“Hybrid Lumber” Grade Stamps Require Special Attention

Lumber properties are influenced by species and growing region. Species with similar design properties are combined into species groups, as shown in Tables 4A, 4B, and 4F of the NDS Supplement[1]. For example, a common species group in the southeastern U.S. is Southern Pine (SP), which includes Loblolly, Longleaf, Shortleaf, and Slash Pine. The same species group can have different design values depending on where it is grown, such as Douglas Fir-Larch which is grown in the U.S. and Canada. In this case, the NDS Supplement lists DF-L and DF-L (North), the latter being grown in Canada.

Lumber mills sometimes process logs from different regions, and this has given rise to a different type of grade stamp. The purpose of this article is to highlight lumber stamps with multiple species/regions, hereafter referred to as “hybrid lumber” stamps. Hybrid lumber stamps are unlike the traditional stamp that component manufacturers (CMs) have relied upon when designing and manufacturing wood trusses and prefabricated wall panels. The hybrid lumber stamp identifies a designated mixture of lumber products (two or more species and/or species groups).

We will explain how to recognize a hybrid stamp, to determine the proper design values for the truss chords and webs, and explain how the design values impact member design and the four metal plate “gripping values.” In addition, we will show how the “diaphragm and shear wall” design values for prefabricated wall panels are impacted by the specific gravity of the framing lumber.

Traditional Grade Mark

As an example, a southern pine grade mark from a 2x8 is shown in Photo 1 [for photo, See PDF or View in Full Issue].

The grade mark format is standardized by the provisions of the American Softwood Lumber Standard, Voluntary Product Standard PS 20-20. Using industry terminology, the grade mark elements depicted in Photo 1 are defined as:

“SPIB” is the Trademark which identifies the agency quality supervision and designates the Southern Pine species group

“No.1” is the grade

“KD19” is the moisture content classification at time of manufacture

“HT” indicates the lumber has been heat treated

“404” is the Mill Identification reference

In the example case, the grade stamp represents only one product, specifically No.1 Southern Pine, that is a species group.

How to Recognize a Hybrid Mark

An example of a hybrid lumber grade mark is shown in Photo 2 [for photo, See PDF or View in Full Issue]. It also gives the same categories of information as reviewed for the traditional grade mark.

Referring to excerpts from the 2018 NDS Supplement, Table 4F on “Non-North American” dimension lumber (Figure 1) [for figure, See PDF or View in Full Issue], one can find two lumber products that match the abbreviations on the mark:

N.SPR-SCOT P. (I)
ROM-UKR KD-HT

The (I) means imported. The key to recognizing a hybrid grade mark is to examine the species part of the mark separated by a hyphen, such as N.SPR-SCOT P., and other text indications that you may not have seen in the past.

Hybrid grade marked lumber can be a combination of two or more products as illustrated by Photo 3 [for photo, See PDF or View in Full Issue].

As for the previous mark (Photo 2), by going to the 2018 NDS Supplement (https://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2018-Supplement-ViewOnly-171027.pdf) Table 4F, it can be determined that:

N. SPR refers to Norway spruce from Germany, NE France, and Switzerland,
SC P. refers to Scots pine from Germany, and
S Fir refers to Silver fir from Germany, NE France, and Switzerland.

North American Hybrid Grade Marked Lumber

Hybrid grade marked lumber manufactured in the United States and Canada is also available for wood construction, truss lumber, and other engineered wood product applications. Some of the U.S.–Canada hybrid grade marked products indicate species combinations of U.S. lumber—DF-L, HF, and SPF(S)—as well as Canadian lumber—DF-L (N), HF(N), and SPF. As discussed in the next section, it’s very important for CMs to check with their plate/software provider if there is any doubt as to the species/species groups indicated on truss or prefabricated wall panel lumber.

How to Calculate Hybrid Lumber Values for Truss Design

The CM’s truss design software typically contains a “lumber file” which includes lumber species and species groups and corresponding design values that are linked to a specific lumber species/species group and grade. For the case of hybrid lumber used for truss fabrication, the design values (DVs) must be calculated by the truss design software for truss member design. In addition, the proper Allowable Lateral Resistance Values, hereafter “gripping values,” must be determined by the plate manufacturer and utilized in the truss design software. The process of calculating hybrid lumber DVs for truss design, as well as for other applications, will be demonstrated by two examples of U.S. and Canada hybrid products—No. 2 SPF-SPF(S) and No.2 HF-HF(N).

For the eight design values in the NDS Supplement, the minimum of each pair of numbers is the correct value for the hybrid product per the American Lumber Standard Committee (ALSC) (http://www.alsc.org/greenbook%20collection/LumberProgram_Facsimile.pdf). A footnote near the bottom of page 9 states:

“When a combination species/species group nomenclature is shown in a grade stamp, the user of the lumber would select the weakest species/species group shown on the grade stamp as the appropriate value for the specific allowable property.”

Using the 2018 NDS Supplement, Table 4A, the tabulated referenced design values for No.2 SPF, SPF(S), HF, and HF(S) are listed in Table 1. Because only two species groups are combined in our examples, the smaller of the eight design values for each (traditional) species group shown on the grade mark determine the proper design values (DVs) for the grade marked hybrid product.

Table 1. The 2018 NDS Supplement referenced DVs for traditional grade marked lumber and calculated DVs for hybrid grade marked lumber. Underlined values represent the smaller value for each component property when they are unequal.

The process of determining DVs for hybrid marked products is tedious; however, on-going work by American Wood Council (AWC) and ALSC is underway to publish DVs for hybrid lumber products in a new NDS Supplement table, likely Table 4G.

Impact of DVs on Truss Design Metal Plate Gripping Values

Specific gravity (G) listed in the last row of the table above is usually used for identifying the gripping design values of all metal plate connected truss joints. In fact, application of plate values or gripping values are based on the “G” of the species/species group for traditional grade marked lumber. The G is the single most important lumber property for connection strength.

Impact of G-value on Metal Plate Gripping Values

Metal connector plates have proprietary design values and are tested per the ANSI/TPI 1 truss design standard. The gripping values for the four loading orientations for each plate type/gauge are recognized by the building code community through “code reports.” For example, in Table 1 of the July 2020 ICC-ES Report ESR-1988 on MiTek truss plates (https://www.mitek-us.com/files/pdf/Code%20Evaluation%20Reports/esr1988.pdf), gripping values are tabulated for different types of truss plates, typical truss lumber species groups, and for the four loading orientations (AA, EA, AE, and EE). The lowest tabulated G-value is 0.42 for SPF and the highest is 0.55 for Southern Pine (SP). For the listed design values in Table 1 of ESR-1988, the ratios of the “allowable lateral resistance values” for SPF compared to Southern Pine are as follows:

Clearly, the G-value of the truss lumber greatly impacts the design strength of connections for the four loading conditions: AA, EA, AE, and EE.

The lowest G-value for traditional species groups is 0.42 for SPF. Some of the species groups used in hybrid lumber stamps have G-values less than 0.42, and recall that the lower DV must be used. For this case, truss plate gripping values would need to be determined by the plate manufacturer and utilized in the truss design software.

Impact of DVs on Prefabricated Wall Panels

Lateral loads on buildings stemming from wind and seismic events are resisted by a system of shear walls and diaphragms designed by the Building Designer of a project. The wall panel designs for the lateral load resisting system specified and contained in the Construction Documents would be based on an assumed minimum G-value of the wall framing lumber along with several other wall panel design variables. The G-value assumed by the Building Designer might be implied by specifying a specific species/species group and grade, such as “Wall framing lumber and plates shall be No. 2 DF-L, or better.” (Per the NDS Supplement, the G-value of DF-L is 0.50.)

For a collection of common and recognized wall construction details, “nominal unit shear design capacities” for shear walls (and wood-framed diaphragms) are published in Chapter 4 of the SDPWS standard[2], and rely heavily on the nailed panel-to-lumber connections. The design data in the tables are based on the assumption that the framing lumber has a specific gravity, G, of 0.50 or greater. For species groupings less than 0.50, the nominal unit shear capacities must be reduced by a Specific Gravity Adjustment Factor (SGAF) = [1 – (0.5 – G)], as specified in the design table footnotes.

For an example, assume DF-L was specified for the wall framing by a Building Designer, but the wall panels were made with hybrid lumber product having a G-value of 0.36. How would the design values for the prefabricated wall panels be affected? Using the adjustment equation from the previous paragraph, the unit shear values would be multiplied by SGAF = [1 – (0.5 – 0.36)] = 0.86, resulting in a 14% reduction in unit shear design capacity.

Another concern with shear walls is the selection of hold-down hardware to resist overturning. Design capacities for hold-downs are typically provided for a range of specific gravities, with G = 0.42 at the low end. The CMs would need to work with connection suppliers for wall framing lumber with specific gravities less than 0.42.

Concluding Remarks

A hybrid product grade mark indicates a mixture of species groupings and/or regions, requiring careful consideration of design values. For each design value, the lowest of the mixture must be used. This includes strength and stiffness values, along with the specific gravity. Specific gravity is especially important as it influences truss plate gripping values, unit shear capacities for diaphragms and shear walls, and hold-down capacities for shear walls.

For CMs interested in using hybrid grade marked lumber, as a first step we recommend contacting your software supplier to update the design software to include the correct DVs for the products to be used in chords and webs and related gripping values or for instructions on how to input the hybrid products for truss designs should the software already accommodate the matter.

The fact that one letter in a species group designation, such as “N” or “S”, or a longer string of text (species/groups) can substantially impact DVs for truss design, manufacturing, in-plant quality assurance, and inspection highlights the need for widespread education of the CM staff within a plant. Timber Products Inspection, Inc. has published an advisory on the subject that can be viewed online at: https://www.tpinspection.com/uploads/file/505/lumber-stamped-w-multiple-species-regions.

Lumber grading-rules writing agencies and SBCA have published educational information for hybrid lumber consumers as follows:

• Northeastern Lumber Manufacturers Association (NELMA, 2020)
  https://sprucepinefir.us/spfs-vs-spf/
• Western Wood Products Association (WWPA, August 2020)
  WWPA.org (Free website registration, go to Technical Support “Western Lumber Resource Library,” select Topic “Design Values,” and the article will appear on the list as “Design Values for U.S. and Canada Species Group Combinations”
• Cheung, K. and WTCA Staff. 2005. Lumber Design Values for U.S. and Canadian Species Group Combinations—Applications to Metal Plate Connected Wood Trusses. SBC Magazine, December, pp. 72, 74, 75, and 76.
  https://www.sbcmag.info/sites/default/files/0512%20designvalues.pdf.

As demonstrated in this article, interpreting hybrid grade and calculating the correct DVs is challenging for the CM and others involved in wood construction, such as building designers, general contractors, framing subcontractors, and building inspectors. For CMs that supply prefabricated wall panels, great care is needed to establish equivalency between the lumber grade/species specified on Construction Documents and the lumber to be used for manufacturing the wall panels.

Frank Woeste, P.E., is Professor Emeritus, Virginia Tech and a wood construction consultant. Frank, along with his colleagues, has developed and presented continuing education programs for more than 30 years. He is a member of the ANSI/TPI 1 Project Committee for revision of the truss design standard and the AWC Wood Standards Design Committee, and serves as the Consumer Representative (Alternate) of the ALSC. Comments are welcome and can be sent to fwoeste@vt.edu.

Don Bender, P.E., Ph. D., is Professor of Civil Engineering and Director of the Composite Materials & Engineering Center at Washington State University. He has taught courses and conducted research on wood engineering for over 35 years. He is a member of the AWC Wood Standards Design Committee responsible for the NDS and WFCM and serves as Consumer Representative of the ALSC. Don also is active in research and standards development in post-frame building design. He holds the Weyerhaeuser Distinguished Professorship at WSU. Email: bender@wsu.edu.