Timber framers commonly call stressed-skin panels the composites of wood, glue and plastic foam that sheathe the roofs and walls of many modern American timber frame buildings. But, properly defined, the term is a misnomer since, with very few exceptions, these panels lack the lumber core essential to the definition as promulgated by the American Plywood Association and accepted by code agencies in the 1960s (1).

A stressed-skin panel is understood to comprise one or two skins firmly bonded by adhesives of specified structural performance to timber members of certain sizes and on-center spacing. The presence or absence of thermal insulation is irrelevant to the definition.

The panels that do generally surround modern American timber frames, and which comprise one or two (usually two) engineered-wood sheet skins chemically bonded to a simple plastic foam core, were originally described by the American Plywood Association as sandwich panels (2). Today, by agreement of the people who build them, such panels are generally called structural insulated panels because they have both insulating and load-bearing abilities.

Both forms of panel rely on composite action, which requires the core and skin or skins to act as a unit, forbidding slippage between them. The adhesives or fastenings must be effective in transferring shear forces and cannot deteriorate over time because of moisture or creep.

While many stressed-skin panels have skins attached to both edges of the framing lumber, a panel can meet the definition with one skin (Figure 2a, below). Since the strength and stiffness of the stressed-skin panel are based on the composite action of the core and skin, an important requirement is that the adhesive be rigid, with known structural performance in both the short and long term.

If for some reason the adhesive between the skin and the lumber core failed to function as intended, the components of the stressed-skin panel acting individually would still safely carry some substantial percentage of the design load. While this scenario is not desirable, it demonstrates that stressed-skin panels (as we define them) are inherently robust with respect to manufacturing deficiencies in the type and application of adhesives used to connect the skin to the core. This virtue is not shared by structural insulated panels.

What are SIPs?

Structural insulated panels (SIPs) consist of a layer of rigid insulating foam, varying from 3 1/2 to 11 1/4 in. thick, sandwiched between layers of 7/16-in. oriented-strand board (OSB), with possibly an interior finish, such as gypsum board or tongue-and-groove paneling, added to one side.

The insulating foam for SIPs can be polyurethane (including polyisocyanurate) or polystyrene (expanded or extruded). Figure 3a shows a cross-section of a common SIP configuration. SIPs, without a core of framing lumber spaced 24 in. or less on center, are substantially different from stressed-skin panels in that 100 percent of a bending moment is assumed for design purposes to be resisted by the tension and compression capacity of the skins.

In addition to relying on the adhesive bond between the two skins and the core for the needed bending strength, the core material must also transfer the shear produced by the bending loads, both in the short and long term. If the adhesive bond between the skins and core fails to function or the core material fails to function, the SIP fails.

Since the structural integrity of the SIP depends entirely upon the glue bonds between the skins and the core and the durability and structural reliability of the core material, it's obviously important for SIPs to be manufactured under accepted standards and that manufacturing procedures and quality control be subjected to third-party inspection by an approved agency. Typically, such inspections involve unannounced, regular visits to the manufacturing facility by representatives of a testing agency such as Product Fabrication Service (Madison, Wisc.), to scrutinize fabrication methods and test random samples of SIPs to ensure that the foam, OSB and the adhesion between the foam and the OSB are adequate. These third-party inspections are required in order to maintain code approval by the International Conference of Building Officials (ICBO).

How can you determine if a SIP meets recognized industry standards for quality and is appropriately designed for published load ratings? First, you can request a code report from the manufacturer, or by searching the web sites of the governing code agency (e.g., www.icbo.org/icbo-es/es-search.html).

The existence of a full code report for a product and its manufacturer is a reliable indication that third-party inspections have been conducted at the SIP plant and that the governing agency, such as the ICBO or the National Evaluation Service, has embraced the product as acceptable. Additionally, code-approved SIN should display a stamp on the OSB that indicates the panel type, the code report number, the manufacturer's trademark or name and the third-party inspection agency's logo and report number.

If SIPs are designed properly for their intended application, manufactured using establish quality procedures and verified by third-party inspection (3), builders can be assured of structural performance similar to the solid-sawn timber and board sheathing constructions that have been used for centuries. -- DAVID CARRADINE, FRANK WOESTE AND SCOTT M. KENT

David Carradine is a graduate research assistant in the department of Biological Systems Engineering, Virginia Tech University, Blacksburg, VA 24061. Frank Woeste PE, Ph.D (fwoeste@vt.edu) is professor in that department and the author of numerous books and papers on wood construction technology. Scott M. Kent P.E. is Quality Manager, Wood Science & Technology Institute, Corvallis, OR 97333.

1. Plywood Design Specification Supplement 3, Design and Fabrication of Plywood Stressed-Skin Panels (updated August 1990), document available in PDF format at www.apawood.org.

2. Plywood Design Specification Supplement 4, Design and Fabrication of Plywood Sandwich Panels (March 1990), available at www.apawood.org.

3. ICBO Evaluation Service (ES) documents AC04 and AC05, acceptance criteria, respectively, for Sandwich Panels and Sandwich Panel Adhesives, are available at www.icbo.org.

 

Figure 1. Bending stresses in a conventional assembly are assumed to be only in the supporting timber.

Figure la shows the cross-section of a nominal 2x8. If the 2x8 is loaded simply as a joist, Figure 1b illustrates the bending stress distribution on the cross-section. Assuming a 12-ft. span and a 50-psf load, the top half of the 2x8 is subjected to a maximum compressive stress of 1,095 psi, and the bottom half to a maximum tensile stress of 1,095 psi. At the center of the 2x8, the neutral axis, there is no compressive or tensile stress. Figure 1c shows the shear distribution on the cross-section.

Maximum shear stress (here 56 psi) is at the mid-height of the section. For a 2x8, the allowable shear stress is called horizontal shear, and ranges from 70 to 100 psi for common construction species. For a beam to function adequately, the compression, tension and shear stresses produced by the external loads must be fully resisted by the material.

Figure 2. In a stressed-skin panel, bending stresses act in both the sheathing and the timber.

Figure 2b shows the bending stresses within a single-skin panel. Compression stress at the top of the skin is 625 psi and 503 psi at the bottom. Tensile stress at the bottom of the 2x8 is 745 psi, and 419 psi at the top. Note that the neutral axis is now not at the center as in Fig. 1, but rather falls 4.6 inches up from the bottom of the 2x8. The maximum bending stress in the 2x8 (745 psi) is significantly less than the 1,095 psi maximum observed for the traditional system. Figure 2c shows the distribution of shear stress in the cross-section. The skin is 5/8-in. plywood (5 plies) with face plies parallel to the timbers.

The critical region for rolling shear in the plywood occurs one ply up from the bottom, where the shear stress is 12 psi. The critical shear stress in the timber at 4.6 inches up from the bottom is 43 psi. All the stresses here are less than allowable values for the plywood and assumed lumber grade and species, so the design is deemed adequate for the design loads.

The significant feature of the stressed-skin panel lies in the fact that applied bending moment is shared. In the example, about 65 percent of the moment is carried by the joist, the rest resisted by the plywood skin. In case of failure of the glue bond between core and skin, one could calculate the percentage of the total design load that would be safely carried by the timber acting alone. Assuming the allowable bending stress for the 2x8 were exactly 745 psi, for this design, span and loading the timber acting alone would safely carry 67 percent of the design load.

Figure 3. In a structural insulated panel (SIP), almost all the bending stresses are in the skins.

Figure 3b shows the compression and tension stresses in the skins due to bending (plus a small amount in the core material that is neglected in design). Figure 3c shows the distribution of shear stress throughout the cross-section. The maximum shear stress throughout the section is 3.4 psi, which is less than the allowable shear stress of the foam, assumed to be 6 psi. Although there is a negligible bending stress in the core material, note however that the foam core must consistently carry stress from top to bottom.