Detailing For The Forces

by Don Friedman on October 17, 2017

A big part of structural design is “detailing,” which is drawing the way various pieces of a building  are connected. The bridge above (click to enlarge all of the photos) is in Wrocław and has a shorter span than the average American suspension bridge of any era. I walked over it a number of times and a few details caught my eye…

Since the towers are on land and the bridge deck elevation is close to land elevation, there was no need for a suspended land span. In other words, the roadway between the tower and the abutment is not hung from the cables. So the cables are straight from the tower top to the abutment. More interestingly, the “cables” are bundled steel plates at the center span and trusses at the land spans. The trusses are the same bundled plates as the chords with small angles riveted between as the webs. You can see this clearly at the tower bearing:

The masonry above that heavy steel bearing is decorative only, and the arched openings make that clear. My favorite detail is the similar bearing at the abutment:

That is simply a beautiful detail, regardless of its purpose. What it’s doing is interesting, though: the anchorage for the plate-cables is buried below ground. Usually there’s a large mass of concrete or masonry on top of the metal anchor at the end of a suspension bridge’s cables. The cables are in tension and the easiest way to anchor them is to sit a weight on top of their ends that would have to move to receive the tension.

If you want to have that pair of forces (the cable pulling against the weight sitting) vertical, then you need to bend the cable’s tension, and that’s what this detail does. The main element has a curve for the plates, similar to the tower bearing and necks down to a hinge to make certain that movements of the cable (from shifting load on the bridge deck, from wind, and from temperature changes) don’t translate into any bending moments on the foundation. Speaking of the foundation, how cool is that? The bend in the cable’s tension creates a diagonal compression halfway between the two legs of the cable, which is the line from the curved bearing top, through the hinge, and at the centerline of the masonry foundation. The designer decided, not reasonably, to treat that compression as if it were a vertical gravity compression, and so we have the sight of a tilted foundation partially buried in the ground.

None of these details had to be done this way. Most suspension bridges are different. But this particular set of details – this design solution – is what gives the bridge its character and makes it infinitely more interesting than the plate-girder bridge that wold be built today.

Previous post:

Next post: