Home > Structures

Q&A: Structural wood design

Q&A: Structural wood design

‘Ask an Expert’ addresses common technical questions related to design and construction of wood-frame and mass timber buildings.

Do you have a technical question related to the structural design of commercial or multifamily wood buildings? Perhaps you’re interested in the questions others are asking. If that’s the case, you may want to check in with the U.S. WoodWorks program. In addition to offering project support (at no cost), WoodWorks – Wood Products Council publishes a monthly “Ask an Expert” Q&A addressing technical questions related to the design and construction of wood-frame and mass timber buildings. The current Q&A can always be found on the home page at www.woodworks.org, or browse or search more than 40 Q&As in the archive at www.woodworks.org/ask-an-expert.

Questions cover the gamut of wood-frame and mass timber design considerations. For example, the ones below cover thermal movement in wood structures and wood diaphragms consisting of multiple layers. Other topics include floor-to-exterior wall detailing, podiums, shrinkage, how to accommodate MEP in exposed mass timber buildings, construction tolerances, and much more.

Q: Do wood-frame buildings need to account for thermal movement?

A: The general consensus is that designers of wood-frame buildings do not need to account for thermal movement, as thermal expansion is offset by the shrinkage of wood due to increased temperatures and moisture loss — which designers do need to consider.

While concrete and steel buildings are typically designed with expansion joints to account for thermal movement due to environmental temperature fluctuations, wood has a significantly lower coefficient of thermal expansion. Wood can experience dimensional movement with temperature changes. As noted in Chapter 4 of the USDA Forest Products Lab’s Wood Handbook (www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf), “The thermal expansion coefficients of completely dry wood are positive in all directions; that is, wood expands on heating and contracts on cooling.” However, moisture fluctuations impact dimensional movement of the wood at the same time.

Wood is hygroscopic, meaning it has the ability to absorb and release moisture. As this occurs, it also has the potential to change dimensionally. Note that the above excerpt from the Wood Handbook references completely oven-dry wood (0 percent moisture content, or MC). Wood used for the structure of a building isn’t completely oven dry during or even after construction. As wood experiences an increase in temperature, it may experience some thermal expansion, but there is also a loss of MC. The shrinkage due to reduced MC is more significant than the expansion due to increased temperature; therefore, the net result is shrinkage.

The following section from Chapter 4 of the Wood Handbook explains this: “Wood that contains moisture reacts differently to varying temperature than does nearly oven-dry wood. When moist wood is heated, it tends to expand because of normal thermal expansion and to shrink because of loss in moisture content. Unless the wood is very dry initially (perhaps 3 percent or 4 percent moisture content or less), shrinkage caused by moisture loss on heating will be greater than thermal expansion, so the net dimensional change on heating will be negative. Wood at intermediate moisture levels (about 8 percent to 20 percent) will expand when first heated, and then gradually shrink to a volume smaller than the initial volume as the wood gradually loses water while in the heated condition.”

Wood shrinkage/expansion occurs most notably perpendicular to the grain, meaning that a solid sawn wood stud or floor joist will change in width and depth. Longitudinal dimensional change due to moisture change is negligible, meaning the length of a stud or floor joist will essentially remain unchanged.

As the Wood Handbook notes: “Even in the longitudinal (grain) direction, where dimensional change caused by moisture change is very small, such changes will still predominate over corresponding dimensional changes as a result of thermal expansion unless the wood is very dry initially. For wood at usual moisture levels, net dimensional changes will generally be negative after prolonged heating.”

Section 4.4 of the American Wood Council’s National Design Specification (NDS) for Wood Construction Manual (www.awc.org/pdf/codes-standards/publications/archives/lrfd/AWC-LRFD2012-Manual-1211.pdf) also provides commentary on this subject and includes coefficients of thermal expansion for a number of wood species.

While accommodating thermal movement isn’t generally considered necessary, it is recommended that designers of wood-frame buildings account for expansion during construction due to increased moisture exposure. It is especially important to consider expansion of wall, floor, and roof sheathing. Because panel products start at a low moisture content (approximately 8 to 12 percent MC) and are directly exposed to the elements during construction (in many cases increasing to greater than 19 percent MC), the expansion of these products is likely to be more pronounced. Panel buckling, which occurs when there is no room for panel expansion, is prevented with a standard 1/8-inch space between all sheathing panel edges and end joints as recommended by APA in Technical Note D481N Minimizing Buckling of Wood Structural Panels (www.apawood.org/publication-search?q=d481n). In larger buildings (more than 80 feet in length), it is recommended to increase the gap between panels and take additional construction sequencing precautions to avoid panel buckling. The WoodWorks paper, Accommodating Shrinkage in Multi-Story Wood-Frame Structures (www.woodworks.org/wp-content/uploads/Accomodating-Shrinkage-Multi-Story-Wood-Frame-Structures-WoodWorks.pdf), discusses this, as does technical note U425 Temporary Expansion Joints for Large Buildings (www.apawood.org/publication-search?q=U425&tid=1) from APA.

Q: What design and detailing considerations exist when installing a layer of wood structural panels over a lumber deck diaphragm to achieve higher diaphragm capacities?

A: When designing wood-frame structures with lumber floor and roof decking, either new construction or modifications and rehabilitations of existing construction, the diaphragm capacity or aspect ratio of the lumber decking system may be inadequate and require reinforcing measures. The capacities of lumber diaphragms are given in Table 4.2D of the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS). Additional options to evaluate the seismic capacity of existing lumber decking can be found in ASCE 41-13. A common method of increasing diaphragm capacities in this condition is to install a layer of wood structural panels (WSP) — i.e., plywood or OSB — on top of the lumber decking. In this condition, the lumber decking is no longer used as the structural diaphragm; the WSP layer is attached to the lumber decking and acts as a blocked diaphragm, using the capacities in SDPWS Table 4.2A.

While the WSP layer is resisting the diaphragm forces, the decking is acting in the same function that wall studs would in a shear wall or floor joists would in a diaphragm — providing common framing members for two adjacent panel edges to attach to in order to provide shear load transfer between panels. Field fasteners attach the sheathing to the decking to keep panels from buckling out of plane.

SDPWS Sections and provide specific requirements when using the WSP over lumber decking diaphragm condition. As noted, the diaphragm capacity of the decking is not used — only that of the blocked diaphragm consisting of the sheathing laid on top. Lumber decking, particularly when perpendicular to the supports, is a much more flexible diaphragm system than WSP diaphragms and does not effectively share load with the WSP diaphragm. One key detailing and construction item to keep in mind is that the sheathing panel edges must be offset from joints in the decking. This will require careful layout of the decking placement (in new construction) or careful layout of panels (in existing construction).

Another consideration is whether minimum nail penetration values into the decking can be met. For example, if the lumber decking is only 1x (3/4-inch actual thickness) or 2x (1-1/2-inch actual thickness), nails used to attach the WSP to the decking may only be 1-1/2 inches long, meaning the minimum nail penetration of SDPWS Table 4.2A may not be able to be met. APA Report TT-097

(www.apawood.org/publication-search?q=TT-097&tid=1) provides a summary of considerations and APA Report TT-061 (www.apawood.org/publication-search?q=TT-061&tid=1) provides information for the condition of inadequate nail penetration. This report suggests that only 1.0 inch of penetration is required for 8d nails to achieve full diaphragm capacity. As this may not meet the minimum nail penetration of SDPWS Table 4.2A, discussion with the building official may be prudent.

Once the standard diaphragm capacity has been determined and fastener schedules set, the boundary of the diaphragm must be designed. At these locations, the diaphragm forces need to be taken out of the sheathing, through the decking, potentially into the boundary framing members (depending on how the floor/roof is framed) and then into the vertical lateral force-resisting systems below. The attachment of the decking to the diaphragm boundary framing members (or directly to the vertical lateral force-resisting system) is designed for the unit diaphragm reaction along that line, or the chord force, whichever is greater. This is typically done with common fasteners (e.g., nails, screws, etc.). The required minimum spacing of the fastener being used is determined using the diaphragm shear load or diaphragm capacity per unit length of the connection.

Information provided by WoodWorks, which provides free technical support to design professionals as well as education and resources related to the code-compliant design of commercial and multifamily wood buildings. WoodWorks staff have the expertise to assist with all aspects of wood building design, including (but not limited to) allowable heights and areas/construction types, structural detailing of wood and hybrid systems, fire resistance and acoustical-rated assemblies, efficient and code-compliant lateral system design, alternate means of code compliance, and energy-efficient detailing. For assistance on a project, visit www.woodworks.org to contact a regional director or email help@woodworks.org. View a calendar of education events across the country at https://www.woodworks.org/events-calendar/upcoming. Follow WoodWorks on Twitter (@WoodWorksUS), LinkedIn (@WoodWorks), or Instagram (@WoodWorksUSA) to be alerted when a new Ask an Expert is released.