Gaining Urban Space: Platforms Over Rail Yards

Land-strapped cities are starting to erect massive steel overbuilds on top of rail yards to spur much-needed urban development
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Sponsored by The Steel Institute of New York

Columns, Trusses, and Girders

It’s important to note that a typical 4-inch, A572 plate produced in the United States has a yield strength of 50 ksi. So in order to deliver the required 65 ksi yield strength with a 4-inch plate, a large European steel plate manufacturer was tapped to leverage a fabrication process to produce the laminated plates.

The column structures vary in diameter from 1 foot to 5 feet 6 inches and are drilled into the bedrock beneath the railroad tracks at an average depth of 40 feet below the surface. Approximately 3,300 tons of solid steel cores, the largest of which was 30 by 30 inches, were fabricated from the 4-inch-thick plate. The longest is 87 feet, and the heaviest weighs an amazing 71 tons.

When placed between the tracks, the columns often have size limitations imposed by the “train dynamic envelope,” which is the clearance required for unobstructed train movement, explains Patrick Chan, senior vice president, building structures, WSP, New York.

Photo courtesy of Hudson Yards New York/Geoff Butler

Shown is column, truss, and girder construction at Hudson Yards.

“Moreover, even the shape of the columns is sometimes dictated by this clearance where the tracks are not straight but curved,” he adds. “The fact that these columns often are tall and slender—usually there is no room for any type of bracing inside the yards—also adds to limitations imposed on column design.”

Ultimately, the goal is to lay out the columns to fit the track alignments while creating as uniform a grid in one direction as possible to allow for the standardization of many of the deck elements, explains Gottlieb.

To illustrate the structural and spatial dynamics of the platform, Robbins explains that much of the support is actually created by walls, as opposed to the individual columns.

“What you essentially end up doing is creating walls in between the tracks which act as a kind of continuous column to support the deck,” he says. “Because those structural elements run parallel to the tracks, it means that you have very good structural support in one direction, and it tends to dictate the orientation of the building.”

As a result, the buildings on top of the platform end up running perpendicular to the tracks so that they get the most support.

Spanning in between the columns are steel plate girders and steel trusses. For Hudson Yards, steel plate girders support the plaza area and vary from 35-foot spans up to 120-foot spans, with depth ranging from 4 feet to 6 feet.

Photo courtesy of Hudson Yards New York/Geoff Butler

Shown is the truss for Hudson Yard’s retail structure under construction.

“These girders also are designed to incorporate the mechanical plenums for smoke and air ventilation of the yards as well as incorporate all the other MEP services within their depth,” Gottlieb says. “By providing regular openings, similar to castellated beams, the girders were an ideal way to stack all the structural and mechanical systems together and minimize the depth of the entire system sandwich.”

The plate girders were also used where vertical space was limited and were coordinated in dimension to incorporate all the platform ventilation systems, including fan rooms and ducting, while minimizing the vertical dimension from rail clearance to the top of the finish grade, he adds.

Generally speaking, trusses and beams need to be shallow enough to provide the vertical clearances required by railroad authorities, and the roof over the yards should meet street-grade requirements. In many cases, this can be challenging, as rail yards built at the turn of 19th century are relatively shallow. Using steel helps address this shallow depth issue, in addition to meeting lightweight, long-span, and high-capacity construction needs.

At Hudson Yards, the platform’s base structure clears the tracks by at least 17 feet and ranges in thickness from less than 3 feet to up to 7 feet where special features have been incorporated into the design. For instance, in many areas, the platform houses a network of tubing carrying cooling liquids that will buffer the plaza’s landscaping from the heat of the train yard below, which can reach up to 150 degrees Fahrenheit.

Mega-Transfer Truss

Trusses were used for the largest spans and where there is a lack of interior/intermediate support for overbuild construction.

“A prime example of this type of condition is closely spaced tracks within the yard that do not allow for column placement between them,” explains Jeffrey Smilow, F.ASCE, executive vice president and director of building structures, WSP, New York.

In the case of Hudson Yards, four tracks on the east side branch out into 30 tracks on the west side, and within this track configuration, switches are used to navigate trains in between the tracks. Because the railroad couldn’t tolerate any disruption to the switches, the structural engineers had to span these long spaces between the foundations with large trusses.

The trusses were also coordinated to be the “basement” of the retail building and incorporate services, mechanical rooms, and storage as well as the loading docks for the project.

One advantage with these trusses is that the absence of support columns minimizes construction and time and coordination inside the rail yard. However, this lack of intermediate supports increases the depth of the truss. So ultimately, the fabrication, transportation, and installation is more complex and costly compared to a simple beam/girder system.

As noted, when building these platforms, there’s a misconception that the deck is like a skirt around the building, with a rail yard going through the building basement. In reality, the skirt around the building acts like an extremely large basement, which is the rail yard, explains Robbins.

 

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Originally published in Architectural Record

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