Curtesy, again, of the Wurts Brothers, we have the underside of the Jamaica elevated (now the J train) running over Broadway at the south end of Bushwick, in 1942:
You’ve got a nice slice of life from 80 years ago, but I want to focus on one detail in the elevated structure: the bent on the right, spanning over Grove Street. First: in this context, a bent is an upside-down, squared off U consisting of two columns and a girder, with moment connections at the top. Bents are very useful in that they give you resistance against lateral load without cross-bracing and without needing any moment resistance in the foundations. So, in the photo we see the western column and the girder of the bent spanning over Grove. The column is a built-up, laced box; the girder is a very deep built-up I section, and the moment connection is, in part, a big curved bracket.
The Grove Street bent looks very much like the bents that are the regular supports crossing Broadway, turned 90 degrees. Which, when you thunk about it, is a little odd since it’s loaded very differently. But that leads to the key to the whole system: why are the bent columns not in line with the heavy rail loads? Look at the column near the center of the photo, in front of Jack Harolds Haberdashers. The top of the column is in sunlight because it’s not under the rail structure, it’s a few feet outboard of that and so closer to the buildings. This geometry isn’t a function of the driving lanes, as the column is well short of the curb.
It seems to me, that the driving factor in the design of the el structure was keeping the girders supporting the tracks as regular as possible. The Broadway line has three tracks, and each sits directly above a big girder running longitudinally, along the line. We see four of those girders in this photo: a tiny diagonal bit of one in the upper left corner, then the longer diagonal just to its right that runs from the left edge to left of the center of the top edge, and then the two for the northmost track that run from the center of the left edge to near the upper right corner. Exact repetition of the girder spans and connections is good: it makes fabricating the steel easier, it makes erection easier, and it makes design easier. But the conditions on the ground are variable: there are cross streets like Grove, there are sewers and water mains below the street, there are vaults below the sidewalk… If the columns were in line with the outer two longitudinal girders, they would be forced to be irregular once in a while, maybe often. So the design solution was to have transverse bents carrying the longitudinal girders, and let all the variation happen in those bents. The longitudinal girders – like all structure – is non-sentient, and all that matters is that it’s supported at the same regular interval; whether the nature of that support changes is only noticed by us. The transverse girder can even end, not at a column, but at a 90-degree turned secondary bent cross Grove Street, without affecting the longitudinal girders. The transverse girders virtualize the support of the longitudinal ones, eliminating, to the extent possible, any changes relating from geometry or other structures at the ground.
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