Wood shape/direction and strength

[mjoekingz28]

I was just thinking, say you have a 2x4.

If you lay it flat and wide it will probably flex and break if you suspend it by the ends and sit on it. But,if you set it tall and narrow it seems to me it could handle alot more load.


Is this have to do with the grain, shape?

What if you stacked multiple boards? Would the tall stack handle more than the wide stack?


Is this physics, structural engineering or what?


Do you think the concept is consistent in all materials: concrete, rebar, metal angle iron, etc?

 

[jimgood]

I don't know if you're being serious or not but...

I'm not an engineer so this is my layman's understanding of how it works. I think it has to do with both the fibrous structure of the wood (grain) and the cross section.

The grain is made up of long fibrous tubes layered and bonded together by whatever magical properties trees produce. In order for one length of fiber to bend, it must push/pull on the adjacent fibers. The resistance of each fiber to bending is multiplied.

Now, when the board is laying flat, the cross section is shorter so there are fewer fibers to resist bending. When you lay it on edge, the cross section is taller and there are more fibers to resist bending.

Another way to think about it would be to imagine a flat bar of metal; say 1" wide by 1/8" thick. You can easily flex a 3' length of that bar by hand. When you flex it, the outer side of the bend wants to stretch and the inner side wants to compress. If you take two of these bars exactly the same length and hold them together loosely and do the same thing, you'll notice two things; First, it will be harder to bend them. Second, the ends of the bar on the inside of the bend will extend past the ends of the bar on the outside. That means that the two bars, because they aren't bonded together, are sliding past one another as they bend. So even though it is a little harder to bend them, the two bars are not really working together to resist bending. Each is independently resisting and that combined resistance is only doubled (maybe).

Now, if you weld them together along their edges and try to bend them, you have their combined resistance to bending PLUS, because they can no longer slide past one another, you have more stretching and compressing that has to take place in order to achieve the same bend radius. I'm guessing that this amounts to much greater resistance than simply adding the resistance of each by itself.



Edit: one other thing to consider is mechanical leverage. The length of the board matters. If you support an 8' 2 x 4 at each end and sit in the middle, it will deflect maybe 1 inch. If you do the same to a 16' board, it might deflect several inches. If you bond (with glue) several 2 x 4 boards together, the resistance to bending is multiplied in the same way as it would be for the metal bars welded together.

 

[fury9]

It has to do with shape, make it a 4x4 and it'll be the same

 

[mjoekingz28]

Thanks. And yes I am serious. And curious.



Here is another. I saw a guy once, before pouring concrete, take a saw and cut many slots about an inch about for a length on ne side of a board. He then was able to nend it greatly and make a form (or mold) for the pour to shape into. It was pretty neat.



About that 4X4. I guess if you took a MLB slugger and gave him a 4X4 and he swung and hit a punching bag with it. Then he picked up a 2X4 and swung it with the wide cross-section, then swung a third time with the narrow cross-section.


I am guessing beside inertia (aka why we need different weight hammers and why a 10oz carbon fiber bat wont slug the way a 2lb wood bat will), the two swings with 4" of wood behind it will hit identical, and then the other swing (the 2X4 hit flat) will be weaker.





It just seemed to me there was some 'magic' by driving with the 4" side of the board versus driving with the 2" side of the board.......so much so that I thought, say, a 2X4 could be stronger than a 4X4 since the board had that 'edge'.

Put another way, say you made a table. You took a 2x4 and placed each end thru a cinder block then stood on it. It seems you would get over twice the weight capacity (before breaking) if you stood on it with the board standing. It even seemed, to me, that doing that would even be stronger than having a solid 4X4.





I tried to comprehend the fibrous flexing and structure above and maybe thst is what makes it work. But taking a metal rectangle and doing the same would seem, in my mind, to give the structure exponentially more load bearing capacity by standing it up versus laying it flat.....and maybe 1.5-2 times the strength of a squared structure.

 

[derosa]

A 4x4 has twice the strength of a 2x4. While a 2x6 on edge is stronger then a 4x4 with less volume, as mentioned above relating to the wood fiber's ability to move. With your hitting analogy it depends on force. Assuming a machine applies equal force to both the 4x4 will apply damage over a wider area while the level of damage should be the same. Since I could swing a 2x4 easier then a 4x4 I can get more momentum behind it resulting in greater damage with the edge. The flat of the 2x4 with equal force may do as much damage as the 4x4 depending on the level of force applied. Remembering that the 2x4 has less stiffness hitting with the flat and may give or break lessening the damage.