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Box beams from rough cut lumber

Started by coalsmok, September 28, 2017, 09:04:02 AM

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coalsmok

Contemplating building few sheds and want/need to keep the front sides as open as possible to make them better suited for what I'm planning on using them for.  I have been looking at box beams but would rather not buy plywood if I can use the lumber I cut myself to skin them. Poking around the web hasn't  brought anything to the surface yet so I'm asking here.
  I can look around the farm and see the sheds my dad and grandfather built. The ones using just a single 2x8 to span the openings have sagged, but the ones where they used a second 2x8 or 2x6 a couple feet below the one supporting the rafters with board and batten siding nailed on are still straight.  So I'm wondering now if doing that created a box beam type situation that helped support the rafters.
I'm only wanting to span 12'-14' between post on the open side to make getting the tractor in and out with hay faster/easier.

Don P

that is a modest enough span there's probably no real need to think outside the box. it should work with a solid or built up conventional beam, rough sawn would be fine. sketch up what you're thinking of and what species you have if you'd like some help with sizing.

tule peak timber

Hi Coal, To answer your question-yes-from rough cut lumber. We do no use them for support though, just cosmetic covering.  Rob

  

 
persistence personified - never let up , never let down

coalsmok

Don I may take you up on that once I get the dimensions nailed down a little better. 
Rob I guess I am trying/wondering if I can make structural box beams using rough cut as the skins instead of plywood.
   I think it may not be as strong as a plywood skinned beam but still give it added strength. I have some wide short boards that are wide enough to skin the box beam the same as plywood.  I could buy the 4 2x? Wide enough to span the openings just curious if what I have can be made work. 
Something holds the roof of a Jen Lynn style structure up and by the span tables it ain't the stringers it's all nailed to. 

tule peak timber

coal , lot's of caution. In general- engineered wood can be a lot stronger than just wood. The problem is that the engineered stuff is strong in what it is engineered to do and may be weaker than just plane old wood in some aspects. Can't you just put up a big solid beam ?Sure don't want to see you get hurt !!!!Joints in structural stuff need good attention to joinery , proper glues, likely the right press (pressure), and the right material selection. Don't let me discourage you to experiment, bu
t before you make a final decision try some testing to be sure things will hold together. I destructive test all the time !
persistence personified - never let up , never let down

Don P

shear is the problem with what you have described. it is the difference in the cross laminated plies of plywood or oriented strand board and the straight grained thin boards you would be using.

as the beam is bending the top edge is trying to get shorter, the bottom edge is trying to get longer. between those two opposing forces at the two extremes, the material between those two edges needs to resist the horizontal shearing force that sets up. there is probably not enough cross section of relatively weak in shear boards to resist that.

not enough section using the shear strength of wood along the grain. much harder to split firewood from the side...

there were various forms of keyed beams made to build up vertically deep enough sections for very large beams. one method was to stack the core of 2 or more large beams up vertically then nail diagonal sheathing boards on the outside, canted opposite directions on the 2 faces. many,many nails and they were getting some strength but if memory serves it was about 60% the strength and stiffness of what the same solid sawn section would have been.

trusses are another way to go

coalsmok

Well I looked online at the ag extensions plans and they say a double 2x8 is fine for 16' on center post to support trusses across a opening for a farm shed.  I can't cut anything longer than that right now anyway.  When I look at span tables for housing it seems to take bigger beams for the same distance.
Yes I can and probably will cut a few bigger beams, just my curiosity and observations had me wondering.
Don that last part is kind of what I was wondering about. Seemed strange that a box beam just jumped up out of nowhere after plywood was invented. And seeing how the old sheds with a few feet of board and batten siding nailed from the support header to another 2x member spanning the opening didn't sag as much made me curious.

Not so important on small hay sheds but I have a small barn and hopefully a new work shop to build in the next few years.  Lots of trees but so far I haven't found money growing on any of them. Anything I can cut and build for myself helps.

Don P

there is much good to be said for a portal frame type of entry with the lower dropped header. you can pick up lateral bracing for that big opening and it can be trussed.

sizing a solid sawn beam for that would be my first try though, see if it is in the realm of a simple beam.
this sketch shows a 16' beam supporting half the span of a set of 16' rafters.

there is a beam at the rear supporting the other end of each rafter, so half the rafter span is bearing on the rear beam, half of each rafter span is bearing on the front beam... 8'. the beam is spanning a 16' opening. these measurements are taken on the horizontal plane, from straight above not along the angle of the rafter. that's where I was trying to be fancy and draw in the horizontal measurement plane in yellow. for ease of discussion, 8'x 16', the tributary load are of the beam is 128 square feet.
design loads are 10 psf, lbs per square foot, dead load, the weight of the roof itself+ 25 psf minimum design live load, snow, I'm guessing on your snow load. 128 sf x 35 psf=4480 lbs, might as well say 4500 lbs uniformly loading the beam.

going here;
http://www.timbertoolbox.com/Calcs/beamcalc.htm
load is 4500
span 192"
width of beam, try a 3 ply true 2x12 in yellow poplar, enter 6"
depth 12
fb, #2 tulip poplar 925
e 1.3
fv 166
hit show result

that's about the minimum size I would use on an opening that large.

coalsmok

Don thanks for that. I spent some time playing with it last night. Looks like I am going to put some more post in the back to cut down on the large timbers needed to span everything. I just need access from the one side anyway.  Might just give the truss company up the road about 10 miles a call and see what it would cost for two 16' trusses to span between the front post, a 6x12x16 is heavy.

Don P

I was using three 2x12x16' pieces to make the 6x12
cheaper than a truss for an opening like that using an engineered component would be an lvl... one of the plywood looking beams.
You can check the size of one of them on the same calc, keep the spans and loads the same. it is higher strength than sawn lumber,
for Fb plug in 2800
E=2
Fv=280

Those come in planks 1-3/4" thick and 14, 11-7/8, 9.5, and 7.5" wide.
I would check different 2 ply combinations like 3.5 x 9.5 with those higher strength numbers. Generally for a simple beam under not so heavy a loading these come in cheaper than a truss.

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