Do You Design with Wood (Shrinkage) in Mind?

Wood is a unique building material because its characteristics are dependent on the environment and its moisture properties, which can vary over time. Often, sawn lumber is delivered to the jobsite with relatively high moisture content. Over the life of the building, moisture content will decrease until equilibrium has been achieved. As moisture content drops, the wood members shrink. Most wood shrinkage occurs during the first six months of the building life and it’s an important design consideration.

Wood is directionally dependent on anisotropic, meaning it shrinks different amounts in each grain direction. The greatest amount occurs perpendicular to the grain, such as in sole plates, top plates and solid sawn lumber joists. Shrinkage occurring in the longitudinal direction, such as in studs, is very small and often not considered during design. Structural composite lumber (SCL) shrinkage – I-joists, LVL, LSL and PSL – is also usually considered negligible because the initial moisture content is low.

In multistory construction where continuous tiedown systems are used, wood shrinkage can impact shear wall performance because of gaps occurring between the wood and the tiedown system. To compensate for these gaps, and eliminate added shearwall drift due to shrinkage, shrinkage compensating devices are usually the solution.

There are various types of shrinkage compensators available, and each device functions differently in their installed location and the type of loads they obtain. To see animations explaining the various shrinkage compensation devices and how they function in a multi-story building, click here.

– Paul

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Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day: Q.   What is your mom/dad's job? A.   Goes and gets coffee and sits at his desk Q.   What does your mom/dad actually do at work? A.   Walks in the test lab and checks things When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems. I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.

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