From Ohio History Central
S-Bridges were a type of bridge used on the National Road. The National Road was one of the first paved (gravel) roads to cross the Appalachian Mountains. The United States Congress authorized construction of the road in 1806. In 1811, the Congress awarded contracts to private builders to construct the road. The War of 1812 prevented construction from beginning until 1815. Construction began in Cumberland, Maryland, and the contractors completed the road to Wheeling, Virginia (modern-day West Virginia), in 1817. Eventually, the federal government extended the road to near St. Louis, Missouri. Construction of the National Road occurred in Ohio from 1825 to 1838.
The S-Bridge, when viewed from above, resembles the letter "S." Workers made the bridges out of cut stone, which proved to be a more durable material than wood. The reason for the unusual shape was to make construction easier for the workers. When the National Road crossed a creek or stream at an angle, the workers built the bridge's supporting arches at a right angle to the stream. This process allowed water to flow through the arches more easily and also allowed workers easier access to build the bridges from each side of a creek or stream. Some people claim that the S-Bridges were designed to stop runaway horses, but there is no supporting evidence for this claim. A total of five S-Bridges existed along the National Road in Ohio.