state of Tennessee. It is the seat of Shelby County in the southwest part of the state; it is situated along the Mississippi River. With a population of 633,104 at the 2020 U.S. census, Memphis is the second-mo ...

Wylam Railway Bridge Wylam Railway Bridge (officially West Wylam Bridge, also known as Hagg Bank Bridge and locally as Points Bridge and Half-Moon Bridge) is a footbridge and former railway bridge crossing the River Tyne at Hagg Bank, approximately west of Wylam ...

is an early through arch bridge upstream of the Tyne Bridge. File:Chaotianmen Bridge-1.jpg, The Chaotianmen Bridge, China, the 2nd longest steel arch bridge in the world. File:Hell Gate Bridge ca 1917.png, The

, New York, shortly after its completion. File:High BB from Bayonne jeh.jpg, The

, the 6th longest steel arch bridge in the world. File:Tyne_Bridge.jpg,

, opened in 1928. File:AhwazWhiteBridge.jpg, The Ahwaz White Bridge, built in the 1930s. File:Sydney Harbour Bridge from Circular Quay.jpg,

, the 9th longest through arch bridge. File:Douglass Bridge 090621.jpg, Frederick Douglass Memorial Bridge in Washington, D.C.

Arch rib arrangements

The through arch bridge usually consists of two ribs, although there are examples like the

Hulme Arch Bridge The Hulme Arch Bridge in Hulme, Manchester, England, supports Stretford Road as it passes over Princess Road, and is located at grid reference . The construction of the bridge formed part of the regeneration of the Hulme district of Manchester ...

of through arches with a single rib. When the two arches are built in

parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster o ...

planes, the structure is a parallel rib arch bridge. When the two arch ribs lean together and shorten the distance between the arches near the top, the span is a basket handle arch bridge.

Tied-arch bridges

Many

tied-arch bridge A tied-arch bridge is an arch bridge in which the outward horizontal forces of the arch(es) caused by tension at the arch ends to a foundation are countered by equal tension of its own gravity plus any element of the total deck structure such grea ...

s are also through-arch bridges. As well as tying the side-loads of the arch, the tension member is also at a convenient height to form the bridge deck, as for a through-arch. The converse is not true: through-arch bridges do not imply that they are tied-arch bridges, unless they also provide the deliberate tension member that is the key to a tied-arch. Although ''visually'' similar, tied- and untied- through-arch bridges are quite distinct structurally and are unrelated in how they distribute their loads. In particular,

cast iron Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuri ...

bridges such as the Stanley Ferry Aqueduct may resemble tied-arch bridges, but as cast iron is weak in tension they are not structurally a tied arch.

Construction sequence

Frederick Douglass Bridge construction 2020b

In some locations it is not practical to support the arch from beneath during construction. In modern construction, temporary towers are erected and supported by cables anchored in the ground. Temporary cables fly from each side to support arch segments as they are constructed. When the arches are almost complete a jacking bridge is placed over or beneath the gap to force the arches apart, whence the final section is constructed in place or lifted into position. Runcorn-Widnes bridge - geograph

In some cases, this type of arch has been created by constructing

cantilever A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cant ...

s from each side, with the shoreside ends bolted securely down into heavy piers. The incomplete channel ends are then constructed toward each other and either filled by construction or by lifting a prefabricated center section. This type of construction was used in the

illustrated above, with the supporting cables to the higher side of the arches removed after completion.

References