Above: Woodstock Vermont Covered Bridge
Covered wooden bridges once dotted the landscape of the United States, numbering 14,000 at its 19th-century peak. The timber-truss-covered bridge is a passage for vehicles and pedestrians, spanning rivers, gullies, or other impediments. Beyond its frequent appearances in cultural narratives and artwork, the covered bridge holds substantial importance in engineering history. The purpose of its distinctive roof and walls, which typically form an enclosure around most of the bridge, is to shield the wooden components from weather-related deterioration. In structural terms, a "truss" refers to a fundamental design wherein elements are assembled in triangular configurations, furnishing the necessary stability for the bridge.
In the United States, the first such bridge, known as the Waterford Bridge, was constructed by Theodore Burr in Connecticut in 1804, crossing the Hudson River in New York and standing for over a century. Subsequent bridges were erected in Oregon City, Oregon, in 1851 and 1852, but they succumbed to floods the following year. Before these bridges, river crossings were dominated by ferries that transported people, horses, and carriages, often operated by local merchants who profited significantly from the ferry fees. This monopoly led to public demand for bridges, financed initially by tolls, eventually providing free passage for all.
Initially, the primary focus of bridge construction was functionality; however, aesthetic considerations became increasingly important over time. While Connecticut is recognized for the first American-covered bridge, Pennsylvania is often the state most celebrated by enthusiasts. It was home to at least 1,500 bridges built between 1820 and 1900, and it continues to host the most extensive collection of covered bridges in the country, with 219 across 40 of its 67 counties. The record for the longest covered bridge ever built goes to one constructed in 1814 in Lancaster County, Pa., which measured an impressive 5,960 feet, extending over a mile, before its unfortunate destruction by ice and floodwaters in 1832.
The golden age of the American covered bridge building was between 1825 and 1875. By the latter part of this period, it had become standard practice to roof and enclose these structures at the time they were built to shield the trusses and decking from the elements. In addition, the covered spaces had other benefits: they prevented horses from panicking at the sight of the water below, offered travelers shelter from storms, and served as venues for a wide variety of community activities, including political and religious gatherings, town meetings, casual socializing, and even raucous celebrations. While an uncovered bridge typically had a lifespan of about 20 years, a covered bridge could be expected to stand for a century.
The National Historic Covered Bridge Preservation (NHCBP) Program is administered by the Federal Highway Administration. This program includes preservation of covered bridges that are listed or are eligible for listing on the National Register of Historic Places. It also includes research for better means of restoring and protecting covered bridges and includes an education component to disseminate information as a means of preserving our cultural heritage. In addition, the National Center for Wood Transportation Structures publishes the World Guide to Covered Bridges, which is a comprehensive guide for the traveler to wooden bridges across the United States.
The thrust of recent preservation efforts to save historic covered bridges has been the establishment of the National Historic Covered Bridge Preservation Program. One of the most significant preservation issues has been identifying preservative treatments. During the era when covered bridges were built, there were no chemical treatments available to prevent fungi and insects from attacking and colonizing the wood. Although covering a bridge keeps the wood dry and helps to prevent wood components from deterioration, over time, moisture will eventually lead to suitable conditions for colonization by wood-destroying organisms.
Strengthening historical covered bridges to carry modern traffic loads has been another aspect of the preservation effort. The preference when restoring covered bridges is to keep them open for today’s vehicular traffic even though they were not designed as such. This is often done by keeping the external appearance but reengineering the structure.
Finally, like all wooden structures, fire retardant treatments for covered bridges have been of concern. While many of the covered bridges are constructed of large timbers with inherent fire-resistive characteristics due to their size, engineers sometimes deem additional protections necessary.
While they may be architectural relics, covered bridges also represent a bit of Americana that are still visible for the traveler to enjoy.