specific gravity of lumber

[SLTA]

ok, folks. Why is the specific gravity of dense select structural the same as the specific gravity of No. 3 and Stud? Pick up the same length of the same size of those and the DSS is CLEARLY much heavier, and therefore more dense. So why is this not reflected in the NDS?

Please remember: we're not all guys!

 

[TehMightyEngineer]

As a guess I would say it was just a conservative assumption for simplicity of connection design. However, I really don't know and that is a good question. Is there a caveat in the NDS that allows you to use the actual measured specific gravity if it exceeds the graded values?

Maine EIT, Civil/Structural.

 

[SLTA]

TehMightyEngineer, haven't seen one. I called the Southern Forest Products Assn since it's all SYP down here, and was argued with that the density doesn't change from grade to grade. It seems for dead loads, this could potentially be an issue. Or, I may be getting lost in the details and not seeing the big picture...

Please remember: we're not all guys!

 

[woodman88]

Time and money.

From the this link,

http://www.astm.org/Standards/D1990.htm.

"1.3 Due to the number of specimens involved and the number of mechanical properties to be evaluated, a methodology for evaluating the data and assigning allowable properties to both tested and untested grade/size cells is necessary. Sampling and analysis of tested cells are covered in Practice D2915. The mechanical test methods are covered in Test Methods D198 and D4761. This practice covers the necessary procedures for assigning allowable stress and modulus of elasticity values to dimension lumber from In-Grade tests. The practice includes methods to permit assignment of allowable stress and modulus of elasticity values to untested sizes and grades, as well as some untested properties. The practice includes procedures for periodic monitoring of the species or species group to quantify potential changes in the product and verification of the assigned design values through, evaluation, and reassessment."


Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[SLTA]

Yay Garth! I was hoping you'd find this question.

So does that mean that I should just relax about it? I am following the code, but the clear difference in weight between grades still has me perplexed.

Please remember: we're not all guys!

 

[woodman88]

I did some specific gravity testing on #1 DF-L about 15 years ago (well maybe 20 years ago). Just looking at this one grade we got sp from .5 to .79 and we checked over 500 pieces. As we needed only the .5 sp lumber to test the truss plates with.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dhengr]

SLTA:
The problem is that there is no ‘one’ perfect (exact) value for many of the properties on such a variable product. What Garth was testing for, he wanted the min. S.G. or density within the product group so his truss plate testing (or design values) would be conservative. He used the lesser value and thus most of the testing should have proved-out to be better than the min., on all the higher density pieces. In your case, you are concerned about DL, and the conservative approach is to use the higher S.G./density so as not to under estimate the dead load. Knowing what you know, nothing prevents you from adding a pound or two to your DL estimates before you start your design. I’ve never been a great believer in skinning the design loads to the bone before I start my design. Soon enough, you will wish you had a few extra pounds of design load to fall back on, when some guy/gal asks you to rationalize a bull they shot during construction.

 

[msquared48]

SLTA...

Kinda depends on what you are designing. A house is not so critical, but not so with a boat. Just a thought.

Mike McCann, PE, SE (WA)