Small And Cute

Like a few other pictures of bucolic scenes I’ve posted here, this is the Bronx River, probably a little bit after 1900. The countryside is not quite as untouched as it might seem: that waterfall is actually a weir, and this area was already part of Bronx Park. But there’s nothing wrong with an urban park imitating the wilderness, as the north end of Central Park shows to this day.

In a city full of navigable rivers, small islands, and bays, the Bronx River was never much used for transportation. In its natural state, it was too shallow and rocky to be of much use; the various dams thrown across it (three exist today) were more for creating water supplies and hydropower than for navigation.

What really catches my eye on this photo is the pedestrian bridge (as far as I know, long gone) downstream of the weir. Here’s a blow-up:

The couple on the right seem to be enjoying the view. This appears to be a mostly-wood gable truss. The bottom chords are continuous for the full span, and the verticals are steel rods, connected to a cross-deck girder that passes below the bottom chords. I am reluctant to call this a king-post truss because calling rods “posts” doesn’t work for me. In a real king-post truss, where the verticals are roughly the same size as the chords (and therefore look like posts), they can carry tension or compression, as opposed to the rods here that can only take tension. Note that not only are rods unsuited to compression, but the rod connections look one-sided, and may be able to move if subjected to compression.

What this form of truss really reminds me of is a tension-rod girder, upside down. A tension-rod girder reinforces a beam by adding a lower tension chord tied to the beam at the ends and at one or two intermediate points; here we have a beam reinforced by adding an upper compression chord tied to the beam at the center. That comparison may seem odd, but it’s just another way of saying we categorize structures according to how we see them, and there are only so many ways for the forces within them to act.

In any case, cute little dam, cute little bridge.

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