Ambient Technology

The trap when talking about things is to talk about the superlatives: the biggest ship, the longest bridge, the tallest tower, and so on. Those things are interesting but they get talked about all the time. As much as I like the Empire State Building, I’m pretty sure that if no one wrote anything new about it for a few years, its fame and reputation would remain intact. In writing about early skyscrapers I found myself drawn to those buildings that are not famous, because they told me something about how the new technology of steel skeleton framing was used in ordinary circumstances.

Here’a a bridge example of that phenomenon. The bridge above is, according to the Library of Congress, “Enloe Bridge No. 90021, Spanning Red River of North between Minnesota & North Dakota on County State Aid Highway 28, Wolverton, Wilkin County, MN.” The documentation says that Enloe is a small settlement near the bridge on the North Dakota side, but it seems to be so small as to be a name without much else attached to it. This is a small road bridge over a small (at this location) river, in a rural area, constructed in 1917. This is not a place where cutting-edge design and construction techniques would be used, and therefore represents the diffusion of steel-truss technology over the decades before construction. A through Pratt truss design is about as generic as gets.

Given the short span and the light loads, the truss elements are very light, with rod tension and wind-bracing diagonals and built-up box compression members. The deck was always just wood plank.

The bridge was designed and built by the Great Northern Bridge Company, out of Minneapolis. The actual work on site somehow manages to feel both practiced and quite casual:

The small size of the river allowed the use of temporary supports sitting on the river bed, which certainly made erecting the superstructure easier. For a company like Great Northern, this bridge was as easy as work could get: a familiar design at a small scale. It was arguably past its expiration date. By 1917, steel and concrete girder bridges were beginning to displace truss designs for short spans like this. My internal definition of “vernacular architecture” is what people build when they don’t think about what to build; if there’s such a thing as vernacular bridge design, this is probably it. This bridge was built with technology that was at that time roughly 60 years old (in the US) and at a small scale.

This story has a happier ending than most: the bridge was closed in 1990 after being damaged by a flood the previous year. It was recognized as a (locally) historic structure, so it was moved off the river in 1994 rather than being demolished; after sitting around for 14 years, it was relocated for use as a footbridge on a trail.

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  1. Pingback: Ambient Technology — Old Structures Engineering – The Bridgehunter's Chronicles

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