Union Arch Bridge

The Union Arch Bridge, also called the Cabin John Bridge and the Cabin John Aqueduct, was constructed from 1857 to 1864 as part of the Washington Aqueduct. It was the longest span stone masonry arch in the world at that point, and it held that record until 1903. It is still the 33rd longest in the world, and the longest in the United States. The Union Arch Bridge is also still an active part of the Washington Aqueduct.

The bridge was originally designed and built by the U.S. Army Corps of Engineers as an aqueduct to bring drinking water from Great Falls, Maryland to Washington D.C. The main arch ring is built of cut and dressed granite, and the secondary arch ring is radially laid sandstone. The facade is also constructed from ashlar sandstone and hides the main structure of the bridge. There are five spandrel aches along the west approach, and four spandrel arches along the east approach. These spandrel arches transmit the load to the main arch, and the space between them is actually hollow!

The site of this arch created many challenges during construction. The right-of-way for the aqueduct was very narrow, making overland transport very difficult. Therefore, the creek was dammed, and a lock was built to connect the flooded valley to the nearby C & O canal. Barges were used to transport the massive arch ring stones from the quarry to the bridge site.

The bridge was built to carry the aqueduct, not traffic. It originally did not have a parapet or any safety measures, but people began using the bridge to reach the nearby town of Cabin John. A parapet was added in 1872, and daredevils would ride bicycles across the top of it. In 2001, the roadway was renovated due to vehicles damaging the unreinforced stone parapet. A concrete curb and wrought iron railings were added, as well as a concrete barrier between the pedestrians and traffic. The Capital Beltway was constructed under the Union Arch Bridge in the 1960s, but it did not require any modifications to the bridge.