Tag Archives: section modulus

Flat Use Factor (Cfu) for Dimensional Lumber

Flat Use Factor (Cfu) for Dimensional Lumber

For those who have not had their eyes glaze over from my recent articles on S (or Sm) Section Modulus or Cf (size factor adjustments), I bring, in all its glory, Cfu (Flat Use Factor).

Although produced in a factory environment (a saw mill), wood is a non-uniform material and doesn’t behave identically when bent about different axes (yes, this is plural of axis). Tabulated strength values are always based upon strong axis (x-x direction), so a modifier was created based upon actual physical testing, to adjust for loads placed about a board’s weak axis (against wide face, or y-y).

Look at it this way, if a board has a knot in one spot, and is bent about its strong axis, said knot may have a significant effect on overall strength, since this knot can alter effective depth (d).

Lay board flat and a knot only effects a portion of width (b), so effect on flexural (bending) strength is less pronounced.

Allowable bending strength (Fb) is based upon strong axis bending and takes into account statistical variation of flaws and their effect on “d” (depth). When used flatwise, Fb values can be increased to account for this less pronounced effect.

Looking at this example, if this was a joist (or purlin) bent about its strong axis, this knot will significantly affect allowable tension or compression stress, in joist’s extreme fiber. However, used flatwise, affected area is only a portion of extreme fiber and thus defect is less important. Wood allowable stress values are based on some amount of defects and thus they are already reduced for strong axis bending. In weak axis bending this assumption is taken out.

For 2×4 and 2×5 Cfu = 1.1, 2×6 and 2×8 = 1.15, 2×10 and wider = 1.2.


Why Not Use 6×6 or 8×8 Posts Up North?

Reader DARRELL in LUCEVILLE asked this question and included photo below.

While this photo is not of a Hansen Pole Building, I can comment upon it. Featured in this building photo are glulaminated columns – they are a great product, high strength to weight ratio, straight, highly resistant to warp and twist. They are strong because they are most generally manufactured from high strength materials, most three ply 2×6 columns have a Fb rating (Fiberstress in bending) of roughly 1900 psi. Your local lumber dealer or big box store will gladly sell you a 2×6 #2 with a Fb rating of anywhere from 1000 to 1170 (depending upon lumber species, with SYP lowest and Douglas Fir highest), so a glulam’s three members start off being about equal to five every day individual 2×6.

What about strength comparisons to solid timbers?

To determine bending strength of a member, multiply Fb X Sm (Section Modulus). A three ply 2×6 glulam would be 1900 X 19.86 = 37,738 in-lb. A 6×6 #2 SYP would be 850 X 27.73 = 23,570 in-lb. A 6×6 #2 Hem-Fir (treated species of choice in Western U.S.) has a base Fb of 575 with a reduction for incising of 20% (X .80). 575 X .80 X 27.73 = 12,755 in-lb.

Clearly, when picking for strength, glulam columns are going to be a better choice.
When it comes to practicality on a jobsite, would you prefer to carry a 20 foot long glulam weighing roughly 100 pounds, or hefting a same length 6×6 tipping your scale at 180 to 300 pounds? Not much to think about there!

Glulams columns are more prevalent in northern states due to locations where they are manufactured – primarily Pennsylvania, Ohio, Wisconsin and South Dakota. We do offer them as an option on any Hansen Pole Building. Give a call to a Building Designer today at 1(866)200-9657 for your post frame building design solution.