NDS Flat Use Factor Rationale 1

[EngineerMary]

Hello;

This is my first post here; although I have used the search feature on the site several times and found many helpful answers.

My question involves the flat use factor for allowable bending in wood design. I know how and when to use it, but I was wondering if anyone knows the rationale behind it?

Any information you can provide would be appreciated!

 

[JoshPlumSE]

It just relates to testing. Wood is a non-uniform material and doesn't behave the same when bent about the different axes. Since the tabulated values are always based on the strong axis, they give a modifier for when they are bent about their weak axis.

 

[JAE]

If you have a knot in one spot on a board, and you bend that board in its strong axis, the knot may have a significant effect on the overall strength since the knot can alter the effective depth, d, which is significant for bending.

If you are laid flat, a knot affects only a portion of the width, b, so the effect on flexural strength is less pronounced.

Think of strength affected by d^2 vs. b.

The allowable bending stress, Fb, is based on strong axis bending and takes into account the statistical variation of flaws and their effect on d (strength).
With a flat-use, you can factor up the values of Fb to account for the less pronounced effect.

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[JoshPlumSE]

I understand that this is a little counter-intuitive. You might think the rating system may allow some degree of imperfections at the mid-depth (where it affects strong axis stress less) and that this should DECREASE the weak axis allowable stress.

 

[TehMightyEngineer]

To expand on JAE's summary:

Looking at the wood below, if this was a joist bent about the strong axis that knot will significantly affect the allowable tension or compression stress in the extreme fiber of the joist. However, if used flatwise the affected area is only a portion of the extreme fiber and thus the 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 you take this assumption out.

One important note! Decking and other boards intended for flat-use already have the flat use factor taken into account as that is the intended use. Do not adjust the allowable stresses or strengths of decking or other tongue and groove wood planks for flat use.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[EngineerMary]

Thank you all for the answers; it makes a lot of sense.

I was always hesitant to use the factor because I didn't understand the reasoning behind it. After reading this, I am much more confident using that factor.

I design formwork/shoring/reshoring so I generally have a pretty healthy duration of load factor in my design anyway, but sometimes that extra 10 or 11% would come in handy. (And, ten four on the comment about decking.)

Thanks again!

 

[Ingenuity]

strong axis that not will significantly

“knot” not “not”. Nots everywhere

 

[TehMightyEngineer]

“knot” not “not”. Nots everywhere

Knot again... Its alright, I no better then that.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/