Unlike almost every other engineering material, the strength of a wood structural member depends on the total duration that the member is subjected to the peak expected load.
The implications of this are staggering: wood structural designers need to consider the total amount of time over the lifetime of any given structural member it will see load at a particular magnitude. Much like the cumulative damage fatigue brought on by cyclic loading, wood members suffer from extended time in a loaded state.
Independently determining this cumulative load time for each loading case would quickly exhaust even the best designer, but luckily the American Wood Council’s National Design Specification for Wood Construction (NDS) has codified a simpler approach for design.
The NDS “Load Duration Factor” is a multiplier applied to most Reference Design Values for wood as an adjustment to account for wood’s interesting property of carrying substantially higher loads for short durations than the loads it can carry for longer durations. The NDS Reference Design Values assume loads are applied for 10 years over the life of the member, while for extremely short durations these strengths may up to double.
Not to be confused with creep (increasing deflections under a constant load), which wood also exhibits, the Load Duration Factor is a totally separate, strength-only phenomenon. We’ll examine long-term deflections in wood in another article.
Load Duration Effects
When a structural wood member is loaded for a short period of time, it can hold substantially higher loads than if it was subjected to that load permanently. The reference design values provided in the NDS Supplement are all calibrated to correspond to 10 years of cumulative loading over the life of the structural member, corresponding to a floor Live Load.
Note that for anything less than a 10-year load duration, this factor would technically be optional, and conservative to ignore, as it can only increase the strength of the structural wood member under consideration.
Extensive research at the USDA Forest Products Laboratory in Madison, WI yielded the below “Madison Curve”, on which all of these load duration factors are based.
Ironically enough, the researcher who originally published the below “Madison Curve” in his 1951 “Relation of Strength of Wood to Duration of Load” was actually named Lyman W. Wood. No doubt this is why it came to be known as the Madison Curve, rather than by his last name.
Mr. Wood investigated several clear Douglas Fir beams under a variety of load durations, and fit “an empirical hyperbolic equation” to his trends data, which survives largely unchanged to this day. Much further experimentation, both at the Forest Products Lab and at research institutions across the country, has helped to confirm his findings, and extend them for use in all wood species.
For design purposes, this equation and curve have been sampled and reduced to the table below for key durations and load duration factors.
NDS Load Duration Factors
Every NDS Reference Design Value utilizes the Load Duration Factor, with the exception of the Modulus of Elasticity (E), the Adjusted Minimum Modulus of Elasticity for Stability (E.min), and the Compression Design Value Perpendicular to the Grain.
The NDS provides a table, Table 2.3.2, which informs designers of the expected cumulative load durations of various load types over the life of a structure, and recommends appropriate Load Duration Factors for each, according to the Madison Curve above.
When used with load combinations, the shortest duration load in the combination governs which Load Duration Factor is applied to the whole combination.
Load duration factors are limited to 1.6 maximum for pressure-treated lumber, as well as for connection design, both owing to the inherent brittleness of those particular aspects of design limiting these effects at extremely short load durations.
| Load Duration | C.D | Governing Design Load | ASD Load Combos |
|---|---|---|---|
| Permanent | 0.9 | Dead load | D |
| Ten years | 1.0 | Occupancy Live Load | D+L |
| Two months | 1.15 | Snow Load | D+S, D+L+S |
| Seven days | 1.25 | Construction Load | D+L.r |
| One day* | 1.33* | NO LONGER USED | *Used prior to 1987 |
| Ten minutes | 1.6 | Wind/Earthquake Load | D+W, D+E, D+L+W, D+L+W |
| Impact | 2.0 | Impact Load | Often not combined |
Roof live loads from construction are generally assumed to have a load duration of around a week, which puts them in the 1.25 Load Duration Factor. For quick re-roofings infrequently over the life of a building, this is probably appropriate, but do recall that these are durations of the load over the lifetime of the building, so slow roofers or frequent re-roofing could push the designer into using the two months, 1.15 Load Duration Factor.
Finding the Governing Load Combo / Load Duration Factor
Structural designers are used to juggling numerous load combinations for each component in a design, but having different allowable stresses for the various load combinations adds an extra layer of complexity.
Especially when a designer is newer to wood, it’s easy to lose track of which factors combine with which combinations, but there’s an old algorithmic approach to helping simplify that all.
Warning: This approach is only totally valid for fully-braced members, as the beam and column stability factors also depend on the load duration factor, which then takes away the clean linear relationship relied on by this algorithmic approach!
- For members not subject to buckling, an easy way to find the controlling load case is to simply divide each load combination by the appropriate load duration factor, based on the shortest-duration load in the combination.
- Then grab the highest combined load from there and run your normal design.
Again, this approach isn’t totally valid for anything with buckling present, though it can still be used as a rough guess to help narrow down which load cases might control.
Summary
Wood members can hold higher loads for shorter durations, and engineers designing according to the ASD philosophy use the Load Duration Factor (in combination with numerous other adjustment factors) to adjust reference design values for this effect. The corresponding LRFD factor is λ, the Time Effect Factor.
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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