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Technical Background – Engineered Wood Structural Floor Assemblies

critical with today’s larger 12x12 in. (305x305 mm) and greater tile modules, as some of the stress induced by the increased curvature can be absorbed by the softer and more frequent grout joints of smaller tile module installations across the same increased joist spacing. Similarly, while I-Joists have the ca pacity to support concentrated loads within the prescribed de flection criteria, the typical subfloor-panel thickness is not suf ficiently stiff to support concentrated loads without excessive curvature. Therefore, the stiffness and curvature of the subfloor panel are more critical than the span and deflection-related curvature of the I-Joists. When installing tile over the increased spacing and more vibration-prone I-Joists, it is recommended that the builder or tile contractor provide a subfloor panel, in combina tion with an underlayment panel, that has a greater stiffness than is required by minimum building code requirements. The TCNA Handbook methods provide recommendations for sub floor and underlayment panel construction to provide adequate stiffness to resist deflection at increased I-Joist spacing. While a tile installation bonded directly to the subfloor, that utilizes a more flexible adhesive or a bonded flexible interlay er/membrane, will minimize the shear stress induced by in creased deflection tile remains susceptible to cracking if the loads exceed the tile’s flexural bending or breaking strength. A non-bonded tile installation that has the capability of distribut ing concentrated loads to the joists (full mortar bed), is much less susceptible to distress as long as the I-Joists are designed using realistic dead loads of the non-bonded mortar bed sys tem. Differential Deflection EW I-Joists located adjacent to and parallel to an end wall or in ternal load-bearing wall have significantly different curvatures under load than those located over the continuous support of the rim joist or bearing wall. This is also true of sawn lumber joists. Differential deflection, especially at longer I-Joist spans, may result in torsion due to the difference in deflection over a rela tively short distance, adjacent to the perimeter support or inter nal support. It is advisable to compensate with framing details (stiffer joists, closer joist spacing, outriggers, or blocking). Effects of Moisture on Engineered Wood Products Exposure to moisture can affect the physical performance of all wood products, and engineered wood products are no ex ception. EW products, including plywood, generally have an equilibrium moisture content (MC) that is slightly lower than conventional sawn dimension lumber. I-Joists and OSB are manufactured with MC in the range of 4 to 6% and reach an equilibrium moisture content of about 10-12% in a typical home. Sawn lumber, on the other hand, may have an MC at installation of over 20% and then dry to 10-12%. The net effect is that engi

about 0.008 in. under a total load of a 40 psf, the code-pre scribed minimum live-load capacity. A 300-pound point load, such as that from a hand-truck wheel or heel of a person carry ing a large load, can produce a deflection of about 0.12 in. be tween the joists. Neither deflection is very much and is unlikely to be noticed by the occupants or the tile installer. The curvature induced by the concentrated load, however, is approximately 9 times that induced by the uniform load. It’s a bit ironic that the building codes traditionally prescribe an L/360 uniform live load deflection limit when a floor is almost never subjected to a uniform live load. The live loads imposed on a tile floor are generally the concentrated loads of various sizes, ranging from high heels and loaded hand-truck wheels to heavy furniture or fixtures. Increased Joist Spacing and Deflection of Subfloor Panels The spacing of I-Joists is one of three variables that must be balanced to satisfy the basic requirements of an engineered wood structure. The span and depth (size) of the I-Joists are the other variables. Increasing the spacing between joists is typ ically the prime consideration, as there is a tangible labor and material saving by eliminating one or two joists per 96 in. (2.4 m) length of subfloor panel. In reality, decreasing the joist depth, or increasing joist span while maintaining traditional joist spacing of 16 in. (406 mm) on center, may have equal but less tangible benefits. The issue with joist spacing is the effect that increased joist spacing has on the stiffness and resulting curvature of the subfloor and underlayment between the joists. Conventional dimension lumber joists are typically spaced either on 16 in, (406 mm) or 12 in. (305 mm) centers because of the more limit ed bending capacity and stiffness of sawn lumber at maximum spans compared to EW I-Joists. Wood I-Joists introduced and popularized the concept of 19.2 in. (488 mm) and 24 in. (610 mm) on-center floor-joist spacing, primarily because of the increased structural load capacity and stiffness of these en gineered wood products. The 19.2 in. (488 mm) and 24 in. (610 mm) spacings correlate with the modular length 96 in. (2.4 m) of typical subfloor panels, thus maximizing economy and perfor mance by eliminating one or two joists per 96 in. (2.4 m) length of wood structural panel. At the conventional floor-joist spacings, wood structural sub floor and underlayment panels are typically governed by bend ing strength, so curvature caused by deflection isn’t as big an issue. With wider joist spacings, however, deflection is more frequently the governing factor and it can lead to greater cur vature. It is the curvature that is likely the biggest structural source of tile failures. Increased I-Joist spacing increases the curvature of the sub floor panels, which in turn may induce excessive flexural and shear stress in a tile adhered to the subfloor. This is even more

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates