The picture above is best viewed with the picture below, to explain the change in focus. (If you were ever a fan of The Outer Limits, “We are controlling transmission….We will control the horizontal. We will control the vertical. We can roll the image; make it flutter. We can change the focus to a soft blur or sharpen it to crystal clarity…”)

The pictures compare the reality of Bridge 28 in Central Park with some of the field notes taken by Tim Michiels and Mel McCloy during their conditions investigation. The bridge overall is in pretty good shape, although there are some nagging issues that will have to be addressed. Part of how we determine if a particular defect is a problem is to look at the load path and see if the defect interferes with the load-carrying capability of any member or if the bridge as a whole. In this case, we were asking a pretty basic question: is this an arch?

It looks like an arch, but does it act like one? In the top picture, where the actual bridge is in focus, you can see some decorative cast iron right at the peak of the curve. It hides a joint: the main arch was cast in pieces and (it seems) erected in two halves, with the halves meeting at that joint. On one side of the bridge deck – on one arch – that joint was tight, with the cast iron meeting at a hairline. On the other side, there was a gap with only a small splice bar attached with a single bolt connecting the two half-arches.

Here are a few possibilities: It’s an arch but the compression is so small that the splice bar is enough to carry the force across. It’s not an arch but rather a pair of cantilevers that meet at the center. It was meant to be an arch but the cantilever is accidentally strong enough to carry the load when one side was erected misaligned. It’s a cantilever when lightly loaded but the gap closes when heavily loaded (or when thermal expansion does its trick) allowing it to act as an arch. There are more possible explanations but I’ve probably worn out my welcome here.

Things to keep in mind with this type of discussion: The bridge is functioning with no sign of non-weathering overstress, so whichever load path it has is empirically adequate. The bridge is quite thoughtless – cast and wrought iron, and a wood deck, but no neurons – so it doesn’t care what models we make of it. The actual state of stress in the iron is difficult to figure out without disassembly. Our repair scope depends on having a pretty good idea of the state of stress.

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