Wood, like soil, is able to hold varying amounts of moisture within its porous structure. Also like soil, the mechanical properties of wood vary based on the moisture content.
Totally separate from any moisture-related decomposition, the strength and stiffness of wood members vary greatly from the “fiber saturation point” of fully-saturated, fresh-cut wood down to about 12% moisture content. Engineers need to factor in this variation in properties during design, but how can they hope to capture this wide range of variation?
The reference design values in the American Wood Council’s National Design Standard for Wood Construction (NDS) are based on sawn lumber with less than 19% moisture content, and glulam timber with less than 16% moisture content for its design life. When structural wood members are subject to higher moisture contents for “an extended time period”, the Wet Service Factor is used to reduce the mechanical properties from the reference design values to account for wood’s lower strength at higher moisture content.
Various wet service factors are applied to the different Reference Design Values from the NDS Supplement, depending on how strongly each design value is affected by moisture. Species and grading also play somewhat of a role, with larger timbers assumed to have a higher moisture content throughout their design life, which causes many of the reductions to disappear for them.
How does Moisture Content Affect Wood Strength?
The USDA Forest Products Laboratory (FPL) in Madison, WI has been bending and crushing wood samples of all sizes, species, and moisture contents for decades. One of the noteworthy variables they’ve spotted in all this data is moisture content at the time of testing, which affects most of the tested properties.
In their “Wood Handbook: Wood as an Engineering Material”, the FPL lays out expected design values for hundreds of species of wood, both domestic and exotic, at both “green” and 12% moisture contents. Consistently throughout this huge dataset, we see a big hit to the strength and stiffness properties from the “dry” (12% moisture content) to the green condition.
Keep in mind that “green” (the natural water content of freshly-harvested wood) wood is not a particular moisture content as such, but varies substantially from species to species, anywhere from 30% to upwards of 200%. There’s often large moisture content variation between the heartwood and sapwood of the same trunk of a given tree.
As an example, here are the mechanical properties of domestically-grown Longleaf Pine, one of the species in the popular “Southern Pine” grade designation. These are not recommended design values, as they have no safety factors incorporated, and are based on specimens of exceedingly good quality.
| Property | Dry (12%) | Green | % Reduction |
|---|---|---|---|
| Modulus of Rupture (psi) | 14,500 | 8,500 | 41% |
| Modulus of Elasticity (x10^6 psi) | 1.98 | 1.59 | 20% |
| Work to Maximum Load (in*lbs/in³) | 11.8 | 8.9 | 25% |
| Compression Parallel to Grain (psi) | 8,470 | 4,320 | 49% |
| Compression Perpendicular to Grain (psi) | 960 | 480 | 50% |
| Shear Parallel to Grain (psi) | 1,510 | 1,040 | 31% |
| Tension Perpendicular to Grain (psi) | 470 | 330 | 30% |
Looking through the table above, we can pretty clearly see that the moisture content of wood is a big deal, and needs to be accounted for in design. Years ago, engineers thought that the strength of wood just kept increasing with ever-lower moisture contents, but further research has found that these properties actually plateau around the 6-8% moisture content level, with further drying not appreciably impacting the properties.
Wood also stops getting weaker after a certain point, typically around 30% moisture content, which engineers call the “fiber saturation point”. Any added water past this point pretty much just adds more weight to the wood, without changing the strength or stiffness, though it may also promote decay, which would obviously serve to decrease the strength over time.
NDS Wet Service Factors for Design
The Reference Design Values provided in the NDS Supplement (2018) are based on assumed “normal” moisture contents that wood is expected to experience inside a structure.
As wood can take up moisture from and expel moisture into the air, local humidity typically drives a seasonal variation in seasoned wood products. In the American Southwest, low prevailing humidity and high heat tend to dry wood to very low moisture contents (6-8%), while in more Oceanic climates, humid summer months can push wood up into the 17-19% range with ease.
For structural wood design, the American Wood Council set the moisture content for the Reference Design Values at assumed maximum equilibrium moisture contents to cover the majority of the US, and introduced the “Wet Service Factor” to adjust these values down whenever the structural member is expected to exceed these moisture contents for “an extended period of time”, which of course they leave undefined.
Most discerning designers opt to include the Wet Service Factor whenever they have reason to believe that their structural members will be exposed to conditions driving higher moisture contents for any time at all, given the total lack of definition as to what “extended” means.
Timbers, having a nominal thickness (lesser cross-sectional dimension) of at least 5″, are much harder to dry out from their green condition. Based on the generally-higher moisture content of these members, most properties are actually based on a higher service moisture content than the 19% listed below, and then effectively throw out the Wet Service Factor modification for those properties by defaulting those values to 1.00.
Due to the way glue tends to block moisture uptake in glulam members, those members have their reference design values based on a 16% moisture content, while sawn lumber has design values based on a 19% moisture content.
When members exceed these moisture levels, the designer needs to pick up the appropriate Wet Service Factor corresponding to the Supplement table where the Reference Design Values are found, as the values differ for sawn dimensional lumber, sawn timbers, and glulam timbers. These values are summarized in the table below:
| Property | Sawn Lumber MC > 19% | Sawn Timber MC > 19% | Glulam Timber MC ≥ 16% |
|---|---|---|---|
| F.b | 0.85* | 1.00 | 0.8 |
| F.t | 1.00 | 1.00 | 0.8 |
| F.v | 0.97 | 1.00 | 0.875 |
| F.c⊥ | 0.67 | 0.67 | 0.53 |
| F.c | 0.8** | 0.91 | 0.73 |
| E & E.min | 0.9 | 1.00 | 0.833 |
Summary
The strength and stiffness of wood structural members increase with decreasing moisture content. In order to account for the reduction in strength that comes with service at a higher moisture content, the NDS provides a number of Wet Service Factors to reduce the provided Reference Design Values, which are based on sawn lumber with a moisture content of 19% or less, and glulam timbers with a moisture content of less than 16%.
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 (I’m entirely self-taught out of this one, having needed to drop my wood design class in favor of Wastewater Treatment Plant Design to get through my Capstone Design project…), “Design of Wood Structures” by Breyer, Fridley, Cobeen, & Pollock.
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