I’ve written little about the Eads Bridge in St. Louis, largely because it’s a well-researched topic and I don’t see that I have much to add. That said, the picture above shows an important aspect of not just that bridge, but many similar wrought-iron and steel arch bridges: they may be shaped like masonry bridges, but their structural action is quite different.
The main structure fo the Eads Bridge is a set of tubular arches at the bottom of the superstructure, with each tube riveted together from steel plate. The tube section is a good one for arch compression, being resistant to lateral buckling. Everything above that – the top vehicle and pedestrian deck, the lower rail deck, and the lattice spandrel structure – basically sits on the arches. The spandrel structure was designed with some resistance to lateral load (although, again, it ultimately relies on the arches) but nothing much in terms of longitudinal load-carrying capacity. A close-up of the spandrel (with, serendipitously, the monumental Gateway Arch in the background) makes this clear:
The built-up spandrel vertical struts have their strong axis facing across the deck, to resist lateral wind pressure, and are very slender parallel to the deck. They’re also pinned (for longitudinal rotation) to the arches, which would reduce their efficiency if they were intended to act as pieces of a supplementary moment frame.
In a masonry arch bridge, the spandrels are to some degree part of the main structure. Depending on how the bridge is built (whether or not it has a rubble or fill core as opposed to solid ashlar or brick masonry, whether or not the deck has any strength of its own or is simply sitting on the masonry below) the spandrels may be integral with the strength and stiffness of the arch or may simply be carrying a portion of the load. The easiest way to describe the difference is a little mental demolition: the main structure of the Eads Bridge, the arches, would be just as strong if the spandrels were removed (assuming that the arched tubes were still braced against lateral movement), while removing the spandrels on most masonry arch bridges weakens the bridge. It’s impossible to separate the discussion of strength from a discussion of stiffness: the Eads spandrels do not stiffen the arches for the deck load because the spandrel has so little longitudinal stiffness of its own, while in a masonry bridge, the spandrels are very stiff in-plane and therefore may contribute to the arch stiffness.
In short, an arch isn’t just an arch. Ductile-metal arches work differently than masonry arches.
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