As the home-building industry slowly comes back from the “Great Recession” of 2007–2009, the urban infill market stands out as a bright spot. City living is attractive to singles, young families, and downsizing baby boomers. And in markets where undeveloped land is scarce, scattered infill lots offer some of the last available building sites around.
Portland, Maine, offers a good example. In the city’s trendy East End, teardown and infill projects are sprouting up on almost every block. The project shown here, in the city’s Munjoy Hill neighborhood, is a four-unit building squeezed into a tight lot between an older, existing wood-frame house and a brand-new four-story mixed-use condominium building.
The Design Problem
Architect David Lloyd of Archetype Architects in Portland (archetype-architects.com)
told JLC that a change in city zoning rules last year allowed his clients, who also own the old wood-frame house next door, to build a separate structure on the property. (The earlier rule would only have permitted a structure adjoining the existing house.) But the required setbacks still limited Lloyd’s design options. The front setback from the street had to fall at the average of the setbacks of the existing buildings on either side. And required clearance on the sides and back of the long, narrow lot also applied a squeeze to Lloyd’s vision.
Lloyd chose to place his new four-unit building as close as possible to the boundary with the new condo to its north and to leave space on the south side, to gain the benefit of sunshine on the new building’s windows and balconies, facing due south. Lloyd located the balconies toward the front of the building, near the street, allowing a view over the open street toward the waterfront. A garage under the building, with doors facing south, provides four parking spaces—although the location’s tight confines, Lloyd said, barely left room for a driveway.
A Structural Solution
With the form of the building tailored to the lot, the structural design was a secondary problem—which landed in the lap of Portland engineer Aaron Jones, of Structural Integrity Consulting Engineers (structuralinteg.com). Jones chose steel I-beams for garage headers, and parallel-strand lumber (PSL) beams running across the width of the building and cantilevering out to support the balconies.
The pair of two-car garage doors falls under a stepped overhang, in a profile similar to a garrison colonial house—a familiar concept to most builders. But with the overhanging upper-story walls carrying three levels of roof and floor loads down onto the cantilevered trusses of the first occupied floor above the garage, the loads involved are heavy. That’s what drove the choice of steel I-beams for the garage headers, Jones explained. Engineered wood might have been able to make the spans, but steel offered better control of deflection across the wide garage door span.
The steel beams for the two garage-door openings were the same, even though one of the headers also has to carry one of the PSL balcony supports crossing the steel beam near mid-span. (Compared with the main floor loads, Jones explained, the balcony loads are relatively minor.)
PSL members can easily handle the weight of the balconies, Jones said. But he beefed up the PSLs in order to enhance the feeling of stiffness underfoot for building occupants. “Cantilevers are a big concern to all of us who are designers,” Jones said, “because we’re trying to make them not bounce. To make everybody happy with them at the end of the day is always a battle, because we can put so much structure in there trying to make that diving board feel less springy.” In this case, the PSLs that were lined up with partition walls inside the buildings were only 3 1/2 inches thick. But the beams that ran through the floor, away from any floor or wall loads that would stiffen the beams, were sized at 5 inches thick to maintain a stiffer feeling underfoot for people using the balconies.
Detailing the two stories of balconies will be interesting for contractor Shawn Geyer, of Geyer Construction, who is responsible for all the building’s exterior details as well as the framing. The projecting PSL beams will have to be cut down with a taper to shed rain and equipped with outer beams and an infilled floor frame (also tapered for drainage). Then the deck structure will have to be waterproofed, tapered sleepers will be applied to restore a level plane, and finally, the decking and a railing system will be installed. The underside of the balconies will receive a tongue-and-groove board ceiling.
But Geyer’s immediate task was just to get his materials off the delivery truck and onto the site—a task made easier by having a large crew of framers on the job. Setting the steel I-beams, however, required more than muscle power: After bracing off the wall, Geyer called for a crane to lift his beams into place. The crew bolted 2x6 plates onto the underside of the beams before setting them, then marked and fastened an upper plate to the beams in place, allowing wood-to-wood connections between the steel beams and the walls below and floor framing above.
With the steel headers in place, the next task was to set the first level of Parallams—again, a job for the crane, along with plenty of skilled hands. On the upper levels, the crane was even more useful, as it came in handy for booming bundles of floor trusses and clips of sheathing up onto the higher stories.