SKILLS, STREETS, SOILS, AND SPEED

“The key with affordable housing,” says structural engineer David Odeh, SE, PE, “is getting the most good-quality housing with the most density that we can get, so we can maximize the yield of projects, whether it’s an 80/20 mix or 100 percent affordable housing.” Odeh leads the national building structures practice at WSP, the national firm that his Providence-based firm Odeh Engineers merged with two years ago, expanding the scale and geographic range of projects it can work on. Providence, Boston, and additional New England cities differ from others, he notes, in their higher reliance on structural steel for high-rise construction in residential, commercial, and industrial construction. “Probably the biggest factor is just economics and the fact that steel is very cost-competitive in New England.”

Odeh attributes this condition to several intersecting features of the local construction environment: availability of both material and expertise, scheduling, climatic robustness, and zoning requirements. “New England has rapid availability of steel,” he notes, plus “the availability of labor to install those systems; we have a pretty robust market of steel fabricators.” In addition, “steel has a distinct advantage of being friendly towards winter conditions in New England.... Steel’s a material that can be erected in almost any weather.”

Another aspect of older cities in New England, Odeh notes, like other urban settings whose history extends to pre-automotive eras, is the presence of “a warren of streets” rather than the wide thoroughfares of younger cities or the gridded networks found in New York and similar cities. Narrow, irregular streets and tight staging conditions pose challenges for cranes, concrete trucks, and other deliveries of construction materials; “the larger the elements that you’re trying to prefabricate and bring to a site, whether it’s in Providence or Boston or Worcester or any of our cities in New England, the more challenges you have.”

Soil quality is poor in downtown Boston, particularly in the artificially filled Back Bay. “Providence has organic silts because it sits at the confluence of rivers,” Odeh says, requiring deep foundations using driven piles or drilled shafts (caissons). Minimizing building weight is of paramount importance on such sites in any city with similar sedimentary soil conditions, he notes: “That is an area where steel shines: steel buildings tend to be lighter.” A typical New York apartment building has eight-inch-thick concrete slabs, weighing 100 pounds per square foot per floor, whereas “a comparable structural-steel building might have a slab that’s much lighter than that, only 50 pounds per square foot.” In Manhattan, “the gift of bedrock” allows for taller buildings and the weight of concrete, but steel’s greater strength-to-weight ratio has clear advantages when that gift is absent.

With strict planning and zoning requirements, particularly in Boston and Providence, that limit the heights of buildings, flexibility with floor-to-floor heights strikes Odeh as another area where steel is competitive. Although concrete is associated with low floor-to-floor heights, certain steel systems can achieve them as well. “Building structural depth is a big factor in construction and the architectural decision to use steel versus another system, or a hybrid system,” he says. “The lower the floor-to-floor height of the building, either the more stories you can fit inside of a zoning envelope or the higher ceilings you can have in your building.... Structural engineers in New England have been pretty creative at coming up with so-called low-floor-to-floor-height structural systems that can use structural steel by doing things like, for example, using longer-span steel decking.” Steel beams supporting floor slabs are farther apart, are placed inside walls, or are shallower; in some hybrid systems, beams do not stick below the floor socket, allowing a thinner floorplate. “There are some systems we’ve employed in Boston that are using precast concrete in combination with steel beams that have a very low floor-to-floor height.”

Even in cities where nearly all residential construction uses concrete, Odeh says, steel is preferable under certain conditions. In the Washington, DC, area, “it’s very unusual to use steel, but we actually did a steel-frame residential building there, a barracks building for the Army Corps at Fort Myer.” Though the building was initially proposed as a cast-concrete slab, Odeh and a contractor achieved the same floor-to-floor height with a girder-slab system of steel beams and precast hollow-core concrete planks. “It’s a very thin structure,” he says, “because the steel beams are actually embedded in the depth of the planks. There are upturned T-shaped beams, so they have a bottom flange and a web that sticks up between the planks, [which are] erected like a kit of parts, just sitting on the bottom flange of the beam. And then it’s grouted into position, so it becomes a composite system. The thickness of that system is only about eight inches,” about the same depth as standard flat-plate reinforced concrete, but with the advantages of offsite prefabrication and faster, less expensive construction.

Speed of construction with structural steel can have dramatic effects on construction costs, Odeh points out. In Boston, where many projects require underground parking–typically one or two levels, but occasionally four or five–excavation is a time-consuming and expensive stage of construction. “A big advantage of structural steel,” he says, is that “while they’re building the basement, they can be fabricating the structural steel, and when they get to the bottom, grab the piles and do the foundations. You then come back and immediately start erecting columns and beams and building the structure, as opposed to a concrete building, where you have a lot of formwork that has to be put in place.”

Fireproofing is another factor that Odeh sees as driving material choices. Taller buildings require fire-rated structures, capable of withstanding a designated number of hours in a fire while remaining structurally sound; steel with fire-resistive coatings is noncombustible for one to three hours and “can be used in larger structures compared to wood or timber, which is often the system of choice for lower-rise buildings because of its cost.” Considerations of sustainability, he adds, make steel a compelling choice for larger projects, since well over 90 percent of structural steel in the United States is recycled content, made with the electric arc furnace process (see “Structural Steel for Low-Carbon-Emission Lightweight Frames,” Architectural Record Continuing Education Center, March 2024). “Steel is a very sustainable way to build,” Odeh comments; “that’s driving a lot of decisions on housing projects meeting the demands of sustainability that owners really have now. We’re also exploring a lot of hybrid systems that use structural steel with timber,” combining steel columns and beams with floor and roof slabs of cross-laminated timber (CLT). One new student housing project at Brown University combines the strength and flexibility of steel with the visual appeal of CLT (see Case Studies, “Brook Street Residence Halls”).

REINVENTED CLASSICS AND HYBRID VIGOR

“A nice advantage of steel is its ability to be modified over time,” Odeh adds. “When we think about future-ready structures and the unknowns associated with the future, steel is well known as a material that can be easily adapted to different situations: you can make steel members stronger by welding plates to them or adding elements to them.” In a resource-conservation-conscious era when more architects and clients favor adaptive reuse, particularly commercial-to-residential conversions where feasible, over demolition, the durability, recyclability, and flexibility of structural steel are all points in its favor. Odeh cites the theme of the recent Council on Tall Buildings and Urban Habitat conference in London, “New or Renew,” and points to a prominent steel-framed Art Deco building in downtown Providence, Walker and Gillette’s 1928 Industrial Trust Tower (the state’s tallest, also known as the Industrial National Bank Building and as the “Superman Building” for its resemblance to the Daily Planet headquarters in the 1950s televised series), as a candidate for residential conversion. It has been vacant since its sole tenant Bank of America moved out in 2013; its current owner High Rock Development has engaged WSP to work on its adaptation.

Eugene Flotteron, AIA, partner and director of architecture at CetraRuddy in New York, notes that older steel-framed office buildings are strong candidates for conversion, which accounts for the majority of his firm’s experience with steel in residential work. “The only steel building we have ever done for residential was out in Boston,” he recalls, a million-square-foot midrise; otherwise, CetraRuddy is among the firms that use steel for conversions and rely on reinforced concrete for pure residential buildings. The conversion sector has become one of its specialties, a growing one, Flotteron notes. “All the old office buildings were made out of steel, and we’re converting a bunch of them right now. We’re probably one of the few companies in the country doing the largest amounts of these.... It’s a key topic in the country now,” he says, recounting his conversations at this year’s AIA Convention, where he repeatedly heard colleagues from other cities ask, “‘What do we do with our downtown office market? The buildings are more than half vacant. They’re all going into distress. How do we make this work?’”

One way Flotteron and colleagues make conversions work is through expertise with the structural additions and subtractions that help a building purpose-built for offices meet residential needs and contemporary codes. Postwar, pre-1970s buildings, he notes, “weren’t designed to the current wind loads. When we convert these buildings, we sometimes have to carve the area out of them. We have to take a 40,000-square-foot office floorplate, and we may have to carve one or two courtyards to make the light and air work for residential use.” When a conversion involves reducing area, zoning regulations sometimes allow adding comparable area on top–“You take useless space and put it on the most valuable space, on top of the building”–yet today’s wind codes require that “anything more than an increase of 5 percent of surface area of any elevation will require rebracing the whole building for wind.” Analyses of these restructured steel buildings by CetraRuddy and its structural engineers, Flotteron says, identifies a tipping point where the added area would pass that 5 percent level. The overbuilds for most clients are limited to one or two stories to avoid having to “trigger reinforcing the building.... Even if there’s an extra 70,000 square feet of floor area they can sell, the numbers almost don’t work until it gets higher than that.” (One current amenity-rich conversion CetraRuddy is working on, however, too early in the process to be named yet, involves a 10-story overbuild and two newly cut courtyards, with the building braced from top to bottom.)

“Depending on the vintage of the building,” Flotteron continues, apartment layouts require fine-tuning to manage the complications of navigating a column grid and locating openings in steel-framed buildings, which from the 1970s onward may include metal decking and reinforced concrete slabs. Long chases with back-to-back bathrooms organized with the grain of the decking, he notes, save miscellaneous steel costs, offering flexibility in the size and position of openings. “If we cut our openings for new ductwork and plumbing chases with the decking, we can cut long openings and a certain amount of width. If we go against the decking, we have to separate those openings so far apart they really don’t really work very efficiently. So we actually have to plan, or there’s a tremendous cost to miscellaneous steel to make those openings that you’ll need for your new plumbing chases and your new ductwork, and the only way these projects even pencil out [is] you’ve got to be really tight on the construction cost.”

Postwar buildings were often built with “five-foot exterior wall/window grids, which aligned with the module of office furniture,” Flotteron says. “The five-foot grid was ideal for offices, center line to center line, to do basically a 9’6” clear office to make that work. That is not ideal for residential. 9’6” is a little too narrow for a living room; you’re going to want minimally 10, ideally 12.... You would ideally like to design it on the grid line; that would be perfect, the partitions on the grid line. But it doesn’t usually work that way, because then if you go to a 15-foot width module and you end up with a 50-foot-deep apartment, in some cases, the proportions are all off.” Planning around columns, making sure plumbing openings work with beamlines, occasionally jogging walls at the perimeter, and maintaining alignment with the grid, he says, makes successful conversion a complicated game.

Usually, an overbuild component will use the same structural material as the existing building, Flotteron says–steel on steel or concrete on concrete–but not always. In certain buildings, he and his colleagues have investigated using concrete in new segments. “It’s basically a height issue,” he says, “because then we can go lower floor-to-floor and get higher clear ceiling heights without the beam issues that you have to deal with.” In a transition from a beam up to a concrete structure, he says, “We look at it so we don’t have to do a 12-foot floor-to-floor to get a 10-foot ceiling; we could do a 10’8” floor-to-floor to get a 10-foot ceiling.” Over a number of floors, this could lead to building one story less, thus changing the weight of the overbuild. “Concrete inherently is heavier, so it’s a balance, adds Flotteron. “Steel is lighter, but then we have to go taller.” Zoning height limitations may also be involved, so concrete’s lower floor-to-floor height could make two stories possible where only one would fit with steel.

All things considered, Flotteron says, in the residential sector (with the Boston exception), “we have not done a new high-rise building out of steel. We’ve looked at a couple of them; they haven’t penciled out yet.” Conversely, he finds steel preferable in offices (for “big, hulking floorplates,” long spans, and other familiar features) and for conversions. He is aware of no conversions of concrete-frame buildings to date. If COYHO pushes the date of allowable conversions to buildings from the 1990s, these are still “modern steel buildings with modern curtain walls. They’re more energy-efficient, a little bit different to deal with.” The chronic challenge with conversions remains the prevalence of five-foot grids, he says, though “every once in a while you get lucky, and you find a building on a six-foot window grid; that project is perfect. Then you’re on a 12-foot module for rooms, and you can make that work all day long.”

“The vast majority, if not all, of the residential high-rise construction that we’ve worked on—and there’s more than a million square feet and over 10,000 units in the last decade–have almost all been in cast-in-place concrete or block-and-plank concrete, but none in structural steel,” says Toby Snyder, AIA, LEED AP, senior associate and design director at FXCollaborative (FXC) in New York. The projects where FXC has preferred steel, in his experience, have been conversions and hybrids, where programs combine residences with schools or offices in different segments of the building (see Case Studies, “35XV”).

Another combination, a residential tower atop a school and retail, occurs at 42 Trinity Place/77 Greenwich Street, a.k.a. Jolie, a 42-story tower incorporating and renovating a landmarked four-story Federal-style townhouse dating from 1810, the Robert and Anne Dickey House. FXC has come to view such hybrids as a specialty, solving business problems for institutions that partner with developers to add a profitable component, requiring structural improvisation to accommodate the mix of programs.

FXC’s residential buildings typically use eight-inch-thick cast-in-place concrete slab construction with concrete column spans of about 22 feet, Snyder says, “so you get about two rooms wide for every bay of space.” Ceiling heights of eight to 10 feet are important in residential units, and long spans are not the norm. In contrast, “what a school or an office building needs is much longer spans for classroom spaces or large lease spans. And so that’s what steel really offers... Steel, of course, with the greater depth in the beams, does have some limiting factors for ceiling height, which is not as much of an issue in schools and in office districts, but in residential districts, where a lot of residential buildings are built, they’re under a lot of pressure to keep the height low. There are fewer restrictions on height in office districts, plus they need long spans; steel makes sense.”

Outside the mixed-use sector to date, despite increasing attention to carbon metrics and other factors favoring steel, “the delta in time savings and in methodology and the programmatic needs of residential have yet to tip the balance between using steel versus concrete in most high-rise residential construction in the United States,” Snyder notes. “We are certainly starting to do life-cycle analysis on a lot more of our projects, and actually the biggest component of that, and the part where steel is coming into place, is when we’re looking at office-to-residential conversions.” One such project at 95 Madison Avenue (the Emmet Building, a 1913 steel structure by John Stewart Barney and Stockton Beekman Colt, landmarked since 2018) requires reworking of the core and floorplans; steel makes sense here, Snyder says, as it did on the “renovation of St. Vincent’s Hospital into what’s now called the Greenwich Lane Apartments. There are a lot of those older hospital buildings [with] steel structures where we basically had to reconstruct parts of the building using structural steel.”

In conversion projects, Snyder continues, where floorplates suitable for an older program must change to meet residential requirements, there are “times when you have to cut away at a certain part of the building... some places, you cut away from one area, and then you build more area on top.” The usual practice is to use the same structural material found in the existing building for the remediation as well. An occasional exception, he says, is a gut renovation in a smaller century-old building with brick load-bearing walls and damaged wooden floors and joist systems, where light-gauge steel is used for 20- to 25-foot spans. “It would be overkill to use a concrete slab in a lot of those, so that’s a place where the steel makes sense.”