COVERED BRIDGE TERMINOLOGY

While researching the covered bridge topic we chose for this issue of The American Antiquities Journal, I was having difficulty understanding a lot of the terminology used in describing covered bridges. So in order to assist our readers in understanding those terms, I have compiled a list of terminology and descriptions.

One of the first, and most used, terms in describing a covered bridge is the word "truss." Truss is a common and general word used in the construction trade meaning a structural framework that provides support. We usually see trusses used on the roofs of houses, however they are used in many different applications. Trusses are typically (but not necessarily) composed of triangles because of the structural stability of that shape and design. A triangle is the simplest geometric figure that will not change shape when the lengths of the sides are fixed and stress is applied.

One way that a covered bridge is identified and described is by the type of truss used in its construction. Truss design determines how long a span the bridge can have and how complex it is to build. Local residents can often construct simple designs that cross small streams. More complex and multi-span bridges require more experienced builders. Types of trusses are usually named for the men who design them.

The oldest covered bridge design is the kingpost. It is the simplest, most common, yet most limited (40-ft span) type of truss construction. The kingpost truss, however, can be used in multiples to span longer lengths.

The queenpost truss forms an elongated, topless triangle with support posts at each end. Also a simple design, it allows for greater bridge lengths than the kingpost.

Theodore Burr of New York patented the Burr Arch truss in 1804. It incorporates reinforced arches that tie directly into the bridge abutment (the concrete or rock pillar that supports the ends of a bridge at the shoreline) with a series of triangular support posts. It allowed bridges to span lengths over 100 feet for the first time.

Colonel Stephen H. Long developed what became know as the "X" truss.

Connecticut architect Ithiel Town patented his truss of crisscrossed diagonals or lattice in 1820.

Builder William Howe introduced iron into wooden truss design by substituting adjustable iron rods for the vertical members of Long’s truss.

Robert W. Smith developed trusses in three variations of his basic design in 1867 and 1869.

Rueben L Partridge received a patent for a design that was very similar to Smith’s truss.

Horace Childs’ truss simply added diagonal iron rods to a multiple kingpost design.

Two Englishmen patented a truss using isosceles triangles and called it the Warren Truss.

There is also a Warren Truss Plus Arch.

As early as 1805 German designer extraordinaire, Lewis Wernwag constructed a single span bridge across Pennsylvania’s Schuykill River, a span of 340 feet. The bridge was so enormous that rumors spread all across Philadelphia that the bridge would collapse as soon as the scaffolding was removed. Thousands of people lined the river to watch the catastrophe. To demonstrate faith in his project, Wernwag removed the supports himself. The bridge proudly stood for 26 years until it was destroyed by fire.

At least two covered bridges make the claim of being the first built in the United States. Town records for Swanzey, New Hampshire, indicate their Carleton Bridge was built in 1789, but this remains unverified. Philadelphia claims a bridge built in the early 1800s on 30th Street and over the Schuylkill River was the first, noting that investors wanted it covered to extend its life

According to Covered Bridges Today (Daring Books, 1989) by Brenda Krekler, as many as 12,000 covered bridges once existed in the United States, but that number dropped to under 1500 by the 1950s. The National Society for the Preservation of Covered Bridges was formed in 1950.