Thetford Center's "ingenious and ambitious" Sayre Bridge

The years between 1825 and 1875 saw the heyday of covered bridge construction in the U.S. Indeed, Vermont is still blessed with many surviving covered bridges. Our state places third in the nation for its current number of bridges (100) after Pennsylvania (227) and Ohio (143). Between the U.S. and Canada, about 20,000 wooden bridges, most of them covered, had been built by 1958, and around 1,100 are still in use today. The rest have been destroyed by floods, fires, neglect, and, sadly, vandalism.

The use of wood for bridges dates back to when the U.S. was first colonized. Almost everything the settlers built was made of wood, a material that was abundantly available, lighter than stone, and far cheaper than iron or steel. Wood is also very strong, although it is prone to rot if left exposed.

In this age of high-tech, we think of covered bridges as simple, antiquated, and nostalgic. However, their construction paved the way (no pun intended) for the development of engineering innovation in the U.S. The engineering challenge that builders faced was how to cross rivers with bridges that were long enough and strong enough to be of practical use. While engineering schools began to appear in the 1800s, a major contribution to bridge design came from master builders and craftsmen who drew on empirical knowledge of what worked based on personal experience and history.  

While it is assumed that any bridge with a roof is a covered bridge, the term actually refers to a specific type of construction defined by a timber truss. A truss is a frame composed of triangles – a shape that is inherently rigid and strong. Each side of a covered bridge is one truss, typically containing many triangles. Bridge components have to stand up to several forces, the chief ones being compression and tension. In a covered bridge, the timber running along the top of the truss (known as a chord) is in compression, while its counterpart chord at the base of the truss is in tension. Wood is very strong under tension, but long pieces of wood can fail under compression (buckling). The job of the truss is to distribute compression forces to other parts of the structure so the weight of the bridge deck is safely supported. In many cases this is accomplished by overlapping triangles that distribute forces more efficiently and enhance overall stability. This design could handle heavy loads and long spans in excess of 200 ft. It was a huge advance over simple beam bridges. The inherent weakness of wood beams over long distances means they are best suited for short to medium spans, and they cannot handle heavy loads or high traffic. 

As bridge building progressed, the complexity of truss design and the discipline of structural engineering evolved as one. Engineers patented new truss designs, critiqued the competition, and worked steadfastly towards improvement. They analysed existing bridges for deflection (sagging), changes in camber, and wear and tear on structural connections, all to improve the design of wooden (and soon metal) trusses. The National Society for the Preservation of Covered Bridges lists no less than 17 different truss designs used in covered bridges. The simplest truss is the kingpost truss, dating back to the Middle Ages, followed by the queenpost truss used in the Italian Renaissance. Then, from the 1770's on, came a flurry of patented designs.

At the very bottom of the list, there is a category "Unique Trusses."  Here we find two entries, one being the Sayre Bridge "in Orange County, VT." 

What makes the Sayre Bridge unique is its fusion of different truss configurations. 

The book "Covered Bridges of the Northeast" comments that some Vermont bridges do not rely on patented designs "but instead demonstrate the creativity of Vermont's carpenter craftsmen." It speculates that "a desire to experiment may have been irresistible." 

The Sayre Bridge, built around 1840, has been variously thought to incorporate elements of the Haupt Truss, designed by Colonel Herman Haupt (1817-1905), builder of the U.S. Military Railroad in the South for Union forces; the Lattice Truss designed by Ithiel Town (1784-1844); and the truss with arch, designed by Theodore Burr (1771-1882). 

Another  book. "Covered Bridges of Vermont." describes the Sayre Bridge as "an ingenious and ambitious structure.” It goes on to remark, “Perhaps the massiveness of the bridge was overly ambitious. In 1963 four steel beams were added underneath to help it carry heavy traffic ...."  plus a concrete pier in mid-river and a nail-laminated deck. Note that by 1963 the Sayre bridge had stood for well over 100 years. And it was never designed to carry the ten-wheeler trucks loaded with gravel that now cross it on a routine basis. "In any event, there is certainly no other bridge in Vermont, or any of the other New England states, with a truss that begins to resemble this one." Only one other bridge with a similar truss exists, in North Carolina.

The defining roof of a covered bridge was a feature that prolonged the bridge's life. Without it a wood bridge would rot in about 20 years. But with a protective roof, bridge longevity was extended to upwards of 100 years. The roof is more than a cover; it also serves to reinforce the trusses, allowing for a longer bridge. There was one disadvantage however. In the winter, many people traveled by horse-drawn sled. In order for sleds to glide through a covered bridge, snow had to be shoveled into the bridge interior.

At the time of building, the vicinity of the Sayre Bridge was alive with water-powered industry, harnessing the force of the waterfalls directly downstream. Saw mills, grist mills, and a woolen mill operated at various times. The bridge’s name derives from a nearby business founded in 1883 by brothers Oramel Sylvester Sayre and Charles Wesley Sayre, who crafted extension tables and offered “carriage & sleigh repairs, custom planing and jobbing.”  

Today, the Sayre bridge is still a vital component of Thetford's transportation infrastructure. It carries upwards of 900 vehicles a day over the east branch of the Ompompanoosuc River on the busy road connecting Rt 132 to Rt 113 and the all-important Interstate. We should be grateful to the unknown builder whose ingenuity, and perhaps ambition, have bestowed on us this unique reminder of our cultural and engineering heritage.