NDS Size Factor, CF
NDS Size Factor, CF
(OP)
It appears that the size factor, CF, allows for Fb of a 2x4 to be increased by a factor of 1.5. (2x6's & 2x8's get bumped up by 1.3.) Am I reading that correctly? It seems like a lot. Also, why is it needed? Why aren't the referenced design values simply increased in the first place? What am I missing? Thanks.






RE: NDS Size Factor, CF
RE: NDS Size Factor, CF
RE: NDS Size Factor, CF
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: NDS Size Factor, CF
RE: NDS Size Factor, CF
Clear as mud?
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: NDS Size Factor, CF
RE: NDS Size Factor, CF
Back then most engineers specified a minimum lumber grade on their plans (such as Douglas Fir #1 or better) and didn't go much further than that. Then someone mentioned that the Glu Lam industry used a Volume Factor to modify the section modulus of a member (indirectly influencing the stress level) and wondered if sawn lumber should have a similar factor. This resulted in the In-Grade-Testing program.
What they found was the smaller members failed (broke) at higher stress levels than the deeper members. Applying statistical analysis methods they came up with a percentage factor (Cf) to be applied to 2x and 4x memberse between 4" and 16" depths. Back then, if I remember correctly, a 2x8 was selected for having a Cf = 1.0. I don't know if it's still the same.
The intent was for the engineers to account for this stress skew in their calcs, without having to specify (on their plans) the grade of every size member they used.
Note the In-Grade-Testing program only covered 2x and 4x members. What about 6x and larger, have they done a testing program for those? Food for thought.
From a practical standpoint you'll find most of your designs will be governed by 2 X dead load deflection well before you start having stress problems. Still, your spreadsheets should use Cf just to show the plan checker that you are aware of it.
LonnieP
RE: NDS Size Factor, CF
Thanks for the history behind it. That makes it make more sense to me.
RE: NDS Size Factor, CF