Kings and Queens

The short version: a king-post truss is basically a gable with a post in the center; a queen-post truss has two posts and with a rectangular panel between them. There are all sorts of variations on both themes, but here’s a pair of bridges to provide a nice visual comparison. First, the 1918 Marlow Road Bridge, over some railroad tracks in Clinton, Tennessee:

In most forms of king-post truss, the central vertical isn’t really a post: it’s more often than not in tension rather than compression. But “king-hanger truss” sounds bad. Here’s the 1930 (!)Taylor Bridge, carrying Bayfield Road over the Middle River in Amnicon Falls, Wisconsin:

Again, the verticals are hangers.

Note that both bridges are pony trusses – king-post bridges are almost always pony trusses – and use extended cross-deck beams as a way to mount diagonal braces to stabilize the tops of the trusses.

Neither HAER description mentions an engineer, although the Marlow bridge was constructed by a railroad. It’s entirely possible that either or both of these were built by carpenters based on older rural bridges or roof trusses and no explicit structural design was involved. Since neither bridge was removed because of overload, there’s at least empirical evidence of adequate design.

There’s a fuzzy line in structural engineering for buildings: when you’re talking about small enough buildings, the engineering can be done entirely empirically. For example, ordinary houses use wood joist, rafter, and stud sizes from tables and typical details that have been handed down and refined for decades. We all accept that the structural engineering of such buildings doesn’t require detailed design and for the most part things work out okay. It never occurred to me that a similar fuzzy line exists for bridges, but maybe it does. These small timber bridges are okay for short spans and light loads.

Regarding the map above: the city of Brooklyn, conterminous with Kings County, is the far west end of Long Island, left of the first heavy red line. In 1898, twelve years after this map was drawn, Brooklyn and the Queens County towns of Long Island City, Newtown, Flushing, and Jamaica joined into New York City. Eventually, Queens County was split into two pieces, and the towns of Hempstead, North Hempstead, and Oyster Bay were organized into Nassau County. There is, to my knowledge, zero relationship between king- and queen-post trusses and the names Kings and Queens counties.

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  1. Pingback: Kings and Queens — Old Structures Engineering – The Bridgehunter's Chronicles

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