ToolBase.org logo
The Home Building Industry's Technical Information Resource

Back to Standard View
Search TechnologiesAbout Technology Inventory
 
Browse by Building System


Symbol Legend
Adobe Acrobat Reader required for PDF documents

PDF documents require the free Adobe Reader.


All PDF documents open in a new browser window. Close the browser window to return to the site.

Steel-Framed Modular Housing

Steel is gaining a foothold on wood's long-standing dominance in residential framing.

Find manufacturers of this PATH Technology Inventory item

If you're concerned about the quality and strength of framing lumber and its vulnerability to insects and rot, you might consider switching to steel. Steel [Residential Light-Gauge Steel Framing] is gaining a foothold on wood's long-standing dominance in residential framing. As the price and consistency of lumber grows less reliable, some homebuilders are taking a cue from commercial builders, finding fast ways to produce steel-framed products. And with its lightweight and stress-resistant frame, steel lends itself to the rigors of the road.

Capsys Corp. is installing hundreds of two-story, steel-framed modular townhouses in Brooklyn, as described in West of Pennsylvania. At 18' wide, these units stack vertically, with no horizontal service connections, to create a spacious townhouse. Steel modular is the logical solution for Capsys, which builds mostly in dense New York City, where the construction class often calls for fire-rated assemblies. With a local factory, Capsys fabricates steel-framed units as wide as 20', such as the three-story, two-family homes being delivered in another project in Brooklyn. The short, non-interstate distances allows Capsys to deliver wide sections. Capsys now offers three townhouses designs: two two-story plus basement models and a two-story plus cellar model.

Advanced Modular Concepts (AMC) makes narrow modules for a wider geographic market. Varying from 8' x 10' to 8' x 52', the units are sized for easy shipping and can even go by flatbed rail. Developed in response to South Florida's tropical storms of the 1980s, a 4" x 4" x 3/8" rigid tubular frame serves as a post-and-beam superstructure, to which a 14-gauge sheet metal envelope is welded. An inch of expanded polystyrene insulation is applied around this as a thermal break. The cavity between exterior columns is filled with either 4" of rigid foil-face insulation or 1" rigid plus 3" batt (if installed in US). AMC finishes the walls with stucco, siding, or brick on the outside, and gypsum board (or 5/8" veneer plywood on furring strips) on the inside. The floor structure is wood 2 x 4s set on end and butted together within a steel channel frame, which is set flush within the beams. The wood is steam treated, salt cured, and wax coated to make it water- and pest-resistant. The whole package is shrink-wrapped to protect it for shipping.

Deluxe Homes of PA, Inc. builds with either steel framing or wood framing. Deluxe routinely fabricates steel modular housing in multistory (up to seven) configurations and lower-rise multiple dwellings, including ancillary elements like stair towers and elevator shafts. For fireproof, multistory construction, Deluxe uses steel wall framing with steel/poured concrete decking; low-rise uses light gauge joists in a steel channel perimeter band, usually C10x25. For mid-rise, the quantity or thickness of steel members is beefed up to handle the increased load. Deluxe generally uses a structural tube column system with 18 gauge infill studs.


Quality and Durability

Steel framing members are consistently straight and square, resulting in straight walls and square corners. This eliminates the need to cull or sort through a pile of studs of varying quality. The consistent material quality is a result of production in accordance with national standards. Steel is dimensionally stable; it does not expand or contract in reaction to moisture in the environment; it does not rot, warp, split, crack, or creep; nor is it vulnerable to termites or any type of organism.


Kind of difficult

Because tooling for steel home production is more exacting than that for wood, and the market is limited, manufacturers typically don't do one-off designs, preferring to do multiple units.

As with any building with steel-framed exterior walls and roof, investigate how the manufacturer addresses thermal bridging. Thermal bridging is less of an issue in warmer climates. The wall detail must however address potential condensation; rust and corrosion can severely limit life expectancy of the wall assembly.

At 8' wide, AMC modules fall within standard requirements for shipping, so they don't entail the expense and scheduling of safety pace cars for oversize loads. As such, they can be shipped throughout the US for $800 to $2,000. They have been trucked as far as Texas and Florida from the factory in North Dakota. An eight-year-old company, AMC's orders have skyrocketed from two per year in 1997 to thousands in 1999, with most of these going to tropical storm areas overseas. Panama recently solicited an RFP for 240,000 units over six years. AMC attributes the sales explosion to their colorful website.

Capsys, on the other hand, manufactures exclusively for the NYC market, reflected in the width of its units (since delivery is local, the Brooklyn factory location allows for shipping extra-wide units) and adherence to New York City fire codes. Capsys is working on new five- and six-story designs with concrete floors which they expect to release early 2000. Deluxe, an older company with a more extensive product line that includes wood-framed and mid-rise designs, delivers throughout New England, the mid-Atlantic states, and the Caribbean.


AMC has built one-off units for as low as $42/sf (with inexpensive finishes, not including sitework) and as high as $255/sf.


Not Applicable


According to AMC, lack of US demand is due to HUD's not yet approving methods such as 4" of rigid insulation and ABS piping in lieu of electro-mechanical tubing (EMT) for electric conduits. Steps such as replacing some of the rigid with fiberglass and the ABS with EMT will allow approval, according to AMC. Because of the many innovations included in the homes, a design approved in one state might not be approved in another state, requiring building code research for that area.

Requirements for noncombustibility or fire-rated construction class for attached or multiple dwellings are often a driving force behind choosing steel framing.


Quality Homes of the Pacific (QHP), Hawaii and R-Anell Housing Group, Denver, NC.


Capsys constructs its "Saratoga" attached townhouse modules on a perimeter chassis of 12" x 3½" cold-rolled channels, which are welded to a steel plate embedded in the concrete foundation. Exposed walls are infilled with R-13 batt insulation installed with the kraft paper facing the interior as a vapor barrier. Studs, including the 16"o.c., 20-gauge studs used in the front and rear walls, have BX cable running through them, although code allows Romex. Eighteen-gauge studs are used where a side wall is exposed, like at the corner of a city block. At the party wall, two ½" layers of type-X gypsum board are applied on 20-gauge studs, with 5/8"on the inner and outer sides of the front and back walls. Half-inch furring strips are applied where aluminum siding will be installed.

Capsys aims to have as little work as possible done on site. Setting up seven homes in one day requires four people removing the protective tarpaulins, three "erectors" with guide ropes during craning, one crane operator, and two transporter drivers. Since all vertical loads are transferred through the square-section corner columns, the only structural connections between boxes involve slipping the four top-floor columns over exposed tapered tabs projecting from the first-floor columns, and welding them the next day. Then the upper half of the stair is installed and trim is applied around the stair opening. Service connections are similarly easy. A subpanel on the second floor means there is only one electrical connection. Hot/cold water, waste, vent, and flue typically involve a single connection through an access door. Siding and/or brick is installed onsite by the GC.

AMC orders superstructures manufactured by a major steel shape fabricator. All connections are welded and braced, the steel shell and structure is coated with zinc chromate (a corrosion inhibitor), and any cuts or holes are cold galvanized with powdered zinc. For any earth-bermed sections of the building, steel surfaces are additionally coated with asphalt. To make the units strong and airtight, AMC welds the seams of its sheet steel envelope and applies mastic along the seams of the outside rigid insulation. All electrical circuits are fully grounded.

The complete units are placed 90% finished on the foundations (and stacked if multistory), which are typically slab-on-grade and sometimes piles. Once delivered, AMC's responsibility ends. The builder makes connections between boxes and to outside services. AMC says it is easy to connect the boxes, and supplies couplings, additional piping, glue, and primer for the water, sprinkler (every room is sprinkled), and electrical connections accessed through the dropped ceiling. AMC claims its single-story homes are so easy to assemble the owner can do it and AMC will still honor its warranties (although not the construction warranty).

Deluxe makes sure the units are weathertight the same day they are set. Although most assembly occurs in the factory, Deluxe may contract to do some interior work following installation, including joints along mating walls; electrical, plumbing, and mechanical connections between floors; and carpeting. Steel-framed structures are often clad with an exterior insulation finishing system (EIFS). The EIFS includes foam insulation, of which Deluxe opts for 1" or 2". Structural members are welded together in the factory (non-loadbearing members are either welded or fastened with screws); lower units are welded to the above structural frame onsite. An advantage of this approach is ease of crane transport, as the rigid frame can withstand lifting from various points along the top.

Deluxe often incorporates hybrid modular techniques such as panelizing dormers and garages. The scale of Deluxe's operations allows it to maintain its own fleet of trucks and transporters, resulting in greater scheduling control and long-term savings. In its factory operations, Deluxe prides itself on resource efficiency. All excess steel, copper, and aluminum is sold to a recycler (standard practice for steel framers). Any scrap lumber is collected in a large container. Pallets are sent back to suppliers. Cardboard packaging is run through a binding machine and sent off to a recycler along with office paper.


Varies with manufacturer.


The clash between environmentalists and the logging industry, not only a green issue, underscores the dwindling supply of quality old-growth timber. Any carpenter who was an apprentice in the '70s knows that era's standard lumber is premium now, and variable prices reflect the unreliability of the supply. Though residential light-gauge steel framing is often more expensive than conventional wood framing in site building, trained factory crews hone their technique to close the gap.

Since steel is impervious to insects, it is a good choice in areas prone to infestation. Much stronger in tension and compression than wood, steel uses much less material, resulting in a stiff, lightweight structure, with level and true walls less prone to warping. Another advantage of its light weight is in earthquake zones, where steel applies less seismic load, especially compared to heavy wood trusses. Even in these regions, four-story buildings can be designed with light-gauge steel load-bearing walls. Sheet metal sheathing makes lighter, stronger, and stiffer shear walls.

Capsys can set the modules for their two-story, single-family townhouse model on a basement for less than $45/sf builder cost. Capsys claims the modules are 95% complete, but all sitework-foundations, services, and landscaping-are not included. Real savings come in time, where seven houses have been set before 2PM, with a crew of seven plus drivers. Deluxe installs six to a dozen modules in a day, typically eight.

AMC has built one-off units for as low as $42/sf (with inexpensive finishes and, again, not including sitework) and as high as $255/sf. AMC claims its steel frame can support 273,000 lbs dead weight. It also claims up to R-54 walls, but thermal bridging will neutralize some of those benefits. AMC offers a 40-year warranty on its modular units, and a hurricane warranty with the precondition that roll-down shutters be installed at the factory.

Almost all steel framing can be recycled after demolition, and the light gauge steel you build with has about 25% recycled content, whereas structural steel contains as much as 66%.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.