Monocoque Structure for an Arctic House
In the North, differential displacement due to permafrost degradation and frost heave are major problems. This can cause severe racking and subsequent damage to the foundation, building frame, window and door openings, and finishes of a house. The loss of airtightness and the constant need for repairs mean costly maintenance.
Several new foundation designs are being developed to combat this problem. An alternative approach is to construct a stiff structure that is able to withstand foundation movement. This is the concept of the monocoque, an outer shell which bears all or most of the stresses, much like an airplane fuselage or an automobile in which the body and chassis are one unit.
CMHC commissioned Gower Yeung & Associates to build a prototype structure in North Vancouver, designed for Arctic conditions and suitable for full-scale testing. The structure had to be built with commonly used building materials and techniques. Most components of the house were prefabricated, reducing the need for on-site construction and ensuring the quality of the structural components.
The building has 93 m2 (1,000 sq. ft.) of main floor space and 46 m2 (500 sq. ft.) of second storey space. It was supported on four footings rather than on a conventional concrete perimeter foundation. To build the house on these four supports, the longitudinal exterior walls had to be rigidly connected to the roof and the main floor. This formed a single-body tube or monocoque. The footings and the monocoque allowed the building to withstand differential movement. Unlike conventional concrete perimeter foundations, the footings greatly reduced the area exposed to frost heaving and discontinuous permafrost. The stiff structure beared some of the stresses normally transferred to the foundations and resisted bending which can damage the frame and window and door openings.
Two structural A-frames (Figure 1) transferred the wall and beam loads down to the four foundation bearing points. The A-frames provided the structural stability, and also permitted openings through that plane. Because of its location, the rear frame carried 75% of the overall load, and therefore was constructed as a double frame: two frames bolted together which in turn are bolted to the floor beam. The frame was 21 cm (8.3 in.) thick to resist compression buckling. It enabled the side walls to transfer their loads to the four vertical members of the rear frame through double rows of nails. The main floor was hung from the side walls.
The design used 19 mm (3/4 in.) spruce plywood because calculations showed that this thickness would form an adequate monocoque shell. Double framing (i.e., framing on both sides of the plywood) was needed to provide effective nailing of the plywood. Rather than using conventional trusses for the 9.2 m (28 ft.) floor span, the designer chose an I-section for the floor beams, which allowed the tongue of the beam to be firmly secured between the framing lumber by nails acting in double shear (Figure 2).
This detail was used throughout to connect floors, walls and roof framing members. It provided significant end connections for all members.
I-section beams are prone to buckle under some load conditions. To counteract this, the design requires that the members brace one another when possible.
Because the studs, rafters and floor beams were doubled up, the need for economy required the framing to be spaced at 1.2 m (4 ft.) on centre. Pieces of 20 gauge galvanized sheet steel are used to reinforce the connections at the corners of the plywood panels and to ensure the continuity of the stresses.
The load on the foundation was estimated to be 45.4 tonnes (50 tons). This included the structure as built, the anticipated finishes, upper and main floor occupancy and snow loads. The footings were sized to withstand 7325 kg/m2 (1,500 lb./sq. ft.) of pressure. In order to minimize the stresses in the monocoque, two footings were placed along the wall of one end and one each about three-quarters of the way down each side wall (Figure 1). Both upper and lower beams projected beyond the footing to allow for the use of a pair of hydraulic jacks to level the building.
In order to resist high winds, the house was designed to sit close to the ground, with low side walls, a steeply pitched roof and no roof overhangs. Additional resistance to uplift and overturning could be provided by auger anchors.
The ground floor is almost clear of partitions with the exception of the rear frame. Four rooms could be placed behind the front frame and an additional room behind the rear frame. The second floor space is open but head room is restricted at the side walls.
The need for the external sheathing to transfer stress limits the number and size of wall openings. The end walls can accommodate reasonably large openings if they avoid structural framing members.
Shop drawings were produced for prefabrication of most components. On-site drawings were limited to overall dimensions and connection details. Shop drawings were invaluable as the frames would have been very difficult to build if the builders had to use typical house plans.
After preparing the foundation, the floor beams are set on temporary beam supports, blocking and bracing installed, sheet metal reinforcement placed, and the plywood sub-floor nailed in place.
The floor forms a platform on which the frames could be nailed or bolted together. Pockets are cut around the perimeter of the floor to accept the frames and studding. The fully assembled main frame is too heavy to lift by hand, so a crane lifts all the frames into position. When the frames and studs are in place, wall sheathing is installed.
Great difficulty was encountered in setting the attic beams (which required a crane), rafters and upper floor joists. This was due in part to dimensional inaccuracies in the frames (a jig was not used) and the absence of a traditional upper wall plate. This experience showed that it was critical that the floor beams be accurately positioned and that the attic beams be erected before the side walls.
The project had a tight budget of $15,000. This budget and the need for clear floor space (requested by the organization that was taking over the building) forced the designer to create an unorthodox, but practical and economic structure. The actual costs, in 1989, for the foundation and structural frame were:
Two simple tests were performed after construction. When the temporary supports of the rear wall were removed, the deflection of the rear wall of the building was noted with the use of string lines set along the side walls. The immediate deflection was less than 6 mm (0.25 in.) and after five months had increased to 11 mm (0.46 in.).
One of the two posts supporting the front wall was raised by a hydraulic jack 38 mm (1.5 in.) resulting in a 6 mm (0.25 in.) deflection of the other front post. After five months this had increased to 25 mm (1.0 in.).
The monocoque structure is an example of form adapted to function - the exposed floor beams, steep roof pitch, lack of overhangs and limited wall openings are intended to meet the functional requirements of an Arctic environment at minimum cost and without sophisticated technology. However, the size and shape of the building is limited and the location of openings is critical.
A bonus of the open floor plan is the flexibility to arrange the interior to the individual's needs. The only constraint is the rear frame (i.e. the interior frame) which separates the front two-thirds from the rear one-third of the building.
In the spring of 1990, a monocoque structure was constructed at the Yukon College in Whitehorse and finished as a residence to determine the livability, comfort, and performance of this type of construction. About four years after construction, a representative from Yukon College who was responsible for building operations reported that the monocoque structure had been used as an office, and that there had been no complaints or problems concerning the structural performance of the building.
Research Report: Structure for an Arctic House, February 1990.
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