6x6 Post Strength

[Djs5017]

 Good Day everyone.

I'm building a timber/stick hybrid cabin, and am having second thoughts about the post sizing I've chosen in a couple spots. I've got 7 foot 6x6in (6x5.5), red pine posts supporting my structural ridge beam on my gable ends. The post are off my mill, but of a good grade I believe. Few, small knots, evenly distributed.

I've see a few different charts and tables regarding their estimated vertical bearing strength, but I'm unsure if those tables are weight ratings for timbers integrated into a shear-wall (which one of mine is not since there will be two large windows on either side of it).

With a worst case scenario for snow loads in my region, I'm anticipating the post bearing about 13k pounds with both dead and live loads combined.

Below is a link to a chart I used. This chart puts the strength of the post in the 25k realm, so we'll clear of my needs. I just want to double check with someone more knowledgeable.

I'm sure this is common info for any engineer in here, so please and THANK YOU!


https://www.engineeringtoolbox.com/amp/wood-columns-safe-loads-d_1834.html

[Don P]

No engineer here.

That was a Dougfir-Larch table but better than a poke in the eye. One quick check is to see if it is a "short post" which is 1' or less tall for each inch of thickness. If you can keep a 7' post as a 7x7 then the only check that needs to be done is crushing parallel to grain (as far as the post is concerned). The post is too thick to buckle. As it becomes taller, then you need to start docking that strength according to the slenderness ratio, it will fail by buckling out of plane. So there was one way when possible, to make the check easy, short, fat posts.

Then there's the other 90% of the time  :D
2 column capacity calcs are in the toolbox here;
This one is set up normally;
Column Capacity Calculator (forestryforum.com)

This one is more of a walk through the equations and has a drop down species list with red pine in it;
figuring a simple column (forestryforum.com)
I get a pass but closer than you thought and a #2 dougfir 6x6 plays out at ~18,000lbs.

edit:
I just poked around on a wood beam calc on that site. Throw that link as far away as humanly possible!  :D  If they don't know where to get basic design data from, what else don't they know.

[Djs5017]

Don, Thanks!

The those are great calculators.

I looked up on MatWeb what the modulus of elasticity of Red Pine is, and found 1630ksi once it's somewhat dry. I'm not sure how that number translates to the data entry into calculator 1 where it asks for a unit I'm not familiar with. Any thoughts on that?

I found another snow load calculator which allows me to enter roof pitch, and I'm getting more modest snow load measurements anyway.

My central support post is a fully 7x7 at 7 feet of length, so that falls into your "short post definition," which rocks. I wasn't as worried about that one even though it's going to carry twice the load as the smaller gable posts.

The second calculator has my head spinning, but I'll keep at it.

Dustin

Edit:
If KSI is for kilograms, I could simply take 1630 and divide it by 2.2 to find psi?

With data I'm pulled from MatWeb, I'm passing with a capable load of 19700 on the posts.

[reride82]

Dustin,

KSI in reference to structural calculations is an abbreviation for 1000 pounds per square inch or kilopounds. Kilopounds - kilo meaning one thousand. A kip is also equal to a 1,000 pounds of load/force. It is easier once you are used to it.

Levi

[Don P]

^That would be the real engineer 

The correct source for allowable design strengths of wood is the Supplement to the NDS (Nat'l Design Standard for Wood Construction). The NDS and supplement are under the publications tab at awc.org. Someone has been sprucing up over there, you'll have to poke around but they are there. What I saw on the engineers toolbox link was they were using average ultimate strengths in their calcs, half the sticks broke below average.
Don't build using the Wood Handbook numbers... which is most of the wood strength numbers you see flying around the net. We need the safe working load strengths not the ultimate strengths.

For #2 red pine Posts&Timbers
Fc parallel (Allowable fiber stress in compression parallel to grain) is 475 psi (pounds per square inch maximum allowed) or, you can put about half a kip on a square inch of endgrain all day long and do no damage to the wood. a ton, 2 kips, would take a little over 4 square inches of post top to be safe in compression, crushing... for the post. The side grain of the wood on top of the post is probably not as strong as the end grain of the post, just be aware we are just checking the post here but that is not the only thing to check.

E, stiffness, .9 million psi ... uhh yeah, don't try to wrap your head around the units, it just makes the math work. Get a sense of the range and what that means stiffness wise as you progress. .9 is pretty low in stiffness but in the right range, the upper end of wood we would be dealing with would probably be around 1.5 x 10⁶psi (which is another way you'll see it written, same thing, 1.5 million psi,  1,500,000 psi, 1500ksi)

I think that is enough to get you going on the first calc. Since Levi is reading.. I wrote that normal looking first calc using the "old" column equations and the second calc uses the modern ~'05 math, you'll see Emin, they felt the need to insert an "allowable" stiffness into the equation "Emin" rather than the old "E". If you followed what I was saying in the first paragraph, this does make sense, I guess  :D. At the end of the day, the results of these two should be within a gnat's hindquarters of each other. 

[Djs5017]

Thanks again Don, and Thanks Levi!

I've learned a ton in the last 18 months about sawing. Only in the last 4 years have i been an amateur DIY builder on my property, and since I'm finally nailing together my own place, I've tried absorbing as much as I can about engineering. This has been helpful and humbling, which is great because it gives me a new perspective. I'm glad my intuition was good enough to size the posts correctly for the application. 

Here are some pics 



 

 

Dustin