Determining Recommended Allowable Design Values for Lumber and Timbers in Structures.
One of the most common questions that I get as a wood science consultant is, "if I send you a piece of wood from a building can you tell me how strong it is?" The answer is yes, I can tell you how strong it is, but it probably won't answer the real question: "what are the recommended allowable design values for the lumber and timbers in the structure?"
Design professionals commonly use the National Design Specification for Wood Construction (NDS) for wood engineering design. Provided in the NDS are the most current and acceptable practices and procedures for design using a variety of wood and wood-based products, including dimension lumber and timbers. The reference design values for the commercially important species and species groupings of dimension lumber and timbers are listed in several tables provided in the NDS Supplement - Design Values for Wood Construction. These are the current reference design values for dimension lumber and timbers and in most cases will likely apply to any modifications made to a structure. One common mistake in evaluating an older structure is to research the reference design values that were applicable at the time of construction. However, these historical values do not always apply to older structures due several modifications that have been implemented to the reference design values over the past few decades.
The first step for determining allowable design values is to determine the species of the lumber or timbers in the structure. This can be accomplished by removing a small splinter from an representative element that is only about 1" long and about 1/2" in cross section. Small, thin sections of the sample are removed using a razor blade and are then viewed using a light microscope to verify the species using a dichotomous key.
The next step is to have the material in the building inspected in-situ to determine the visual grade. The visual grade of the material is what ultimately determines the allowable design value. Dimension lumber is graded according to the National Grading Rule (NGR) for Dimension Lumber. Timbers are graded in a slightly different manner. The grade rules that apply to timbers are published by the grading agencies and depend on the species of the material. For example, southern pine timbers are graded according to the Southern Pine Inspection Bureau (SPIB), Douglas-fir timbers are graded according to the West Coast Lumber Inspection Bureau (WCLIB) or the Western Wood Products Association (WWPA) and hardwoods are graded according to the Northeast Lumber Manufacturers Association (NeLMA). A qualified inspector is required to grade the material according to the applicable rules.
Split in timber reveals a 1:7 slope of grain resulting in a No.2 visual grade.
There are several characteristics used to determine a visual grade including; checks, knots, shake, splits, wane, pitch streaks, and warp. For purposes of strength, the two characteristics that are generally most import are knots and slope-of-grain. When grading knots, the size, location, and frequency are examined and measured. Slope-of-grain is the angle that the grain deviates along the length of the member and is measured as the distance along the length of the member that it takes the grain to deviate 1". For example, a slope-of-grain measurement of 1:8 indicates that the grain deviates 1" along 8" of length. Slope-of-grain is one of the most common characteristics that was often overlooked several years ago and is one of the more important strength reducing characteristics. Even in lumber and timbers that appear to be of high quality, "clear" wood, elements could in fact have steep slope-of-grain that results is a lower visual grade. In some instances slope-of-grain will be revealed by a check or split, as shown above. Alternatively an inspector must always be attentive to the grain and other characteristics on all the exposed surfaces of the element in a structure. If needed, the inspector may use a slope-of-grain indicator to mark the grain similar to a scribe and then measure the deviation to the determine the value.
Wide-face knot with distorted grain measuring 3 1/2" diameter in a 2x10, resulting in a No.2 visual grade.
In mills, visual graders examine a piece on all four sides along the entire length to determine the grade controlling characteristic, and ultimately, the visual grade. Grading in-situ cannot be performed in the same manner as that performed by a grader in a mill. In fact, it is generally not possible because only three sides of a piece are exposed. This occurs in beams and joists where columns generally have all sides, or surfaces, exposed. Anything less than three exposed surfaces along the length of a member means that the inspector must use their judgement, which requires a lot of experience. Also, if there is a film forming finish on the materials this can conceal important characteristics used for determining the visual grade. When this happens, a conservative approach to grading is often the best concept.
Photo shows painted timber with a depression in the coating that revealed a 1:6 slope of grain as a result of a split. The large split to the right of the ruler was the initial indicator for the presence of slope of grain. Rockefeller Hall. Cornell University, Ithaca, NY.
Accessibility is a very important issue to consider. Scaffolding, lifts and staging may be required to have adequate access to inspect lumber and timbers in a building. The better the access, the better the field data and visual grading. If access is limited then the inspector will be forced to make conservative judgements for the appropriate visual grades.
Staging for access into the Highland Presbyterian Church roof system. Photo courtesy of Senler, Campbell and Associates, Inc. Staging installed by Schaeffer General Contracting Services. GEM Engineering also provided materials testing consulting.
There are some important issues that also need to be taken into account when inspecting dimension lumber and timbers in buildings which include: identifying areas with wood decay, insect attack, mechanical damage, or improper modifications. If wood decay is found, then the allowable properties do not apply to those areas due to the significant effect that wood decay has on allowable properties. Wood decay can be further examined with a resistance drill or increment cores can be removed and examined microscopically. The same applies to areas with insect attack. Termite damage can be quite extensive, whereas carpenter ants are generally isolated. In some cases there are large notches removed from members for plumbing which result in cross sectional loss. Some members can be mechanically damage, such as from fork lifts. In extreme cases, some members are already exhibiting failures. All of these issues need to be identified and documented by the inspector as well.
In summary, determining the design values of dimension lumber and timbers in structures is fairly straight forward process but requires an experienced individual with knowledge of the grading rules for dimension lumber and timbers as well as the experience of in-situ grading. The wood species of the elements need to be determined, which can often include different species for different elements. The elements need to be visually graded and inspected so that the appropriate reference design values can be determined and reported to the engineer. Once determined, the engineer can then apply all the applicable adjustment factors in the NDS to the reference design values. Quite often destructive testing of lumber and timbers is unnecessary in older structures as long as any areas exhibiting wood decay, insect attack, mechanical damage, modifications or failures are properly documented and handled appropriately.