strength of wood fine grain VS coarse grain

[phil]

I just had a chance to speak to an engineer who is testing various samples of wood. I had to fix the test equipment so I got to see up close how they were doing the testing and the results..
basically they are compressing blocks of wood which are cut with a notch so when they fail by compressing the samples they are shearing the wood. they are also measuring the forces needed to shear the wood.

they did a series of fine and coarse grained wood. the results are that there really isn't much difference in the shear strength of coarse vs fine grained wood in this application.

In woodworking one would normally choose finer grained woods over coarse grained and material selection is a part of building anything. Its more highly regarded and looks nicer to most. might be a bit more predictable for finishing etc.
but the interesting part is that the wood , at least when examining shear strength, did not show a lot of difference weather it had 50 growth rings per inch or 8 growth rings per year

they are also compressing wood samples to see the force needed , This would be like how much weight a short 2x4 can support.. again not a lot of difference between the fine and coarse grain materials..

I found it interesting , I dont' know if anyone else would.. the samples were all fir.
Phil

[Mick_VT]

I would have thought the same as you - old growth tends to be more dense - were thery trying acroos ther grain as well as along it?

[Mick_VT]

I do recall that when the rebuilt the covered bridge opposite my house, the engineers would not use local wood like the original was built with as they claimed new growth would not be strong enough. I seem to recall that they cited both the grain density and the prevalence of knots. Instead they used a non-native species of wood brought from the south.

[phil]

the engineer noted that when they do this testing on larger beams , they have less predictability as there can easily be hidden imperfections , for example cracks and sap inclusions. smaller stuff like 2x4's s easier to grade. any imperfections are often either caught or the others spread the load and help the weak ones, but I guess with something like a bridge or archway the posts might be 8x8 and acting more independently than they would on a 2x4 wall for example.

in the samples I saw they were only compressing the wood with the forces acting upon the endgrain. the blocks that were subject to the shear test had a notch cut out of one end but not both ends. this caused a fracture parallel to the grain and the force to fracture it is accurately measured. he pointed out that although the fine grained samples would often be thought to be more perfect what happens is that there are more shear surfaces with the finer grained lumber and the chain is only as strong as the weakest link.

Yes I'd be inclined to believe old growth wood is stronger due to less knots , the trees grew in competition so they didn't have as many branches on the lower half of the tall stands of fir trees here. they were also quite wasteful in the early days, they would burn a lot of material rather than carting the second rate stuff to the mill. fast forward to now they use all they can even finger-jointing a lot of it or grinding it and reconstituting it into other products.

moisture comes into play too, damp wood is more bendy, dry wood cracks easier. I have a bunch of firewood Ive been splitting, it is juniper I think , it's some sort of softwood much like pine. man the stuff is impossible to chop and not the best to burn either. It doesn't want to split because it has sort of an elastic nature to it, plus it has a lot of knots.. I refuse to give in but some of those chunks I have to hit many time and use wedges.. It takes some determination. Fir is easy to split by comparison. the big rounds are easy to split just a bit heavy to handle.

[Lily left the valley]

Thanks for sharing this info, Phil. It's not what I would have thought either, but at the same time, it makes sense. It's still the same type of wood, just grew differently as to duration.

[Casey]

Bring me up to speed on "8 growth rings per year" ??

[phil]

the number of growth rings per year might be one or two. the age of wood can be detected by analyzing the weather history. the rings may be larger in years of more growth so it's a bit like a fingerprint of time. I think it may depend on the seasons but I dont think you can actually just count the rings and say that it is this number of years. It might depend upon the weather patterns.

I'm not sure where we got 8 per year or if it is a typo but the engineer was doing testing on some pretty fancy testing equipment. it will stress the sample and measure accurately how many newtons of force it took before it broke and this is graphed. upon doing multiple specimens they can have more reliable data.

old growth refers to places like around my house where the timbers were tight and crowded so growth rings were tiny and you may easily see 50 lines per inch across the grain. Newer growth trees have more light since they arent' in tightly packed forests,, especially with clearcutting, so the rings are considerably larger and that wood is less sought after especially if you were making furniture.

but when strength of the wood is concerned the results seemed to show the wood was really not significantly stronger if it had finer grain..even thought I always thought it was quite significant. the test seemed to at least shed more doubt on the idea that old growth is stronger.

if the wood is sheared parallel to the grain then there are more surfaces where they wood can shear and it will shear on the weakest layer just like a chain will break at it's weakest link. more shear surfaces might be like a chain with more links , more likely to be a weak one in there..

Im not really clear on the testing of breaking strength across the grain. I will try to clatrify that when I m back in there.
I do think there is a relationship between density and the strength. if you pick through a lumber pile you may notice the cracked boards may sort of have a ring to them and that some are heavier. the weight can be wood density or moisture content so there is that second variable. in a project I would normally try to select out the best lumber for the most crucial parts so if it were a staircase you'd want good wood for the stringers , they are harder to change and more critical. a cabinetmaker might select grain patters he likes for the face of his furniture and use the rough stuff for the carcass. wood that is not near a knot is stronger and things like cracks and inclusions of sap can cause hidden weak spots. Of course no two boards are identical but choosing the right lumber for a structure and it's strength ratings are important when looking at things like roof spans and so the engineers get an understanding of how these factors get used in a practical sense such as a professional lumber grader stamping the lumber and then it being used to carry weight. Of course if you build a span some boards are weaker but by averaging things out they can arrive at some numbers to use for predictability. and a safety factor is added to compensate for any unpredictability. The engineers aren't carpenters of cabinetmakers , but they need to understand how the strength of wood is calculated. it would be unusual for an engineer to be breaking samples like this but he/she would use the data tables and its important they understand what the different properties are.

some woods act differently for example cedar isn't super weak as a board but if you chop it you soon see it splits very easily. so I guess how the load is being carried is important. Pine is a weak wood but it doesnt' usually crack so easily when you drive nails through it. some species like cedar may split more easily.

I asked a carpenter what I needed to build a scaffold and he said spruce pine or fir #2 grade or better. my question was can I build a cedar scaffold because I could then use the wood on my fence later, but the answer was no I can't use cedar for this.. I guess it's considered too unpredictable for that application. Of course with a huge safety factor I would think it would be possible, it just wouldn't meet code.. so Ill use what the code wants in case an inspector should come checking.. or in case there is an accident.. the moisture content also comes into play as wood is more springy if it isn't dried right out. so maybe the dry wood could fail suddenly. If you cut most of the way through a wet tree bough and try to break it you'll find the living wood to be more elastic.

a cabinetmaker making a table with long slender legs may carefully select his wood and he might look for a knotty area for it's strength, maybe if the leg had a "knee" a furniture maker may play with that perhaps he'd want to incorporate the grin direction into his product. the bent knee might be near a knot where he wants this sort of strength. knowing what to expect if he were to make a table with long thin elegant legs. so these strengths aren't just used in carpentry but a cabinetmaker would depend on experience and not so much on tables showing the span for various species and things like that.

I think if you were to use a long fir board with even grain patterns it would indicate the wood was cut from a long trunk without a lot of lower branches. wood like that is more predictable than if the same plank were cut from a knotty tree with a lot of internal stress. the knotty board might be heavier so wood density isn't everything.

In reality if I pick a pile I'm looking at how bent boards are, the bent ones aren't as predictable and you'll have more waste. Ill also look at the ends of each piece. the ones with rings that go more or less flat across the board looking from the end indicate it came from nearer the core so they are sort of like edge grain.. You don't usually see so much quarter sawn but you may notice the end grain running at angles to the board surface. the ones in the pile that were cut from the edges of the log will show a cirve in the grain pattern (looking at the end of the board and observe the endgrain. these boards with a curve in the endgrain will cup.. the way to remember which way they will cup is if you imagine that the growth rings will try to straighten out.. that might not be actually quite true but it helps me remember which way to place a board. if it cups down that's not as bad but when the cup is up then the board rocks and pulls its nails due to the motion. if I'm at the lumberyard and I need 3 boards from the stack Ill try to get the ones from the middle that show a more concentric edge grain grain pattern.. these ones will stay straighter.. the ones with the curvature in the endgrain have more internal stress causing them to bend and twist more as they dry. Of course you won;t find a perfect one ever but it's nice to select the best for where it shows or where straightness is important so you might for example save the straighter ones for near a door and use the twisty ones for studs that won't influence the squareness, fit and strength so much. at the end of the day it is usually about working with what you have to it's greatest advantage. one thing I found about looking at a lumber pile from the end is that you can see where the good ones are before you start shifting the pile and many others would not notice this at all , they just eyeball for the straight ones and reject cracked ones etc. once the banding is taken off and the pile released, it will start moving. If it normalizes then the boards look much less straight and the lumberyard wants to sell them all.. but if you notice a pile is not straight and it hasn't been shifted much sometimes its because the boards are becoming more conditioned to the environment and moving they way they want to go. th ones that want to bend the most will be the first to show the bend.. unless the middle of the stack just didnt' see air yet and so it didnt' give off moisture.

the bend is very important if you are cabinetmaking. in order to straighten the normalized but now bent board, material has to be lost. you can't really straighten a board easily with any other means or it will find it's way back to where it wants to be. sometimes you can lay boards out on your lawn and let the sun hot one side and the damp ground hit the other and this will at least temporarily help you straighten them at least until you nail them in place.
If the board needs to be long this can be a signicicant amount of the final thickness. If he needs short boards and if he can cut them to rough length then the bent board can be straightened with a lot less loss of material. you can pretty much eyeball any board and see how far it's bent and see how much you'd need to remove to straighten it..

I'm passing some of this along because it took me considerable time to see what is actually happening. I don't think this is all immediately apparent if no one shows you, and my apologies if I'm rambling and repeating myself. maybe I am just a little fascinated with trying to really understand it.

I usually try to reject boards that are noticeably lighter. I'm not sure what is going on there sometimes =but you find the odd one that is like half the weight.. why ? open grain? maybe dry rot? anyway I leave those ones alone if I can as I don't trust them as much..I think if they are unusually light as compared to the others there is a reason why.. even though they don't always look that unusual or excessively crooked by comparison.