A barrel vault, also known as a tunnel vault or a wagon vault, is an architectural element formed by the extrusion of a single curve (or pair of curves, in the case of a pointed barrel vault) along a given distance. The curves are typically circular in shape, lending a semi-cylindrical appearance to the total design. The barrel vault is the simplest form of a vault: effectively a series of arches placed side by side, i.e., one after another. It is a form of barrel roof.
As with all arch-based constructions, there is an outward thrust generated against the walls underneath a barrel vault. There are several mechanisms for absorbing this thrust. One is, of course, to make the walls exceedingly thick and strong - this is a primitive and sometimes unacceptable method. A more elegant method is to build two or more vaults parallel to each other; the forces of their outward thrusts will thus negate each other. This method was most often used in construction of churches, where several vaulted naves ran parallel down the length of the building. However, the outer walls of the outermost vault would still have to be quite strong or reinforced by buttressing. The third and most elegant mechanism to resist the lateral thrust was to create an intersection of two barrel vaults at right angles, thus forming a groin vault.
Barrel vaults are known from Ancient Egypt, and were used extensively in Roman architecture. They were also used to replace the Cloaca Maxima with a system of underground sewers. Early barrel vault designs occur in northern Europe, Turkey, Morocco and other regions. In medieval Europe the barrel vault was an important element of stone construction in monasteries, castles, tower houses and other structures. This form of design is observed in cellars, crypts, long hallways, cloisters and even great halls.
Union Station is a former train station in Seattle, Washington, USA, constructed between 1910 and 1911 to serve the Union Pacific Railroad and the Milwaukee Road. It was originally named Oregon and Washington Station, after a subsidiary line of the Union Pacific. Located at the corner of S. Jackson Street and 4th Avenue S. in the Pioneer Square neighborhood, the station opened on May 20, 1911. The Milwaukee Road discontinued passenger service to Union Station 50 years later, on May 22, 1961, and the Union Pacific followed suit on April 30, 1971. With no passenger rail service serving Seattle from Union Station, the building remained largely empty. After nearly 30 years of sitting idle, the station finally experienced an expansive renovation supported by Nitze-Stagen with financial backing from Paul Allen. The Union Station renovation was the winner of the 2000 National Historic Preservation Award. It now serves as the headquarters of Sound Transit; its grand hall is rented out to the public for weddings and other events.
In Seattle, the term Union Station refers not only to the main station building, but also to the several adjacent office buildings at 505, 605, 625 and 705 5th Avenue South. Until 2011, Amazon.com was a major tenant of these properties, all but one owned by Opus Northwest, and the other by Vulcan. The entire complex is earthquake-proofed by an underground ring of rubber.