The Nevius Street Bridge – Member Design

See here for part 1 of this dive into archaic bridge design.

In a pure truss, all of the members are loaded only in axial tension or axial compression. No bridge is composed only of pure trusses – the deck beams that support the roadbed from the side trusses are loaded in bending – but the designers of old truss bridges went to some length to see that the main trusses work this way. That’s the reason for the pin connections.

A pure tension member can be a simple metal bar, so the eyebars at this bridge have rectangular or round cross sections. They are very weak and flexible when subjected to any load other than axial tension: I was able to visibly move one of the diagonal round rods by pushing on it with my hand. Besides being a fake macho feat of strength, it’s a good illustration of how weak the bars are when subjected to lateral load. In addition to the main-truss diagonals, the wind bracing at the top of the trusses, tying the two side trusses together consists of really slender rods.

The compression members are a different story. They have to be designed to resist buckling, and the cross-section shapes available in 1886 were limited. There were no wide-flange H shapes, which are the most common compression members used in steel today, and there were no tubes, which are the most efficient compression members. So the designer and builders built up boxes (more or less square times) using channels and plates. The plates were mostly arranged as lacing – as you can see in the connection photo above). In some cases (the sloped top chords at the ends of each of the two spans, for example) the box is laced on one side and has a continuous plate on the other. In terms of analysis, that’s a bad idea, as it makes the member asymmetrical and thus introduces bending. The continuous plates are always on the top side, and may have been a way to try to protect the inside of the built-up boxes from the weather. (Wait for tomorrow’s thrilling conclusion of this discussion!)

My favorite member though is the lattice portal frame, as seen in the photo immediate above. Part of the wind bracing for the trusses is to have a rigid frame at the each end of each span, to keep the trusses from freely twisting sideways. Since (a) the top chords are relatively small built-up boxes and (b) commonly-available structural welding was about 80 years in the future, the portal here is a lattice of small bars, riveted to the boxes and to each other. It does the job with a very small amount of material, but a large amount of effort. (Again, more in the conclusion.)

It’s worth mentioning that the members are probably steel – by 1886, steel had mostly driven out wrought iron – but it doesn’t matter for this discussion, as the design methodology then or now would be basically identical. The only real difference in terms of design between the two metals is that steel is stronger.