Timber grading

[Ljohnsaw]

I need to understand the codes on the beam calculator.  When I look at the list of species to choose from, I see codes after the names: SS, B+S, and P+T.  I think SS is surfaced 2 sides, right?  What do the other two mean?  I understand the #1 and #2 (how many/where knots are located, bark areas, etc).  When you are using your own trees, what do you usually get out of pines, #1 for beams since you are pulling them from the center?

Thanks

[Jim_Rogers]

SS does not mean surfaced two sides that's S2S

SS means structural select.

B+S means Beams and stringers.

P+T means posts and timbers.

The difference between B+S, and P+T is two inches.

What I mean is if a timber is an 8x8 it's in the P+T category. If a timber is 8x10 it's also in the P+T category. But as soon as the second dimension goes to more than 2" then it is assumed that the timber will be used as a beam (example an 8x11). So it's in the B+S category.

Beams mean the timber is horizontal. There is no such thing as a vertical beam.
Stringers as you all know are the pieces of lumber that hold up stair treads. And they must be strong or the stairs will fail. Rafters are also in the "stringer" family.

Timbers that are in the P+T category have lower grade rules due to the fact that most of them could be used as Posts. And wood is strong in compression. Knots in compression don't fail, usually.

But, knots in beam can be a problem if they are big. When we look at pieces of wood to be used as beams we always look for "crown" the sweep in the piece from end to end. And we put the crown up so that gravity, and the load on the beam will push it down, towards being straight. If it was crowned down and then the load is applied the bow would be even more. This would make a very un-even, un-level floor system.

Also, when we are inspecting beams that are fairly straight, no crown, we look at the size and location of the knots. And roll the beam over so that the knots are near the top surface of the beam. Knots are strong in compression. But weak in tension. The lower side of the beam would be in tension. The upper side would be in compression.
Some where in between these two "zones" of the beam is the area known as the "neutral axis" where there is neither compression or tension. It would be best if the joints/mortises could be cut in the neutral axis area. As these would not effect the strength of the piece.

I hope that helps you to understand which values to use when you select your values in the calculators.

Jim Rogers

If not keep asking questions.

[WindyAcres]

Also, when we are inspecting beams that are fairly straight, no crown, we look at the size and location of the knots. And roll the beam over so that the knots are near the top surface of the beam. Knots are strong in compression. But weak in tension.

Cool, I did not think about that, good point, thx.

Just use common sense, get nice trees for your beams + overbuild, then you dont have to worry about things.. 

[canopy]

No matter how much overbuild I think it is still good practice to be in the habit of carefully inspecting each member individually, optimizing knots, crown, and other attributes to allow each to perform at its absolute best.

[Jim_Rogers]

No matter how much overbuild I think it is still good practice to be in the habit of carefully inspecting each member individually, optimizing knots, crown, and other attributes to allow each to perform at its absolute best.

Well said.....