Simple Conceptually, Difficult To Build

That’s an action shot of the construction of the Lethbridge Viaduct by the Canadian Pacific Railway, in southern Alberta, sometime between mid-1908 and early 1909. Everything about this is fantastic, in both meanings of the word.

The viaduct, like most such structures, is quite simple. A series of braced towers sit on foundations in the valley, and relatively short simple-span girder bridges span between the towers. In this case, the maximum height of the viaduct is 314 feet above the Oldman River; there are 33 towers and 67 bridges (ranging from 67 to 107 feet in length) for a combined length of over a mile. (If the count seems funny, keep in mind that there are bridge sections over the top of each tower, spanning from one face of the tower to the next.) This impressive viaduct was built to replace a lower valley crossing, which was longer and had steep grades.

Bootstrapping the first few towers must have been difficult, dragging steel up and down a hill. Once they were built, and the bridges connecting them were in place, the crane seen above could run out and provide a lifting point directly over the towers and bridges. That didn’t make construction simple or easy, but having that overhead support made the steel erection more straightforward. In the photo, all of the apparent “truss work” above the girder bridge elevation is part of the track-mounted crane.

Each tower consists of two vertically-oriented and almost-triangular sides, with the legs sloping in from the sides to meet the girders, and two rectangular sides that are sloped inwards. The rectangular sides are broken into five panels by horizontals; the triangular sides have those same five panels but the top one is divided again into a main panel and a small top panel. All of the panels are cross-braced by built-up latticed members. (The tower legs are also built-up, but only partly latticed.) Each tower is therefore braced to carry wind and gravity load on its own, and the only connections from one tower to the next are the girder bridges at the top. If you think about it, this presents some difficulty in construction: how do you bring up the tower steel?

The picture makes it clear: the frame under construction on the far left is one triangular side of a tower that has otherwise not yet been started. The thing at the top that looks vaguely like triplane’s wings is a scaffold for the steel workers. The detail that won my heart is the use of temporary braces just above the permanent horizontals of the completed tower to the right. These braces are square-section built-up lattices and have a relatively fat middle and taper down at both ends. This is a near-ideal section for compression: because there ae only temporary braces on one side and the wind can blow in any direction, these braces might have to take compression or tension at any time. (Wind parallel to the tower frame, perpendicular to the line of the viaduct, can be taken by the permanent cross-bracing already competed in the frame.) The tapered ends are a physical manifestation of a pin in design, and are there to make sure that the temporarily-braced frame doesn’t impart locally-high moments that could warp or buckle the tower legs.

An overly dramatic but not fake view of the completed bridge, which is still in use by the railroad:

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