Many engineering materials suffer from decreased mechanical properties at elevated temperatures. Aside from the obvious melting problem, even metals that don’t rely on heat-treatment processes often lose large portions of their strength as they approach their respective melting points, and carefully-tempered alloys can be “reset” to their annealed state at shockingly low temperatures.
Wood doesn’t melt or have a crystalline structure to soften, but it can burn. Even at temperatures as low as 150 °F (65.5 °C), some of the more volatile organic compounds in wood’s structure can start to react away, permanently damaging the strength and stiffness of the wood. Wood also experiences reversible loss of strength and stiffness at elevated temperatures (above room temperature)
The American Wood Council’s National Design Specification for Wood Construction provides Reference Design Values based on testing at room temperature. For structural members used above room temperature, but below 150 °F, the Temperature Factor is used to reduce those reference design values.
The design of wood structural members intended to be used above 150 °F is specifically beyond the scope of normal wood design.
How Does Temperature Affect Wood Strength?
Between 32 °F and 150 °F, almost all mechanical properties of wood decrease approximately linearly with increasing temperature, and these effects are reversible. These effects have been determined from numerous studies at the USDA Forest Products Laboratory, and by other researchers around the world.
The moisture content of wood has a huge influence on how changes in temperature drive changes in wood’s mechanical properties.
Below 32 °F, freezing of water in the wood can remarkably jump up the mechanical properties, especially in wood with a moisture content above the fiber saturation point. These effects are largely reversible, but repeated freeze-thaw action can degrade the wood over time. Temperature effects in cold temperatures are very sensitive to moisture content, so designers in ordinary structures are advised not to rely on them, barring extremely controlled conditions.
Above 150 °F, the wood starts to break down and experience permanent damage. The Forest Product Lab’s Wood Handbook discusses this at some length and cites FPL–RP–631 “Durability of Structural Lumber Products After Exposure at 82 °C and 80% Relative Humidity” as determining much of the chemical breakdown process beyond that temperature. Here too, moisture content drives the effects to a tremendous degree, as does the abundance or absence of oxygen to react with the wood.
The hemicellulose, which makes up much of the wood’s cellular structure, is attacked by weak acids in the water bound up within the wood, breaking down to form more acid, accelerating the process as time goes on.
Within the range of temperatures between about 20 °C and 40 °C, the effects on wood’s mechanical properties are not generally pronounced enough to bother with adjusting the reference design values.
However, between 100 °F and 150 °F, they are somewhat more pronounced, and some of these effects are fairly dependent on the moisture content.
NDS Temperature Factors for Design
The NDS is typically vague on when exactly designers must apply these adjustment factors. It simply states that when wood is expected to see “…sustained exposure to elevated temperatures, less than 150 °F…” that we need to apply the Temperature Factor.
Let’s unpack that a bit:
- The Commentary tells us that “elevated temperatures” are anything between 100 °F and 150 °F
- “Less than 150 °F” is the hard limit, as prolonged exposure to higher temperatures than this can cause irreversible damage, which is beyond the scope of the Reference Design Values provided. Special consideration is needed for design above this temperature.
- No guidance on what exactly constitutes “sustained exposure” is given anywhere in the NDS, the Commentary, the Appendices, the USDA Forest Product Laboratory’s Wood Handbook, nor in any of the referenced research articles in any of those works.
- However, as this is an adjustment factor to account for the immediate and reversible temperature effects only, it would stand to reason that we only need to apply this factor when there’s a reasonable chance the wood member will see design-level loading concurrently with elevated temperatures.
- NDS Appendix C even supports this, stating “In recognition of these offsetting factors, it is traditional practice to use the reference design values from this Specification for ordinary temperature fluctuations and occasional short-term heating to temperatures up to 150°F.”
- A member in an unheated building that only sees substantial loading from snow or ice could probably be designed without the Temperature Factor used, and then rechecked for the lower loading conditions with the Temperature Factor applied.
The Commentary does direct us that the Temperature Factor is typically not used in ordinary roof support applications. Though attics can become quite warm, experience has shown that they seldom exceed 140 °F, even in the warmest of climates. Some special cases do of course exist, in which case special design provisions will be necessary.
Research has shown that moisture content greatly influences the magnitude of these reversible thermal effects, as well as the irreversible effects at greater temperatures. As such, the bending, shear, and compression values receive different Temperature Factors for wet and dry service conditions. Axial tension and the modulus of elasticity are not as sensitive, so have not been differentiated.
| Reference Design Values | In-Service Moisture Conditions | T ≤ 100°F | 100°F < T ≤ 125°F | 100°F < T ≤ 125°F |
|---|---|---|---|---|
| F.t, E, E.min | Wet or Dry | 1.0 | 0.9 | 0.9 |
| F.b, F.v, F.c, and F.c⊥ | Dry | 1.0 | 0.8 | 0.7 |
| F.b, F.v, F.c, and F.c⊥ | Wet | 1.0 | 0.7 | 0.5 |
Summary
At elevated temperatures, but below 150 °F, wood reversibly loses some strength and stiffness, and the NDS Temperature Factor accounts for this reduction in properties. Above 150 °F, permanent losses in properties are expected, and these require special design beyond the scope of the NDS.
For more information about structural wood design, check out my other articles on wood design, or grab a copy of by far the best wood design textbook on the market, “Design of Wood Structures” by Breyer, Fridley, Cobeen, & Pollock.
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