I also tried to design a sawmill shed and had a few problem with the main beam where the unsawn log go through.

First problem: my sawmill is 21 feet long. I can cut lumber up to 18 feet long. I want the openning to be 25 feet wide: how can I get 25 feet long pieces from a sawmill that can only cur 18 feet long pieces. I'll get back on that later.

You asked if bolting 2 3x12 together would do. What spiecies are they? Their strength varies with the spiecies and the grade. Let's say that you use canadian SPF #2. Bending Strength is 875 psi (from

www.cwc.ca). From tables, I see that a single 3x12 can support 1318 lbs with a Fb=900 psi. So for 875 psi we get:

W=1318lbs*875/900 = 1281 lbs.

If you put 2 of them together, you'll get twice the strength, so 2562 lbs.

Now what is the load? Normally, you would divide the load on the roof (dead+live) between the various supports. Let say that the shed is 24x10. So each 24 beam support half the load of the roof, so 5 x 24. If you use metal roof (with a high enough pitch so no snow can stay on the roof), you can neglect the snow load. You would have to estimate what will be the effective weight of the material on the roof (rafter+metal). Let's say 10 dead + 10 live: So the weight on the beam would be:

5 ft x 24 ft x 20 lbs/sqft = 2400 lbs.

So the load is less than the capacity of the 2 beams so that would work. I don't know how composite (metal + wood) would behave: I'm also building around my land with my own wood, but the thing that grow here is mostly wood: I don't get many metal tree so...

WARNING: Here are the assumption I used in the calculations above, if any assumption doesn't apply to your case, then the end result doesn't apply either:

A) wood: Canadian SPF. Don't you say you wanted to mill your own? What speicies?

B) grade: #2. I can't tell you how to grade your lumber. There is a grading document on

www.nelma.org.

C) load: what are the actual dimension of the roof where the 24 foot beam will be.

D) slope: even with metal roof, there must be enough slope for the snow to fall. If I remember right below some angle (something like 25 degrees) all the snow stay on the roof. Above another angle (something like 60 degrees) no snow stay on the roof. And in between, the load goes linearily from all the load to no load depending on the actual angle. So maybe you have to include a snow load in the calculation. On the other side, when you add the snow load, you can increase the strengh of the wood (Fb) by 15% because of the duration of the snow.

E) this is a pretty long beam. You have to make sure that it will not buckle. You have to make sur it will not rotate on its extremities. Also it has to be laterally supported on all it's length. There are ways to compute a beam stability factor which will affect the Fb (I used 875 psi) depending upon how the beam is attached to the rest of the structure.

I mentionned previously about the fact that a mill can't cut long enough to cover itself completely: I solved that issue by using a truss. In a truss all the member are working in compression or in tension, not in bending, so you can make a connection between 2 lumbers, as long as the connection is stronger than the compression or tension load.

jf