A Three-Point Spaceframe Foundation for Houses in the North
Buildings in northern communities may experience problems because of "frost heave", the seasonal swelling and shrinking of the soil caused by freezing and thawing of water. The heaving soil can shift foundations, crack drywall, affect windows and doors so that they no longer open and close properly, and can cause separation of walls and roof. Buildings shift and settle, adding stress to the structure. Repairs can be costly, and sometimes can be so extensive that it is more cost-effective to demolish the building.
Canada Mortgage and Housing Corporation contracted Triodetic Building Products Ltd. to design a substructure for northern houses that would withstand major shifting and settling. This report concerns the prototype, a three-point spaceframe foundation that acts like a floating slab. It permits movement of the soil and footings but still maintains a level building. The prototype was installed in 1985 under an existing house in Rankin Inlet, Northwest Territories, that was experiencing significant foundation distress.
Preliminary studies, mostly theoretical, indicated a three-point spaceframe foundation would eliminate problems caused by ground movement. Differential settlement or uplift of one point out of three would not cause racking stresses to the building because three points always define a plane and therefore the building would always be stable. Any vertical displacement would simply tilt the plane, and the design of the foundation system was such that the house can be made level again at any time.The basic structural parts of the spaceframe are tubes, called a Achords@, and connectors, called a Ahubs@. Mechanical joints are made by inserting the ends of the chords into slots in the hubs (see Figure 1).
To determine the sizes of the chords and hubs needed for the spaceframe, a computer model was used to simulate expected loadings (from the house, its occupants and snow) and resulting stresses on the foundation system. A grid layout corresponding to the overall dimensions of the building was then generated (see Figure 2).
The spaceframe was designed to be placed on three main supports. The supports securely anchored the frame but also allowed some flexibility in movement, for example, when soil conditions change. The grid configuration also allowed any movement to be easily measured.
The weight of the spaceframe, including timber purlins (floor supports) placed directly underneath the building, was 4,500 kg. Shipping volume was 4 m3 and the crates were not more than 2 m long. This permitted easy shipment by air cargo.
The original plan was to assemble the frame and then slide it under the building. However, existing site conditions dictated that the frame be constructed underneath the house. The house was raised on five jacks and then the frame was assembled. The chords were driven into the hubs with leather-faced hammers to prevent damage to the aluminum components. The chords and hubs were large enough for easy handling while wearing Arctic mittens, and light enough for one person to handle.
At the time of assembly, the three jacking units had not yet been delivered to the site. Therefore, the spaceframe was assembled on eight timber pads for support.
After the assembly was completed, the purlins were fastened to the clips provided at the top chord. The house was then lowered onto the spaceframe. The undersides of the floor panels bear on the purlins; the floor beams bear on the top chords of the spaceframe.
Assembling the frame and lowering the house onto the frame took three workers and one working supervisor 120 person-hours (two and a half days) to complete. The frame was easy to install and no specialized labour was needed for the work.
The spaceframe was installed in 1985. Deflection measurements since then indicate the spaceframe has met the design criteria. The frame has worked remarkably well, preventing damage to the house. There are no cracks in the drywall, and windows and doors operate properly.
Placing the frame on three supports puts a lot of stress on the structure near the footings. To alleviate this, additional support chords were installed around the three main anchors. Adding these reinforcement chords takes more time when building the spaceframe and detracts from the overall uniform construction of the frame.
A building using a three-point system is limited in size due to the size of the framing members required to support the large concentrated loads. As well, this system is restricted to buildings that are rectangular. If homeowners want to add on to buildings they must do so by adding to the spaceframe grid, much like an extension to a house.
The three-point spaceframe foundation system is a success at eliminating racking in housing. The system can be used for new and renovation projects and can even be used when relocating a building.
The system is easy to install, and does not require specialized labour. Because the installation of the spaceframe was a prototype, an accurate cost comparison could not be made. However, based on the costs available, the system is cost competitive.
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