Light-frame Trusses
A truss is a structural frame relying on a triangular arrangement of webs and chords to transfer loads to reaction points. This geometric arrangement of the members gives trusses high strength-to-weight ratios, which permit longer spans than conventional framing. Light-frame truss can commonly span up to 20 m (60 ft), although longer spans are also feasible. The first light-frame trusses were built on-site using nailed plywood gusset plates. These trusses offered acceptable spans but demanded considerable time to build. Originally developed in the United States in the 1950s, the metal connector plate transformed the truss industry by allowing efficient prefabrication of short and long span trusses. The light-gauge metal connector plates allow for the transfer of load between adjoining members through punched steel teeth that are embedded into the wood members. Today, light-frame wood trusses are widely used in single- and multi-family residential, institutional, agricultural, commercial and industrial construction. The shape and size of light-frame trusses is restricted only by manufacturing capabilities, shipping limitations and handling considerations. Trusses can be designed as simple or multi-span and with or without cantilevers. Economy, ease of fabrication, fast delivery and simplified erection procedures make light-frame wood trusses competitive in many roof and floor applications. Their long span capability often eliminates the need for interior load bearing walls, offering the designer flexibility in floor layouts. Roof trusses offer pitched, sloped or flat roof configurations, while also providing clearance for insulation, ventilation, electrical, plumbing, heating and air conditioning services between the chords. Light-frame wood trusses are prefabricated by pressing the protruding teeth of the steel truss plate into 38 mm (2 in) wood members, which are pre-cut and assembled in a jig. Most trusses are fabricated using 38 x 64 mm (2 x 3 in) to 38 x 184 mm (2 x 8 in) visually graded and machine stress-rated (MSR) lumber. To provide different grip values, the truss connector plates are stamped from galvanized light-gauge sheet steel of different grades and gauge thicknesses. Many sizes of truss plates are manufactured to suit any shape or size of truss or load to be carried. Light frame trusses are manufactured according to standards established by the Truss Plate Institute of Canada. The capacities for the plates vary by manufacturer and are established through testing. Truss plates must conform to the requirements of CSA O86 and must be approved by the Canadian Construction Materials Centre (CCMC). To obtain approval, the truss plates are tested in accordance with CSA S347. During design, light-frame trusses are generally engineered by the truss plate manufacturer on behalf of the truss fabricator. When light-frame trusses arrive at the job site they should be checked for any permanent damage such as cross breaks in the lumber, missing or damaged metal connector plates, excessive splits in the lumber, or any damage that could impair the structural integrity of the truss. Whenever possible, trusses should be unloaded in bundles on dry, relatively smooth ground. They should not be unloaded on rough terrain or uneven spaces that could result in undue lateral strain that could possibly distort the metal connector plates or damage parts of the trusses. Light-frame trusses can be stored horizontally or vertically. If stored in the horizontal position, trusses should be supported on blocking spaced at 2.4 to 3 m (8 to 10 ft) centres to prevent lateral bending and reduce moisture gain from the ground. When stored in the vertical position, trusses should be placed on a stable horizontal surfaced and braced to prevent toppling or tipping. If trusses need to be stored for an extended period of time measures must be taken to protect them from the elements, keeping the trusses dry and well ventilated. Light-frame trusses require temporary bracing during erection, prior to the installation of permanent bracing. Truss plates should not be used with incised lumber. Contact the truss manufacturer for further guidance on the use of light-frame trusses in corrosive environments, wet service conditions, or when treated with a fire retardant. For further information, refer to the following resources: Canadian Wood Truss Association Truss Plate Institute of Canada CSA O86 Engineering design in wood CSA S347 Method of test for evaluation of truss plates used in lumber joints Canadian Construction Materials Centre
Lumber
Dimension lumber is solid sawn wood that is less than 89 mm (3.5 in) in thickness. Lumber can be referred to by its nominal size in inches, which means the actual size rounded up to the nearest inch or by its actual size in millimeters. For instance, 38 × 89 mm (1-1/2 × 3-1/2 in) material is referred to nominally as 2 × 4 lumber. Air-dried or kiln dried lumber (S-Dry), having a moisture content of 19 percent or less, is readily available in the 38 mm (1.5 in) thickness. Dimension lumber thicknesses of 64 and 89 mm (2-1/2 and 3-1/2 in) are generally available as surfaced green (S-Grn) only, i.e., moisture content is greater than 19 percent. The maximum length of dimension lumber that can be obtained is about 7 m (23 ft), but varies throughout Canada. The predominant use of dimension lumber in building construction is in framing of roofs, floors, shearwalls, diaphragms, and load bearing walls. Lumber can be used directly as framing materials or may be used to manufacture engineered structural products, such as light frame trusses or prefabricated wood I-joists. Special grade dimension lumber called lamstock (laminating stock) is manufactured exclusively for glulam. Quality assurance of Canadian lumber is achieved via a complex system of product standards, engineering design standards and building codes, involving grading oversight, technical support and a regulatory framework. Checking and splitting Checking and splitting Checking occurs when lumber is rapidly dried. The surface dries quickly, while the core remains at a higher moisture content for some time. As a result, the surface attempts to shrink but is restrained by the core. This restraint causes tensile stresses at the surface, which if large enough, can pull the fibres apart, thereby creating a check. Splits are through checks that generally occur at the end of wood members. When a wood member dries, moisture is lost very rapidly from the end of the member. At midlength, however, the wood is still at a higher moisture content. This difference in moisture content creates tensile stresses at the end of the member. When the stresses exceed the strength of the wood, a split is formed. Large dimension solid sawn timbers are susceptible to checking and splitting since they are always dressed green (S-Grn). Furthermore, due to their large size, the core dries slowly and the tensile stresses at the surface and at the ends can be large. Minor checks confined to the surface areas of a wood member very rarely have any effect on the strength of the member. Deep checks could be significant if they occur at a point of high shear stress. Checks in columns are not of structural importance, unless the check develops into a through split that will increase the slenderness ratio of the column. The specified shear strengths of dimension lumber and timbers have been developed to consider the maximum amount of checking or splitting permitted by the applicable grading rule. The possibility and severity of splitting and checking can be reduced by controlling the rate at which drying occurs. This may be done by keeping wood out of direct sunlight and away from any artificial heat sources. Furthermore, the ends may be coated with an end sealer to retard moisture loss. Other actions which will minimize dimension change and the possibility of checking or splitting are: specifying wood products that are as close as possible in moisture content to the expected equilibrium moisture content of the end use ensuring dry wood products are protected by proper storage and handling Fingerjoined lumber Fingerjoined products are manufactured by taking shorter pieces of kiln-dried lumber, machining a ‘finger’ profile in each end of the short-length pieces, adding an appropriate structural adhesive, and end-gluing the pieces together to make a longer length piece of lumber. The length of a fingerjoined lumber is not limited by the length of the log. In fact, the manufacturing process can result in the production of joists and rafters in lengths of 12 m (40 ft) or more. The process of fingerjoining is also used within the manufacturing process for several other engineered wood products, including glued-laminated timber and wood I-joists. The specific term “fingerjoined lumber” applies to dimension lumber that contains finger joints. Fingerjoining derives greater value from the forest resource by using short length pieces of lower grade lumber as input for the manufacture of a value-added engineered wood product. The fingerjoining process utilizes short off cut pieces of lumber and results in more efficient use of the harvested wood fibre. Fingerjoined lumber can be manufactured from any commercial species or species group. The most commonly used species group from which fingerjoined lumber is produced is Spruce-Pine-Fir (S-P-F). Design advantages of fingerjoined lumber Fingerjoined lumber is an engineered wood product that is desirable for several reasons: straightness dimensional stability interchangeability with non-fingerjointed lumber highly efficient use of wood fibre The design and performance advantages of this engineered wood product are its straightness and dimensional stability. The straightness and dimensional stability of fingerjoined lumber is a result of short length pieces of lumber, consisting of relatively straight grain and fewer natural defects, being combined with one another to form a longer length piece of lumber. The grain pattern along fingerjoined lumber becomes non-uniform and random by attaching many short pieces together. This results in fingerjoined lumber being less prone to warping than solid sawn lumber. The fingerjoining process also results in the reduction or removal of strength reducing defects, producing a structural wood product with less variable engineering properties than solid sawn dimensional lumber. The most common use of finger-joined lumber is as studs in shearwalls and vertical load bearing walls. The most important factor for studs is straightness. Fingerjoined studs will stay straighter than solid sawn dimensional lumber studs when subjected to changes in temperature and humidity. This feature results in significant benefits to the builder and homeowner including a superior building, the elimination of nail pops in drywall and other problems related to
Lumber properties
For many years, the design values of Canadian dimension lumber were determined by testing small clear samples. Although this approach had worked well in the past, there were some indications that it did not always provide an accurate reflection of how a full-sized member would behave in service. Beginning in the 1970s, new data was gathered on full-size graded lumber, known as in-grade testing. In the early 1980s, the Canadian lumber industry conducted a major research program through the Canadian Wood Council Lumber Properties Program for bending, tension and compression parallel to grain strength properties of 38 mm thick (nominal 2 in) dimension lumber of all commercially important Canadian species groups. The Lumber Properties Program was conducted as a cooperative project with the US industry with the goal of verifying lumber grading correlation from mill to mill, from region to region, and between Canada and the United States. The in-grade testing program involved testing thousands of pieces of dimension lumber to destruction in order to determine their in-service characteristics. It was agreed that this testing program should simulate, as closely as possible, the structural end use conditions to which the lumber would be subjected to. After the test samples were conditioned to approximately 15 percent moisture content, they were tested under short- and long-term loading in accordance with ASTM D4761. Lumber samples in three sizes; 38 x 89 mm, 38 x 184 mm and 38 x 235 mm (2 x 4 in, 2 x 8 in, and 2 x 10 in), were selected across the Canadian growing regions for the three largest-volume commercial species groups; Spruce-Pine-Fir (S-P-F), Douglas Fir-Larch (D.Fir-L) and Hem-Fir. Select Structural, No.1, No.2, No.3, as well as light framing grades, were sampled in flexure. Select Structural, No.1 and No.2 grades were evaluated in tension and compression parallel to grain. Several lesser-volume species were also evaluated at lower sampling intensities. The in-grade testing resulted in new relationships between species, sizes and grades. The dimension lumber database of results was examined to establish trends in bending, tension and compression parallel to grain property relationships as affected by member size and grade. These studies provided a basis for extending the results to the full range of dimension lumber grades and member sizes described in CSA O86. In Canada, both the CSA O86 and the National Building Code of Canada (NBC) have adopted the results from the Lumber Properties Program. The data has also been used to update the design values in the United States. The scientific data resulting from the Lumber Properties Program demonstrated: close correlation in the strength properties of visually graded No.1 and No.2 dimension lumber; good correlation in the application of grading rules from mill to mill and from region to region; and a decrease in relative strength as size increases (i.e. size effect) – for example the unit bending strength for a 38 × 89 mm (2 x 4 in) member is greater than for a 38 × 114 mm (2 x 6 in) member. Following the testing program, the consensus-based ASTM D1990 standard was developed and published. Data for bending, tension parallel to grain, compression parallel to grain, and modulus of elasticity continue to be analyzed in accordance with this Standard. Unlike visually graded lumber where the anticipated strength properties are determined from assessing a piece on the basis of visual appearance and presence of defects such as knots, wane or slope of grain, the strength characteristics of machine stress-rated (MSR) lumber are determined by applying forces to a member and actually measuring the stiffness of a particular piece. As lumber is fed continuously into the mechanical evaluating equipment, stiffness is measured and recorded by a small computer, and strength is assessed by correlation methods. MSR grading can be accomplished at speeds up to 365 m (1000 ft) per minute, including the affixing of an MSR grade mark. MSR lumber is also visually checked for properties other than stiffness which might affect the suitability of a given piece. Given that the stiffness of each piece is measured individually and strength is measured on select pieces through a quality control program, MSR lumber can be assigned higher specified design strengths than visually graded dimension lumber. For further information, refer to the following resources: Canadian Lumber Properties (Canadian Wood Council) ASTM D1990 Standard Practice for Establishing Allowable Properties for Visually-Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens ASTM D4761 Standard Test Methods for Mechanical Properties of Lumber and Wood-Based Structural Materials National Lumber Grades Authority (NLGA)
Mass Timber
Advancements in wood product technology and systems are driving the momentum for innovative buildings in Canada. Products such as cross-laminated timber (CLT), nailed-laminated timber (NLT), glued-laminated timber (GLT), laminated strand lumber (LSL), laminated veneer lumber (LVL) and other large-dimensioned structural composite lumber (SCL) products are part of a bigger classification known as ‘mass timber’. Although mass timber is an emerging term, traditional post-and-beam (timber frame) construction has been around for centuries. Today, mass timber products can be formed by mechanically fastening and/or bonding with adhesive smaller wood components such as dimension lumber or wood veneers, strands or fibres to form large pre-fabricated wood elements used as beams, columns, arches, walls, floors and roofs. Mass timber products have sufficient volume and cross-sectional dimensions to offer significant benefits in terms of fire, acoustics and structural performance, in addition to providing construction efficiency.
Mid-Rise Buildings
When it comes to wood construction, many people think of basic 2×4 framing, panels or flooring for single-family homes. However, advances in wood science and building technology have resulted in stronger, more sophisticated and robust products that are expanding the options for wood construction, and providing more choices for builders and architects. The Canadian Wood Council’s support for mid-rise construction is not unique In Ontario, Home Builders, through organizations such as RESCON, BILD and the Ontario Home Builders Association are also highlighting this opportunity. Mid-rise buildings made of wood are a new construction option for builders. That’s good news for main-street Canada, where land is so expensive. The net benefit of reduced construction costs is increased affordability for home buyers. In terms of new economic opportunity, the ability to move forward “now” creates new construction jobs in cities and supports employment in forestry communities. This also offers increased export opportunities for current and innovative wood products, where adoption in Canada provides the example for other countries. This also reflects a new standard of engineering in that structural, fire and seismic concerns have all been addressed by the expert committees of the Canadian Commission on Building and Fire Codes. In the end, when occupied, mid-rise buildings fully meet the same requirements of the Building Code as any other type of construction from the perspective of health, safety and accessibility.
Mid-Rise Buildings
In the early 1900s, light-frame wood construction and heavy timber, up to ten-storeys in height, was commonplace in major cities throughout Canada. The longevity and continued appeal of these buildings types is apparent in the fact that many of them are still in use today. Over the past decade, there has been a revival in the use of wood for taller buildings in Canada, including mid-rise light-frame wood construction up to six-storeys in height. Mid-rise light-frame wood construction is more than basic 2×4 framing and wood sheathing panels. Advances in wood science and building technology have resulted in stronger, safer, more sophisticated engineered building products and systems that are expanding the options for wood construction, and providing more choices for builders and designers. Modern mid-rise light-frame wood construction in incorporates well researched and safe solutions. The engineering design and technology that has been developed and brought to market is positioning Canada as a leader in the mid-rise wood-frame construction industry. In 2009, via its provincial building codes, British Columbia became the first province in Canada to allow mid-rise buildings to be made from wood. Since this change to the British Columbia Building Code (BCBC), which increased the permissible height for wood frame residential buildings from four- to six-storeys, more than 300 of these structures have been completed or are underway with BC. In 2013 and 2015, Québec, Ontario, and Alberta, respectively, also moved to permit mid-rise wood-frame construction up to six-storeys in height. These regulatory changes indicate that there is clear market confidence in this type of construction. Scientific evidence and independent research has shown that mid-rise wood-frame buildings can meet performance requirements in regard to structural integrity, fire safety, and life safety. That evidence has now also contributed to the addition of new prescriptive provisions for wood construction, as well as paved the way for future changes that will include more permissible uses and ultimately greater permissible heights for wood buildings. As a result of this research, and the successful implementation of many mid-rise wood-frame residential buildings, primarily in British Columbia and Ontario, the Canadian Commission on Building and Fire Codes (CCBFC) approved similar changes to the National Model Construction Codes. The 2015 edition of the National Building Code of Canada (NBC) permits the construction of six-storey residential, business, and personal services buildings using traditional combustible construction materials. The NBC changes recognize the advancements in wood products and building systems, as well as in fire detection, suppression, and containment systems. In relation to mid-rise wood-frame buildings, several changes to the 2015 NBC are designed to further reduce the risks posed by fire, including: increased use of automatic sprinklers in concealed areas in residential buildings; increased use of sprinklers on balconies; greater water supply for firefighting purposes; and 90 percent noncombustible or limited-combustible exterior cladding on all storeys. Most mid-rise wood-frame buildings are located in the core of smaller municipalities and in the inner suburbs of larger ones, offering economic and sustainability advantages. Mid-rise wood-frame construction supports the goals of many municipalities; densification, affordable housing to accommodate a growing population, sustainability in the built environment and resilient communities. Many of these buildings have employed light-frame wood construction from the ground up, with a five- or six-storey wood-frame structure being constructed on a concrete slab-on-grade, or on top of a concrete basement parking garage; others have been constructed above one- or two-storeys of noncombustible commercial occupancy. Mid-rise wood buildings are inherently more complex and involve the adaptation of structural and architectural details that address considerations related to structural, acoustic, thermal and fire performance design criteria. Several key aspects of design and construction that become more critical in this new generation of mid-rise wood buildings include: increased potential for cumulative shrinkage and differential movement between different types of materials, as a result of the increased building height; increased, dead, live, wind and seismic loads that are a consequence of taller building height; requirements for continuous stacked shearwall layouts; increased fire-resistance ratings for fire separations, as required for buildings of greater height and area; ratings for sound transmission, as required for buildings of multi-family residential occupancy, as well as other uses; potential for longer exposure to the elements during construction; mitigation of risk related to fire during construction; and modified construction sequencing and coordination, resulting from the employment of prefabrication technologies and processes. There are many alternative approaches and solutions to these new design and construction considerations that are associated with mid-rise wood construction systems. Reference publications produced by the Canadian Wood Council provide more detailed discussion, case studies and details for mid-rise design and construction techniques. For further information, refer to the following resources: Mid-Rise Best Practice Guide (Canadian Wood Council) 2015 Reference Guide: Mid-Rise Wood Construction in the Ontario Building Code (Canadian Wood Council) Mid-Rise 2.0 – Innovative Approaches to Mid-Rise Wood Frame Construction (Canadian Wood Council) Mid-Rise Construction in British Columbia (Canadian Wood Council) National Building Code of Canada Wood Design Manual (Canadian Wood Council) CSA O86 Engineering design in wood Wood for Mid-Rise Construction (Wood WORKS! Atlantic) Fire Safety and Security: A Technical Note on Fire Safety and Security on Construction Sites in British Columbia/Ontario (Canadian Wood Council)
Mid-Rise Buildings – Research
Studies General “The Historical Development of the Building Size Limits in the National Building Code of Canada“, by Sereca for CWC (2015) (17 Mb) Reports produced for the Province of British Columbia’s 2009 building code change to permit 5- and 6-storey residential wood buildings Structural & Seismic Vertical Movement in Wood Platform Frame Structures (CWC Fact Sheets) Basics Design and detailing solutions Movement prediction Design of multi-storey wood-based shearwalls: Linear dynamic analysis & mechanics based approach A Mechanics-based Approach for Determining Deflections of Stacked Multi-storey Wood-based Shearwalls Design of Stacked Multi-storey Wood Shearwalls using a Mechanics Based Approach Linear Dynamic Analysis for Wood Based Shear Walls and Podium Structures Design of wood frame and podium structures using linear dynamic analysis, by Newfield, G., Ni, C., and Wang, J., Proceedings of the World Conference on Timber Engineering 2014, Quebec City, Canada (2014) Fire “Case Studies of Risk‐to‐Life due to Fire in Mid‐ and High‐Rise, Combustible and Non‐combustible Buildings Using CUrisk“, by Xia Zhang and George Hadjisophocleous of Carleton University, and Jim Mehaffey of CHM Fire Consultants Ltd. (March 2015) (2.3 Mb) Fire Safety During Construction for 5 & 6 Storey Wood Buildings in Ontario: A Best Practice Guideline Testing Fire Research Research for Wood and Wood-Hybrid Mid-Rise Buildings Project National Research Council Canada (2011-2015) Fire safety summary: fire research conducted for the project on mid-rise wood construction, A1-004377.1 Full-Scale Apartment Fire Tests Apartment Fire Test with Encapsulated Lightweight Wood Frame Construction (Test APT-LWF-1), A1-100035-01.9 Apartment fire test with encapsulated cross laminated timber construction (Test APT-CLT), A1-100035-01.10 Full-scale apartment fire test with lightweight steel frame construction (Test APT-LSF), A1-100035 01.11 Second Apartment Fire Test with Encapsulated Lightweight Wood Frame Construction, A1-004620.1 Full-Scale Standard Fire Resistance Tests Full-scale standard fire resistance tests of wall assemblies for use in lower storeys of mid-rise buildings, A1-100035-01.8 Full-Scale Standard Fire Resistance Test of a Wall Assembly for Use in Lower Storeys of Mid-Rise Buildings, A1-004691.1 Encapsulation Time Data from NRC Fire-resistance Projects, A1-100035-01.14 Full-Scale Exterior Wall Fire Tests (CAN/ULC-S134) Full-scale standard fire test for exterior wall assembly using lightweight wood frame construction with gypsum sheathing (Test EXTW-1), A1-100035-01.4 Full-scale standard fire test for exterior wall assembly using a simulated cross-laminated timber wall assembly with gypsum sheathing (Test EXTW-2), A1-100035-01.5 Full-scale standard fire test for exterior wall assembly using lightweight wood frame construction with interior fire-retardant-treated plywood sheathing (Test EXTW-3), A1-100035-01.6 Full-scale standard fire test for exterior wall assembly using a simulated cross-laminated timber wall assembly with interior fire-retardant-treated plywood sheathing (Test EXTW-4), A1-100035-01.7 Fire test for rainscreen wall system (Test EXTW-5), A1-100035-01.15 Intermediate-Scale Furnace and Cone Calorimeter Tests Intermediate-scale furnace tests with encapsulation materials, A1-100035-01.2 Ignition of Selected Wood Building Materials, A1-100035-01.12 Cone Calorimeter Results for Encapsulation Materials, A1-100035-01.1 Cone Calorimeter Results for Materials used in Standard Exterior Wall Tests, A1-100035-01.3 Cone Calorimeter Results for Acoustic Membrane Materials Used in Floor Assemblies, A1-100035-01.13 Other Reports Final Report – Full-scale Mass Timber Shaft Demonstration Fire (including the National Research Council test report as an Appendix), by FPInnovations (April 2015) Full Scale Exterior Wall Test on Nordic CLT System, by the National Research Council (January 2015) Client Report A1-005991.1 – Fire Endurance of Cross-Laminated Timber Floor and Wall Assemblies for Tall Wood Buildings, by the National Research Council (December 2014) Report No. 101700231SAT-003_Rev.1 – Report of Testing Cross-Laminated Timber Panels for Compliance with CAN/ULC-S101 Standard Methods of Fire Endurance Tests of Building Construction and Materials: Loadbearing 3-ply CLT Wall with 1 Layer of 5/8″ Type X Gypsum Board – 1 hr FRR, by Intertek for CWC (November 2014) Report No. 100585447SAT-002B – Report of Testing Cross-Laminated Timber Panels for Compliance with CAN/ULC-S101 Standard Methods of Fire Endurance Tests of Building Construction and Materials: Loadbearing 3-ply CLT Wall with 1 Layer of 5/8″ Fire-rated Gypsum Board (60% load) – 1 hr FRR, by Intertek for CWC (December 2013) Report No. 100585447SAT-002A_Rev.1 – Report of Testing Cross-Laminated Timber Panels for Compliance with CAN/ULC-S101 Standard Methods of Fire Endurance Tests of Building Construction and Materials: Loadbearing 3-ply CLT Wall with Attached Wood-frame Partition – 1 hr FRR, by Intertek for CWC (January 2012) RR No. 184: Results of Fire Resistance Tests On Full-Scale Floor Assemblies – Phase II, by the National Research Council (March 2005) IR-833: Results of Fire Resistance Tests on Full-Scale Gypsum Board Wall Assemblies, by the National Research Council (August 2002) IRC-IR-764: Results of Fire Resistance Tests on Full-Scale Floor Assemblies, by the National Research Council (1998) Acoustics Research Research for Wood and Wood-Hybrid Mid-Rise Buildings Project National Research Council Canada (2011-2015) Acoustics summary: sound insulation in mid-rise wood building, A1-004377.2 Acoustics: sound insulation in mid-rise wood buildings, A1-100035-02.1 Other Reports & Guides RR-331: Guide to calculating airborne sound transmission in buildings (Second Edition), by the National Research Council (April 2016) IRC RR-169: Summary Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission and Impact Insulation Data, by the National Research Council (January 2005) IRC IR-811: Detailed Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission and Impact Insulation Data in 1/3 Octave Bands, by the National Research Council (July 2000) IRC IR-766: Summary Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission Class and Impact Insulation Class Results, by the National Research Council (April 1998) Building Envelope Research Research for Wood and Wood-Hybrid Mid-Rise Buildings Project National Research Council Canada (2011-2015) Building Envelope Summary: Hygrothermal Assessment of Systems for Mid-rise Wood Buildings, A1-004377.3 Mid-rise wood constructions: specifications of mid-rise envelopes for hygrothermal assessment, A1-100035-03.1 Climatological analysis for hygrothermal performance evaluation: Mid-rise wood, A1-100035-03.2 Mid-rise wood constructions: investigation of water penetration through cladding and deficiencies, A1-100035-03.3 Mid-rise wood – characterization of hygrothermal properties, A1-100035-03.4 Benchmarking of the advanced hygrothermal model hygIRC – large scale drying experiment of the mid-rise wood frame assembly, A1-100035-03.5 Hygrothermal modelling benchmark: Comparison of hygIRC simulation results with full scale experiment results, A1-100035-03.6 Mid-rise wood constructions- hygrothermal modelling and analysis, A1-100035-03.7 Visit Think Wood’s Research Library for additional resources
Moisture and Wood
The durability of wood is often a function of water, but that doesn’t mean wood can never get wet. Quite the contrary, wood and water usually live happily together. Wood is a hygroscopic material, which means it naturally takes on and gives off water to balance out with its surrounding environment. Wood can safely absorb large quantities of water before reaching moisture content levels that will be inviting for decay fungi. Moisture content (MC) is a measure of how much water is in a piece of wood relative to the wood itself. MC is expressed as a percentage and is calculated by dividing the weight of the water in the wood by the weight of that wood if it were oven dry. For example, 200% MC means a piece of wood has twice as much of its weight due to water than to wood. Two important MC numbers to remember are 19% and 28%. We tend to call a piece of wood dry if it is at 19% or less moisture content. Fiber saturation averages around 28%. Fiber saturation is an important benchmark for both shrinkage and for decay. The fibers of wood (the cells that run the length of the tree) are shaped like tapered drinking straws. When fibers absorb water, it first is held in the cell walls themselves. When the cell walls are full, any additional water absorbed by the wood will now go to fill up the cavities of these tubular cells. Fiber saturation is the level of moisture content where the cell walls are holding as much water as they can. Water held in the cell walls is called bound water, while water in the cell cavities is called free water. As the name implies, the free water is relatively accessible, and an accessible source of water is one necessity for decay fungi to start growing. Therefore, decay can generally only get started if the moisture content of the wood is above fiber saturation. The fiber saturation point is also the limit for wood shrinkage. Wood shrinks or swells as its moisture content changes, but only when water is taken up or given off from the cell walls. Any change in water content in the cell cavity will have no effect on the dimension of the wood. Therefore, wood only shrinks and swells when it changes moisture content below the point of fiber saturation. Like other hygroscopic materials, wood placed in an environment with stable temperature and relative humidity will eventually reach a moisture content that yields no vapor pressure difference between the wood and the surrounding air. In other words, its moisture content will stabilize at a point called the equilibrium moisture content (EMC). Wood used indoors will eventually stabilize at 8-14% moisture content; outdoors at 12-18%. Hygroscopicity isn’t necessarily a bad thing – this allows wood to function as a natural humidity controller in our homes. When the indoor air is very dry, wood will release moisture. When the indoor air is too humid, wood will absorb moisture. Wood shrinks/swells when it loses/gains moisture below its fiber saturation point. This natural behaviour of wood is responsible for some of the problems sometimes encountered when wood dries. For example, special cracks called checks can result from stresses induced in a piece of wood that is drying. As the piece dries, it develops a moisture gradient across its section (dry on the outside, wet on the inside). The dry outer shell wants to shrink as it dries below fiber saturation, however, the wetter core constrains the shell. This can cause checks to form on the surface. The shell is now set in its dimension, although the core is still drying and will in turn want to shrink. But the fixed shell constrains the core and checks can thus form in the core. Another problem associated with drying is warp. A piece of wood can deviate from its expected shape as it dries due to the fact that wood shrinks different amounts in different directions. It shrinks the most in the direction tangential to the rings, about half as much in the direction perpendicular to the rings, and hardly at all along the length of the tree. Where in the log a piece was cut will be a factor in how it changes shape as it shrinks. One advantage of usingdry lumber is that most of the shrinkage has been achieved prior to purchase. Dry lumber is lumber with a moisture content no greater than 19%; wood does most of its shrinking as it drops from 28-19%. Dry lumber will have already shown its drying defects, if any. It will also lead to less surprises in a finished building, as the product will stay more or less at the dimension it was upon installation. Dry lumber will be stamped with the letters S-DRY (for surfaced dry) or KD (for kiln dry). Another way to avoid shrinkage and warp is to use composite wood products, also called engineered wood products. These are the products that are assembled from smaller pieces of wood glued together – for example, plywood, OSB, finger-jointed studs and I-joists. Composite products have a mix of log orientations within a single piece, so one part constrains the movement of another. For example, plywood achieves this crossbanding form of self-constraint. In other products, movements are limited to very small areas and tend to average out in the whole piece, as with finger-jointed studs.
Nails
Nailing is the most basic and most commonly used means of attaching members in wood frame construction. Common nails and spiral nails are used extensively in all types of wood construction. Historical performance, along with research results have shown that nails are a viable connection for wood structures with light to moderate loads. They are particularly useful in locations where redundancy and ductile connections are required, such as loading under seismic events. Typical structural applications for nailed connections include: wood frame construction post and beam construction heavy timber construction shearwalls and diaphragms nailed gussets for wood truss construction wood panel assemblies Nails and spikes are manufactured in many lengths, diameters, styles, materials, finishes and coatings, each designed for a specific purpose and application. In Canada, nails are specified by the type and length and are still manufactured to Imperial dimensions. Nails are made in lengths from 13 to 150 mm (1/2 to 6 in). Spikes are made in lengths from 100 to 350 mm (4 to 14 in) and are generally stockier than nails, that is, a spike has a larger cross-sectional area than an equivalent length common nail. Spikes are generally longer and thicker than nails and are generally used to fasten heavy pieces of timber. Nail diameter is specified by gauge number (British Imperial Standard). The gauge is the same as the wire diameter used in the manufacture of the nail. Gauges vary according to nail type and length. In the U.S., the length of nails is designated by “penny” abbreviated “d”. For example, a twenty-penny nail (20d) has a length of four inches. The most common nails are made of low or medium carbon steels or aluminum. Medium-carbon steels are sometimes hardened by heat treating and quenching to increase toughness. Nails of copper, brass, bronze, stainless steel, monel and other special metals are available if specially ordered. Table 1, below, provides examples of some common applications for nails made of different materials. TABLE 1: Nail applications for alternative materials Material Abbreviation Application Aluminum A For improved appearance and long life: increased strain and corrosion resistance. Steel – Mild S For general construction. Steel – Medium Carbon Sc For special driving conditions: improved impact resistance. Stainless steel, copper and silicon bronze E For superior corrosion resistance: more expensive than hot-dip galvanizing. Uncoated steel nails used in areas subject to wetting will corrode, react with extractives in the wood, and result in staining of the wood surface. In addition, the naturally occurring extractives in cedars react with unprotected steel, copper and blued or electro-galvanized fasteners. In such cases, it is best to use nails made of non-corrosive material, such as stainless steel, or finished with non-corrosive material such as hot-dipped galvanized zinc. Table 2, below, provides examples of some common applications for alternative finishes and coatings of nails. TABLE 2: Nail applications for alternative finishes and coatings Nail Finish or Coating Abbreviation Application Bright B For general construction, normal finish, not recommended for exposure to weather. Blued Bl For increased holding power in hardwood, thin oxide finish produced by heat treatment. Heat treated Ht For increased stiffness and holding power: black oxide finish. Phoscoated Pt For increased holding power; not corrosion resistant. Electro galvanized Ge For limited corrosion resistance; thin zinc plating; smooth surface; for interior use. Hot-dip galvanized Ghd For improved corrosion resistance; thick zinc coating; rough surface; for exterior use. Pneumatic or mechanical nailing guns have found wide-spread acceptance in North America due to the speed with which nails can be driven. They are especially cost effective in repetitive applications such as in shearwall construction where nail spacing can be considerably closer together. The nails for pneumatic guns are lightly attached to each other or joined with plastic, allowing quick loading nail clips, similar to joined paper staples. Fasteners for these tools are available in many different sizes and types. Design information provided in CSA O86 is applicable only for common round steel wire nails, spikes and common spiral nails, as defined in CSA B111. The ASTM F1667 Standard is also widely accepted and includes nail diameters that are not included in the CSA B111. Other nail-type fastenings not described in CSA B111 or ASTM F1667 may also be used, if supporting data is available. Types of Nails For more information, refer to the following resources: International, Staple, Nail, and Tool Association (ISANTA) CSA O86 Engineering design in wood CSA B111 Wire Nails, Spikes and Staples ASTM F1667 Standard Specification for Driven Fasteners: Nails, Spikes and Staples
National Model Codes in Canada
On behalf of the Canadian Commission on Building and Fire Codes (CCBFC) the National Research Council (NRC) Codes Canada publishes national model codes documents that set out minimum requirements relating to their scope and objectives. These include the National Building Code (NBC), the National Fire Code (NFC), the National Energy Code for Buildings (NECB), the National Plumbing Code (NPC) and other documents. The Canadian Standards Association (CSA) publishes other model codes that address electrical, gas and elevator systems. The NBC is the model building code in Canada that forms the basis of most building design in the country. The NBC is a highly regarded model building code because it is a consensus-based process for producing a model set of requirements which provide for the health and safety of the public in buildings. Its origins are deeply entrenched within Canadian history and culture and a need to house the growing population of Canada safely and economically. Historical events have shaped many of the health and safety requirements of the NBC. Model codes such as the NBC and NECB have no force in law until they are adopted by a government authority having jurisdiction. In Canada, that responsibility resides within the provinces, territories and in some cases, municipalities. Most regions choose to adopt the NBC, or adapt their own version derived from the NBC to suit regional needs. The model codes in Canada are developed by experts, for experts, through a collaborative and consensus-based process that includes input from all segments of the building community. The Canadian model codes build on the best expertise from across Canada and around the world to provide effective building and safety regulations that are harmonized across Canada. The Codes Canada publications are developed by the Canadian Commission on Building and Fire Codes (CCBFC). The CCBFC oversees the work of a number of technical standing committees. Representing all major facets of the construction industry, commission members include building and fire officials, architects, engineers, contractors and building owners, as well as members of the public. Canadian Wood Council representatives hold membership status on several of the standing committees and task groups acting under the CCBFC and participate actively in the technical updates and revisions related to aspects of the Canadian model codes that apply to wood building products and systems. During any five-year code-revision cycle, there are many opportunities for the Canadian public to contribute to the process. At least twice during the five-year cycle, proposed changes to the Code are published and the public is invited to comment. This procedure is crucial as it allows input from all those concerned and broadens the scope of expertise of the Committees. Thousands of comments are received and examined by the Committees during each cycle. A proposed change may be approved as written, modified and resubmitted for public review at a later date, or rejected entirely. For further information, refer to the following resources: Fire Safety Design in Buildings (Canadian Wood Council) Codes Canada – National Research Council of Canada National Building Code of Canada
Non-Pressure Treated Wood
Non-Pressure Treated Wood For most treated wood, preservatives are applied in special facilities using pressure. However, sometimes this isn’t possible, or the need for treated wood was not apparent until after construction or building occupancy. In those cases, preservatives can be applied using methods that do not involve pressure vessels. Some of these treatments can only be done by licensed applicators. When using wood preservatives, as with all pesticides, the label requirements of the Pest Management Regulatory Agency (in Canada) or the EPA (in the USA) must be followed. Five categories of non-pressure treatments Treatment during Engineered Wood Product Manufacture Some engineered wood panel products, such as plywood and laminated veneer lumber (LVL) are able to be treated after manufacture with preservative solutions, whereas thin strand based products (OSB, OSL) and small particulate and fibre-based panels (particleboard, MDF) are not. The preservatives must be added to the wood elements before they are bonded together, either as a spray on, mist or powder. Products such as OSB are manufactured from small, thin strands of wood. Powdered preservatives can be mixed in with the strands and resins during the blending process just prior to mat forming and pressing. Zinc borate is commonly used in this application. By adding preservatives to the manufacturing process it’s possible to obtain uniform treatment throughout the thickness of the product. In North America, plywood is normally protected against decay and termites by pressure treatment processes. However, in other parts of the world insecticides are often formulated with adhesives to protect plywood against termites. Surface pre-treatment This is anticipatory preservative treatment applied by dip, spray or brush application to all of the accessible surfaces of some wood products during the construction process. The intent is to provide a shell of protection to vulnerable wood products, components or systems in their finished form. One example would be spraying house framing with borates for resistance to drywood termites and wood boring beetles in some cases. Such treatments may also be applied to lumber, plywood and OSB to provide additional protection against mould growth. Sub-surface pre-treatment (Depot treatment) This is preservative treatment applied at discrete locations, not to the entire piece, during the manufacturing process or during construction. The intent is to pro-actively provide protection only to the parts of the wood product, component or systems that might be exposed to conditions conducive to decay. One example would be placing borate rods into holes drilled in the exposed ends of glulam beams projecting beyond a roof line. Supplementary treatment This is preservative treatment applied at discrete locations to treated wood in service to compensate for either incomplete initial penetration of the cross section, or depletion of preservative effectiveness over time. The intent is to boost the protection in previously-treated wood, or to address areas exposed by necessary on-site cutting of treated wood products. One example would be the application of a ready-made bandage to utility poles that have suffered depletion of the original preservative loading. Another example is field-cut material for preserved wood foundations. Remedial treatment This is preservative treatment applied to residual sound wood in products, components or systems where decay or insect attack is known to have begun. The intent is to kill existing fungi or insects and/or prevent decay or insects from spreading beyond the existing damage. One example would be roller or spray application of a borate/glycol formulation on sound wood left in place adjacent to decayed framing (which should be cut out and replaced with pressure-treated wood). Formats of non-pressure treatments Non-pressure treatments come in three different forms: solids, liquids/pastes, and fumigants. Unlike pressure-treatment preservatives, which rely on pressure for good penetration, these rely on the mobility of the active ingredients to penetrate deep enough in wood to be effective. The active ingredients can move in the wood via capillarity or can diffuse in water and/or air within the wood. This mobility not only allows the active ingredients to move into the wood but can also allow them to move out under certain conditions. This means the conditions within and around the structure must be understood so the loss of preservative and consequent loss of protection can be minimized. Borates, fluorides and copper compounds are particularly suitable for use as solids, liquids and pastes. Methyl isothiocyanate (and its precursors), methyl bromide, and sulfuryl fluoride are the only widely used fumigant treatments. Methyl bromide was phased out, except for very limited uses, in 2005. Solids The major advantage of solids in these applications is that they maximize the amount of water-soluble material that can be placed into a drilled hole, due to the high percentage of active ingredients contained in commercially-available rods. The major disadvantage is the requirement for sufficient moisture and the time needed for the rod to dissolve. The earliest and best-known solid preservative system is the fused borate rod, originally developed in the 1970s for supplementary and remedial treatment of railroad ties. These have since been used successfully on utility poles, timbers, millwork (window joinery), and a variety of other wood products. A mixture of borates is fused into glass at extremely high temperatures, poured into a mould and allowed to set. Placed into holes in the wood, the borate dissolves in any water contained in the wood and diffuses throughout the moist region. Mass flow of moisture along the grain may speed up distribution of the borate. Secondary biocides such as copper can be added to borate rods to supplement the efficacy of the borates against decay and insects. While all preservatives should be treated with respect, many users feel more comfortable dealing with borate and copper/borate rods because of their low toxicity and low potential for entry into the body. Fluorides are also currently available in a rod form. The rod is produced by compressing sodium fluoride and binders together, or by encapsulation in a water-permeable tubing. Fluorides diffuse more rapidly than borates in water and may also move in the vapour phase as hydrofluoric acid. Zinc borate (ZB) is a powder
Oriented Strand Board (OSB)
Oriented Strand Board (OSB) is a widely used, versatile structural wood panel. OSB makes efficient use of forest resources, by employing less valuable, fast-growing species. OSB is made from abundant, small diameter poplar and aspen trees to produce an economical structural panel. The manufacturing process can make use of crooked, knotty and deformed trees which would not otherwise have commercial value, thereby maximizing forest utilization. OSB has the ability to provide structural performance advantages, an important component of the building envelope and cost savings. OSB is a dimensionally stable wood-based panel that has the ability to resist delamination and warping. OSB can also resist racking and shape distortion when subjected to wind and seismic loadings. OSB panels are light in weight and easy to handle and install. OSB panels are primarily used in dry service conditions as roof, wall and floor sheathing, and act as key structural components for resisting lateral loads in diaphragms and shearwalls. OSB is also used as the web material for some types of prefabricated wood I-joists and the skin material for structural insulated panels. OSB can also be used in siding, soffit, floor underlayment and subfloor applications. Some specialty OSB products are made for siding and for concrete formwork, although OSB is not commonly treated using preservatives. OSB has many interleaved layers which provide the panel with good nail and screw holding properties. Fasteners can be driven as close as 6 mm (1/4 in) from the panel edge without risk of splitting or breaking out. OSB is a structural mat-formed panel product that is made from thin strands of aspen or poplar, sliced from small diameter roundwood logs or blocks, and bonded together with a waterproof phenolic adhesive that is cured under heat and pressure. OSB is also manufactured using the southern yellow pine species in the United States. Other species, such as birch, maple or sweetgum can also be used in limited quantities during manufacture. OSB is manufactured with the surface layer strands aligned in the long panel direction, while the inner layers have random or cross alignment. Similar to plywood, OSB is stronger along the long axis compared to the narrow axis. This random or cross orientation of the strands and wafers results in a structural engineered wood panel with consistent stiffness and strength properties, as well as dimensional stability. It is also possible to produce directionally-specific strength properties by adjusting the orientation of strand or wafer layers. The wafers or strands used in the manufacture of OSB are generally up to 150 mm (6 in) long in the grain direction, 25 mm (1 in) wide and less than 1 mm (1/32″) in thickness. In Canada, OSB panels are manufactured to meet the requirements of the CSA O325 standard. This standard sets performance ratings for specific end uses such as floor, roof and wall sheathing in light-frame wood construction. Sheathing conforming to CSA O325 is referenced in Part 9 of the National Building Code of Canada (NBC). In addition, design values for OSB construction sheathing are listed in CSA O86, allowing for engineering design of roof sheathing, wall sheathing and floor sheathing using OSB conforming to CSA O325. OSB panels are manufactured in both imperial and metric sizes, and are either square-edged or tongue-and-grooved on the long edges for panels 15 mm (19/32 in) and thicker. For more information on available sizes of OSB panel, refer to the document below. For more information on OSB, please refer to the following resources: APA – The Engineered Wood Association National Building Code of Canada CSA O86 Engineering design in wood CSA O325 Construction sheathing CSA O437 Standards on OSB and Waferboard PFS TECO Example specifications for oriented strand board (OSB) Oriented Strand Board (OSB) Grades Oriented Strand Board (OSB) Manufacture Oriented Strand Board (OSB) Quality Control Oriented Strand Board (OSB) Sizes Oriented Strand Board (OSB) Storage and Handling
