Lumber Grades

[MountainDon]

This is not really a part of the IRC, but as most of us use a lot of wood in our projects I thought a brief look at softwood lumber grades, as used in construction, would be useful. Hopefully this conveys the idea that there is more to building something than having an idea and then making a quick trip to the big box store. All lumber is not created equal, either in species or grades within that species. You should have a minimal understanding of species and grades commonly available in your area, so you can plan accordingly.

A few quick helpful definitions:

Heartwood and Sapwood—Heartwood does not need to be taken into account in stress grading because heartwood and sapwood have been assumed to have equal mechanical properties. However, heartwood is sometimes specified in a visual grade because the heartwood of some species is more resistant to decay than is the sapwood; heartwood may be required if untreated wood will be exposed to a decay hazard. On the other hand, sapwood takes preservative treatment more readily than heartwood and it is preferable for lumber that will be treated with preservatives.

Knot—That portion of a branch or limb that has been surrounded by subsequent growth of the stem. The shape of the knot as it appears on a cut surface depends on the angle of the cut relative to the long axis of the knot.

Loose Knot—A knot that is not held firmly in place by growth or position and that cannot be relied upon to remain in place, in effect this is a void in the member and will not carry any loads or stresses.

Sound Knot—A knot that is solid across its face, at least as hard as the surrounding wood, and shows no indication of decay or looseness.

Spike Knot—A knot whose face is on the edge of a piece of lumber and "spikes" toward the middle of the piece.  A spike knot can greatly reduce the strength of the lumber if it is very large. It may not be readily visible at first look. An example of a spike knot in a floor joist is shown below, and vividly shows the danger of having these types of knots on the tension edge of a bending member. This joist had to have another joist sistered on along side it, to take its place.



Skip—An area on a piece that failed to surface clean. Skip is caused when the width or thickness of a piece is too small to allow the planer to remove all the rough surface.

Slope—Slope of grain is the deviation of the line of fibers from a straight line parallel to the sides and edges of the piece of lumber. Along with knots, slope of grain is an important strength-reducing characteristic. Slope is expressed as a ratio, such as 1:12. That means the grain can not deviate sideways across the width (depth) of the piece more than 1 inch in 12 inches of lineal measurement. A knot causes localized cross grain with steep slope as well; a knot cluster is even more of an issue. The less the deviation from straight grain parallel to the edge the stronger the piece of lumber.

Shake—A separation along the grain, the greater part of which occurs between the rings of annual growth. Usually considered to have occurred in the standing tree or during felling.

Wane—Bark or lack of wood from any cause on edge or corner of a piece except for eased edges.

Warp—Any variation from a true or plane surface. Warp includes bow, crook, cup, and twist, or any combination thereof.

The Southern Pine Inspection Bureau has a glossary available online; http://www.spib.org/glossary.shtml#s


Lumber is graded visually or by non destructive machine stress rating. The lumber we buy in big box stores and lumber yards is visually graded as a rule. Machine stress rated lumber will be stamped 'machine rated' or 'MSR'. MSR lumber is a premium material and goes to special markets, such as the metal plate truss industry, GlueLams, I-Joists, etc., all engineered products.

Below are a couple examples of visual grading stamps and an explanation..


From the best grade downwards we have:

Select: Sound, firm, encased and pith knots are to be tight, well spaced and limited up to 7/8". Unsound or loose knots or holes are limited to ¾", one per 4 feet lineal. Slope of grain is limited to 1:12 or less.

No. 1: Knots must be of same type as in Select grade, up to 1 ½". Unsound or loose knots or holes are limited to up to 1", one per 3 feet lineal. Wane is allowed.

N0. 2: Well spaced knots of any quality up to 2", with one hole up to 1  ¼" per 2 feet lineal. Wane and skips are included. . Slope of grain is limited to 1:8 or less.

No. 3: Knots of any quality can be up to 2  ½", with one hole up to 1  ¾" per lineal foot. Wane and skips are included.

Note: knots are not usually allowed near or at the edges. Also note that because of permissible knot placement, resawing into two or more, narrower sticks can take a piece of lumber down to a lower grade, or even into the scrap wood category.



Stud Grade: One of the main differences between stud grade and No. 3 is the slope of grain. Slope of grain may be as high as 1:4. That greatly reduces the bending strength. Stud grade should only be used in compression applications, never as a primary bending member.

There are grades that are inferior to stud grade; we are not interested in them.

Prior to uniform National grading (American Lumber Standards Committee's National Grade Rule) each mill made their own grade rules and they varied considerably. Today we have uniform rules governing lumber grading which makes it easier when purchasing lumber for use. Remember when working from plans use the grade specified, or one that is better. There are differences between species too; it is not good practice to substitute a species or grade for what is spec'd unless one knows what the differences are.

At the big box you'll often see another stamp on the lumber, such as "Prime". This is not a grade, it is a visual sorting for appearance. After watching the buying patterns of the public, and seeing what was left behind in the bins after consumer picking they developed appearance grades within the structural grades. Consumers will reject a board that has wane much faster than one that has a structural defect like a large knot. The prime grades limit things like wane tighter than the grade dictates.(actually in many cases that outer board is a clear select structural that was limited to a #2 by wane and is far stronger than one in which a knot was the controlling defect.



Wood from an Engineers Perspective
1. From an engineering standpoint we generally think of steel as a homogeneous and isotropic material. Homogeneous means a consistently uniform material composition and structure, a consistent microscopic grain structure in the case of steel. Isotropic means identical mechanical properties in any direction. Wood is neither homogeneous or isotropic; it has early wood and late wood and many variations in its cell and fiber structure, and it has distinct mechanical properties parallel to its grain or cell structure, and still other mechanical properties radially, perpendicular to the growth rings, or tangential to the rings, these latter two are perpendicular to the cell structure, or grain. In steel we think of tension and compression as normal stresses, along with shear stresses, and these work the same in any direction at a point in the material due to its uniform material structure, although their magnitudes may change as a function of orientation or direction. Whereas, in wood each of these have greatly different properties in different directions, because of the varying nature of the woods cell structure.

Wood products and sawn lumber are graded into 'use classes' as a function of their structural qualities and intended use, and through the NDS (National Design Specification for Wood Construction) and the various agencies involved in the lumber grading process, we basically boil these down to the following:

A.) F_b, bending stress at the extreme fiber in PSI (lbs./sq. inch), that is the bending stress at the top or bottom edges of a joist, header or built-up girder;
B.) F_t, tension stress parallel to the grain or long axis of the member in PSI, as might exist in a truss member, or a rafter tie;
C.) F_v, shear stress in PSI, this is most commonly thought of in terms of horizontal shear stress, a shear stress parallel to the grain which is the weakest orientation for this stress condition;
D.) F_{c  perp} , compression perpendicular to the grain in PSI, this might be the condition of a stud bearing on a sill plate, or a joist bearing on another beam or foundation wall.
E.) F_c, compression parallel to the grain in PSI, this might be compression in a truss chord member, or a stud or a column or pier;
F.) E, is the modulus of elasticity for the wood species, or group or grade, in PSI, it is a measure of the stiffness of the piece or grade;
G.) G, specific gravity of the piece or species, or density.
H.) Some stresses and force directions such as tension perpendicular to the grain are specifically not allowed because any capacity is so low as to be negligible, and the failure is a sudden rupture and can be dangerous.

Factors of Safety, and various adjustments are applied to these values and then they are used in the actual structural design and analysis. Examples of the adjustment factors are; Load duration factor, Wet service factor, Size factors, Repetitive member factor, Bearing area factor, to name a few. Because of the variability of the material within species and even within grades, the Factors of Safety are fairly high with respect to their ultimate strengths; much higher than for steel or concrete because of the lack of homogeneity and isotropy.

In a prescriptive code like the IRC, all of these things are taken into account for you in the various tables and charts or code sections. But you must read and understand all of the footnotes and how to utilize the table properly, because they can be easily misinterpreted or misused. The code committees are conservative in the way they set these things up, such as using the lowest grade of material covered by the table or code section, or to use the max. load condition covered by the table or code section.

2. Generally speaking knots are not allowed in or near the edges of the better grades of members graded for bending, since they disrupt the uniform flow of the maximum bending stresses. Smaller, solid, tight knots are somewhat less problematic near the compression edge of a bending member, but we don't know that the builder will always orient the 2x that way, thus they are just not allowed near the edges in the middle half of the length of the piece. Other defects or lesser material qualities may be allowed to varying degrees, depending on their location in the bending member and how they may detrimentally affect the predominant stress in that area of the bending member.

3. Grain slope and orientation of the grain are very important as relates to the predominant stress orientation and type of stress. And, the extreme might be high grain slope near the tension edge of a bending member to the point that you were approaching tension perpendicular to the grain, a very weak condition.

4. Members graded for compression are graded by a completely different set of rules since the predominant stress is compression parallel the grain or the long axis of the member. Tight, sound, knots are generally not a problem with respect to compressive stresses. Obviously, loose or missing knots are a problem, they are just like a notch; they actually reduce the net area of the member available to carry the load. The slope of grain can be greater without affecting the compressive stress picture, but the extreme is Fc perp which is quite a bit lower than compression parallel to the grain, and you can have a compressive failure which acts like horizontal shear stress failure under the right conditions. Knot location, and size within reasonable limits, is less significant for these members. The above leads to a wise use of materials, but some of the characteristics which are acceptable for compression, are a failure mechanism in a bending member. Thus, these members graded for compression still do have an allowable bending stress, but it is much lower than that for a member graded for bending.


Wood References and Resources:

Wood Handbook, Wood as an Engineering Material, from the US Forest Service. It is available for download online.
http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

National Design Specification: Design Values for Wood construction  
http://www.awc.org/pdf/2005-NDS-Supplement.pdf

[MountainDon]

For more on knots and how they affect lumber grading, here are images made from some PDF documents Don_P shared with me. The actual PDF documents may be downloaded at the bottom of thois message if wanted.  Click on the pdf file name to download the document; it will not display on the screen.

[duncanshannon]

Very interesting Don! Thanks for posting... The first part at least!  2nd part was a little too intense for my level of experience (low).

[rwanders]

 [cool]Great post Don! 

Just when I thought I had things figured out--

you pulled me back!

(apologies to Mario Puzo and Michael Corleone)