Help building a structure for my mill

[Snag]

I would like to build a structure to put my mill under so I can saw in the shade, out of the rain, etc...  I would like to build it approx 10' wide x 30' long.  My mill will take about a 20' log so I would like to have an opening on one side somewhere around 18'.  I will be sawing my own lumber for it.  I was thinking of 6x6's for the uprights but really am clueless beyond that  ???.  My question is, what do I need to to use to support the rafters in the 18' span.  I plan on using a metal roof and can put it at whatever pitch necessary to keep snow loads off it.  Can I just put a 6x6 on top of the uprights, or 2x12's or ????  I appreciate any help you can provide.   Thanks.

[Raphael]

  An 18' clear span is certainly within the realm of possibility but I'd give serious thought to making sure it was engineered to handle the snow load rather than counting on the pitch of the roof to shed the snow.  Get the right slushy ice storm and that roof will pick up a good 6000+lbs regardless of the pitch.

  When it comes to just 'putting a 6x6 on top of the uprights', there are a couple things you need to concern yourself with.  First is limit any lateral thrust on the beam as it will tend to roll off the top of the posts.  Second even if the (static) lateral forces are zero, make sure it's tied down securely in case there is lateral loading or uplift from wind.  Do you get much wind load up there?

  If you post some rough drawings we can probably give you more specific input that will help you come up with a good solid final design, but I think eventually you'll want to have an engineer take a look at it.  I paid my $170/hour engineer about $600 for evaluating the foundation, the 9'x36' shed and the 16'x18' office frames for my house and that includes a couple alternate bent designs for the office.  It's probably worth a couple hundred bucks to protect your investment in the mill.

[Tim]

Make sure you pitch the roof to run snow and rain away from your 18' door. Nothing like getting a call for some sawing in the winter and having a 7'  block of ice in front of the door. Even keeping that door clear in the winter is going to be enough of a pain that you'll be ripping the roof off your mill in the spring.

[Don P]

Raphael and Tim are right on with their comments.

How about a simple shed roof sloped to the backside, the high side is on the log door side. That would allow room for a relatively deep beam over the 18' log opening. Those openings take a deeper beam than most of us realize.

I went and found some snow loads for NY, don't know how correct they are though, (35 psf?). For fun I figured up a beam with 18' span and a loaded area of 6' ( half the 10' depth, the other half is on the back wall, + 1' of overhang out front). Coming up with 108 square feet x 45 pounds per square foot (35 snow+10 dead) = 4860lbs

Using some very rough wood strength numbers I'm coming up with a beam in the neighborhood of a 6x14.

You know how we like pictures  ;D.

[Raphael]

Using some very rough wood strength numbers I'm coming up with a beam in the neighborhood of a 6x14.

  I was playing around with the numbers as well; my thinking was increase the posts to 8x8's, then the beam would be from 8x10 to 8x12 depending on load and species.  I think this would look more proportional and give more meat for a secure attachment at the post tops.

[Corley5]

Use the I-beam from a mobile home frame.  Easily doubled too 

[Snag]

Thanks for all the suggestions.  8x8 uprights is no problem.  I would rather err on the side of safety.  This is what I have for species on my property that I could use: white pine, red pine, hemlock, red oak, black birch, poplar, beech.  Which would be the best to use or where can I find a table of their respective strengths?

Don, I promise pics.  Any suggestions from anyone on "niceties" I should encorporate.  This will be my first mill structure and I am sure I will wish I had done some things differently.  The shed roof is a great idea.

[Don P]

I haven't looked at strength but Raphael has described a more stable beam. Greg makes a good point with the trailer frame too, Jim has posted pics of a floor truss used there before also.
I need to update to newer numbers but there is a link on this calculator to a list of wood design values as of 2001. Scroll halfway down the page and click on "click here for wood design values" in blue. https://forestryforum.com/members/donp/beamclcNDS2.htm

[Snag]

I understand the max shear and modulus of elasticity, but what is max fiberstress??  BTW- my last post said 8x8 is "now problem".  It was supposed to say (and now edited) "no problem".

[Norm]

I have my mill in a three sided shed now. The one thing I hate most about sawing in there is the dust just swirls around no matter what direction the wind is out of. I think if it had an opening on the front and back side it would help move it out.

10' wide is pretty narrow and would maybe suggest you think a bit wider if possible.

[thecfarm]

I use hemlock for any time that I need strenght,such as your 18 foot opening. Built my wife's horse barn out of all hemlock.I would use pine for the uprights if I was trying to save the hemlock.I want to put mine saw uncover too.I would keep the pitch away from your door as Tim said.My other house that I bought had it so the water run down in front of the garage door.Was a pain in the winter.We had this one built right.

[Don P]

I understand the max shear and modulus of elasticity, but what is max fiberstress??  

Hopefully I won't muddle this.
Fb is used for "extreme fiberstress in bending". The extreme there means outermost, so they are really talking about the bottom edge if the beam.

The best example I can think of right now would be to take a popsickle stick and support its ends on something. push down on the middle slowly bending it till it starts to break. It will fail (I hope) by the fibers on the bottom edge tearing somewhere around the middle of the span.

The Fb number is the amount of force in pounds per square inch that a particular species and grade of wood can safely handle along that bottom edge without tearing those bottommost fibers apart.

The load applied and the shape and orientation of the beam determine how many pounds per square inch end up concentrated on the bottom edge. If you apply the same force that broke the popsicle stick to a 2x4 of the same length it shouldn't break, those extreme bottommost fibers are under much less stress.

If a beam fails to check out in bending it needs to be bigger or shaped differently. Making it deeper makes it stronger faster than making it wider. Making a beam twice as wide makes it twice as strong, making it twice as deep makes it 4 times as strong. Stand the popsickle stick on edge and push down again, should be much stronger. There comes a point though where the beam is too tall and skinny to be strong.

An open web floor truss is another good example of how this works, you know the kind with a 2x4 flatways top and bottom and W bacing in between. A truss 19" deep can span 24 feet as a joist (not a beam a joist!). When they spread the top and bottom 2x4 chords further apart they can span 32' in a 24" deep truss. They got the bending stresses down to where a 2x4 could handle it by spreading them further apart. If I were to stack and fasten together two 24' 2x4's on top of each other with no space between and then walked out on them I have no doubt I'd have a fiber failure.
When you saw and install wood, inspect the bottom edge for spike knots or other defects that might want to tear more easily as the building pushes down on them. The real kick in the pants is that usually that spike knot ends up making the board bow. You want to put crown up and in doing so the knot ends up down ... DanG it  :D

In a perfect world the beam can handle at least 2.1 times the design value without failing. Those Fb numbers are the safe load numbers.

[Raphael]

I understand the max shear and modulus of elasticity, but what is max fiberstress??

AKA: max fiberstress in bending, bending strength and IIRC modulus of rupture.
In this case it's the value to be most concerned with as it's the one that tells you if the beam will 'break' under a full load.

I don't have the values for Hemlock & Poplar handy but of the other species you listed I'd look at the Beech & Birch first (they are grouped with Hickory in the tables Fb=900) then the Red Oak (Fb=875).  Pine isn't going to make it at the sizes we've discussed so far.  Depending on the values for Hemlock it might be a good choice for it's weather resistance and lighter weight.

One nicety you can add that serves some purpose is long arched knee braces that connect from low on the post, 5 - 7' or more below the beam, to roughly 3½ - 5' or more out on the beam.  Aside from adding visual interest they do help channel a percentage of the roof load from your beam to your post.  You can also make the post below this brace 8x10 if the brace is sufficiently massive to warrant it or place a down brace on the opposite side of the post (a variation on this would be an X brace or angled strut).  Will your logs be rolling in low or coming in raised on forks?

Of course the I beam is a good idea too, just a little tough to bore a mortice into.  :D

[Snag]

Thanks for all the suggestions.  I think the first thing I will do is pick up a book on timber framing.  I have done a descent amount of conventional framing, but have no idea how to do it otherwise.

[mark davidson]

I recently built a similar shed for my mill with a 22 foot opening. I used a 32 foot 10x10 hemlock.
the beam was morticed for two posts and four braces and I'm sure it will hold up fine, as the roof is only 12 feet long, with another shed holding the other side.
In your case, if you make the shed 10 feet wide, the beam is only holding up 5 feet of building, although I would agree with a comment higher up in the thread.... "make it wider"  I would go at least 12 and 14 wide would be better. An eavestrough will help with water and snow problems, better yet would be a dormer to keep everything away from the opening that your logs are coming through.
the thing I don't like about my new shed is that its hard to "spin" the logs now(end to end)... before I could spin the log on the mill bed, but now the shed is in the way. Still, It is very sweet to saw on a rainy day and stay dry.

[red]

my favorite sawmill shelter is from Shamus

https://forestryforum.com/board/index.php?topic=13749.msg193192#msg193192