That was the 1912 Winkley Bridge across the Little Red River at Heber Springs, Arkansas. Between that name and the ivy, it seems rather twee, but this story has a very bad end. (And please stop reading now if that’s a problem for you.) The bridge was photographed by HAER in August 1988, and it collapsed abruptly in October 1989. It had been closed to vehicular traffic since 1972, but was a recreation and tourist draw, including being used as a place to sit and fish in the river below.
The collapse killed five people and injured eighteen others. Given that this was a short-span bridge near a town with a population of fewer than 10,000, that shows the popularity of the bridge as a promenade spot. The cause of the collapse hinges on three facts, each of which is by itself disturbing. First, the cables were not wrapped, so that water entry was a regular occurrence. The bridge had been inspected, but you can only see the surface strands in a bundle, and there was every reason to believe that the interior strands would be damaged. The post-collapse review showed extensive rusting of the cables.
Second, there was pretty much no bracing in the deck against movement. The cables supported vertical suspenders, which supported deck beams running side to side, which supported stringers running longitudinally, which supported the wood deck plank. There was no vertical deck stiffening of any kind, and nothing to brace the cables against lateral motion. What did this mean in practice? Another local name for the structure was “the Swinging Bridge.”
Note that a wood deck like this one has pretty much no inherent stiffness. A modern deck – a steel grating or a concrete slab, for example – would have been much stiffer in the horizontal plane. That in itself is not enough, as there would still be no deck stiffening in the vertical direction, but it would have helped. Even a tongue-and-groove or splined plank deck would have been better.
The third fact is the most disturbing: people deliberately made the problems worse because they didn’t understand them. The reports from the collapse was that a group of people intentionally got the bridge swinging from side to side; that precipitated progressive collapse by over stressing the rusted strands in one or both cables. As each partially-rusted strand failed, more load was thrown into its neighbors. Unlike in movies, where this kind of failure proceeds with each strand breaking at a roughly even pace until the last strand takes longer to snap (see, for example the opening scene with an elevator in Speed), in reality the failure accelerates as it goes along, because the load in the remaining strands keeps getting bigger. After the first few strands break, the rest may happen so quickly that it seems to observers like they all break simultaneously. I don’t blame a random collection of people for not understanding structural mechanics, but I have to wonder at people who are okay with one bridge that moves so easily when other bridges in their experience do not. I guess I’m too much the engineer to understand someone thinking “it’s called the swinging bridge but it’s not moving…let’s make it swing.”
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