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Pre-engineered buildings offer cost effective ways to design energy efficiency.
Truss Manufacture
The factory manufacture of light frame trusses is demonstrated in Figure 3. Since wood trusses are custom made, the variety of roof pitches and location of lumber members entail complex cutting patterns. Each member must fit snugly in place. The computer design of trusses generates fabrication instructions. These indicate the size and grade as well as the precise cutting patterns for each chord and web member. The type, size, location and orientation of the connector plates are also indicated. Once the pieces have been cut and arranged using a template, identical truss plates are placed on opposing faces at the joints and pressed into the lumber using hydraulic presses or rollers. When the pressing of the plates has been completed, the trusses are checked for plate tooth penetration and transported to a storage area.
Performance
Since they were first introduced in the 1950’s, metal plate connected wood trusses have demonstrated an excellent track record and are recognized in Building Codes throughout North America.
Roof Trusses
• Wood trusses eliminate on-site framing problems. Accurate fabrication and constant quality control assure that trusses are uniform in size and shape and provide required structural integrity to a building.
• Wood trusses can be constructed and spaced to optimize lumber strength and conserve timber resources. For example, smaller dimensioned lumber are used in the truss webs and the typical roof truss spacing of 600 mm on centre optimized roof framing.
• When wood trusses are used as the principal framing members, constructing the roof or floor system is simplified. Wood sheathing can be easily attached to the top chord to provide the underlay for the roofing membrane or floor finish. Ceilings can be readily connected to the truss bottom chords and insulation is easily installed in the truss cavity. In this way, the process is reduced in complexity and extent.
Floor Trusses
• Top chords of floor trusses provide a wide 89 mm surface for easier nailing and increased glue contact area with the sheathing material. This helps build a floor system that is stable and quiet for the life of the structure.
• Additional stiffness can be built into the floor truss and floor system to reduce floor vibration.
• Design requirements for fire safety in buildings are specified in the Building
Codes. Fire-resistance ratings, based on standardized tests, are a measure of the fire resistance of the roof and floor assemblies. Depending on sheathing, ceiling construction, and insulation, truss assemblies have achieved fire resistance ratings up to 2 hours. Not all truss assemblies require a fire resistance rating, though. The building occupancy, the building size, the number of exits and the use of sprinklers will determine what fire resistance rating is required.
• Floor truss assemblies can also be optimized to reduce sound transmission. In apartments, this limits noises from the upper or lower units. For further information on fire and sound performance refer to the “Fire Resistance and Sound Transmission in Wood-Frame Residential Buildings.”
