Structural Steel Meets Passive House in Modular Construction

To view the figures and tables associated with this article, please refer to the flipbook above.

In the rapidly evolving landscape of New York City construction, where performance standards grow more demanding and housing needs remain urgent, Bethany Senior Terraces in Brooklyn represents a forward-looking model. It is the city’s first modular affordable senior housing building designed and constructed to meet Passive House standards, a project that integrates volumetric modular construction, structural steel framing, and PHIUS+ 2015 certification to deliver 58 units of sustainable and affordable housing for low-income and formerly unhoused seniors.

Developed by RiseBoro Community Partnership and located in the East Flatbush neighborhood of Brooklyn, the five-story, all-electric building demonstrates how structural engineering can lead innovation, not only by solving technical problems, but by helping define a replicable path toward low-carbon, high-quality, resilient housing in urban settings.

Bethany Terraces includes 58 studio and one-bedroom apartments, all constructed to Passive House criteria. Communal spaces like a test kitchen, greenhouse, and outdoor gardens were included to foster social engagement and wellness. In terms of energy performance, the building is certified under PHIUS+ 2015, DOE Zero Energy Ready, and Enterprise Green Communities, powered in part by a 130-kW solar PV array that meets 100% of common area energy needs and about 80% of the building’s total annual usage.

What distinguishes this project structurally is the combination of volumetric modular construction and Passive House performance, delivered with a steel-framed structure.

Volumetric modular construction is an advanced off-site construction technique in which fully enclosed, three-dimensional building modules are fabricated in a controlled factory environment. These modules are typically delivered to the site with a high level of completion, including integrated mechanical, electrical, and plumbing (MEP) systems, interior finishes, and in some cases, fixtures and furnishings. Unlike panelized systems that involve assembling separate components such as walls or floors on-site, volumetric modular construction delivers structurally independent units that are assembled on site to merge into the complete building.

Bethany’s structural system was designed around a cast-in-place concrete base, which included the foundation (spread footings), cellar floor, a series of piers, and the perimeter wall forming the support system for the modular building above. The remainder of the superstructure, floors 1 through 5 and the rooftop penthouse, was constructed from 47 volumetric steel-framed modules—accounting for 75% of the building's structure—each fabricated off-site by Whitley East LLC in Pennsylvania and delivered to Brooklyn.

Each module measures up to 14 feet wide, up to 60 feet long, and 11 feet high, weighs up to 43,000 pounds and is sized to accommodate two fully finished studio apartments. Modules were fabricated under factory-controlled conditions and delivered at approximately 90% completion, including interior finishes, triple-glazed windows, integrated MEP systems, and envelope thermal insulation.

Each module was designed to support:

In modular construction, each module consists of a six-sided surface, meaning when the upper module is stacked on top of the lower module, there is a floor-ceiling “sandwich.” Furthermore, when two interior modules are placed and installed adjacent to each other, there is a double wall. We refer to these two junctions (vertical and horizontal spaces) as module matelines which define and distinguish the gap between two or more modules. The mateline is a critical location within the module layout, and connections are required to transfer loads across each mateline to complete load paths for gravity and lateral loads. Module matelines are typically between ½ inch to 1 inch, with the latter providing more construction tolerance in the field.

The modules were framed using:

The modules were stacked in five levels over the foundation system and welded together at the floor and wall matelines to create continuity for both the vertical and lateral load paths.

Special care was taken to define gravity and lateral load paths, particularly:

Loads imposed at the site-built portion of the project such as the stepped terraces on the south facade, the greenhouse roof structure, and the rooftop solar canopy were transferred into the modular frames. This required reinforcing module beams and girders with web stiffeners.

To achieve a 2-hour floor fire rating, the UL-H501 assembly was enforced. This assembly consists of a ¾-inch thick nominal structural cement board fastened on top of the structural floor framing. Although it is not required for the H501 assembly, an additional ¼-inch fiber rock was provided to enhance durability for this project. At the bottom of the ceiling framing, one layer of ⅝-inch nominal gypsum board is secured to ½-inchx25ga resilient channels that are fastened to the ceiling framing. In this assembly, individual protection (by means of intumescent paint or applied fireproofing) for the primary floor framing is not required. Not having to place concrete slabs on the floors eliminates the time needed for concrete to cure and also reduces the weight of the modules.

The typical 1-hour wall fire rating was accomplished with UL-U419. This assembly consists of one layer of ⅝-inch type X gypsum board in the interior side of the module. The wall stud cavity was filled with 1-½-inches of acoustical fire mineral wool batt insulation. At the vertical mateline gap, the joint was firestopped with mineral wool. The stair and elevator walls were fire rated for two hours by providing an additional layer of ⅝ inches gypsum on each side of the module.
Modular construction achieves an increased sound transmission class (STC) rating and better noise reduction/transmission due to the thicker floor-ceiling and double wall assemblies.

Meeting Passive House standards in a modular senior housing development introduced unique challenges. The building must achieve high thermal performance, excellent indoor air quality, and accessibility, all within strict energy targets. While Passive House buildings often rely on heavy timber or concrete for thermal mass and airtightness, Bethany Senior Terraces demonstrated how structural steel can be effectively used in Passive House applications, with the right detailing.

One of the most stringent Passive House requirements is achieving less than 0.6 air changes per hour at 50 Pascals (ACH50). Modular construction adds complexity due to the many joints between units.

Each module underwent factory blower door testing before shipping. Final building-level testing occurred after full stacking and mateline sealing. Critical structural connections, such as steel joists spanning from one module into another had to be detailed with air barriers, gaskets, and pre-installed membranes. Mateline gaskets were selected based on thermal expansion coefficients, expected deflections, and durability over time.

Transitions between modular components and site-built elements including the greenhouse, rooftop terrace framing, solar canopy, and stair towers were particularly challenging. These transitions had to meet both structural and Passive House airtightness requirements, which often conflicted. Structural steel also poses a challenge for Passive House designers due to its high thermal conductivity.

To mitigate thermal bridging:

The result was a steel-framed modular building that passed Passive House testing without compromising load path efficiency or constructability.

A key engineering success was the integration of mechanical systems into the structural design. MEP risers were routed through modular corridors and shaft walls, with chase alignment modeled in 3D prior to fabrication.

The all-electric systems include:

CFS joist layouts were coordinated with MEP routing to minimize interference, and all penetrations were detailed with thermal breaks and air seals.

Constructability challenges, such as the interface between modular and site-built components, were resolved through early-stage planning using Revit-based Building Information Modeling (BIM). Structural, architectural, and mechanical systems were tightly coordinated across disciplines to ensure seamless integration, reducing potential conflicts during construction.

Digital coordination tools included a fully integrated 3D Revit model for structural modeling and coordination between other consultants. Moreover, the fully integrated model assisted in the module weight verification, crane pick analysis, and shipping loading constraints.

BIM coordination was also essential to ensure:

Modules were transported over 170 miles from Pennsylvania, requiring planning for size, route, and timing. They were staged in New Jersey and delivered via truck to the site, where a mobile crane stacked all 47 modules over an 11-day install sequence.

The challenges included:

Each module included embedded connections with pre-drilled holes for module installation. The construction team used alignment pins and laser measurements to set each unit within tolerance before welding crews completed horizontal and vertical joints.

Bethany Terraces offers valuable takeaways for engineers pursuing modular or Passive House projects:

Modular + Passive House = Coordination First
Integrating steel modular systems into Passive House requires early design-phase alignment across all trades. Structural engineers must lead in defining how modules interact with airtightness and thermal performance criteria.

Steel in Passive House Is Feasible
With attention to thermal breaks, envelope detailing, and integrated modeling, structural steel can meet Passive House goals, even in modular construction.

Define Modular and Site Responsibilities Early
Clearly delineating responsibilities (between site contractor and modular builder) avoids clashes over fieldwork, reduces change orders, and streamlines the construction process.

Transport and Lift Planning Is Structural Work
Every module is a moving building. Engineers must account for lifting stresses, transport deflection, and pick point reinforcement.

BIM Is Not Optional
Without 3D modeling and real-time coordination, tolerance errors and connection misalignments are likely to derail off-site projects. Model everything, including matelines, and risers.

The project exemplifies how modular Passive House construction is viable at scale. Efficient use of materials and labor was integral to the project’s success. Repetitive, standardized unit layouts—primarily studios and one-bedrooms—enabled economies of scale in fabrication, and transport. Offsite construction reduced material waste, improved labor conditions, and accelerated timelines. Site work and modular fabrication occurred concurrently, shortening the overall project duration and minimizing neighborhood disruption. Use of modular construction allowed the project to avoid significant on-site labor costs typically associated with New York City prevailing wage requirements. It serves as a replicable model for public and supportive housing across the U.S., particularly in urban areas facing cost, labor, and climate constraints.

From a sustainability perspective, Passive House design minimizes operational energy use, while modular construction reduces embodied carbon and material waste. The fully electric, solar-powered design ensures long-term affordability and operational savings. Durable finishes and resilient systems provide thermal comfort and backup capacity in the event of outages or extreme weather. This development is designed with its residents and community in mind, offering spaces that are not simply amenities but extensions of the building’s mission to foster community and provide comfort and dignity to its occupants.

Bethany Senior Terraces has been recognized with:

The use of steel-framed volumetric modules within a Passive House envelope required precise engineering, advanced coordination, and construction discipline. As urban housing, energy efficiency, and prefabrication continue to converge, structural engineers are increasingly challenged to deliver integrated, high-performance solutions. Projects like Bethany Senior Terraces provide a tested roadmap for achieving these goals effectively, efficiently, and at scale. ■

About the Authors

Michael Lynch, PE, SE, has over 35 years of experience in the construction industry, specializing in both conventional and modular structural systems across residential, commercial, educational, and healthcare projects. As Director of Modular Design and Construction at Murray Engineering, he leads the firm’s modular portfolio, including several multi-story housing developments, schools, hotels, and the largest off-site modular healthcare hospital in the United States.

Jimmy Liang, PE, is a Project Engineer at Murray Engineering with ten years of experience in the structural design of steel, concrete, and wood buildings. He has led the design and coordination of complex multi-story residential, commercial, and institutional projects for both conventional and modular off-site construction.