Size Factor in the Wood Design Manual 1

[Eng_Girl]

Hi everyone,

In the Wood Design Manual there's a factor Kzc which is used to determine compressive capacity of sawn timber columns. The equation is as follows:

Kzc = 6.3(dL)^-0.13 <= 1.3 or Kzc = 6.3(bL)^-0.13 <= 1.3 which ever is more

This factor is to account for the size of the member to check for buckling in both axis of the member.

The thing that is confusing for me is that if the factor exceeds 1.3, intuitively I would decrease my member length, but this makes the factor larger do to the negative exponent.

For example,

Using a 14ft (4268 mm) 2 x 6; b=38 mm, d=140 mm
Kzc = 6.3(38x4268)^-0.13 = 1.32 > 1.3 NG

So if I lessen the member length to 4000 mm
Kzc = 1.335

Is this right?

 

[driftLimiter]

We (in the US) use both a size factor and a column stability factor. The stability factor accounts for buckling and the size factor accounts for variation in the design strength based on the dimensions of the sawn lumber. I can't speak to the CA code put perhaps that is also what is going on.

 

[Eng_Girl]

Yes, we also have a slenderness factor Kc which is used to consider buckling. So the size factor is there because the longer members have less strength?

 

[Craig_H]

1.3 is the upper limit for the value, but exceeding that value is not a failure or "NG" value, it merely means that Kzc = 1.3. As you increase your member length, Kzc will become less than 1.3, in which case the calcuated value is to be used.

 

[BridgeSmith]

The size factor, at least in the US codes, accounts for the statistical probability of a (hidden) defect in the wood that substantially affects its capacity. The larger the piece, the more likely there is a hidden defect that will adversely affect the strength or stiffness. Try it with a 6x6 (d=140mm)that's 14ft long, and you'll find it drops to 1.12.

 

[Eng_Girl]

Okay that makes sense now. Thank you!