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.”