Timber strength question.

[Donn]

I have to span a ~12' distance with a rafter that will be helping to support a fairly light load ( a few hundred#). It'll be supported at it's ends, with joist-hangers.

I have a nice 2x12.

Is the 2x12 stronger in this application, than ripping it in half and gluing/screwing up a beam with the two halves?

[StevenBauer]

It's stronger as a 2 x 12. No worries.

Steven

[ssor]

I wouldn't depend on a single 2x12 12 feet long to carry a few hundred pounds for very long. I would double it and place jack studs under the ends. Floors are generally designed for sixty pounds per square foot. That is a uniformly distributed load. Concentrated on the center of a 12 foot span that would be 720 pounds per linear foot down the middle of the floor.

[P.I. Stazzer-Newt]

I assume that the 12" will be vertical.

[Donn]

There will be two vertical 4x4 posts adding support at each end of the rafter/beam. The whole framework will be holding up a lightly constructed compost bin roof, 2x2's and cedar shakes. Two more center 4x4's will add support to the roof, front and rear.

[Gary E]

Donn,
Your gona be fine with the 12 in dimension up. But before you go doing something ya really dont know much about test it, here's how.

Place it standing the correct way on concrete blocks at each end. Measure how much daylight is under the middle. Now go find the heaviest guy you can, maybe fat Albert that lives near by can be inticed to stop buy for a cheezeburger and a sacka fries. Have him stand in the middle and measure the daylight while he stands there.

But really ya gona be fine, we used to pull Chevy V8's out with the trans attached with a lot less of a beam.

[George.]

Will the load move or sway at all? If not, 2" by 12" should be fine, and stronger than 4" by 6". But if it will be a moving load, generating sideways forces, 2" may be too narrow - in the long run it could crack, depending on the wood.

[Donn]

Originally posted by Gary E:

Place it standing the correct way on concrete blocks at each end. Measure how much daylight is under the middle. Now go find the heaviest guy you can, maybe fat Albert that lives near by can be inticed to stop buy for a cheezeburger and a sacka fries. Have him stand in the middle and measure the daylight while he stands there.
ROFL! My kind of science!

I tried it, and with 220# precariously balanced in the middle (I tacked a 4" x 16" piece of plywood to the edge), the amount of daylight didn't change at all.

[Mike Vogdes]

Make sure you crown your rafter/beam befor installing it. Crown up...

[Donn]

When sitting on edge, one edge has about a 1/16th clearance to the floor at the middle of the board. I assume the other edge is the "crown up?"

[Bruce Hooke]

This is not a completely simple problem, because, as you may have guessed, the issue is that the 2x12 could, in theory, twist because unlike a floor joist it will not have a floor above it (and attached to it) to constrain it. I believe that this would be required by the building code so officially what you are planning to do does not fly. However, leaving aside that code requirement, here is what I came up with:

I assumed that your 2x12 is construction grade spruce-pine-fir and that it would be kept dry (i.e., that it is not outdoors and unprotected). On that basis, without considering the posibility of twisting, the load capacity of a 2x12 comes in at 857 pounds for a point load at the center of the beam. With the possiblity of twisting factored in this drops to 414 pounds. Cutting the 2x12 in half and making a 3x6 out of it yields a capacity of 419 without the twisting factor and 413 with it (as you can see, a wide beam is much less prone to twisting). So, as you can see, with the twisting factor a 2x12 is about as strong as a 3x6, which is somewhat surprising.

Do make sure the ends of the 2x12 are well locked in place so that the whole beam cannot rotate. Also, as others have noted, be careful about shifting loads that would cause the beam to twist. Finally, given that you do not have a lot of capacity to spare (at least in theory) you may want to double check that your load figure of "a few hundred pounds" is correct. For example, is snow load an issue? That could add a LOT of weight.

Now, there are a couple of caveats. On the plus side, the rated strengths of construction lumber are pretty conservative. Also on the plus side, I assumed that the load would be consentrated at the center of the beam. If the load is more uniform then the capacity goes way up -- to around 720 pounds with the twisting factor (but the 3x6's capacity is 822 pounds with a uniformly distrubted load).

On the minus side, I ain't no engineer and this advice is worth what you paid for it! :D

[Mike Vogdes]

If you sight down the edge of the board you will notice it is either humped or sagging, in or close to the middle of the board. You want the humped edge (crowned) to be the top of the rafter. Its good practice to sight both edges of all your rafters and joists and mark the crowned edges befor building.

[George.]

What Bruce said. The crux is how the weight is borne. If it is evenly spread out over a couple of feet of beam, with no twisting momentum whatsoever, then fine.

If it is hung by a bolt that goes through a hole in the middle (6") of the timber...

or if it is hung by some sort of saddle that imparts an uneven, i.e., twisting, force on the timber...

or if it is placed on top of the timber and is not perfectly balanced to either side...

Then you twist the timber as well as push it down, and that's where the 2" will get you in trouble, even if the 12" is plenty.

[Donn]

Bruce...the board is ~20 year old construction grade, in excellent shape. It was indoors for most of it's life, but has been outdoors for ~2 years. It's very slightly cupped, and has a couple of small checks in one end, which I will gusset with galvanized plates.

It's going to spend the rest of it's life outside, sitting over a compost bin, and under a ~6' x ~12' roof consisting of a frame of 2x2 or 2x4 PT, covered with cedar shakes.

George...It will be fastened in to side rails that consist of a 2 x 6 topped with a 2 x 12, ripped corner-to-corner, to provide slope for the bin roof. The fasteners are galvanized joist-hangers which are like saddles that support most of the width (height) of the rafter. Galvanized screws will run into both the rafter and side rails, through the joist-hangers. The roof will be in two sections. The lower, front section will be supported on the side rails described, the rafter and five 4x4 posts. The back, higher section will be able to be moved away to access the bin. It will be supported by the side rails, the rafter, and five 4x4 posts.

Snow load is not an issue, because I can clean snow off it in a minute or two. Code is also not an issue. It's a compost bin, for heaven's sake! The front and both sides are clad in 1x6 cedar, and the back will be cedar lattice, slotted into the rear posts for easy access. I'm doing it in cedar on the outside for acceptability in a semi-urban garden, and reinforcing the interior of the frame with 2x bones. Within 2 years, it will be virtually hidden behind vines, ornamental grasses and shrubs.

If it lasts 10 years, I'll be satisfied, and build a new one. I really only wanted to know if ripping the 2x12 in half and making a 3x6 out of it would be stronger than just using the 2x12. I thought someone might have "the book" that tells you about timber span strength and such.

[Bruce Hooke]

Originally posted by Donn:
Bruce...the board is ~20 year old construction grade, in excellent shape. It was indoors for most of it's life, but has been outdoors for ~2 years. It's very slightly cupped, and has a couple of small checks in one end, which I will gusset with galvanized plates.

It's going to spend the rest of it's life outside, sitting over a compost bin, and under a ~6' x ~12' roof consisting of a frame of 2x2 or 2x4 PT, covered with cedar shakes.It does not matter much where it HAS been stored in the past (since it is still in good condition) what matters is if it will be "wet" in use. Since it will be under a roof I think you are OK on this count. If the moisture content of the wood would normally exceed 19% then the allowed load is reduced to 0.85 of the dry load limit.


George...It will be fastened in to side rails that consist of a 2 x 6 topped with a 2 x 12, ripped corner-to-corner, to provide slope for the bin roof. The fasteners are galvanized joist-hangers which are like saddles that support most of the width (height) of the rafter. Galvanized screws will run into both the rafter and side rails, through the joist-hangers. The roof will be in two sections. The lower, front section will be supported on the side rails described, the rafter and five 4x4 posts. The back, higher section will be able to be moved away to access the bin. It will be supported by the side rails, the rafter, and five 4x4 posts.Am I understanding that the 2x12 in question will be supported by joist hangers attached to 2x6's? I'm not really understanding the 2x12 that tops the 2x6's. It would be best if the joist hanger did not hang down below the beam it is hanging from (the side rails).


Snow load is not an issue, because I can clean snow off it in a minute or two. Code is also not an issue. It's a compost bin, for heaven's sake! The front and both sides are clad in 1x6 cedar, and the back will be cedar lattice, slotted into the rear posts for easy access. I'm doing it in cedar on the outside for acceptability in a semi-urban garden, and reinforcing the interior of the frame with 2x bones. Within 2 years, it will be virtually hidden behind vines, ornamental grasses and shrubs.I realize the building code is not an issue in the legal sense, I cited it simply because it is the most available source of data on what will work when it comes to building things from construction lumber.

Regarding snow load, remember that planning to clean the roof off to avoid snow load means going outside in the middle of a blizzard to clean off the snow occassionally before it gets too deep! That said, it sounds like only a small part of the 6' x 12' roof will bear on this beam so the amount of snow load you will be dealing with is not that great.


If it lasts 10 years, I'll be satisfied, and build a new one. I really only wanted to know if ripping the 2x12 in half and making a 3x6 out of it would be stronger than just using the 2x12. I thought someone might have "the book" that tells you about timber span strength and such.Well, "the book" I referred to is the Wood Engineering and Construction Handbook and the results are what I posted. The reason for all the add-ons are that (as is usual with this sort of comparisons) there are lots of factors that can come into play in ways that affect the outcome.

All in all, given the safety factors built into the building code I'd be surprised if you run into strength problems with the 2x12.

If you want some added safety factor then look for ways to brace the 2x12 against twisting. This can be as simple as a post coming down from the roof that is fastened securely to the side of the 2x12 to hold it vertical. A horizontal brace running on the diagonal from the top of the 2x12 to the side rails would be another way to brace things.

[ssor]

If all you're do is making a compost pile why put a roof over it? The stuff won't decay without water, we get that for free when it rains. Just rake the sides down and pile the loose on top add a little water and a handfull of fertilizer once in a while and the bugs and fungus will do the rest. If you want to store hay put it in a shed. :D

[Gary E]

Donn,
I shuda warned ya that the gang with the inginneering PHD's would sho up and peee on the parade, but maybe that's not all bad ifin they crap in the compost pile.

It sounds like it's gona be built plenty strong. But just to passsify them, maybe CPS it? or isit Epoxy it??

[Donn]

I think I'll go with it as it is, a 2x12. If it needs it, I can always sister in another.

The side rail is as follows:

A 2x6 sistered inside the 1x6 cedar top rail on each side, toe-screwed into the 4x4 corner posts. On top of that 2x6 will be a 2x12, ripped corner-to-corner, spanning the 4x4 corner posts and gusseted to the 2x6 with galv plates.

The joist hangers will span the whole 2x6, and some of the triangular 2x12. The entire joist hanger will be screwed to the side rails, and the 2x12 rafter.

[Donn]

SSOR...It needs a roof, because it sits under a huge filthy white pine. If I leave it topless, the compost will be filled with 3-4" pine needles in no time. They take forever to break down (which is why I rake them up to use as pine straw mulch). I'll be watering the bin regularly, probably with a soaker hose on top of the pile.

[pipefitter]

If you double it with a rip of ½" ply sandwiched in the middle it will fit the whole dimension of the 4x4 and it wont be as prone to sagging or twisting. If it was to be load bearing, thats so it doesn't sag over time. Even singular timbers exceeding the load will still sway over time.

[George Roberts]

Donn ---

From your description a 2x8 would be enough.

[ssor]

Donn,
As an answer to your original question: The strength of a beam varies as to the third power of the depth and the forth power of the span. If you rip a 2x12 you end up with two 2x6's one half as deep and one eighth as stiff times two beams so the product will be one forth as stiff as a 2x12.

[ssor]

Donn,
By now you know that on this forum if you ask the time you will receive detailed discriptions of how a clock works, the history of the derivation of time and some where along the way you might learn the time at that moment. :D :D

Ross in Bel Air

[George Roberts]

ssor ---

Cutting construction lumber lengthwise requires regrading for a reason.

While your formula is correct, the location of defects affects the load carrying ability of the lumber. In particular many 2x12's have knots along the center. These knots would be along the edge after resawing. That loaction is not desirable.

[Bruce Hooke]

Originally posted by ssor:
Donn,
As an answer to your original question: The strength of a beam varies as to the third power of the depth and the forth power of the span. If you rip a 2x12 you end up with two 2x6's one half as deep and one eighth as stiff times two beams so the product will be one forth as stiff as a 2x12.Hmmm...while forum responses may go into too much detail, it is also worth remembering that there may be a reason in many cases. For example, while what you said above is techincally correct as far as it goes, in the overall picture of the problem at hand your information would likely lead one to a very incorrect conclusion about the real relative strength of a 2x12 versus a 3x6.

[ssor]

There also exist formulae for determining the effects of lateral loads, rolling loads, whether or not the beam is fixed at one or both ends, ad infinitum.