# NDS Size Factor (Cf) for Lumber

NDS Size Factor (Cf) for Lumber

Continuing explaining terms used in a decade old article of mine (https://www.hansenpolebuildings.com/2014/08/lumber-bending/) is Cf (size factor, also known as width adjustment factor).

NDS (National Design Standards)design values for SYP (Southern Yellow Pine) are specified by size due to actual, physical in grade testing, whereas other lumber species’ design values have a base value adjusted by width of member.

Prior to 1957 no size adjustment equations for deep bending members were given in the NDS, only other publications were referenced based upon tests of bending members up to 12 inches deep. From 1957 to 1968 specifications included an adjustment factor for deep bending members.

Back in the day, most engineers specified a minimum lumber grade on their plans (such as Douglas Fir #1 or better) and didn’t go much further. Then someone mentioned glu-laminated timber industry was using a volume factor to modify a member’s section modulus (S or Sm) indirectly influencing stress carrying capacities. It was wondered if sawn lumber should have a similar factor, resulting in actual in grade physical testing.

It was found smaller members failed (broke) at higher stress levels than deeper members. Applying statistical analysis methods a percentage factor (Cf) was to be applied to 2x and 4x members between four and 16 inch depths.

The intent was for engineers to be able to account for this stress skew in their calculations, without having to specify on their plans grade of every size member being used.

For two inch thick lumber, visually graded as #3, #2, #1, #1 and better or Select Structural. Fb (fiberstress in bending) value was assigned a Cf of 1.0 (or no adjustment). 2×2 through 2×4 Cf = 1.5, 2×5 = 1.4, 2×6 = 1.3, 2×8 = 1.2, 2×10 = 1.1 and 2×14 and wider = 0.9. These values, as well as adjustment factors for Ft (fiberstress in tension) and Fc (fibrstress in compression) can be found in the NDS.

# Are 2x6s Stronger Than 2x12s?

The following article first appeared in JLC (jlconline.com)

Q: Recently I needed structural design values (E, Fb, Fv) for treated southern yellow pine.  According to the Southern Pine Council’s (southernpine.com) latest design values, SYP’s E (modulus of elasticity) and Fv (allowable shear stress) remain constant, while F (bending strength) values grow smaller as the lumber dimension grows larger. why does the Fb change with the lumber size?

A: Frank Woeste, P.E. a professor emeritus and adjunct professor of sustainable biomaterials at Virginia Tech, responds:

The phenomenon that the allowable Fb value decreases as the depth increases has been proven by extensive testing. The current span tables–for all lumber species–reflect that testing. As to why it is so, there is no conclusive answer, but several theories have been offered. My own theory is based on probability–under ASTM test protocols, more knots and other imperfections are likely to occur in wider pieces of lumber.

In the ASTM test for determining allowable Fb, a wood joist is subjected to stress until it breaks. The test standard requires a span-to-joist ratio of 17. For a 2×6, the test joist is 93.5 inches long (5.5 in. x 17), while a sample 2×12 would be 191.25 inches long (11.25 in. x 17). In the test, only the center third of the span is subjected to the full stress level. For a 2×6, this would be 31.2 inches long; for a 2×12, the middle third is 63.75 inches long.

Knots and other natural characteristics control the strength of lumber, for each grade, there is a maximum allowable knot size. If you look at a piece of pine lumber, you’ll notice the knots are usually clustered a couple of feet apart. Based on typical frequency of knots, it’s likely that there will be more knots in a 63.75 inch section than in a 31.2-inch section, and more likely that the maximum allowable knot will occur in the longer section. So in the test, a 2×12 is more likely to fail at a relatively lower stress level than a 2×6.

# Protecting Posts from Rot

Protecting Posts From Rot

Based upon a Journal of Light Construction article by Grant Kirker, research forest products technologist at USDA’s Forest Products Laboratory in Madison, WI

Posts rot when decay fungi find wood they can digest. Insects such as subterranean termites can also cause posts to fail, but they aren’t common in cold climates, whereas fungi are widespread. Posts often rot at ground level and break off simply because this is where conditions are most conducive for decay to occur, as well as being where highest physical stress occurs. Here, fungi find those three basic things they need to grow and survive: moisture (from soil), oxygen (from air), and food (post itself).

While some wood species—such as eastern white cedar and black locust—are naturally resistant to decay fungi, their performance is highly dependent on extractive content in heartwood, and can be variable. Preservative treatment is a more controlled process resulting in more predictable performance, especially in soil contact. Wood preservatives have historically been formulated to be broad spectrum so they protect from a wide range of organisms. Most waterborne preservative systems commonly used today employ a metallic component (usually copper) combined with co-biocides to improve resistance to copper-tolerant fungi, molds, and bacteria. Our studies have found yellow-pine posts treated with several industrial wood preservatives (including CCA, ACA, pentachlorophenol, and creosote) have remained highly durable even after 50 years of field exposure in a harsh environment.

U.S. Forest Service Harrison Experimental Forest test plot in Saucier, Miss.—classified as a severe decay hazard according to AWPA’s Fungal Decay Hazard Map—is filled with longleaf-pine posts.

U.S. Forest Service A post is assessed by giving it a lateral pull of 50 pounds. If a post breaks at the ground line, it fails; if it doesn’t break, it passes. Tests have been conducted on these posts using this protocol since they were installed in 1964.

In order for a preservative to be effective, wood must be treated to proper retention level and penetration. If wood is treated only on the surface, any cracks or splits open up the treatment envelope and expose untreated wood, and can be readily eaten by fungi and insects. Some wood species (southern yellow pine, for example) are easy to treat and take up preservatives readily, while others (such as Douglas fir and lodge-pole pine) are more difficult to treat due to their wood cell orientation or heartwood presence. These species are often referred to as “refractory” and may require additional preparation (incising, steaming, and so on) to open up wood so it better accepts treatments.

When choosing wood for posts, check the end tag to confirm lumber is pressure-treated in accordance with either, American Wood Protection Association (AWPA), or International Code Commission (ICC). On the label, look for the product’s designated Use Category, or UC. The AWPA’s Use Category system specifies target retention levels for different preservative types to meet specific applications; UC 4B lumber (with a 0.60 pcf retention level for CCA, ACZA, and ACQ or 0.23 pcf for MCA) is required for harsh below-ground exposure in wet areas or regions with high decay hazard (like Southeast or Hawaii).

It’s not necessary to special-order heavy-duty marine-grade PT lumber. Marine pilings are typically treated to retention levels as high as 2.5 pcf CCA (chromated copper arsenate – generally no longer available for residential use) to ward off marine animals such as limnoria, teredo, and phloads, since some of these pests have been found to be copper tolerant. But for soil exposure, higher loadings aren’t necessary and just increase product cost.

U.S. Forest Service In addition to long-term industrial wood preservatives field testing, Forest Product Laboratory conducts research to develop new and improved treatment schedules for a variety of wood species. FPL’s state-of-the-art wood-treating pilot plant, constructed in 2010, offers five different preservative treatment retorts and can accommodate samples up to 12 feet long.

If you have to cut a PT post, be sure to dress any field cuts with a copper-naphthenate preservative containing at least 1% elemental copper. Examples include Copper-Green’s Wood Preservative (coppergreen.com), Tenino copper naphthenate (coppercare.com), and Woodlife Coppercoat (rustoleum.com). Cutting, drilling, or notching PT lumber exposes wood inner faces possibly not treated to the same retention as outer surfaces.

Some installers report wrapping post base with sheet copper or galvanized steel prolongs post life. While post wraps and barriers seem to offer some increased longevity, any gaps, holes, or voids behind the barrier or wrap will compromise the barrier and make it less useful. Coating post base with asphalt roofing cement, driveway sealer, tar, or a bituminous self-adhesive flashing tape are fairly common practices. In concept, these practices would seem to block some moisture transfer into wood, but there isn’t any research to suggest it increases longevity.

Finally, setting posts in concrete offers several advantages. First off, it reduces lateral post movement once it sets, making installation balance much easier. For square posts in foundations, it eliminates some shifting and settling. A recent European study evaluating concrete vs. gravel vs. dirt fill found concrete fill was a best option in regards to longevity and durability, but again, proper pressure treatment is key to long-term field performance. If fully encasing posts in concrete, be sure to bring concrete sleeve above grade and slope top surface away from post to shed water, as recommended in most local building codes.

# Pole Barn Lumber: Southern Pine Updates

Southern Pine lumber has been popular since Colonial times for a wide variety of applications. Favorable growing conditions, wise forest management, and efficient manufacturing ensure a continuous supply of high-quality Southern Pine products for future generations. Southern Pine consists of four main species — shortleaf, longleaf, loblolly, and slash — and has been the preferred choice for today’s design/build professionals.

Visually graded North American lumber is subject to periodic sampling and testing, to verify the product being provided to the public, meets the published design values. These design values are used by architects, engineers and Building Officials to ensure new wood construction is adequate to support the loads which will be imposed upon the structure.  If you’ve ever looked at the end of a piece of lumber – this is where you might find “SYP” stamped on it, for Southern Yellow Pine, which is the same as Southern Pine.

The most recent testing of Southern Pine lumber resulted in a technical proposal by the Southern Pine Inspection Bureau to significantly lower design values. This submittal was reviewed by the Forest Product Laboratory.

As a resultant of this review, the American Lumber Standard Committee (ALSC) has ruled the design values which apply to visually graded Southern Pine and Mixed Southern Pine sized 2” to 4” wide and 2” to 4” thick (2x2s through 4x4s) in No.2 and lower grades (No.2DNS, No. 2, No. 2NonDNS, No.3, Stud, Construction, Standard and Utility), will be reduced as much as 30% as of June 1, 2012.

What does this mean?  This would be like waking up some morning and finding out the dollar you had is now worth 70 cents.  The lumber used in your building, which sizes and grades were chosen based upon your building location and design, now possibly will not meet the building code. The issue here is the design date and your lumber need to match.  In other words – if you purchased 2×2, 2×3 or 2×4 size lumber prior to June 1st, it may not be large enough for your construction design.

Down the line, this will affect lumber larger than 2×4’s, but we don’t know when, or “how much” it will be affected.  The original SPIB proposal submitted to the ALSC included a reduction in design values of 25-30% for all grades and sizes of visually graded Southern Pine.

The wholesale lumber market has substantially adjusted pricing of Southern Pine to compensate for the lower strength values. In many markets, the now stronger Douglas Fir, Hem-Fir and Spruce-Pine-Fir have gained popularity.

For the end user, the recommendation would be to use larger dimension Southern Pine conservatively, until such time as new values are accepted and published. In using Southern Pine for critical members such as floor joists, rafters, beams and headers it would be prudent to go to the next larger size member than what is shown in span tables.

You can be sure I will have my ear to the ground.  I will again blog about Southern Pine, should I find updated information.