The ingenious NYC subway


    The great blizzard of 1888 prompted two great infrastructure projects in New York City: Bury the tangled and almost opaque web of the fallen electrical and utility lines underground and create a subterranean railroad system to replace the street trolleys paralyzed by the storm.

    With a mandate to complete these projects as expeditiously as possible, while maintaining the flow of street traffic, the first subway routes used a construction method called “cut and cover.” Essentially, a trench is dug in the street, the excavation is braced, timber decking is used to temporarily cover the cut to allow street traffic to continue to flow, perimeter walls are assembled and waterproofed, foundations are poured, tracks are laid, columns are erected, a roof is built and the construction is buried with the new utility lines laying on top and the repaved street above. It is a perfect example of American pragmatism: fast, inexpensive and practical.

    At the same time, in the late 1880s, new advances in steel technology and safety elevators made it possible to build taller and taller buildings on the bedrock foundations of Manhattan. To make these structures economically feasible, large populations of people were needed to occupy them. Only the subway could supply the sufficient quantities necessary, helping to create the unique economic and architectural paradigm that is the density of Greater New York City. Concurrently, the construction of subway lines in the outer boroughs and the areas beyond midtown Manhattan was the irrigation that allowed new neighborhoods to grow and flourish in what was then only farmland.

    The NYC subway system runs 656 miles, by far the most extensive system in the country. With an annual ridership of 1.7 billion it is ranked 7th in the world. It opened in 1904 with 28 stations; there are now 468. Built in spurts, by three private companies starting in the 1890s until the 1940s, with construction still ongoing, the original design is predicated on the engineering required to swiftly erect and run the system. This engineered configuration, like other infrastructure or industrial constructions, creates a unique architecture – a “decorated functionalism.” Mosaic tiles in conjunction with the ubiquitous and eponymous “subway tile” adorn what is essentially a bare-bones post and beam construction.

    The structure, for the typical cut and cover train line, is composed of exposed steel members with slightly vaulted concrete ceiling panels (jack arches) supported by the overhead steel beams. The steel columns are 5 feet on center along the tracks and 15 ft. on center along the station platforms with diagonals bracing columns and beams. Using frequently spaced, relatively small steel members saved time in fabrication and installation and allowed the construction to proceed swiftly. These sectional bays of steel column, beam and brace, called bents, form the colonnades that are the hallmark of the passenger stations.

    Building the system over many years in different sections leaves a motley collection of designs, each with a different look based on the means and methods of construction, unlike, for instance, Washington, D.C., where there is a consistent design for all beautiful tunneled stations, which were built simultaneously. While tunneling, an alternate method of design and construction, does not disturb the surface as intensely, it requires the excavation to go deeper, and by virtue of its form creates grand cathedral-like spaces where wall and ceiling are one. Cut and cover is unlike tunnels, which are dangerous, time-consuming and laborious to build, requiring great pieces of industrial equipment and blasting.

    The subway and its stations are an integral part of NYC, an invisible below grade parallel universe that operates 24 hours a day, every single day of the year. Between 550 and 660 feet long, some stations along the tracks and within their connecting corridors contain newsstands, shoe shines and barbershops – acting as the original shopping malls.

    While buried beneath the street, the typical subway entrance is as close to the surface as it can be. One descends its single flight of stairs almost as if entering a residential basement rec room. Being so close to the surface has its advantages: the distances to enter and exit are minimal. Ventilation can be obtained with grilles in the sidewalk above without use of ducting or fans; and with these open steel grates one can hear the approaching train from the street, causing many a rider to go barreling down the steps, even though another will be along in two to 3 minutes. But the shallow depth also leaves the system vulnerable to flooding and damage from above. In addition, the existing streams and creeks that were part of wild Manhattan are still flowing beneath the streets at the same level as the subway and require constant vigilance in trying to prevent leaks and withstanding water pressure.

    Linking the furthest points of the northern Bronx to the beaches of Far Rockaway in Queens, all for a single fare, the experience of riding the subway, at a minimum, consists of essentially three spaces: the station one waits in, the subway car one hurtles through the dark tunnels in, and the station one exits through. The first is experienced statically, standing, waiting in the light, completely anticipatory. The second is in a usually crowded car zipping along the miles of dark tracks that comprise the system, as if travelling in a duct, cut off from the world above, until one reaches the next illuminated station from which to exit, get reoriented and return to the life of the city.

    Structural engineer
    The original construction of the NYC subways occurred through several contracts, under private entities, essentially a public-private enterprise. The originally competing companies that built the subways, such as the IRT (Interborough Rapid Transit) and BMT (Brooklyn-Manhattan Transit) were eventually folded into the present-day Transit Authority under the MTA of NY in the 1950s. One can still catch old-timers refer to the IRT or Lexington Avenue Train that others call the 4/5/6 or tourists may call “the green line.”

    The 1913 contracts, known as the “Dual System of Contracts,” greatly expanded the tributary lines of the outer boroughs that fed into Manhattan, further expanding city growth. The technology of this contract was highlighted by a series of articles published in Engineering News in 1914. Available as a reprint today, it is a fascinating and detailed, yet succinct read, covering the structural design, ventilation, drainage, waterproofing, excavation, tunneling, underpinning, and elevated portions of the subway. We recommend it as a good quick inspirational read, particularly for those in our field involved in historic structures or major infrastructure projects.

    The subway system brought myriad problems to solve. From a simplistic point of view, the main issue was how to open up the streets of NYC, keep the city operational, and build tunnels and tracks to allow the passage of trains, and ventilation, without undermining the existing and in some cases fragile buildings that were already there.

    The cut and cover method, mentioned above, consisted of a rapid street opening and then temporary cover to allow workers to brace, excavate, and then build the new subterranean infrastructure below, under the feet of the people of New York. The basic structure of most subways consists of closely spaced steel bents with concrete arches spanning between. Steel framing was judged to be most appropriate, since it could be brought in small standardized sections and rapidly field-riveted and then immediately loaded. The steel columns were supported on concrete footings where soil conditions permitted. Many areas of the subway are below the ground water table and have a mat foundation to resist water pressures.

    Elevated portions, or “Els,” typically consist of bents of 5-foot-deep built-up plate girders with stiffeners, supported by built-up columns with curved or scrolled decorative and structural stiffeners. The heavy riveted construction created moment connections that resist 30 psf (pounds/square foot) wind loads and impact loads from trains. Some elevated stations have noticeable lateral drift as trains pull in. However, they have functioned very well for many years (seismic loads were not a consideration until modern times).

    While waiting for subways to arrive in stations all around the city, one can study the construction and guess at certain things. Material was probably more expensive than labor, therefore cover plates on built-up plate girders are cut off following the moment diagram and many columns are built-up composite angles with battens (latticed).

    The design criteria was developed by Henry Seaman and published in an ASCE publication. The structures below the roadway and sidewalks were designed for 600 psf, a criterion still in use today for NYC sidewalks.

    The tunnels are, for the most part, naturally ventilated with air exchanges taking place by the “piston like” effect of a passing train. One can feel the breeze of leading air on the platform as an express train roars through. Certain lengths of the subway system were not cut and cover but excavated as true tunnels through the Manhattan bedrock, requiring various forms of heavy temporary timbering.

    Station design represents a subset of work that was more analogous to building design, requiring architectural input as well. Many stations have hung mezzanines that allow for transfer to “uptown” or “downtown” trains, platforms, stairs, and other “architectural components.” In more recent years, elevators and other amenities for disabled passengers have been added. The decorative motifs at some stations, such as the Astor Place beaver or Bleecker Street tablet, are more artwork than mundane signage.

    Of the many side stories to tell, one that we found particularly fascinating, relates to the underpinning portion of the work. By the time any major city embarks on an underground transit system, there is already a great fabric of existing city structures and, unless they have deep basements, underpinning is required. From individual columns to load-bearing masonry walls, the underpinning work was intricate.

    One of the leading engineering and foundation firms of the time was Spencer, White, and Prentis. They summarized much of their work and knowledge in a book titled, “Modern Underpinning,” which in a later edition (1950) was simplified to “Underpinning.” Now out of print, we found this book to be amazingly relevant, as the technology of underpinning has essentially remained the same for the last 100 years. “Underpinning” gives many direct examples of this firm’s work in underpinning the buildings along the subway route. From pit and pier methods to piles and caissons, the methods and building and soil conditions were varied and original, including a patent for a pretest pile used to underpin heavy loads in poor and wet soils.

    Today, the work continues as the City of New York is building a long-awaited new line along the underserved 2nd Avenue. The ventilation will not be naturally provided and the tunnel is being bored by a TBM (tunnel boring machine), but the groundwork of the subway forefathers is ever present.

    Ciro Cuono, P.E., LEED AP,is a principal at Cuono Engineering PLLC, Port Chester, NY. He can be reached at
    Michael Wyetzner, AIA, LEED AP, is a partner at Michielli + Wyetzner Architects, New York. He can be reached at