Wood-based structural panels are thin, flat, composite materials capable of resisting applied loads in specific applications. Structural panels fall into three basic categories based on the manufacturing process used: plywood, mat-formed panels (oriented strand board, or OSB), and composite panels.
Plywood a flat panel built up of sheets of veneer, called plies. These are united under pressure by a bonding agent. The adhesive bond between plies is as strong as or stronger than solid wood. Plywood is constructed of an odd number of layers with the grain of adjacent layers perpendicular. Layers may consist of a single ply or two or more plies laminated with parallel grain direction. Outer layers and all odd-numbered layers generally have the grain direction oriented parallel to the long dimension of the panel. The odd number of layers with alternating grain direction equalizes strains, reduces splitting, and minimizes dimensional change and warping of the panel.
Mat-formed panel any wood-based panel that does not contain veneer but is consistent with the definition of structural-use panels, including products such as waferboard and oriented strand board.
Oriented strand board an engineered structural wood panel composed of compressed wood strands arranged in layers at right angles to one another and bonded with fully waterproof adhesive.
Composite panel any panel containing a combination of veneer and other wood-based materials.
A structural panel may contain either softwoods or hardwoods. Panels approved for use in building-code-regulated construction carry the trademark of a codeapproved agency, such as APA The Engineered Wood Association. Most construction grades have either an Exterior or Exposure 1 durability classification and are made with fully waterproof adhesives. Exposure classifications are defined as follows:
Exterior panels that are suitable for permanent exposure to weather or moisture.
Exposure 1 panels that are suitable for uses not permanently exposed to the weather but may be used where exposure durability to resist effects of moisture due to construction delays, high humidity, water leakage, or other conditions of similar severity is required.
Exposure 2 panels that are suitable for interior use where exposure durability to resist effects of high humidity and water leakage is required.
Interior panels that are suitable for interior use where they will be subjected to only temporary, minor amounts of moisture.
Standards for Structural Panels
Structural panels approved for building-code-regulated construction are manufactured under one or more of three standards:
1. U.S. Product Standard PS 1-95 for Construction & Industrial Plywood (PS 1). It applies to plywood only. This voluntary product standard covers the wood species, veneer grading, glue bonds, panel construction and workmanship, dimensions and tolerances, marking, moisture content, and packing of plywood intended for construction and industrial uses. Also included are test methods to determine compliance and a glossary of trade terms and definitions. A quality certification program is provided, whereby qualified testing agencies inspect, sample, and test products identified as complying with the standard. Information regarding generally available sizes, methods of ordering, and reinspecting practices also is provided.
2. Voluntary Product Standard PS 2-92, Performance Standard for Wood-Based Structural-Use Panels (PS 2). It applies to all types of wood-based panels (typically plywood, OSB, and composite). It establishes requirements for assessing the acceptability of wood-based structural-use panels for construction sheathing and single- floor applications. It also provides a basis for common understanding among the producers, distributors, and users of these products. It covers performance requirements, adhesive bond durability, panel construction and workmanship, dimensions and tolerances, marking, and moisture content of structural-use panels. The standard also includes test methods to determine compliance and a glossary of trade terms and definitions. A quality certification program is provided, whereby qualified testing agencies inspect, sample, and test products for qualification under the standard.
3. APA Performance Standards and Policies for Structural-Use Panels (PRP 108). It is similar to PS 2 but also includes performance-based qualification procedures for siding panels.
Plywood Grades
Plywood grades are generally identified in terms of the veneer grade used on the face and back of the panel; for example, A-B, B-C, . . . , or by a name suggesting the panels intended end use, such as APA Rated Sheathing or APA Rated Sturd- I-Floor.
Veneer grades define veneer appearance in terms of natural, unrepaired-growth characteristics and allowable number and size of repairs that may be made during manufacture (Table 10.17). The highest quality veneer grades are N and A. The minimum grade of veneer permitted in Exterior plywood is C grade. D-grade veneer is used in panels intended for interior use or applications protected from permanent exposure to weather.
Panels with B-grade or better veneer faces are always sanded smooth in the manufacturing process to fulfill the requirements of their intended end use applications such as cabinets, shelving, furniture, and built-ins. Rated Sheathing panels are unsanded since a smooth surface is not a requirement of their intended end use. Still other panels, such as Underlayment, Rated Sturd-I-Floor, C-D Plugged, and C-C Plugged, require only touch sanding for sizing to make the panel thickness more uniform.
Unsanded and touch-sanded panels, and panels with B-grade or better veneer on one side only, usually carry the trademark on the panel back. Panels with both sides of B-grade or better veneer, or with special overlaid surfaces, such as High- Density Overlay, usually carry the trademark on the panel edge.
Plywood Group Number
Plywood can be manufactured from over 70 species of wood. These species are divided on the basis of strength and stiffness into five groups under U.S. Product Standard PS 1-95. Strongest species are in Group 1; the next strongest in Group 2, etc. The group number that appears in the trademark on some APA trademarked panels, primarily sanded grades, is based on the species used for face and back veneers. Where face and back veneers are not from the same species group, the higher group number is used, except for sanded panels 3â„8 in thick or less and decorative panels of any thickness. These are identified by face species if C or D grade backs are at least 1â„8 in thick and are not more than one species group number larger. Some species are used widely in plywood manufacture, others rarely.
OSB panels, being composed of flakes or strands instead of veneers, are graded without reference to veneers or species, and composite panels are graded on an OSB performance basis by end use and exposure durability. Typical panel trademarks for all three panel types and an explanation of how to read them are shown in Fig. 10.5.
The Design/Construction Guide Residential & Commercial, APA The Engineered Wood Association, Tacoma, Wash., contains a comprehensive summary of plywood grades and trademarks and their applications.
Span Ratings for Panels
APA Rated Sheathing, APA Rated Sturd-I-Floor, and APA Rated Siding carry numbers in their trademarks called span ratings. These denote the maximum spacing, in, c to c of supports for panels in construction applications. Except for Rated Siding panels, the span rating in the trademark applies when the long panel dimension is across supports, unless the strength axis is otherwise identified. The span rating in the trademark of Rated Siding panels applies when they are installed vertically.
The span rating in Rated Sheathing trademarks appears as two numbers separated by a slash (Fig. 10.5a), such as 32/16 and 48/ 24. The left-hand number denotes the maximum recommended spacing of supports when the panel is used for roof sheathing with the long dimension or strength axis of the panel across three or more supports. The right-hand number indicates the maximum recommended spacing of supports when the panel is used for subflooring with the long dimension or strength axis of the panel across three or more supports. A panel marked 32/ 16, for example, may be used for roof decking over supports 32 in c to c or for subflooring over supports 16 in c to c. An exception is Rated Sheathing intended for use as wall sheathing only. The trademarks for such panels contain only a single number similar to the span rating for APA Rating Siding and Sturd-I-Floor.
The Span Ratings in the trademarks on APA Rated Sturd-I-Floor and APA Rating Siding panels appear as a single number. Rated Sturd-I-Floor panels are designed specifically for single-floor (combined subfloor-underlayment) applications under carpet and pad. They are manufactured with span ratings of 16, 20, 24, 32, and 48 in.
APA Rated Siding is available with span ratings of 16 and 24 in. Span-rated panels and lap siding may be connected directly to studs, or over nonstructural wall sheathing, or over nailable panel or lumber sheathing (double-wall construction).
Panels and lap siding with a span rating of 16 in may be applied directly to studs spaced 16 in c to c. Those bearing a span rating of 24 in may be connected directly to studs 24 in c to c. All APA Rated Siding panels may be applied horizontally directly to studs 16 or 24 in c to c, if horizontal joints are blocked. The span rating of APA Rated Siding panels refers to the maximum recommended spacing of vertical rows of nails rather than to stud spacing when the panels are applied to nailable structural sheathing.
Availability of Panel Grades
Some panel grades, thicknesses, span ratings, or species may be difficult to obtain in some areas. Check with your supplier for availability or include an alternative panel in specifications. Standard panel dimensions are 4 x8 ft, although some mills also produce plywood panels 9 or 10 ft long or longer. OSB panels may be ordered in lengths up to 24 ft in some market areas.
APA Rated Sturd-I-Floor
APA Rated Sturd-I-Floor (copyrighted name) is a span-rated product designed specifically for use in single-layer floor construction beneath carpet and pad. The maximum spacing of floor joists, or span rating, is stamped on each panel. Panels are manufactured with span ratings of 16, 20, 24, 32, and 48 in. These assume the panel continuous over two or more spans with the long dimension or strength axis across supports (Fig. 10.6). The span rating in the trademark applies when the long panel dimension is across supports unless the strength axis is otherwise identified.
Glue-nailing is recommended, though panels may be nailed only. Application provisions for both methods are given in Table 10.18. Uniform live loads are given in the APA Design/Construction Guide Residential & Commercial.
Panel Subflooring
The limiting factor in design of floors is deflection under concentrated loads at panel edges. Nailing provisions for APA panel subflooring (Fig. 10.7) are given in Table 10.19. Other code-approved fasteners, however, may be used. The span ratings in Table 10.19 applied to Rated Sheathing or sheathing grades only and are the minimum for the span indicated. The span ratings assume panels continuous over two or more spans with the long dimension or strength axis across supports.
Panel subflooring may also be glued for added stiffness and to reduce squeaks if it satisfied nailing provisions in Table 10.18. Long edges should be tongue-andgroove or supported with blocking unless:
1. A separate underlayment layer is installed with its joints offset from those in the subfloor. The minimum thickness of underlayment should be 1â„4 in for subfloors on spans up to 24 in and 11â„32 in or more on spans longer than 24 in.
2. A minimum of 11â„2 in of lightweight concrete is applied over the panels.
3. A 3â„4-in wood strip flooring is installed over the subfloor.
In some nonresidential buildings, greater traffic and heavier concentrated loads may require construction in excess of the minimums given. Where joists are 16 in c to c, for example, panels with a span rating of 40/20 or 48/24 provide greater stiffness. For beams or joists 24 or 32 in c to c, 11â„8-in-thick panels provide additional stiffness.
Wall Systems
Rated siding (panel or lap) may be applied directly to studs or over nonstructural fiberboard, or gypsum or rigid-foam-insulation sheathing. Nonstructural sheathing is defined as sheathing not recognized by building codes as meeting both bending and racking-strength requirements.
A single layer of panel siding, since it is strong and rack resistant, eliminates the cost of installing separate structural sheathing or diagonal wall bracing. Panel sidings are normally installed vertically (Fig. 10.8a), but most may also be placed horizontally (long dimension across supports) if horizontal joints are blocked (Fig. 10.8b). Maximum stud spacings for both applications are given in Table 10.20.
Rated Sheathing meets building-code wall-sheathing requirements for bending and racking strength without let-in corner bracing. Installation provisions are given in Table 10.21 and Fig. 10.9. When 1â„2-in gypsum or fiberboard sheathing is used, APA Rated Sheathing corner panels of the same thickness can also eliminate costly let-in bracing. The APA Rated Sheathing, 15â„32 or 1â„2 in thick, should be nailed to studs spaced 16 or 24 in c to c with 6d common nails spaced 6 in c to c along panel edges and 12 in c to c at intermediate supports. When corner panels are PS 1 plywood, 11â„2-in roofing nails at 4 in along panel edges and 12 in at intermediate supports may be used.
Building paper is generally not required over wall sheathing, except under stucco or under brick veneer where required by the local building code. Recommended wall sheathing spans with brick veneer and masonry are the same as those for nailable panel sheathing.
Allowable Loads for APA Structural-Use Panels
Because it is sometimes necessary to have engineering design information for structural panel products for conditions not specifically covered in other literature, APA publishes separate design-section capacities for the various span ratings for these products. These values are listed in APA Technical Note N375, Design Capacities of APA Performance-Rated Structural-Use Panels. The APA Plywood Design Specification contains load-span tables that apply to APA trademarked structuraluse panels qualified and manufactured in accordance with PS 2-92 or APA PRP- 108, Performance Standards and Policies for Structural-Use Panels. These panels include plywood, composite, and mat-formed products, such as oriented strand board. Loads are provided for applications where the panel strength axis is applied across or parallel to supports. For each combination of span L and Span Rating, loads are given for deflections of L/360, L/240 and L/180, and maximum loads controlled by bending and shear capacity. The values may be adjusted for panel type, load duration, span conditions, and moisture. Table 10.22 is provided to assist in selecting panel constructions for specific span ratings.
Some structural-panel applications are not controlled by uniform loads. Residential floors are a good example. They are commonly designed for 40-psf live load. The allowable uniform floor load on panels with maximum span according to APA recommendations is greatly in excess of typical design loads. This excess does not mean that floor spans for structural panels can be increased, but only that there is considerable reserve strength and stiffness for uniform loads. Actually, the recommendations for panel floors are based on performance under concentrated loads, how the floor feels to passing foot traffic, and other subjective factors that relate to public acceptance. The maximum floor and roof spans for structural panels should always be checked before a final panel selection is made for these applications.
Panel Shear Walls
While the wall systems described in Art. 10.12.8 will provide sufficient strength under normal conditions in residential and light-frame construction, shear walls may be desirable or required in areas with frequent seismic activity or high wind loads.
Shear walls are also advisable in commercial and industrial construction.
Either Rated Sheathing or all-veneer plywood Rated Siding can be used in shear walls. Table 10.23 gives maximum shears for walls with Rated Sheathing, with plywood Rated Siding installed directly to studs (Sturd-I-Wall), or with panels applied over gypsum sheathing for walls required to be fired rated from the outside.
To design a shear wall, follow these steps:
1. Determine lateral loads and resulting shears with appropriate allowances for openings.
2. Determine the required panel grade and thickness and the nailing schedule from Table 10.23. Check the anchor bolts in the sill plate for shear.
3. Check wall framing on each end of the shear wall and design a foundation anchor or hold-down, if required (see Fig. 10.10).
Panel Roof Sheathing
Table 10.24 lists maximum uniform roof live loads for APA Rated Sheathing Exposure 1, and Structural I Rated Sheathing, Exposure 1 or Exterior. Uniform-load deflection limits are 1â„180 of the span under live load plus dead load, and 1â„240 under live load only. Panels are assumed continuous over two or more spans with the long dimension or strength axis across supports (Fig. 10.11). Special conditions, such as heavy concentrated loads, may require constructions in excess of these minimums, or allowable live loads may have to be decreased for dead loads greater than 10 psf, for example, for tile roofs.
Good performance of built-up, single-ply, or modified bitumen roofing applied on low-slope roofs requires a stiffer deck than does prepared roofing applied on pitched roofs. Although Span-Rated panels used as roof sheathing at maximum span are adequate structurally, an upgraded system is recommended for low-slope roofs. Table 10.25 lists maximum spans for low-slope roof decks. Live loads can be determined from Table 10.24, and minimum fastener requirements are given in Table 10.26.
Rated Sheathing is equally effective under built-up roofing, asphalt or glass-fiber shingles, tile roofing; or wood shingles or shakes. Roof trusses spaced 24 in c to c are widely recognized as the most economical construction for residential roofs, particularly when 3â„8- or 7â„16-in, 24/0 sheathing with panel clips is used. However, use of fewer supports with thicker panels, for example, 23â„32- or 3â„4-in, 48/24 panels over framing 48 in c to c, is also cost-effective for long-span flat or pitched roofs.
Live loads are given in Table 10.24. Nailing provisions are given in Table 10.26.
When support spacing exceeds the maximum length of an unsupported edge, as given in Table 10.24, provide adequate block, tongue-and-groove edges, or other edge support such as panel clips. Some types of panel clips, in addition to edge support, automatically assure recommended panel spacing. When required, use one panel clip per span of less than 28 in and two for 48-in or longer spans.
Preframed Roof Panels
Spans of 8 to 12 ft are usually the most practical with preframed panel construction which are typically used in large low slope roof applications, although spans up to 30 ft are not uncommon. Unsanded 4 x 8-ft panels with stiffeners preframed at 16 or 24 in c to c are common. The long dimension of panels typically runs parallel to supports. Stiffeners and roof purlins provide support for all panel edges. Minimum nailing requirements for preframed panels are the same as for roof sheathing.
For preframed panels 8 x 8 ft or larger, the long panel dimension may run either parallel or perpendicular to stiffeners spaced 16 or 24 in c to c. Placing the long dimension across supports may require edge support such as panel clips or cleats between stiffeners at midspan in accordance with Table 10.24.
Deflection limits are 1/180 of the span for total load; 1/240 for live load only.
Nailing requirements for preframed panels are the same as for roof sheathing. See APA Design/Construction Guide Residential and Commercial, APA The Engineered Wood Association, for recommended maximum roof loads.
Panel Diaphragms
With only slight design modifications, any panel roof-deck system described in Arts. 10.12.11 and 10.12.12 will also function as an engineered roof or floor diaphragm to resist wind and seismic loading.
The ability of a diaphragm to function effectively as a deep beam, transferring lateral loads to shear walls, is related to the quality of the connections. Nailing is critical, since shears are transmitted through these fasteners. Common nails provide required strength. Other nail types may be used when their lateral bearing values are considered in the design. Load-carrying capacity is highest when the diaphragm is blocked.
Where 11â„8-in roof panels are desired, such as for heavy timber construction, shear values for 19â„32-in panels are used. Blocked shear values for 11â„8-in panels may be obtained by specifying stapled tongue-and-groove (T&G) edges. Staples should be 16 ga, 1 in long, with a 3â„8-in crown. They should be driven through the T&G edges 3â„8-in from the joint so as to penetrate the tongue. Staples should be spaced at one-half of the boundary nail spacing for Cases 1 and 2, and at one-third the boundary nail spacing for Case 3 through 6, as illustrated in Table 10.27, which gives panel and fastening recommendations for roof diaphragms. Panels and framing are assumed already designed for perpendicular loads. To design a diaphragm, follow these steps:
1. Determine lateral loads and resulting shears.
2. Determine the nailing schedule (Table 10.27). Consider the load direction with respect to joints.
3. Compute the chord stress due to bending moment. Provide adequate splices.
Check deflections. Check the anchorage of boundary framing, for example, to walls.
For situations where greater diaphragm capacities are necessary and framing with a nominal thickness of 3 or 4 in is available, diaphragms may be constructed using heavier nailing schedules, such as that given in the Uniform Building Code.
(Diaphragms, APA The Engineered Wood Association, Tacoma, Wash.)
