Prior to 1850, just a few iron truss bridges were erected in the United States. Truss bridges became a popular style of bridge from the 1870s until the 1930s. There are still examples of these bridges around the United States, but their numbers are gradually decreasing as they are dismantled and replaced with modern constructions. The Norfolk Southern Railway has several hundred truss bridges across its system of rail lines in the East Coast region of the United States.
What is so special about truss bridges? They are simple but very strong because each piece of the structure is tied to at least two other pieces by means of diagonal members called trusses. This type of bridge is especially suitable for railways because their heavy traffic requires a sturdy construction. Also, truss bridges can be built rather quickly since there's no need to nail or weld together separate parts of the frame. Last but not least, they're easy to repair or replace if needed.
Here in Switzerland we have quite some experience with truss bridges. In fact, there are four types of bridges that are based on truss design: road-rail, foot-bridge, cycle-path and tramway bridge. The last one is actually a hybrid between a truss and a suspension bridge design. It uses vertical posts with rubber tires on them instead of horizontal beams under the deck like in a truss bridge. However, it looks like a truss bridge from the outside because it has transverse members connecting the posts together.
In the mid-1700s, Switzerland pioneered modern-style timber truss bridges. In the United States, around 14,000 covered bridges were constructed, the majority of which were constructed between 1825 and 1875. This makes covered bridges the most common type of wooden bridge in the country.
Covered bridges are particularly common in rural areas because they provide protection from the elements for their occupants: people walking or riding horses over rivers, streams, or other bodies of water.
During the American Civil War, many cities across the country ordered the destruction of all covered bridges in an effort to prevent them being used by the Confederate army as fortifications or supply depots. After the war ended, communities began rebuilding their covered bridges, often using materials salvaged from other destroyed bridges. Today, there are more covered bridges in America than at any other time in history.
The first known covered bridge in North America was built in 1764 near present-day Richmond, Virginia. The Carter Family Covered Bridge was made of wood and had a length of about 30 feet (9 m). It was dismantled after only seven years due to severe damage caused by heavy traffic crossing it. No covered bridges were built in North America after this one until 1825 when the Buckner Bridge in Switzerland was erected.
Beginning in the late 1700s, builders in the United States widely developed wooden truss bridges, and by the mid-1800s, this country dominated the globe in wooden truss bridge design (Steinman and Watson 1957:114). A number of reasons led to the United States' rapid increase in wooden truss architecture. First, as technology advanced, so did the ability to manufacture more sophisticated structures. For example, by 1825, Philadelphia-based engineers had developed a completely new type of truss bridge that is still in use today: the Pratt through truss. Second, there was demand for large bridges across large bodies of water, which required heavy timber and strong ties to hold the bridge together.
By contrast, Europe's early truss bridges were mostly made from steel, because heavy timber was scarce there. But when steel became available in larger quantities after 1850, it quickly replaced wood as the material of choice for truss bridges.
The American wooden truss bridge has two main types: the bowstring truss and the Warren truss. Both are formed of orthogonal members connected with joints that have pins or webs. The most important difference between them is the way they connect to the land: the Warren uses stile and rail, while the bowstring relies on a single vertical post called a kingpost that connects to the road surface with a horizontal member called a stringer.