When Is An Arch Not An Arch?

by Don Friedman on November 15, 2017

That’s the F/G subway, paralleling 9th Street in Brooklyn, where the tracks cross over Fourth Avenue. The train is elevated here because it crosses over the Gowanus Canal a little to the west at the Smith/9th Street station. As the tracks head east, to the left, they stay at roughly the same elevation as the ground rises (the slope of Park Slope) and so head back underground.

There’s some fine art-decoish brickwork at the station, but what grabs my attention is the structure that carried the load of the trains across the 100-foot width of Fourth Avenue. That big green arch is steel, one of I believe four parallel arches at this spot, and serves as a bridge. There’s another, older subway line under Fourth Avenue (the R/N/D), so a vertical section through this spot is quite complicated.

The thing is, that’s not really an arch. It’s an “arch.” An arch carries vertical load through compression, and has both vertical (gravity) and horizontal (thrust) reactions at its base. This is a structural element that used to be called a “one-bay bent”: two columns that are moment-connected to the beam that connects them. The tip-off is the corner, visible (where the arch bends down on the right into the brick of the station entrance) as a widening of the arched member. That widened corner is the moment connection.

Here’s where classification starts to break down. The arch foundations are buried in the ground, so in theory they could be designed for the horizontal thrust. The thrust would be pushing away from the old buried train tunnel, so it wouldn’t harm it. So if we allow for thrust, the arch could be working as an arch. In other words, once we allow for thrust, arching action exists. The proper shape for a uniformly-loaded arch to remain solely in compression is an inverted catenary curve, but the loading here isn’t uniform. A train can be in the station and uniformly loading the arch, it can be approaching from the Manhattan end of the line and loading the west half of the arch, or it can be approaching from the Brooklyn end of the line and loading the east half of the arch. Arches that have to carry radically different loading cases either have to be very thick to allow for the different compression paths through their material or they have to be able to carry local bending. Similarly, if an arch has a less-than-ideal curve to it, it either needs to be thick or it needs to carry local bending.

What would an arch that can carry local bending look like? It would be a material that can take tension as well as compression like, say, steel. And in areas of sharp curvature it would be wider because of the concentration of bending caused by the geometry. In other words, it would look a lot like the arch above.

The division between arches and one-bay bents is based (1) whether the material is capable of carrying bending as well as compression and (2) on how we choose to analyze them. In this case, given the heavy loads from the trains, it makes more sense to look at this as a bent, but that’s really a matter of taste.

An Unseemly But Necessary Growth

November 3, 2017

There’s no mystery as to what that big thing sticking up above the roof of this building* is: it’s the top of an elevator shaft. The windows in line with the thing are all marked “SHAFT WAY,” which is a not-so-subtle hint; the fact that this is a mid-1800s commercial building that is still in […]

Read the full article →

Detailing For The Forces

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 […]

Read the full article →

Road Trip: Hanging

October 15, 2017

That’s a picture of three men on bosun’s chairs cleaning a glass curtain wall in central Ottawa. Bosun’s chairs have fallen out of favor in New York, so I don’t get to see this very often. A fifteen-story or so building like this back home would have either tracks for a scaffold built into the […]

Read the full article →

Road Trip: For Show Or For Blow

October 14, 2017

(Ignore the flying porpoises. I have no idea what they are.) That’s the roof of a shopping mall in Ottawa. Are those trusses real structure or just for show? Their form (diagonals sloped for tension, deepest at midspan) and location (paired on either side of the columns) both make sense as real structure. If so, […]

Read the full article →

Intentionally Tilted Masonry

October 6, 2017

That’s a photograph taken in an attic of a mid-1800s house. The good news is that the wood is dry as a bone, so it’s not rotting. The maybe bad news is that brick chimney…isn’t quite straight. The reason for the chimney running diagonally is, simply, fakery. The fireplace below didn’t, for whatever interior layout […]

Read the full article →

Historic New Structural Detail

September 19, 2017

Even though construction is still in progress – note the blue tarps and the chain fall – that is a beautiful sight that very few people will ever see. That is the inside of a newly-constructed onion dome. The old dome and cupola, which formed the steeple of St. Vladimir Ukrainian Catholic Church in Elizabeth, […]

Read the full article →

A New Form of Sidewalk Bridge

September 18, 2017

First, some terminology: the platforms that move up and down on cables are hanging scaffold (AKA swing-stages); the tall temporary constructions of pipe and metal frames that allow access to a facade are standing scaffold (AKA pipe scaffold); the sheds that protect people on the sidewalk from falling tools and debris are sidewalk bridges (AKA […]

Read the full article →

An Inevitable Move Into A New Market

September 17, 2017

I did not know that LEGO was working on full-sized construction. Click on the picture above to enlarge it.

Read the full article →

Historic and Modern Structural and Non-Structural Detail

September 11, 2017

I hope the title is broad enough…I wouldn’t want to leave anything out. This is a probe in the facade of the penthouse of a 1920s apartment house. The exterior of the masonry facade has been stuccoed, which is why the background is so white. The pipe in the lower right corner is part of […]

Read the full article →