Tag Archives: wood stress

Checks and Splits in Post Frame Lumber

Checks and splits in lumber and timbers, especially timbers, are often misunderstood when assessing a structure’s condition. Checks and splits can form in wood by two means: during seasoning, or drying, and during manufacture. This article is concerned with checks and splits resulting from seasoning after installation. 

Development of checks and splits after installation occurs after a timber has dried in place. Quite often these timbers were installed green. Due to their size, it’s not practical for timbers to be kiln dried. Some are air dried for a period of time prior to installation, but usually they are installed green, and therefore, are allowed to dry in place. This also applies to a lot of dimension lumber.

During the seasoning process, stresses develop in wood as a result of differential shrinkage often leading to checking, splitting and even warping. Separation of wood fibers results in checking and splitting. Due to wood’s innate characteristics, it shrinks and swells differently. This is best explained in the image below. As a general rule of thumb wood shrinks (swells) approximately twice as much in the tangential direction of annual rings as compared to radial direction. Additionally, during the initial drying process outside of a timber inevitably dries quicker than interior, causing differential stresses to develop within a timber. Combined effects of these drying stresses in wood often, and sometimes inevitably, results in formation of checks or splits. Since wood’s weakest strength property is tension perpendicular to grain (similar to how wood is split using an ax), drying stresses can result in a check or split forming in a radial direction across annual rings. However, while these seasoning characteristics may initially appear as problematic, they likely are not. It is important to remember as wood dries, it becomes stronger. Furthermore, development of these seasoning characteristics is, quite often, normal. Most importantly, both are accounted for in derivation of design values for lumber and timbers and are also accounted for in applicable lumber grade rules.

See Figure 4-3 from the Wood Handbook, FPL-GTR-190

Figure 4-3 from the Wood Handbook, FPL-GTR-190






A check is separation in wood fibers across annual rings of a piece of wood and a split is a separation of wood fibers across annual rings but through a piece of wood. A third type of fiber separation, known as a shake, occurs along annual rings and is generally a naturally occurring phenomenon in standing trees, not a result of seasoning. There are several types of checks and splits defined and handled in grade rules for dimension lumber and timbers.

In use evaluation of dimension lumber and timbers normal checks and splits can often be interpreted as problematic by some design professionals with respect to allowable design values. However, in most cases they are not. In the image below, ends of both timbers are exhibiting various sizes of normal checks developed as timbers dried. If these timbers were being examined in a structure they would look similar to each adjacent image.

Upper left image shows two large timbers with visible splits on both ends as a result of seasoning. These are considered normal. Each red arrow points to how these checks would look in use.Therefore, checks and splits are often not an issue with in use lumber and timbers.

In summary, normal checks and splits are often encountered when assessing a structure, but they are accounted for in derivation of design values and handled in lumber grade rules.




History of a National Design Standard

History of the Development of a National Standard of Practice for Wood Design

NDSFrom time-to-time you might see the term NDS® (National Design Specification® for Wood Construction appear in my blogs, and it is referenced on every set of Hansen Pole Buildings plans, as well as within the International Building Codes.

Me, being the curious sort, wanted to know the history of the NDS® and I was able to find it in the 1997 NDS® commentary. I felt it interesting enough to share:

In the early part of the century, structural design with wood was based on general engineering principles using working stresses or design values published in engineering handbooks and in local building codes. These design values were often not in agreement, even for the same species of wood. Further, on most cases, the assigned values were not related to lumbar grade or quality level.

To meet the growing need for a national standard of practice, the Forest Products Laboratory, an agency of the Forest Service, U.S. Department of Agriculture, prepared a guide for grading and determining working stresses for structural grades of timber.

It provided basic working stresses for clear, straight-grained material of the important commercial species and presented strength ratios for adjusting basic strength values for each species for the effect of any size and location of knots or other natural characteristics permitted in a structural grade. The basic stresses and the strength ratios established in Miscellaneous Publication 185 were based on extensive test data for small, clear specimens and structural timbers obtained from over twenty years of testing and evaluation at the Forest Products Laboratory.

In 1934, the National Lumber Manufacturers Association (now the American Forest & Paper Association assembled the information given in Miscellaneous Publication 185 and working stresses derived therefrom together with engineering equations and other technical information on wood in the publication “Wood Structural Design Data” (WSSD) Included as supplements to the publication was design information for timber fastening. A major component of the WSSD was extensive span and load tables for various sizes of timber beams and columns.

With the initiation of World War II, the need for a comprehensive national design standard for timber structures, including wood connections, became more urgent. The Technical Advisory Committee of the National Lumber Manufacturers Association undertook a more than three year effort to develop the necessary specification in close consultation with the Forest Products Laboratory. The result was The Specification for the Stress-Grade Lumber and It’s Fastening. The Specification included allowable unit stresses fort stress graded lumber, design formulas, and design loads and provisions for timber connector, bolted, lag screw, nail and wood screw joints. Also included were guidelines for the design of glue-laminated structural members.

Despite numerous revisions since 1944, the scope of the Specification has remained essentially unchanged.

To an increasing degree in recent years, the results of research conducted at universities and other private and public laboratories, in Canada and other countries as well as the U.S.have been utilized to improve the Specification as a national standard of practice for wood construction.

In 1992 NFPA (National Forest & Paper Association) was accredited as a canvass sponsor by the American National Standards Institute (ANSI). The Specification subsequently gained approval as an American National Standard with an approval date of October 16,1992.

And there you have it, years of testing and revisions to the Specification, which is the basis for the engineering which goes into every Hansen Pole building kit package.

For those who want or need to know more, visit this website which has the full story of the history of a National Standard of Practice for Wood Design: