NDS Size Factor, CF 1

[Archie264]

It appears that the size factor, C_F, allows for F_b 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.

 

[mike20793]

Yes, you are reading that correctly. The values weren't simply adjusted because the size factor is different for varying member widths and depths. The commentary in NDS does a good job explaining why the values are different as the depth and width changes. The 2015 version is free to view on their website.

 

[Archie264]

Thank you. I did read their explanation but I didn't really understand it on account of being too much dumb. I have the 2012 version, I'll try to swing by their website to see if there's additional information in the 2015 version. Thanks.

 

[manstrom]

Basically, when you are visually grading a 2x4, it's easier to see the imperfections so you get a 1.5 bump. On a 2x12, you get a 1.0 because it's harder to see the imperfections.

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

 

[Archie264]

Thanks. But why not then just increase the tabulated values since they're categorized by size anyway? I must be something obvious.

 

[manstrom]

Only Southern Pine is tabulated by size, other species have one values for all member sizes. Make sure you aren't applying CF to southern pine.

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

 

[Archie264]

Ah ha! That's what I was missing. (With wood I work almost exclusively with Southern Pine so I forgot there were other woods out there.) Thanks.

 

[LonnieP]

Archie, the stress increases (Cf) goes way back to the In-Grade-Testing program done by the lumber association.

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

 

[Archie264]

Lonnie,

Thanks for the history behind it. That makes it make more sense to me.

 

[XR250]

We can't even get SYP 2x4's unless they are pressure treated. Everyone uses SPF for framing as it is straighter.