Structural Steel in High-Rise Multifamily Housing  

Photo courtesy of Bedrock

A rendering of the office building and tower at Hudson’s Detroit, picturing a public plaza between the development’s two buildings.

The choice of structural materials and designs for multifamily residential construction depends on factors that vary with building programs, site conditions, local expertise, design strategies, zoning, and economics. For reasons not always closely related to the technical properties of the materials, concrete has historically been the customary choice for high-rise residential work in certain cities, while steel predominates in other locations. Engineers and architects with experience in residential towers, however, point to reasons why the balance of preference in some regions may be on the verge of shifting. Steel’s advantages in strength, carbon footprint, and ease of construction are well known, and the coming need for sharp increases in the national housing stock creates conditions that may be conducive to greater use of this reliable material.

The national imperative for more housing, particularly residences that the majority can afford, is common knowledge. Communities have been exploring ways to incentivize more multifamily construction through rezoning, regulatory changes, and other methods. State-level efforts are even afoot to re-energize the public social housing sector (Jefferson; Natarajan), essentially dormant in the United States since the 1970s; progressive legislators and think tanks are exploring the once-unimaginable idea on the federal level as well (Capps; DeGood et al.). California is poised to experiment with a local multifamily infill program, AB 2011, the Affordable Housing and High Road Jobs Act of 2022, crafted with significant input by New Urbanist pioneer Peter Calthorpe (Pedersen 2023). The Federal Reserve Bank’s 50-basis-point interest-rate cut last September 18, the first such in four years, is also likely to help spur private investment in multifamily housing projects. Whether well-publicized programs like New York City’s City of Yes for Housing Opportunity (COYHO, passed by City Council in amended form on November 21) meet with official approval or undergo revision, the demand for more residential construction remains a priority for both the public and private sectors to meet through increased supply.

Conversion of economically underperforming commercial buildings to residential use, one of the eight components of COYHO and a priority in other cities, is an important subset of multifaceted efforts to promote residential expansion; to some observers, conversions are the primary residential-construction category making significant use of structural steel. A recent report for the Urban Land Institute (Peiser et al.), citing Gensler’s studies of conversion variables and candidate buildings (Gensler 2017, 2022), states that “Gensler estimates that 10 percent of downtown gross floor area will be converted to residential. It is likely that conversions will amount to 20 percent to 50 percent of all new housing in cities.”

Architects with conversion experience recognize that this strategy faces challenges involving floorplan dimensions, different needs for lighting and ventilation, and other qualities. Since the Class B and C office buildings that are frequently optimal candidates for residential conversion commonly have steel frames, and since steel is more amenable to reconstruction than concrete when conversion requires dramatic changes to an existing building’s form, familiarity with structural steel systems is an important aspect of the adaptive-reuse subsector of residential design and construction. Another category where residential towers increasingly include structural steel is mixed-use buildings serving multiple programs, either newly built (see Case Studies, “Hudson’s Detroit”) or converted. Matching structural systems to components of a hybrid building calls for multiple areas of expertise and has become an important niche for specialists.

LOCATION, LOCATION, LOCATION

There is no universal, one-size-fits-all argument for choosing either of the two chief structural materials. “In my experience, it’s mostly related to location, to geography,” says Neil Wexler, PhD, PE, president and chief structural engineer at Manhattan-based Wexler Associates. In New York City, Washington, D.C., Miami, and elsewhere, concrete frames have been the norm in high-rise residential projects. New England cities, in contrast, have more steel residential towers. Local labor practices can exert a critical-mass effect, intertwined with zoning requirements and other variables.

“Some cities have labor availability in concrete; others don’t have so much,” Wexler comments. “If there is a lot of high-rise construction in concrete, then contractors maintain the teams for the duration. They move the large teams from one building to the other. They have a lot of work, and they keep them busy. But if you have other cities where there isn’t much concrete construction, then you cannot maintain the labor busy doing concrete work, so then the concrete trade is not available as much. If you were to build a tall building in Albany today, perhaps you wouldn’t have as much concrete choice, because, for high-rise building, you don’t have the labor. But in New York, you have so much labor available, so much technology, and so much knowledge about high-rise in one material or the other, that theoretically we can do both.”

Where zoning requires low floor-to-floor heights, Wexler says, “It’s easier to do it with flat-plate concrete than structural steel. Even though the technology is catching up, one of the problems we have is education. We can do low floor-to-floor height in steel, but a lot of people are not familiar with this approach. There are now options in structural steel where we can use a long-span metal deck, [including] the beam-in-wall system, where the structural engineers lay out the steel in a way that allows the maintenance of a low floor-to-floor height. So there is education, there is technology, there is zoning, and there is availability of labor: all of those contribute to the type of material we use.”

Wexler is a co-author of the American Institute of Steel Construction’s design manual for staggered trusses, a system developed at MIT in the 1960s comprising prefabricated trusses bolted to columns in the field; they are generally one story deep, located in demising walls with a Vierendeel panel at the corridors, providing lateral bracing with little increase in materials (Wexler and Lin). “Staggered truss is a system that has been used in the past a great deal,” he notes, “and it provides significant savings because the geometry is optimized. The cost of steel construction depends not only on the amount of material that you use but also on the ease of fabrication and erection and the number of pieces that get picked up. The staggered-truss system reduces the number of pieces that get lifted into place, and it uses geometry very optimally,” saving costs, time, and material. “The strength is either obtained through geometry or through throwing more metal into the job and so the staggered truss is able to reduce that.”

In California, Wexler notes, this system “received a bad rap in the earlier days of seismic design, because there was a discussion about staggered trusses having fewer columns than normal construction, and therefore [being] perhaps not as ductile in seismic zones. What we have done is supplement staggered trusses with different designs, such as additional posts or braces in certain locations, such as to increase ductility in the system for seismic use. And so it does not penalize the system very much, because the overall geometry and the overall erection have been maintained properly. We still had a low number of pieces to erect; we still had very good geometry; and the whole thing was erected very fast. So even though we introduced additional braces and additional posts, the overall economy was maintained.”

“Whatever we do in steel, we can do in concrete, and vice versa,” Wexler says. “As far as high-rise is concerned, steel or concrete is really a matter of choice, a matter of design. We should use steel where steel’s properties are most needed. Steel is strong; it’s stiff. So the strength-to-volume ratio and strength-to-stiffness ratio of structural steel are properties that we engineers care very much about, and where these properties are needed, that’s where we should use steel.”

Steel also “excels at long-span structures,” such as residential with parking or retail, projects “where we have to design in and around New York City subways,” and multi-use projects including classrooms or gymnasiums along with housing.

“With residential, structural steel was not a material of choice for a long time, but it didn’t have to be that way,” Wexler says. “We can use structural steel in residential buildings if we design them properly and we educate our customers.” He cites a 12-story residential project in upper Manhattan where his team “combined the beam-in-wall system with a long-span shored metal deck. It was very economical. It went up fast. It was done with a structural steel frame. It was very adaptive to foundations that were difficult, and overall it kept the floor-to-floor height.” At another residential project, Clayton Park in White Plains, NY, apartments were designed above three levels of parking, which were constructed with “standard structural steel posts; beam, girder, and column design with lateral braces; and on top, we put staggered trusses with plank floors,” with the trusses aligning with demising walls in the residential floors to create large column-free spaces and eliminate most transfer girders. For a residential building on East 23rd Street with a beam-in-wall system, “we used regular metal deck, and we were able to do that because the rooms are relatively small, and the regular metal deck was able to span the room size. We used structural steel because the building was built in stages: we had to stop construction during the Christmas holidays [because of] a directive for East 23rd Street to allow through traffic, and so construction was stopped at that time, and structural steel was very adaptable. We designed and built half a building, and then designed and built the other half, and it was easy to continue.”

In New York City, where the housing market is stratified roughly three ways (high-end, market-rate, and affordable), Wexler notes that structural systems can differ among the levels, though not for structural reasons. “This is a definition that applies mostly to developers, not so much to structural engineers,” he says. “We design most of them with the same building codes and the same materials. The reason why we even call them differently is because we are told to call them differently by our clients. We assume that the clients use this definition for other reasons, like finance or zoning. From the structural-engineering point of view, they’re all the same for us.”

A small amount of New York’s affordable housing uses structural steel, Wexler observes, chiefly in the outer boroughs and on small sites; ”they are done with structural steel the old-fashioned way, with metal deck and concrete floors and a hung ceiling.” Larger apartment buildings in the affordable sector are typically plank-and-block, with a few using cast-in-place concrete; up to about a 12-story height, he sees “stud bearing walls with all kinds of floors, sometimes long-span metal-deck floors and concrete,” and structural steel is increasingly chosen for larger buildings. “The merit of using structural steel in affordable housing has to do with time and money. If we, the designers, consulting structural engineers, are able to show that we can significantly reduce the time of construction and the budget for construction, then we have a winner. Oftentimes we are being told what to select.... I think our job has to be in education as well. Even if we are told what to use, we think about the opportunity that we have to educate, and we try to, sometimes with more success than others.”

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.”

CASE STUDIES

Photo courtesy of Bedrock

Figure 1. Hudson’s Detroit, aerial view at sunrise.

Photo courtesy of SHoP Architects

Figure 2. Hudson’s Detroit, skyline view.

Photo courtesy of Bedrock

Figure 3. Topping-out of Hudson’s Detroit tower, April 2024: aerial view showing steel facade support elements, plus Comerica Park (baseball) and Ford Field (US football) in the background.

HUDSON’S DETROIT

In downtown Detroit, on the site where J. L. Hudson’s flagship department store (Smith, Hinchman & Grylls, 1911) underwent controlled demolition in 1998, a new 1.5 million-square-foot mixed-use tower complex is rising to transform the skyline and contribute to the city’s revitalization. The new Hudson’s Detroit–the developer Bedrock is retaining a version of the historic name–will comprise two buildings, the 685-foot Hudson’s Tower with residences, an Ian Schrager/Marriott Edition boutique hotel, and exhibition space, and an accompanying 232-foot midrise with retail, office, and event space. Designed by New York’s SHoP Architects with Detroit firm Hamilton Anderson Associates, Hudson’s Detroit will add about 250 apartments totaling 441,500 gross square feet to the local housing stock. It will also attempt to heal the civic trauma of losing a much-lamented local landmark.

Buro Happold partner and New York co-office director Yasmin Rehmanjee points out that the project’s components called for multiple structural systems and materials. “The structural design for this project focused on efficiently addressing both the vision and programming needs of this iconic development. In high-rise construction, especially for mixed-use buildings, it is crucial to use the right materials in the right places. This principle applies equally to new construction and adaptive reuse.”

The office building has an all-steel structural system that allows “expansive, column-free office and event spaces,” Rehmanjee notes. “This was achieved using long-span beams and trusses at the offices as well as 90-foot-long trusses to create the versatile and open event spaces.” The lowest third of Hudson’s Tower uses structural steel, while its upper sections (including hotel and residential components) use concrete (shear walls, columns, and post-tensioned slabs) and steel facade support elements. “Given [that] steel columns take up far less space,” she says, “employing structural steel in the lower part of the buildings allowed for much significant space allocation for the spa, retail, and restaurant areas, providing great amenity spaces.”

The site holds not only memories for generations of Detroiters but the physical remnants of Hudson’s, once the world’s tallest retail building and a narrow second to Macy’s in square footage. “Many old substructure elements and foundation systems from the previous building remained underground,” Rehmanjee says. “We had to ensure that the new caissons avoided these old structures. Minimizing the size of the new foundation elements was beneficial and facilitated construction. Since structural steel buildings are generally lighter than concrete ones, using structural steel for as much of the project as possible reduced the building’s weight and, consequently, the size of the foundation elements.”

William Sharples, founding principal of SHoP, notes that Bedrock founder Dan Gilbert “envisioned this project from the beginning as a 24/7 hub for Detroit, which led to the dynamic range of uses in the civic building and directly impacted the design and engineering solutions. Structural steel was chosen to address the challenges posed by the need for large event spaces with multiple floors above, enabling 50-foot column-free spans that provide maximum flexibility. Another advantage of using steel, given the site conditions, is its application in the 400,000-square-foot office program at the top of the block. The long-span steel structure we used there allowed for column-free floors, providing greater flexibility and visual openness throughout the office levels while enhancing views into the atrium from every floor.”

Hudson’s Detroit scored a major win in April 2024, the same week as its topping-out, when General Motors announced it would move its world headquarters from Renaissance Center to the new complex as commercial anchor tenant. Residents of the tower, meanwhile, will enjoy hotel-grade amenities, 360-degree views of downtown and the Detroit River, proximity to Library Park and its surrounding cultural district, and a sense of becoming eyewitnesses to, perhaps participants in, their city’s long-awaited and repeatedly announced rebirth. Detroit’s troubles (poverty, population losses, 1960s riots, widespread streetlight outages, municipal and auto-sector bankruptcies) have received more national publicity than its victories (the restoration of Michigan Central Station, the removal or renovation of thousands of abandoned houses, the return of major employers, the unheralded vigor of its Black-owned businesses).

“Bedrock has committed to bringing additional residential units to Detroit, through both new construction and by restoring and maintaining the historic fabric of the city,” says James Witherspoon, Bedrock’s senior vice president of architecture and design. “About 50 percent of current residential units in downtown Detroit were added to the market in the last 10 years, and we continue to see a strong trend of new multifamily construction in the city” (Downtown Detroit Partnership). “We have developed an expertise in adaptive reuse and have converted a number of historic office buildings into residential, adding new units to the Detroit residential market, of which 30 percent is committed to being affordable housing. Where appropriate, structural steel provides the flexibility required to facilitate change of use in these historic buildings.” With Hudson’s again brightening its skyline, Detroit is gaining a prominent symbol of urban nimbleness and strength.

Photo courtesy of TenBerke Architects

Figure 4. Brook Street Residence Halls, Brown University (Chen, left; Danoff, right).

Photo courtesy of WSP

Figure 5. Chen Family Residence Hall, Brown University.

Photo courtesy of WSP

Figure 6. MassArt’s TreeHouse Residence Hall by Stantec’s ADD Architects, with structural engineering by WSP’s Odeh Engineers.

Photo courtesy ofWSP

Figure 7. Interior at the Brook Street Residence Halls, Brown University, with steel frame and CLT ceiling.

BROOK STREET RESIDENCE HALLS, PROVIDENCE, R.I.

The Chen Family and William and Amy Danoff Residence Halls at Brown University use a different form of hybrid structure to offer residents the aesthetics of exposed cross-laminated timber (CLT) while taking advantage of structural steel’s virtues: fast construction, long spans, thin members, compact columns, and strong environmental metrics. Designed by TenBerke Architects (formerly Deborah Berke Architects) with David Odeh’s WSP team as structural engineers, the paired five-story dormitories are similar in form and scale but not mirror-imaged. Angular rooflines and subtle irregularities in the fenestration pattern lend a contemporary touch to their contextual material palette of red, brown, and gray brick, the traditional materials of Providence’s urban fabric.

The intended occupants of the two five-story, all-renewably-powered halls are approximately 350 upperclass undergraduates, who often seek off-campus apartments in a city with a chronic housing shortage. Drawing these residents back to the College Hill campus relieves local housing-market pressure and contributes to Brown’s sense of coherence as an urban academic community. The project, like many academic residential buildings, offers a model of structural efficiency that can logically be scaled up beyond its own midrise level.

Odeh Engineers had already completed a similar steel/CLT building at Rhode Island School of Design, working with Nader Tehrani’s NADAAA Architects on the school’s North Hall (see Architectural Record, March 2, 2020); the firm’s academic residential experience also includes the widely heralded Tree House Residence Hall by Stantec’s ADD for Massachusetts College of Art and Design (MassArt), built of steel with slab on deck, using a braced-frame steel truss system using roof-level hat trusses to support its narrow 21-story volume dramatically cantilevered above a curved stone base. Though the Brook Street residences do not perform such structural acrobatics, their combination of CLT flooring and ceilings with an exposed steel frame reflects Odeh’s recurrent interest in arboreal architectural motifs and concepts.

“People like seeing timber,” he observes; “It’s a beautiful material. It’s called the biophilic effect. Using structural steel in combination with the timber allowed us to span longer distances with thinner members, and use very compact columns so that there are fewer vertical elements in the floor plan than a similar system using all timber, like wood columns with wood beams.” Steel’s light carbon footprint, along with renewable power from an off-campus solar facility, also helps Brown achieve its decarbonization aims: the university has pledged to cut campus-wide greenhouse gas emissions to net zero by 2040.

Copyright David Sundberg / Esto

Figure 8. 35XV, aerial view from west showing residential tower atop Xavier High School segment.

Photo courtesy of FXCollaborative

Figure 9. Diagram of 35XV in section, showing programs by floor.

Copyright David Sundberg /Esto

Figure 10. Transition point between residential and academic components of 35XV, with steel load-transfer structure.

35XV, NEW YORK CITY

FXCollaborative’s 347-foot mixed-use building at 35 West Fifteenth Street serves its community in two ways–a school component (Xavier High School’s Fernandez-Duminuco Hall) on the lowest six floors and 17 stories of residences above–and combines two structural systems accordingly. “The residential portion is built in concrete and the school portion at the base is built in steel,” notes architect Toby Snyder. The glazed residential volume is set back from the midblock streetscape with a separate entrance, cantilevered off a steel platform to hover 17 feet over the school and 36 feet over a rear yard, with a tapering form that preserves sunlight angles to the street, as New York setback buildings have done for over a century. The granite-clad school component includes classrooms, a laboratory, a commons, and a double-height space for Xavier’s music program. The seventh floor includes amenities for the residential component (gym, lounge, children’s playroom, wine cellar, and terrace). With separate vertical circulation for the two components, the residents and students need never mix.

At the building’s design stage, says Snyder, “That was a great moment to ask that question: Why not do the whole thing in steel? Or why not do the whole thing in concrete?’ And I think the reason really comes down to the program. What residential spaces need are rooms that are 10, 15 feet wide, fairly short-span spaces, and they don’t have a lot of need for complicated mechanical systems that go all the way out to the edge of the building envelope. A lot of the rooms that you have at the perimeter of a residential building are bedrooms and living rooms. They can get a lot of their mechanical service from further back in the building, where you can have lowered ceiling height.” Floor-to-floor heights in the school floors are much higher with wider spans, requiring the higher strength-to-weight proportions of structural steel.

Hybrid structures for mixed-use buildings have become something of a signature typology for FXC, Snyder says, calling for expertise beyond the individual components. “You have to understand all of these programs on their own,” he says; “you have to be able to do one on its own, and then you have to know them both truly on their own terms, and then you have to know how to put them together. So it’s like three kinds of knowledge.” In other mixed-use FXC projects involving school spaces, he says, “sometimes it’s a gymna-torium, or a cafe-torium, or a gymna-teria,” all of which require long-span structural members.

“We’ve done other hybrids where there’s some institutional component at the base and some sort of developer component at the top, whether office above school, office above church, residential above church, or residential above school,” Snyder recalls. “But we often make that structural change decision 80 feet up in the air. When it goes from one type of program to a different type, even if it’s concrete at the top part of the building, transferring that load often needs to be done in steel because you’re moving one location of column spans to a completely different one for what’s going on in the base. You can’t necessarily keep the same column grid throughout these different programs. They sometimes have independent cores, so the building can really have a big shift from the top to the bottom... and a lot of that load has to be transferred onto steel just to be able to do it in a shorter span.”

Mixed-use buildings like 35XV, Snyder adds, can be beneficial to “institutions that are potentially land-rich–they own a great asset–[but] cash-poor; they don’t have a great endowment. There’s a developer that wants to partner with them, builds them a brand new facility on their land that they own, and then on top of it, builds their profit-generating portion above it.” Such air-rights arrangements between not-for-profits and private developers call for expertise in the puzzling details of New York City zoning, he notes, as well as the details of matching structural materials to purposes. When the pieces of a hybrid project come together, “it makes business sense and programmatic sense, and for us, it makes architectural sense and urbanistic sense, because it creates a genuine mix of uses.”

CONCLUSION

FXC’s Snyder speculates that if a residential building boom lies ahead, several factors could contribute to a resurgent popularity of structural steel. With steel planking systems, “the erection tolerance is something where, if we need to get tighter with that, steel can provide a certain level of precision that you don’t necessarily always find in concrete. There may be a moment when the labor market changes and that’s more of a factor. The precision of certain prefabricated elements needing to be put into place [is] something where it may make sense to work with steel. There may be a cultural change where building height is no longer considered such a constraint,” if the severity of the housing crisis comes to outweigh antipathy to shadows by community spokespersons. “Maybe as my generation and younger are looking for housing, they’re going to be more willing to accept building height than our previous generation. We might start to have cities that are taller, that embrace height again, in which case some of the benefits that steel provides really make a lot of sense.

“We’re also becoming culturally much more sensitive about carbon footprint and the recyclability of materials,” Snyder continues. “As we think about some of the challenges of purpose-built office buildings of the 1950s and 60s that were so rigidly built to only work as offices—and it’s such a challenge to turn them into residences—we are starting to think about buildings more flexibly,” accommodating multiple programs so that a building “may have to almost be built more like a Lego set, where you imagine that you could build some part of it now and add further components to it later or take further components off.” The accommodation to hybrid work during COVID and the loosening of certain regulations proposed by New York’s City of Yes for Housing Opportunity initiative, he notes, both indicate that “we’re thinking more fluidly about program, and we’re learning to live in different kinds of spaces... So there’s cultural and regulatory and technological and climatological changes that are all taking place. It certainly seems to be a moment where we should not just be doing things the way we’ve always done them before.”

At the urban-planning level, Snyder sees a potential tipping point in various cities’ embrace of transit-oriented development—or, as FXC prefers to conceive it, transit-integrated development—as a strategy for not only replacing auto-dependent sprawl patterns with higher density but placing residences closer to workplaces, retail, schools, and new open-space street networks as well as transit. Near Metro North stations in the Bronx and New Rochelle, N.Y., the subway system in New York City, and the expanding Metro system in Washington, DC, along with its metropolitan region from Fairfax County, Va., to Bethesda, Md., FXC is working on higher-density residential development including multiuse buildings; “we’ve had some newer projects there that are much higher density, sometimes integrated with the transit station.” Steel decking at Manhattan’s Hudson Yards, Sunnyside Yards in Queens, and Philadelphia’s Schuylkill Yards, he adds, allows residential construction directly above transit infrastructure. Residential/institutional hybrids (including the schools that serve residents of newly densified urban centers) and other mixed-use buildings use structural steel as programs and load transfers require.

The housing crisis, as crises often do, is creating conditions that call for a departure from past practices. As the design and construction professions respond to the intertwined problems of housing tomorrow’s population, structural steel is certain to be part of the solution.

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Bill Millard is a New York-based journalist who has contributed to Architectural Record, The Architect’s Newspaper, Oculus, Architect, Annals of Emergency Medicine, OMA’s Content, and other publications.