Wood Trusses Strength, Economy, Versatility INTERNATIONAL Building series NO. 2 Introduction Wood trusses are engineered Wood trusses are widely used in frames of lumber joined single- and multi-family residen- together in triangular shapes tial, institutional, agricultural and by galvanized steel connector commercial construction. Their plates, referred to commonly high strength-to-weight ratios as truss plates. permit long spans, offering greater flexibility in floor plan layouts. They can be designed in almost any shape or size, restricted only by manufacturing capabilities, shipping limitations and handling considerations. Metal plate connected roof wood trusses and increasingly, trusses were first introduced into wood floor trusses are being the North American market in used in residential and the 1950’s. Today, the majority commercial applications. of house roofs in Canada and the United States are framed with Wood truss use is not limited to North America. They are gaining acceptance around the world and are widely used in Europe and Japan. 2 INTERNATIONAL Building series NO. 2 We’ve Come a Long Way The first light frame trusses In the 1950’s the metal connector were built on site using plate transformed the truss nailed plywood gusset plates. industry by allowing efficient These trusses offered prefabrication of short and long acceptable spans but span trusses. demanded considerable time to build. In North America the wood truss industry has grown to the point where more than 60% of residential roofs are now built with wood trusses. In Canada, approximately 95% of new • Economy: Through effi- • Research has led to houses are built with wood cient use of wood and by pro- improved materials, design roof trusses. viding a system that is quickly procedures and manufacturing installed in the field, wood technologies for wood trusses. When the advantages of wood trusses provide an economical trusses are considered, it is framing solution. • Truss plates used to con- not surprising that their use is nect the wood pieces together increasing throughout the world. • Versatility: Complex have been optimized for shapes and unusual designs strength and cost. • Strength: Trusses provide are easily accommodated using a strong and efficient wood wood trusses. The versatility • Lumber manufacturing system specifically engineered of wood trusses makes it an and grading technologies have for each application. excellent roof framing system evolved to allow more efficient in hybrid construction where use of the wood resource. wood trusses are commonly used with steel, concrete or • Computers have been masonry wall systems. widely used to optimize truss design and make the manufac- • Environmental: Wood, turing process more efficient. the only renewable building material, has numerous environ- mental advantages. Wood trusses 3 enhance wood’s environmental advantages by optimizing wood use for each specific application. Improvements in materials, design and manufacturing technologies have increased wood truss competitiveness. WOOD TRUSSES | Strength, Economy, Versatility Wood Truss Technology There are a number of steps Truss Design the building designer or builder In North America, designs involved in the production will contact the truss fabricator are based on the structural and installation of a truss. A The truss design is initiated by who will supply a fully requirements of the Building truss is designed for a specific the building designer who must engineered truss. The truss Codes using design standards application, manufactured specify; the shape and span of plate manufacturer usually referenced in the Building Codes in accordance with the truss the truss, where the truss will be designs the truss on behalf of and approved material properties: design, delivered to the supported and what the loads the truss fabricator. building site and safely on the truss will be. Typically, installed in accordance with the design. FIGURE 1: Truss Nomenclature and Common Truss Shapes Pitched (triangular) Truss Roof Trusses: Light trusses are manufac- Panel length Fink tured to suit virtually any Lateral bracing roof profile. Pitched or flat, Top chord they are only limited to the Webs Panel point load arrangements and the Bottom chord support locations. Truss plate Mono Panel length Clear span Overall lenght Level return overhand Scissors Parallel Chord (flat) Truss Flat Trusses: Panel point Flat trusses, also known as Top chord parallel chord trusses, are an Truss plate alternative to conventional Web 4 wood floor joist systems and Bottom chord Bearing point are a competitive option to Room-in-Attic open web steel joist systems. Panel length Clear span Parallel chord floor trusses Overall lenght may be designed with varying chord and web arrangements and bearing support details. Built-in camber INTERNATIONAL Building series NO. 2 Structural analogues and • Truss bearing and uplift methodology have been requirements, and developed and standardized by the National Associations • Truss member bracing representing the manufacturers requirements. of the metal plate connectors. Lumber design values are Truss Materials determined in accordance with the wood design standards. WOOD Laboratory Testing of Wood Trusses All lumber used in trusses is Truss connector plates are graded using either a visual proprietary and each plate has and 38 x 64 mm for webs. The plate manufacturers carry out a process or machine stress rating different structural properties. size of the members increases series of tests to determine the in accordance with national Design values for truss plates according to loading, span and design properties for each type standards. In the visual grading are developed through tests truss spacing. In some long-span of plate. process, each piece of lumber is and analyses in accordance applications, multi-ply trusses are visually examined and the wood Many sizes and gauges of con- with referenced standards. used to increase capacity. grade is based on the size of nector plates are manufactured. Approval of the design values growth characteristics that could TRUss platES The most common plates use is overseen by National certifi- affect the strength and stiffness 16, 18 or 20-gauge (US cation organizations. of the member. Each piece of The truss connector plates are Standard Gauge) sheet steel, proprietary products. They are Truss design is facilitated by the machine stress rated lumber in widths of 25 mm to 250 mm made of galvanized steel and use of computer software that undergoes a stiffness evaluation and lengths up to 600 mm. manufactured by high speed designs all truss members and in addition to a visual evaluation. stamping machines that punch The stamping results in teeth connections and produces a The minimum size of lumber out the plate teeth and shear the with dimensions varying from design drawing with all the used is 38 x 89 mm for chords plate to the required size. Truss 6 mm to 25 mm. essential truss information. Included on the drawing (see Figure 2) is: • The truss geometry, 5 • The loads used in the truss design, • Species, size and grade of all wood members, • Size and location of all connector plates, Truss Connector Plates WOOD TRUSSES | Strength, Economy, Versatility FIGURE 2: Sample of a Truss Shop Drawing 6 INTERNATIONAL Building series NO. 2 Truss Manufacture Truss Handling, Truss Bracing installed according to specifica- Installation and tions provided by the truss The factory manufacture of light Trusses must be braced to designer or the building frame trusses is demonstrated Storage ensure safety and performance. designer. Permanent bracing in Figure 3. Since wood trusses To do so, trusses are placed provides lateral support to Trusses are strong in the vertical are custom made, the variety according to installation compression web and chord position but can be damaged of roof pitches and location procedures and guidelines members and prevents overall at the plate joints if bent in the of lumber members entails provided by the truss fabricator. lateral displacement of the lateral direction. Trusses should complex cutting patterns. Each During construction, the installer roof assembly. be unloaded in bundles and member must fit snugly in place. provides temporary bracing stored on level ground, but to keep the trusses plumb and never in direct contact with the The computer design of trusses correctly spaced and to prevent ground. Trusses should always generates fabrication instructions. damage or collapse caused be protected from the elements. These indicate the size and grade by lateral loads such as wind. During unloading and erection, as well as the precise cutting Permanent bracing is also patterns for each of the chord and proper lifting equipment must be web members. The type, size, used to ensure safety and to location and orientation of the prevent damage. Trusses less connector plates are also indicated. than 6 m can be installed by hand while trusses over 18 m Once the pieces have been cut use heavy rigging equipment. and arranged using a template, identical truss plates are placed Groups of trusses can be on opposing faces at the joints assembled on the ground and and pressed into the lumber lifted together into position. This using hydraulic presses or prevents lateral strain on the rollers. When the pressing of joints and resists wind loads prior to final installation of sheathing the plates has been completed, FIGURE 3: Manufacture of Light Frame Trusses the trusses are checked for plate or permanent