by Rick Reynolds
After moving to a mountain community outside Anchorage, Alaska, past clients Sean and Laura McLaughlin challenged Bensonwood and site contractor, Dana Foley, whom they had previously worked with on their Massachusetts fun barn, to build their most challenging project to date. The design concept, from SAJJ Architecture, called for a timber and glass stair tower and connector to the nearby garage, that could withstand 140 mph winds, the potential for significant seismic activity, and heavy snow loads.
Given the complexity of the job, the shortness of the building season, and the remoteness of the site, the project demanded the efficiencies and precision of off-site fabrication and rapid on site assembly.
Visually, the client wanted to fully expose the wood, metal, and glass structure. Adding to the challenges of engineering, schedule and budget, the entire assembly had to land on a pre-existing slab, complete with radiant heat. To ensure minimal disruption of the slab, job captain Mark Williston traveled to the site in March with a total station theodolite (TNT), a transit with integrated electronic distance meter (EDM) for taking precise measurements critical to the CAD model.
The project has three distinct structural elements: 1) a large semi-cylindrical timber frame tower enclosing 2) a free-standing spiral wooden staircase, and 3) an attached timber connector. To strengthen the structure, custom steel fittings were created by local New Hampshire artisan, Bob Taylor, and later put through a hot-dipped galvanizing process to best weather the hostile Alaska environment. A customized structural insulated panel (SIP) roof system will cover the connector and stair tower.
The entire assembly will join the existing main house and hanger-like garage, and offer multiple entry points to the house. And to take in the breathtaking views of snow-covered peaks to the back and frontal views looking down the coastal plain to the ocean and distant mountains, the heavy timber structure will be sheathed in triple-paned glazing on site, making it a lantern, in effect.
The large timber tower, the connector, and the roof panels were fabricated at Bensonwood’s Blackjack facilities. Some of the heavy Douglas fir glulam timbers were CNC milled while others were hand shaped. They were then fully-assembled, horizontally, in the timberframe shop and inspected to ensure an accurate fit. Timberframer, Philip Henry, had this to say about the tower:
“The stair tower incorporates a lot of different layers that are interdependent. On the very outside of the tower are the posts and wall purlins. The posts are pie-shaped as viewed from above. The pie-shaped nature of the posts, when connected to the wall purlins, gives us the finished openings for the windows which will be installed on site. Just inside the ring of posts are three layers of laminated curves. Each layer is splined together with cherry splines and pegged with oak pegs. Each curved piece is mated to the posts with a pie-shaped saddle that wraps around the post. When all the pieces are locked together, the structure is very rigid. On site, we’ll assemble the curves, posts, and wall purlins on their sides like a green house. Once assembled and braced, the crane will lift the whole assembly and rotate it to stand up. The stair tower is topped with a ring that accepts the starburst of ceiling joists. I’ve had many opportunities to work with curves—as well as other opportunities to work with hand fabricated posts—but this is a first for me to combine the two. We did a lot of layout on the floor of the shop to mate each curve to the appropriate post. I’m looking forward to pulling all the parts together on site in Alaska in a couple of weeks!”
Simultaneously with the tower fabrication, the Bensonwood Woodworking shop team, headed by Kevin Bittenbender, began crafting the dramatic, free-standing spiral staircase. Describing the project, Bittenbender had this to say:
“Spiral stairs are a fun challenge, as are open timber type stairs…..mix the two and you get a real challenge that gets everyone scratching their heads. Most glulam components that we deal with in woodworking projects typically curve in a flat plane (like a curved edge balcony, or an arched doorway). In those situations, the glue covered, thin laminations are bent over an arched form on a floor or wall to make the shape. Once the glue sets, the arch retains its shape since the laminations can’t “slide.” To make each spiraling part requires a very three dimensional glue-up. Spiral stairs are a little different because you’re bending laminations over a radius (as viewed from above), but then those stringers are also rising in elevation. In essence, you are “cupping” the boards to a certain extent, in addition to bending them around a radius. The form for this glue-up is an arch shape with height, which is essentially a section of a cylinder. The laminations that we used for this project were flat grain boards which look more timber-like on the face, but with the vertical grain on the edges of the boards, the glue lines become much less visible. We executed this forming and glue-up process for every part of the stair since they each have a unique radius. Each handrail and each stringer needs its own custom cylindrical form. The spiraling glue-ups are just the starting point for creating the spiraling structure, and then it becomes a little more free form in the layout process. With the stringers in place, as if installed on site, we reference a layout on the floor, which we transfer up to the correct elevations on the stair stringers to locate all of the supports for the treads and balustrade parts. Fun! Many spiral stairs rely on a wall for support, or a center column that runs from floor to floor, but in the design of this stair it is to be totally free standing, which adds an extra engineering challenge resulting in some pretty specific steel connections at the foot of the stair.”
Once the off-site fabrication is completed, the tower and stairs will be disassembled, and the timbers and roof panels packed for shipping via truck to Alaska for a June 1st site date. Two weeks after that, the structure will be ready for glazing.
Design Developer: Doug Reitmeyer
Project Manager: Tom Olsen
Project Developer: Erik Walker
Engineering Team: Chris Carbone, Butch DeLuca
Visual Fabrication: Jon Criswell
Timberframe: Dennis Marcom, Philip Henry, Chops Polcari
Woodworking: Kevin Bittenbender, Skip Singer, Josh Conley, Adam Meyers, Mike LeBlanc and Scott Frazier
Building Systems: Jay Lepple, Justin Killeen, Shawn Brown, Seth Ashworth, Zach Kurimay
Job Captain: Mark Williston in collaboration with Philip Henry and Justin Killeen.