Going Maybe Too Far

This bridge, opened in 1897 and replaced in 1923, has an appealing name: it was the Free Bridge, carrying Little Rock’s Main Street across the Arkansas River. The name was descriptive of a crossing rather than inspirational: the purpose of the bridge was to provide free crossings. It was replaced, as was so common in that era, because twentieth-century traffic exceeded the design loads.

It’s hard to tell from the picture above, but the main span of the bridge is a Pennsylvania truss. It’s a cousin of the Baltimore truss (and both are named after the railroads that most promoted their use): a Pratt truss with subdivided panels to reduce the unbraced length of the web members. The Baltimore truss is a subdivided parallel-chord Pratt truss; the Pennsylvania truss is a subdivided bowstring Pratt truss. I generally prefer parallel-chord through trusses because I like full wind bracing between the top (compression) chords, but the designers of the Free Bridge, the Groton Bridge Company of New York, cheated. You can see it clearly in this postcard:

The truss form is a compromise between the Baltimore and Pennsylvania forms, with a segmental-curve top chord, but one that never drops very far down until the portal frames at the bridge ends. This is an intelligent design that allows full top-chord wind bracing, but it still feels like cheating somehow.

In any case, the reason the picture at the top caught my eye is that the distinction between the tension and compression members. The compression members are built-up boxes, largely solid at the top and bottom chords and open lattices at the verticals. The tension members are eyebars with rectangular cross-sections. So far, quite average. Where things get weird is that the designers apparently very carefully tracked tension versus compression, so that some of the verticals change from built-up boxes to flat bars at the mid-height location where the sub-dividing diagonals intersect. (Look at the far left of the top photo for an example.) This is not necessary for the structural design, as the built-up boxes can work in tension; it’s simply a way to be more-than-usually efficient in saving steel material and fabrication costs.

It looks odd. And not just because I understand the engineering behind the issue: having the bigger structural element physically above the thinner one looks odd to everybody. I find it hard to believe that, on a project this size, saving the expense of the built-up box on six verticals was even a rounding error in the budget, but someone took the common engineering search for efficiency to an extreme.

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  1. Pingback: Going Maybe Too Far — Old Structures Engineering – The Bridgehunter's Chronicles

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