An Artifact Of Old Developments

From my Portland trip, a view of an abandoned railroad bridge. It wasn’t possible to get a good angle for the photo but there are three trusses and room for two tracks; the bridge deck is gone except for the crossbeams.

Each truss has three main (rectangular) panels and two end (triangular) panels; the connections are riveted, and the bracing pattern forms what’s known as a Baltimore truss. This truss type is a variation on the patented Pratt truss, where the diagonals are sloped away from the bridge midspan to force the verticals into compression and the diagonals into tension. The Baltimore truss “subdivides” the lower half of the diagonals and bottom third with secondary diagonals and verticals. This provides extra bracing and reduces the span of the lower chord segments.

This truss, and its sibling the Pennsylvania truss, were developed by the engineering designers of the Pennsylvania Railroad in the 1870s. At that time, they may well have been the best civilian engineering group in the United States, which is to say that they didn’t do things for no reason. That leads to the question: why this truss form? The subdivided panels are helpful, but they mean a lot of extra riveting and a lot of extra connections at funny angles, particularly when the bridge path is skewed as it is in my road-trip example.

The short answer is the limitations on metal-rolling technology of the mid-1800s. At the time this truss was developed, bridge builders were switching over from wrought iron to steel, but the mills couldn’t yet roll very large beam sections or even very large plates. So heavily-loaded members had to be assembled, built-up, from smaller pieces. The whole logic behind building truss bridges for short spans like this rather than using beam-style bridges was to effectively use small pieces of iron or steel. None of the pieces of steel in that bridge above are very big, which was necessary to work with the iron and steel technologies of the era. We don’t build short-span steel trusses of any kind any more* because we now have big wide-flange beam sections available and because we can use welding to make even bigger wide-flange shapes if we need to.

* Unless an owner or architect asks for that oldey-timey look.