Building an arch bridge isn't easy, since the structure is completely unstable until the two spans meet in the middle. For years, engineers used a technique called centering, in which a wooden form supported both spans until they locked together at the top.
http://www.pbs.org/wgbh/buildingbig/bridge/arch_built.html
The arch is an prime example of Roman technology that is still used world wide even though modern materials are now used. A basic example of the construction of a typical Roman Stone Arch is shown below. As described on previous sheets, a wood frame was first constructed in the shape of an arch.
http://www.technologystudent.com/struct1/roman1.htm
Clad in brilliant stainless steel, the Gateway Arch rises high above the west bank of the Mississippi River at the spot where St. Louis was founded. It’s both the tallest man-made monument in the United States and the tallest arch in the world.
http://wonderopolis.org/wonder/how-was-the-st-louis-arch-built/
Construction began in February 1963 and was completed in October 1965. The total cost to build the Gateway Arch was about $13 million, which would equal about $97 million in today’s dollars. The primary construction materials used to build the Gateway Arch were steel and concrete. The outside skin consists of ¼-inch stainless steel. There is also an inner layer of 3/8-inch structural steel with a layer of concrete between the layers of steel.
http://wonderopolis.org/wonder/how-was-the-st-louis-arch-built/
The first recorded Roman triumphal arches were set up in the time of the Roman Republic. Generals who were granted a triumph were termed triumphators and would erect fornices or honorific arches bearing statues to commemorate their victories. A number of fornices were built in Rome during the Republican era.
https://en.wikipedia.org/wiki/Triumphal_arch
The Romans were the first builders in Europe, perhaps the first in the world, fully to appreciate the advantages of the arch, the vault and the dome.
https://en.wikipedia.org/wiki/Arch
ATTENTION: The Gateway Arch Ticketing & Visitor Center has relocated to the Old Courthouse at 11 N. 4th Street. EACH guest planning to enter the Gateway Arch facility will be required to have a timed ticket.
http://www.gatewayarch.com/about/history/arch-faqs-facts.aspx
The wood frame could then be removed and the arch was left in position. Stone arch technology was used even on Roman monuments such as the Colosseum in Rome. Many of the monuments built with stone arch technology can still be seen today, such is the strength of the arch. Stone arches are not built entirely from stone. Stone is as expensive today as it was in Roman times.
http://www.technologystudent.com/struct1/roman1.htm
Arch of Galerius and Rotunda. The arch was built in 298 to 299 AD and dedicated in 303 AD to celebrate the victory of the tetrarch Galerius over the Sassanid Persians and capture of their capital Ctesiphon in 298.
https://en.wikipedia.org/wiki/Triumphal_arch
Most Roman triumphal arches were built during the imperial period. By the fourth century AD there were 36 such arches in Rome, of which three have survived-the Arch of Titus (AD 81), the Arch of Septimius Severus (203-205) and the Arch of Constantine (312). Numerous arches were built elsewhere in the Roman Empire.
https://en.wikipedia.org/wiki/Triumphal_arch
how were arches built
The primary construction materials used to build the Gateway Arch were steel and concrete. The outside skin consists of ¼-inch stainless steel. There is also an inner layer of 3/8-inch structural steel with a layer of concrete between the layers of steel.