The famous covered bridge trusses have names attached. Theodore Burr had his arch, Ithiel Town his lattice, William Howe his iron rods. Nobody patented the kingpost. It was already old when the first American covered bridge went up, and the National Society for the Preservation of Covered Bridges traces it to the Middle Ages. A county that needed to get a wagon across a 30 foot creek did not need a patent holder. It needed a carpenter.
The kingpost: one triangle, one post
A kingpost truss is a triangle with a post down the middle. Two diagonal timbers lean in from the ends of the span and meet at the top of that post. The diagonals work in compression, pushing the load out to the abutments. The post itself, despite the name, mostly hangs: it works in tension, holding up the middle of the bottom chord and the floor with it.
The limit is the length of timber you can find. The Federal Highway Administration's Covered Bridge Manual puts the simplest version at roughly 25 to 30 feet, and calls 50 feet the more common upper limit for the type. When that manual was published in 2005, it counted only about 30 kingpost covered bridges left in the country.
Packsaddle Covered Bridge in Somerset County, Pennsylvania, shows the form about as plainly as any. It is 48 feet long, crosses Brush Creek on cut stone abutments, dates to 1870 in the National Register record, and still carries traffic.
Photo: CC BY 2.0, via Wikimedia Commons (author details on the file page).
Shorter still is Roddy Road Covered Bridge near Thurmont, Maryland, at 39 feet. What stands there now is a replica. Oversized trucks hit the original twice in a month in 2016, and the timber was too damaged to reuse, so the bridge was rebuilt new with "headache bars" at both ends. A kingpost can carry a farm wagon. It was never asked to stop a box truck.
Builders stretched the idea when they had to. Trostletown Covered Bridge, also in Somerset County, gets 104 feet out of the kingpost by using three spans in a row, with tension rods added later. It sits in the Stoystown Lions Club park and is open to foot traffic only.
The queenpost: pull the triangle apart
Take a kingpost, cut it down the middle, slide the halves apart and join them with a horizontal beam. You now have two posts, called queenposts, and a truss the Federal Highway Administration describes as a stretched-out version of the kingpost. Typical spans run 40 to 60 feet, and the manual counted about 101 survivors, a little over a tenth of all American covered bridges.
Vermont's northern counties are full of them. Power House Covered Bridge in Johnson is a 63 foot queenpost over the Gihon River, first built in 1872. In 2000 a heavy snow load dropped its roof and walls into the river. The deck stayed put, thanks to steel beams added in 1995, and the covered structure was rebuilt and reopened on June 29, 2002.
Photo: Steven Bergeron Srbergeron, CC BY-SA 3.0, via Wikimedia Commons.
Greenbanks Hollow Covered Bridge in Danville, built in 1886 over Joes Brook, runs 74 feet, which is long for the type. Its two trusses are not even the same length: the eastern one is a foot and a half longer than the western.
Telling the two trusses apart sounds easy, and usually is. Then there is Kidder Covered Bridge in Grafton, Vermont. The Vermont Covered Bridge Society lists it as a kingpost, the only one in the state that still has its sway braces. Wikipedia describes the same 67 foot bridge as a pair of queenpost trusses. I have no intention of settling that from a desk. Go look at it and count the posts.
Multiple kingpost: keep adding panels
For a longer crossing, a builder could simply repeat the pattern: a row of vertical posts, each braced by a diagonal leaning toward the center of the span. That is the multiple kingpost, good for spans from 36 to 124 feet according to the manual, which counted about 95 of them.
The best place to see the type is the First Branch of the White River in Tunbridge, Vermont. Arthur C. Adams built Cilley Covered Bridge there in 1883 for $523.35, and he was still at it in 1902 when he put up Larkin Covered Bridge a few miles upstream. Both are 68 feet long, and both still carry one lane of traffic.
Photo: Ahale, CC BY-SA 4.0, via Wikimedia Commons.
Add an arch to a multiple kingpost and you are most of the way to what Burr patented. Taftsville Covered Bridge in Woodstock, finished in 1836, does roughly that across 189 feet and two spans, though its design follows no patent and its resemblance to a Burr truss is only skin deep.
Why they lasted
None of these trusses made anyone rich, and that is the point. There were no royalties to pay and nothing exotic to source, so they kept getting built long after the patent trusses arrived. Adams was framing them in Vermont in the twentieth century. If you want to understand how a covered bridge holds itself up, start with a small one, where you can see the whole idea from the portal.
Cover photo: Greenbanks Hollow Covered Bridge by Magicpiano, CC BY-SA 3.0, via Wikimedia Commons
Sources: FHWA Covered Bridge Manual, Chapter 4, National Society for the Preservation of Covered Bridges: Trusses, Vermont Covered Bridge Society: Kidder Hill Covered Bridge