If you've flown anywhere in the last few years, you probably — unwittingly — walked through a covered bridge, what airlines call a "jetbridge," the moveable corridor that links the terminal with the plane. We don't usually think of a jetbridge as a covered bridge, of course — that's reserved for those lovely single-lane timber structures, mostly built in the 1800s before steel and reinforced concrete became commonplace. Some have survived to the present day. Of about 14,000 covered bridges built in this country, about 700 are still in use. The rest have given way to more durable (and wider) concrete and steel structures.

An uncovered timber bridge might have a lifetime of 20-odd years due to the rotting effects of sun and rain. But covered, a wooden bridge might last for a century or more, since its load-bearing braces and struts are protected from the weather. Some of the oldest extant ones, mostly located in the northeastern states, are still in use, such as the Hassenplug, Hyde Hall and Roberts bridges, in Pennsylvania, New York and Ohio, respectively. They were all built in the 1820s.

Around 1840, bridge designers realized they could increase a bridge's load bearing capacity while lowering the cost by incorporating steel rods into the wooden structure. The members, or main structures, of a truss bridge, which is essentially composed of a series of triangular units, are either in tension or compression. In 1840, William Howe, a contractor in Massachusetts, patented a new and efficient bridge design. A Howe truss consists of upper and lower horizontal chords with triangular truss panels between them, with the addition of vertical ties to counter tension in each panel. With wood being cheap and abundant, a typical Howe truss of the 1800s used timber elements other than for the vertical (tensioned) members, which were wrought iron rods.

Locally, we have three county-maintained covered bridges right here in Humboldt, all of which incorporate Howe's truss design. Local historian Jerry Rohde and I doubt a Nov. 24, 2009 story in the Times-Standard claiming the oldest, Zane's, was originally built across the Elk River in 1910 to serve the lumber town of Falk. In fact, it was built by the Works Progress Administration in 1936. Zane's nearby twin, Berta's bridge, also completed in 1936, also spans the Elk. It has the distinction of being the westernmost covered bridge in the U.S. Zane and Berta are both 52 feet long.

Humboldt's third covered bridge, located just off Jacoby Creek Road, is the 66-foot-long Brookwood Bridge, completed in 1969. Created to link the Brookwood subdivision with Jacoby Creek Road, this was the brainchild of two local residents. James Roscoe, one-time chair of then Humboldt State University's engineering department, and real estate agent Earl Biehn spent five years planning, designing and supervising construction of the structure. It cost $19,600 in 1969 dollars, the equivalent of about $200,000 today.

For the sake of completeness, I should mention Humboldt's fourth covered bridge, a private one in Fruitvale on the Dyerville Loop Road. Do readers know of any that I've missed?

Barry Evans (he/him, barryevans9@yahoo.com) designed bridges in a former life. None made of wood, however.