Newbie Questions About Beam and Post size

[Joe Dumy]

Hi everyone. 

I am trying to build a simple lean to design for my new bandsaw mill. And I got a few questions after reading some books on timber framing. 

First is what size beams and post should I use. I can make any size I want as I have large Douglas Fir on my property. My biggest span for a beam is 19 foot post to post. I am planning on using 6' long Braces at this area, which I think will give this beam support for 8 of the 19 foot span. But I could be thinking of braces all wrong. 

Second question is that most designs show the beams and post the same size. Is this due to trying to center cut the posts and beams for dimensional stability. Would making a 8x12 beam cause an issue with dimensional stability. 

[Don P]

Welcome to the forum

Depending on how large the trees are some of the nicest timbers I've use were free of heart Dougfir, and in rectangular sizes, B&S, Beams and Stringers dimensions. FOHC is generally higher grade, more prone to bowing where Boxed Heart is more prone to a heavier check.

Read the first paragraph here;
https://forestryforum.com/members/donp/beamclc06b.htm
B&S is graded a little more stringently than P&T and usually carries higher allowable strength numbers all else being equal.

If the braces end up actually being tight enough in the finally dry assembly to help support the beam, I consider that to be gravy. There is an adage "Load goes to stiffness". Think about as the beam takes load and bends, which is the stiff element that will support the load, the post or the brace. When one can saw whatever size needed there is usually no penalty for thinking conservatively. The beam span is from post to post.

Give that calc a try and holler if you need help.

[Joe Dumy]

Don thanks to pointing me to the calculator. It spawned some more questions. Looks like an 8x9 would work. I am thinking 8x10 beam sitting on 8x8 Posts with timber framed joinery and the 6 foot braces. Let me know if I used the calculator right. Or if I should just go with a 9x9 or 10x10.  

I do have some big 36" logs to mill. But I wouldnt grade them very high. We will see once I start cutting them. Most of my good trees are in the 24" range. My property was cherry picked about 10 years ago. I have been slowly repairing the damage caused by this the last two years. Thats why I got a saw mill so I can make use out of the leftover trees the logger left and clear the land and replant. 

Are Rectangular beams stable no matter free of heart or box heart? is there a particular cut that should be on the depth of the beam. Like the flat cut side or the rift or quarter sawn side. 

On free of heart do they  tend to bow a predictable direction?

This is really close the the design I am doing. 
https://timberframehq.com/24x24-shed-roof-outbuilding/

[Don P]

Joe,
To check you I would need your dimensioned sketch and local snow load. Pointing to a plansellers offerings and asking for engineering is beginning to become something I'd rather not get into.

Rectangular beams up to a point are no different than square ones from a stability standpoint. When boxed heart the major drying check tends to take the shortest path from heart to bark, which is typically on a wide face. Since FOHC does not contain full rings of tangential grain it has less of that checking stress. It does however contain unbalanced growth stress, the tendency of a board or timber to bow away from the heart along its length. There is slightly less tendency to horizontally shear if flatsawn on the wide face, generally disregarded, but more tendency to bow.

[Joe Dumy]

Don

Thanks so much, between the calculator and your explanation of the cuts, direction of bowing,  I now have the confidence I need to build. 

I have no snow load where I live. It snows once a year and is gone with in a day or two. 

[Don P]

In that case wind controls, the building code says unless it is a special wind zone to use no less than 20 psf live load (wind) + 10 psf dead is usual for 30psf total load.