My working definition of “vernacular” architecture or engineering is “what people build when architects and engineers aren’t there to tell them they’re wrong.” It’s nearly always functional and reasonably safe, because people, apparently, have a desire for functional and safe buildings. There are obviously limits to this type of design and construction, but I don’t often see examples of people exceeding those limits. I came across the bridge above, the Chow Chow Suspension Bridge over the Quinault River in Washington state by accident. I was looking at the “suspension bridge” entries in the HABS/HAER index and there it was. It’s not a suspension bridge using strict categorization: it’s a cable-stayed bridge. But when it was built, around 1950, the term “suspension bridge” was still often used for the newish cable-stayed form.
The design is less strange than it looks at first glance. The side trusses consist of four heavy-timber king-post trusses end to end, supporting a series of heavy-timber deck girders. At the three places where two of the king-post trusses meet, there’s a deck girder supported by a cable that runs over both towers and then down to anchorages past the towers. If, instead of king-post trusses, there were big girders spanning between the cable-supported deck girders, no one would bat at eye at the design. It looks funny because it seems like there’s a top chord missing that should connect the truss peaks, or that the builders thought they could span four times as far as a king-post truss by ganging four of them together.
There is a lot to recommend this design, built by the Native Americans of the Quinault Indian Reservation for their own use. Except for the verticals of the towers, there are no particularly large pieces of wood needed, and no large pieces of steel at all. The king-post trusses have fewer complex connections than a more typical Warren or Pratt stiffening truss would have. The detail of looping the stay cables under the deck girders provides direct support with relatively simple connections.
In case you think the designers were unsophisticated, here’s a view of the underside of the deck.
They combined two heavy timbers for each deck girder, but simply laying one piece of wood on top of the other is much less strong than if the two are actually joined. So they’ve provided little Pratt trusses in steel to join each pair of timbers together.
It did not perform well, and had three “collapses,” which I suspect were partial collapses, and was demolished after the third incident in 1988. A design like this has little damping, and the wood connections, which were vulnerable to rot and insect attack, would be damaged by excessive movement.
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