4 x 12

[Duane Bozarth]

For equivalent weights, yes. See my note back to Edwin.

[Duane Bozarth]

I think what he demonstrated must have been different than what you recall or mine is all awash...see my note to Edwin.

As a practical thought experiment, if it were as you say, you could continue to reduce the wall thickness of the tube until it was paper thin and the result wouldn't be different--that obviously wouldn't be true in reality.

[Duane Bozarth]

Those are some hypothetical maybes that sometimes might, sometimes might not actually help...

I still don't believe an engineering load test of two nailed 2x12's would beat a 3x12 in load on average of several trials of same species and grade material...

Tomorrow if I have time I'll see if I can find anything on the US Forestry site specific to the questin...

[CL (dnoyeB) Gilbert]

Why are bird bones hollow?

[CL (dnoyeB) Gilbert]

Sounds like an urban legend to me. Kinda like hot water freezes faster than cold water.

The wikipedia says stiffness is a measure of resistance to deformation. Solid rod resists deformation more than a hollow rod. It makes no sense that boring out a steel rod would make it stronger. How thin should I make it? The thinner the stronger? And the opposite is true too? The more I fill it, the less stiff it becomes?

[maradcliff]

I'm not going to comment about the pipe/rod issue because I was not familiar with that. However, as far as putting two (or more) timbers together, a good part of the reason is the same as plywood. A natural board has all the grain running a certain way, and that allows for weak spots, such as when you buy a board with a crack in it. (a weak spot). So, with two boards, it's highly unlikely that you will end up with the weak spot in the same place as the other board. Thus one compensates for the weak spot on the other. Then adding the plywood down the center, which is mainly just a spacer, that too adds quite a bit. If the boards were glued together they would be stronger still.

Look at the beams they make now. They are simply plywood beams made to be a 2X12 or whatever, and are 2 or 4 inches thick. They are supposed to be considerably stronger, yet each layer is less than 1/4 inch thick. Alone, the pieces would not hold up anything, but together, they are very strong. I used to think that was bogus till I read up on it. As for the particle board type beams, I still find them inferior, regardless what is said about them. Many will disagree with me, but I wont use them.

One final note. I spoke with someone that was building a huge barn at a county fairgrounds. Instead of using solid posts (uprights), like in the old days where they used a "power pole". Instead they used three 2X8's nailed together. Treated lumber below ground, non treated above, and they consist of pieces with the joints at different spots. The builder said they are stronger than single posts, and because of the height of the building, it's next to impossible to get solid poles that long.

Mark

[maradcliff]

Not a legend. Hot water DOES freeze faster. It's because it dont have as much oxygen. Dont ask me why it has less oxygen, but it does. and thus it freezes faster.

[MG]

A composite beam may be stronger because the defects and local weakness will not likely coincide. There is another factor, the tendency to twist and buckle under load may be improved by the fact that the pieces have different twist in the fibers.

We are talking about wood, here. Not aluminum beams like in an airplane wing.

MG

[Edwin Pawlowski]

There are counteracting forces. Take a thin strip of wood and bend it. Now take two thin pieces of wood, bend them, glue them together, clamp, dry, and they remain bent. Why do you think that is? Laminations for curved materials are often made that way.

Nick was quick to chime in with a one word answer, perhaps he will take the time to talk about the molecular flow of this so everyone can easily understand what happens with laminations when you are pushing one while pulling the other. .

[Edwin Pawlowski]

No formula?

[MG]

You say same species and grade, and add averaging over several specimens. This imply uniformity. The more we assign importance to uniformity the more we make wood an homogeneous material, which is not.

Clearly there is no reason why a composite should be stronger that a single piece of equal crossection IF the material are homogeneous. Wood is less dependable than a steel beam in the sense that a higher safety margin must be applied for wood.

In construction you consider the lowest of all possible breaking loads and stay below that. With wood the ratio between lowest breaking load and typical is a smaller fraction than with steel.

To make a silly extreme case a knot hole at one third of the span in a single beam is worst than two knots symmetrically placed on a composite beam.

In other words the issue is about homogeneity or lack thereof. If you select 2 perfectly grained flawless board and compare it to the same perfect single piece there is no difference.

MG

[Edwin Pawlowski]

Well that explains a lot.

And that even more.

[nicksanspam]

You've always been told what???

Nick

[CL (dnoyeB) Gilbert]

/me shakes head...

[Duane Bozarth]

MG wrote: ....

That's exactly my point--that (which is all one can deal with w/ timber since, as you say, it's an inconsistent material), there's no difference between a composite built of 2-tubaX's and a solid beam of the same actual dimensions.

I don't believe there's any code based on the difference between the combination outlined above based on the assumption of defects cancelling on the composite beam.

Engineered and laminated material is something else entirely...

[nicksanspam]

Say E = 1.1 million psi for Eastern hemlock...

I = bd^3/12 in^4, for a b" wide x d" deep beam. I = 2x6^3/12 = 36 in^4 for a rough-sawn (real) 2x6.

....the total load. Say W = 400 lb.

....in inches. So a 10' rough-sawn 2x6 beam would have a D = 5W(10x12)^3/(384EI) = 0.23" max deflection.

You may be confusing something like the polar moment of inertia (including mass, for dynamics) with the geometric moment of inertia about the neutral axis, eg the horizontal diameter x-axis. That's the sum of the products of each tiny area and the square of the perpendicular distance from that area to the axis. Ix = Pir^4/4 for a disk of radius r, eg Pi2^4/4 = 12.57 in^4 for a 2" radius rod, IMO.

....with the same axis, consider the rod to be a composite area and subtract I = Pi1^4/4 = 0.79 in^4 for the 1" radius bore from 12.57 to get 11.78 in^4 for a 2" radius rod with a 1" radius bore.

So a 10'x2" radius Eastern hemlock rod with a 400 pound total load would have D = 5x400(10x12)^3/(384x1.1x10^6x12.57) = 0.650" max, and the hollow version would have D = 5x400(10x12)^3/(384x1.1x10^6x11.78) = 0.695" max.

If the hemlock weighs 30 lb/ft^3 and the solid rod weighs 26.2 pounds, a 26.2 pound 4" radius rod with a 3.46" radius bore with I = 201.1-113.1 = 88 in^4 would have D = 5x400(10x12)^3/(384x1.1x10^6x88) = 0.093" max.

And a 26.2 pound 4"x8" hemlock "I beam" with 2 4"x1.6" boards bolted onto a 4"x4.9" foamboard sandwich and I = 4x8^3/12-4x4.9^3/12 = 133 in^4 might have D = 5x400(10x12)^3/(384x1.1x10^6x133) = 0.062" max, if nothing slips.

Nick

[Duane Bozarth]

Well, it's true that water w/ less entrained air will freeze faster than that w/ more entrained air--and, heating water will drive out some of the entrained air so there's a kernel of truth in the saw...

[Duane Bozarth]

Might look at...

[Lawrence Wasserman]

That's just not true. A (larger diameter) tube made from the same amout of steel as a solid rod would have more resistance to bending force, but a solid tube of the same diameter as a hollow tube will be stronger. If you don't believe that, try bending a lenght of 1/2" EMT over your knee, then try with a solid 1/2" steel bar.