Portland, Maine, is like many cities on the Atlantic Coast: In the popular parts of town, housing is scarce and property values, along with rents, are rising. In the older, fully built-out parts of the city, developers are looking for high-density solutions. In many cases, the answer is an infill project. Outdated, older single-family homes are being torn down, and the few small remaining empty lots are being developed to support shoulder-to-shoulder multifamily construction.
Last month, JLC showcased the balcony structure of a new four-unit apartment building in Portland, built on a tight lot with close setbacks (see "Cantilevered Balcony Beams," JLC 5/16). This month, we’ll take a look at a nearby building under construction that’s not just close to the neighbor—it’s touching it (well, almost).
We talked with contractor Rob Paisley on site during the framing of the new apartment building. The property is sited on a double lot, adjacent to an existing building that dates back to the 1800s.
A recent change in the neighborhood’s zoning allows the developer to build a new four-story structure directly abutting the older house. But while that zoning change was pending, Paisley renovated the existing house. Now, he’s constructing the new building next door.
“When you do that,” Paisley explained, “the new structure has to be completely isolated from the old one.” To accomplish the required fire separation, Paisley and his framing crew built a typical two-hour firewall (in Paisley’s phrase, a “burn wall”) between the two structures as they framed the new building.
This isn’t the only fire-rated wall in the building. The building’s two stairwells and its elevator shaft are each isolated from the rest of the structure with two-hour fire-rated assemblies. That was a challenging problem, said Paisley. “There is no plate-to-plate connection anywhere in this building,” he said. “It literally has to be broken everywhere, which makes this hard.”
With no wood-to-wood connections, keeping the walls straight and plumb has had its own difficulties. And with four occupied floors above the garage level, topped off by a walkable rooftop patio, resistance to lateral loads is also a concern. “This project encompasses every bit of residential and commercial you’d be likely to see in one building,” said Paisley. “It could be a crash course for somebody who has never done it before.”
Paisley beefed up structural assemblies in several places—for instance, adding shearwalls in the garage level and applying structural sheathing to both faces of some wall assemblies. For the wall backing up the two-hour gypsum-panel assembly, Paisley’s crew installed old-school diagonal bracing between the studs.
As the new building rose next to the old one, the two-hour fire separation assembly had to be pieced together in between. That’s accomplished using light-gauge-steel H-studs and C-runners, attached to the wood framing with aluminum breakaway clips. The 4-foot-by-2-foot pieces of gypsum board slip into the channels of the vertical H-studs and horizontal C-channels, and are secured lightly to the metal with screws. Then the metal framework is attached to the wood framing of the buildings on either side using the aluminum clips.
The gypsum board is tested to hold up for two hours in the event of a fire, allowing time for occupants of adjacent buildings to evacuate and for firefighters to extinguish the fire. If fire spreads into the wood walls on either side of the firewall (and the assembly works as designed), the heat of the fire will melt the aluminum clips only on the affected side, so that collapsing stud walls on that side would not pull down the firewall.
Ted Cushman is a senior editor at JLC.