The Stocky Approach

I’ve mentioned several times, including yesterday, my preference in bridges for through trusses (where the deck is aligned near or at the bottom chords and the trusses are connected to one another top and bottom) or deck trusses (where the deck is aligned near or at the top chords and the trusses are connected to one another top and bottom) over pony trusses where the deck is near the bottom chords but the trusses are too short to be connected at the top. The picture above* shows a bridge over the Illinois River for the Chicago & Alton Railroad, built 1901 or so. The bridge is near Pearl, Illinois, which has never had a population greater than 1000 and was around 700 when the bridge was built.

As you can see from the picture above, the main spans, off in the distance, are through trusses and the approach spans are pony trusses. That kind of differentiation was, and still is, common in bridge design, where engineers choose the most efficient form based on the load and span. It’s the pony trusses on the short spans that caught my eye with respect to this bridge. Here’s a good view**:

This is a heavy and well-designed bridge. Note the diagonal wind/sway bracing under the deck. The problem with pony trusses is that the unbraced top chords can buckle sideways and, being in compression, are likely to buckle sideways unless something is done to prevent that. One way of dealing with this design problem is to greatly oversize the top chords so that the stress is low, which reduces the likelihood of buckling. The other is to provide some form of lateral bracing for the top chord, which is what we have here: each vertical in the Warren truss pattern has a miniature truss outboard of it. Those miniature trusses are designed as cantilevers loaded at their tips (the top chord) by the buckling load of the top chord; their maximum moment is at the deck level and is transmitted into the cross-deck beams.

Those braces represent a lot of fabrication and riveted connections of small pieces of steel, in addition to that already needed for the built-up main truss members. The other option was to make the trusses taller and in a through configuration, like the main spans. That would have allowed the main members*** to be lighter and eliminated the need for the little vertical cantilevers by substituting top-chord bracing. I would not be at all surprised to hear that the option I’m suggesting was considered and this one chosen because it had fewer total pounds of steel.

Some of the bridges I’ve shown here are quite slender in appearance, while this one is stocky. It’s a design choice rather than a black and white issue. The main spans were replaced by a lift bridge in 1978, but the pony approach spans remain in use.

* I’ve cropped the picture. I was going to crop it more to show just the bridge, but decided that the houseboat needed to stay.

** I saw this picture first, then went looking for other information.

*** The top and bottom chords, the verticals, and the diagonals.

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