The primary structural function of a bridge cover is to protect the trusses and deck from the elements. Because exposed wooden bridge superstructures are prone to decay, covering and roofing the bridges shield the trusses from the elements, allowing them to survive longer. Bridge covers also provide protection for people walking or riding under the structure who would otherwise have direct contact with the wood.
Covered bridges are more expensive to build than open bridges because there's no way to use standard construction techniques like piers and abutments, so unusual care must be taken to ensure that the bridge remains stable even while being used by vehicles. The size of the opening in the center allows only one vehicle at a time, which increases the cost of building and maintaining them. However, the overall stability of the bridge is increased because there's no way for any one part of the bridge to fail—if one side sags, the other side pushes it back into place.
Open-spanned bridges can be built using standard construction practices but often lack the protection afforded by a cover. This type of bridge is most commonly found in rural areas where they serve as functional replacements for destroyed or outdated covered bridges. Open bridges are usually less expensive to build than their covered counterparts because they don't require structural changes or additions to the existing roadbed. They do, however, lack some of the safety features provided by a cover frame.
The covering's aim is to shield the wooden structural parts from the elements. Because of the impacts of rain and sun, uncovered wooden bridges normally have a lifespan of about 20 years, but a covered bridge might endure over 100 years. Once frequent, just around one out of every ten survived the twentieth century. Today, with much of its trade lost to auto traffic, many people don't know that America has more than 300 covered bridges left. They are found mainly in rural areas, especially Pennsylvania, New York, and Vermont.
Covered bridges were originally built as shelters for wagon trains crossing the American Frontier. As roads became paved and vehicles increased in number, need for these structures declined. However, they remain popular today with tourists who enjoy viewing their artistry and community with open-air concerts and other events under the coverings.
Most covered bridges were constructed between 1825 and 1885 by local residents using materials available in their communities. Although wood is now used instead, most covered bridges still contain some original material such as timber framing, shingling, and floorboards. Some builders even included their own touches like decorative painting and carving. Over time, these bridges have been extended upward with floors added and new rooms created by members of the public who wanted smaller versions of our home office or school room.
There are two types of bridges covered over: those with flat roofs and those with pitched roofs.
Covered bridges are often constructed using longitudinal timber trusses that serve as the bridge's backbone. Some were constructed as railway bridges, with extremely massive timbers and doubled-up lattice work. Others were designed to carry automobile traffic, with smaller timbers and no special reinforcement in the center.
Before the advent of steel beams, wood was the only material available for large structures. Covered bridges were popular from the 1840s until the 1920s, when they were replaced by modern bridges. Today, fewer than 50 remain in America.
Although many factors contributed to their demise, one of the main causes was the inability to drive over them. Since automobiles did not exist until around 100 years ago, there was no market for roads without vehicle access. Even though bridges may have been built for horses or carts, once they were abandoned they became part of the river landscape and were used by other travelers who came later. Without vehicles to disturb their peace, covered bridges were left alone for generations, slowly decaying into history.
Until recently, historians believed that covered bridges were built using traditional construction methods which included using hand tools and masonary skills to cut and fit the many pieces of wood needed to build a bridge.