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.

Composite Structure

August 21, 2017

That’s the side wall and an oblique view of the front of a small building on Nassau Street. The front appears to be early twentieth century, but the odds are good that this is an older building that has been modified multiple times. If nothing else, the storefront – visible as sheet metal at the […]

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Sidewalk Vaults and True Obsolescence

August 14, 2017

For people unfamiliar with sidewalk vaults, the illustration above, from 1865, might seem plausible, but it’s actually Daniel Badger’s fantasy of how he could sell more iron. The left-hand side is reasonably accurate for mid-1800s vaults: the facade columns extend down past the plane of the sidewalk, marking the separation of the cellar proper from […]

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Weathering Metals

August 9, 2017

Beautiful, right? Thanks to some old alteration work, we get to see the results of a nice little experiment in exposing metals to water. You’re looking at three beams here: a modern steel beam that had been directly supporting a sidewalk (middle left, outlined in purple below), the original cast-iron girder that supports that beam […]

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Self-Contradictory Structural Decoration

August 1, 2017

I thought I was done looking at the Pearl Street bridge that is part of the Brooklyn Bridge approach, but apparently not. This is another view of the new steel of the arch structure that was installed to resupport the original truss bridge. The big beam at the bottom and the two diagonal braces above […]

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Decorating Structure

July 18, 2017

The picture above is an oblique view of the Pearl Street bridge in the Brooklyn Bridge approach, showing the original (1880s) truss above and the newer (probably 1950s) arch below. As previously mentioned, one of the notable differences between the two eras is the type of steel member used: eyebar chains for the bottom chord […]

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Different Structural Forms Combined

July 17, 2017

Now that the weekend’s over, back to the Pearl Street bridge that is part of the approach to the Brooklyn Bridge. First, a minor correction to the first part of my street analysis of this structure: the original truss is not a double-diagonal warren truss. It’s a subdivided pratt truss, as some of the compression […]

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Different Eras With The Same Technology

July 14, 2017

There is so much going on in this picture that it’s going to take me two or three posts to cover it all. In case it’s not intuitively obvious to even the most casual observer* that’s a piece of the Manhattan approach of the Brooklyn Bridge, specifically the bridge over Pearl Street. The Brooklyn Bridge […]

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Riveted Lineage

June 22, 2017

National Geographic articles always have great pictures, and this one on the construction of Yankee Stadium is no exception. The article text tells the basic story; the best of the old drawings is, in my opinion, the 1934 extension steel diagram. (Yankees purists – which I am not – will probably prefer the architectural section […]

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Steel Trusses and Wood Purlins

May 19, 2017

No deep thoughts today, just appreciation for a well-designed roof. It’s an industrial building so finishes were kept to a minimum, and there was no requirement for fire-rating. The trusses are steel because they span about 50 feet; the purlins span 18. The end bays have diagonal rod bracing in the plane of the truss upper […]

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