Procedures for estimating wood shrinkage and detailing options minimize effects on building performance.
By Richard McLain, P.E., S.E., and Doug Steimle, P.E.
In wood-frame buildings of three or more stories, cumulative shrinkage can be significant and has an impact on the function and performance of finishes, openings, mechanical/electrical/plumbing (MEP) systems, and structural connections. However, as more designers look to wood-frame construction to improve the cost and sustainability of their mid-rise projects, many have learned that accommodating wood shrinkage is actually very straightforward.
This excerpt of a WoodWorks technical paper describes procedures for estimating wood shrinkage and provides detailing options that minimize its effects on building performance. For a more detailed discussion of this topic, read the full paper at www.woodworks.org/shrinkage-wood-frame-structures.
Wood moisture content (MC) is the weight of water in wood as a percentage of the completely dry wood weight. During the life of a tree, its MC can exceed 200 percent, meaning the total water weight in a given volume of wood makes up two thirds or more of the total weight.
The fiber saturation point (FSP) is the point at which wood has no free water and yet the cell walls are saturated. The FSP of different species of wood varies, but for most common softwoods is around 28 to 30 percent. The MC of lumber in-service is typically 7 to 14 percent, much lower than the FSP. Wood remains dimensionally stable above the FSP; i.e., it doesn’t change in dimension with an increase or decrease of moisture as long as it remains above the FSP at all times. This is because the water being absorbed or released is largely free water in the cells, not bound water (which is bound to the cell walls). Once the moisture content drops below the FSP (i.e., bound water is being removed), the wood starts to change dimensionally. Since water is now being released from cell walls, both the walls and surrounding structure shrink, causing the overall cross-section of wood to shrink.
In wood-frame construction, the three variables influencing the magnitude of shrinkage are:
- installed MC;
- in-service MC, or equilibrium moisture content (EMC); and
- cumulative thickness of cross-grain wood elements.
Initial MC, or MC at the time of manufacture, is typically specified on a project’s structural drawings. In many parts of the country, the specification would read “a maximum MC of 19 percent.” In order to achieve this, lumber is generally kiln dried. Engineered wood products, including wood structural panels such as plywood and oriented strand board (OSB), glued-laminated timber (glulam) and structural composite lumber (SCL), are manufactured with lower moisture contents, in the range of 2 to 15 percent. Engineered wood manufacturers generally provide initial moisture contents for their products.
After close-in, the framing in a conditioned building eventually reaches its in-service MC, or EMC, in the range of 7 to 15 percent. Until that point, the wood framing is subject to inevitable shrinkage. EMC is a function of temperature and relative humidity as shown in Figure 1. It is worth noting that EMC is a dynamic equilibrium, meaning it can change throughout the year with climate changes. It also varies across the country based on local climatic conditions (see Figure 2). Generally, for wood exposed to exterior (non-conditioned) atmospheric conditions, variations in EMC of 1 to 3 percent exist between the driest months and wettest months. The EMC of wood within a conditioned building follows the same trends but varies with interior conditions. Typically, the average annual EMC for a given project’s climate is used for shrinkage estimation purposes.
Shrinkage code requirements
Section 2304.3.3 of the 2015 International Building Code stipulates when shrinkage consideration is required in wood-frame building design.
Wood walls and bearing partitions shall not support more than two floors and a roof unless an analysis satisfactory to the building official shows that shrinkage of the wood framing will not have adverse effects on the structure or any MEP systems, or other equipment installed therein, due to excessive shrinkage or differential movements caused by shrinkage. The analysis shall also show that the roof drainage system and the foregoing systems or equipment will not be adversely affected or, as an alternative, such systems shall be designed to accommodate the differential shrinkage or movements.
Although there are several ways to calculate the amount of shrinkage in wood members, it is important to remember that, regardless of the equations used, they are simple calculations. With proper planning, the amount of wood shrinkage can be accurately predicted with relative ease.
One simple calculation is to assume a dimensional change of 0.0025 inch per inch of cross-sectional dimension for every 1 percent change in MC. For example, the anticipated shrinkage in a platform-framed, solid sawn lumber floor-to-wall detail that includes a 13.75-inch shrinkage zone (see Figure 4), starting at an MC of 19 percent with an EMC of 12 percent, would be:
Shrinkage = (0.0025)(13.75 inches)(12-19) = -0.24 inch
When utilizing other types of floor joists such as I-Joists and parallel chord trusses, a degree of engineering judgement is required to determine what to include in the shrinkage zone. For example, floor sheathing is often manufactured with a low moisture content that could be near or even below the EMC. The same could be true for I-Joists. There is also a possibility that these materials slightly expand as they take on moisture during construction, only to shrink again after the building is closed in. Although the final determination should take into account how well the framing materials are protected from the weather during construction, some engineers choose not to include these members in the shrinkage zone — i.e., they expect no shrinkage contribution from them. For parallel chord trusses in a platform-framed condition, only the top and bottom chords are typically included in the shrinkage zone. This is because a vertically oriented member usually exists at the end of the truss, separating the top and bottom chords, and no longitudinal shrinkage is assumed in this member.
Of the significant variables affecting shrinkage, EMC is the only one largely out of the designer’s control. It is heavily influenced by the building’s conditioning, including temperature, relative humidity, and local climatic conditions. However, a project’s design and construction team can influence the two remaining variables — initial moisture content and combined thickness of the wood in cross-grain orientation.
While the initial MC of wood can be specified by the design team, it is subject to change before, during, and after construction. The amount of change depends to a large degree on the protective measures taken by the contractor. To minimize moisture accumulation onsite:
- avoid storing material where it is exposed to rain or standing water;
- keep unused framing materials covered;
- inspect building enclosure layers such as weather-resistive barriers for proper installation;
- “dry-in” the structure as quickly as possible; and
- immediately remove any standing water from floor framing after rain showers.
Because cross-grain wood members contribute to shrinkage, reducing the total thickness of cross-grain wood members in the vertical load path is one way to minimize shrinkage and its effects. Traditionally, wood-frame construction has been platform-frame, where floor framing and rim or band joists bear on the top plates of supporting walls. However, switching to a semi-balloon-frame system, where only the floor sheathing bears on the lower walls and floor joists are hung from the walls, can significantly reduce per-floor and cumulative building shrinkage. Figure 5 illustrates the difference. The upper platform detail has a shrinkage zone of 15.75 inches per floor, resulting in shrinkage per floor of 0.28 inches, or approximately 1.4 inches for a five-story building. The lower semi-balloon frame detail has a shrinkage zone of 4.5 inches per floor, resulting in shrinkage per floor of 0.08 inches, or approximately 0.4 inches for a five-story building. These values are based on an initial MC of 19 percent and an EMC of 12 percent.
Shrinkage in multi-story wood-frame buildings is not a new phenomenon, nor is it overly complex to address. It requires an awareness of how and why wood shrinks, close attention when selecting and specifying materials and details, proper material care on the jobsite, and installation that closely adheres to the construction drawings.
For more information on this topic, including details on wood science and shrinkage, rate of shrinkage, accommodating the expansion of wood products and differential movement (e.g., brick veneer and openings, architectural finishes, MEP, and balconies/decks), the complete paper, Accommodating Shrinkage in Mid-Rise Wood-Frame Structures, is available at www.woodworks.org/shrinkage-wood-frame-structures.