Another woodworker recently explained that a round piece of stock is stronger than a square piece of stock (square as in width and thickness are equal), when the diameter of the round stock equals the width and thickness of the square stock (hope that explanation is clear). Intuitively, I guess that makes sense as I think about a square baseball bat and how it would shatter pretty quickly. But, I can’t seem to understand the physics of it – I imagine it has something to do with wood fibers and quartersawn vs. flatsawn. It came up in the context of a batten for a door. He learned that a batten should be a) sliding dovetail and b) tapered to a trapezoid where the base (greater width) rests against the back face of the door. Is this at all clear? I was just hoping someone out there could help me understand this a little better….thanks…
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Replies
It's not a fair analogy- the baseball bat. The roundness of a bat deflects impacts not centered. The same impact exerted to a square would not be as likely to break the wood.
The square stock would be stronger. More is better. Removing stock weakens a piece of wood.
Lee
Edited 4/14/2006 7:01 pm ET by LeeGrindinger
That's what I would think - but apparently, he learned it (also in disbelief) from an older woodworker who had learned it during his apprenticeship. I'm talking about the tapered batten now (the baseball bat thing was just a side note reall) - anyway, apparently he tested it and it was true...
PF, you've been taken by an old wives tale.Test it your self in your mind. How could removing mass make wood stronger? All the tissues lending strength to a dowel are in a square piece. In addition to these tissues is the added strength of the corners absent in a dowel.If you do reach a different conclusion the world science is eagerly awaiting your paper on how removal of mass can strengthen wood.Lee
This entire discussion reads like a Woody Allen comedy bit. If you really want the correct answer, talk to a mechanical engineer, not a woodworker.
Todd
Well, just leave some spare change in the tip jar...,Lee
Well, we are looking for a scientific explanation-can you offer one?Philip Marcou
hey Mr. Lee ,
Not that I am agreeing or disagreeing BUT you asked " how could removing mass make wood stronger " playing my role as the devil's advocate humor me/us and think about this example .
As a student and lover and maker of wood furniture I studied while apprenticing many different chair designs . Now I know a straight stick or dowel is different then a finished chair BUT , what has been learned and proven and I will use Thonet's bent wood cafe chair as an example opposed to say a Mission style chair . The heavier and bulkier the weaker especially when dropped off the roof of a building as was the test .
In theory the extra weight of the piece may actually work against itself especially in a chair . So we know a 1/2" dowel is not as strong as a 1" but in certain applications more is not always stronger .
Sorry couldn't resist the urge to murky the water a bit .
all in fun regards dusty
It seemed that when I was a young man frequenting cafes and bars Thonet's designs were everywhere. They seem have diminished a lot. The failure in those pieces were where the round stock had to be joined to the caned frames, they usually failed there unable to resist the racking, not in the bent wood structures that made up a lot of the piece.Actually, a Windsor or Welsh design would support your argument better, it boils down to simply transferring stresses from sheer to tensile at which wood is stronger than steel pound for pound. Hmm, this used to be true, perhaps the newest alloys have changed this.As to Mission, the failures that occur when you toss a piece off a roof of a building have little to do with the shape of the members as much as how they land. Done carefully I could balance a helicopter on a 3/4" dowel if it was short enough.Lee
Interesting argument... but the chair analogy doesn't hold.
First we have to look at the type of forces you are talking about. Let's use your chair analogy. The force involved in throwing a chair off a roof is very 'impulsive', it's a shock. I don't think that is necessarily the force we are talking about when we say strength. In opposition to this, say he have a 500lb man who sits gently on each chair. Chances are the mission chair with it's big heavy frame will last a little longer then a dainty bent wood.
Second it's doubtful that the big 'beams' of the mission chair break when dropped. Most likely the joinery fails or the smaller parts break, there are the weak points of the chair. This makes sense, the smaller chair will deform when it hits the ground the deformation will take the stress away from the joints.
Finally, you're comparing two amounts of forces. If we go by Newtons second law (? or is it 1 or 3), F=ma. Acceleration is a constant here, gravity. So the force is much greater for the heaver chair. It would be unfair to say that the lighter chair is stronger.
In the end there is a simple experiment. Take the two chairs (if you had them), and add weight slowly 25lbs at a time. See which design will hold more weight. My bet is that the bent chair will start to deform well before the mission, and at a certain point will break.
Anyway, I speak from experience. I have a friend that has managed to break almost every piece of furniture in his house except for a mission style couch...
Buster
If square is stronger, then why were the masts of sailing ships aways round?
If more is stronger, then why is a square box standing on a corner less strong than an egg shape?
Just wondering. :-)
A bad day woodworking is better than a good day working -- yes, I'm retired!
>>If square is stronger, then why were the masts of sailing ships aways round? <<Do the trees grow square where you come from?
Hilmar
Good one Hilmar, took a minute or so before I stopped laughing. Before I read your post I was trying to figure out a good reason why a mast would be round instead of square. This also solved the long time wondering why telephone poles aren't square.
mike
Can you imagine the size of the lathe they must have used? And I'm assuming it must have been a treadle-run lathe, at that.
No lathes. Cut down a tall old growth tree. Take off the sapwood with broad axe, adz, and planes. Blacksmiths made the necessary wrought iron fittings. After the mast was raised riggers added the rope stays (tarred hemp) that braced and held it in place. All high tech, state of the art work (for the time at least).
The lathe comment was tounge-in-cheek.
Dang! That's the problem with the written word. Unless we're literary masters it doesn't show the nuances of expression, body language, or tone of voice. But at this time of year I probably would've mistaken your expression as being Easter-egg-in-cheek mumbling. ;-)
Clarification: chocolate Easter-egg-in-cheek mumbling. Always make sure you have enough for the kids AND you.
(next is the chocolate bunny)
No! Grownups have brown egg hunts! Beer bottles in the garden. Find one and down it before moving on to the next.Warning. Not to be held concurrently with the kid's hunt. Bad influence.
Must remember this and plan to take off Easter Monday next year.Leon
When all else fails, try replacing conjecture with experiment. Take a piece of square stock, cut it in half, then turn one of the halves into a cylinder. Support each one at the ends and start to load it in the center and see which breaks first. Let us know the results.
I'm not sure why I've been recalled to comment in this thread, as I thought my diversion into "Brown Egg Hunting" was about as enlightening as I could be. How ever before I go out to buy a lathe (A treadle perhaps?), I'll just check to make sure that we're embarking on the same test.From your post. "Take a piece of square stock, cut it in half, then turn one of the halves into a cylinder. Support each one at the ends and start to load it in the center and see which breaks first."You want me to take a square section of wood, we can decide on the species later. I'll use a 10" x 10" (100 square inches of cross section) for easy calculating. When I cut it in half I will end up with (let's leave out the saw kerf losses) 2 pieces that are 10" x 5" (50 square inches of cross section each). 1 of which I am to put in my newly purchased lathe for this test, and turn it round into a cylinder 5" in diameter (19.6349540849 square inches of cross section) with I assume a surface finish to match the unturned one to minimize that variable. Then take it to a testing facility (to ensure impartiality) and have the two pieces of wood tested for their bending properties. So we can compare a rectangular section of 50 square inches to a cylindrical section of 19.6349540849 square inches.Ok. I've got it. No! Wait! Do you want the 10" x 5" tested in the vertical 10" x 5" position, or in the horizontal 5" x 10" position?Please let me know if this is what you had in mind, as my qualifications are in structural aircraft part inspection and not in armchair engineering.
I admit that the poster could have been a little clearer... But I think you've gone the wrong way. Unless you were trying to be funny...
He say take a piece of square stock and cut it in half... I assume he meant in length, not rip it in half. (i.e. a 1" by 1" x 4' long, being cut into two 1" x 1" x 2' lengths). Then turn one into a 1" dowel. (All this assumes no loss from cutting).
Assuming the stock is clear of knots, and you align the grain of the two pieces the same you could run a simple test.
***
Given this the square stock would have a cross sectional area of 1 square inch, and the dowel would have a cross sectional area of 0.79 square inches.
Unless you have some invested interest in seeing one result over the other, feel free to do this in your own garage. I'll believe you.
I am not a materials engineer, so take the following with a grain of salt. <!----><!----><!---->
I believe that a cylindrical piece of material will be stronger than a rectangular piece of material of equal weight (assuming the same height). If you take a 4 inch square 2 foot long rectangular piece of wood, it will be stronger than a 4 inch diameter 2 foot long piece. Of course, the cylindrical piece will weigh less (since you removed the 'corners'). If you increased the diameter such that it weighed the same as the 4x4 square piece, I believe the cylindrical piece will be stronger (we'll leave stronger as a vaguely ill-defined notion for now).<!----><!---->
I do not think you can add strength by taking away from an existing structure (ignoring stress riser relief, etc). You can add strength by rearranging mass (i.e. making a bar of steel into an I beam shape), but not by merely cutting away mass without ‘sticking it’ back on somewhere else. That is why a torsion box is strong for its weight; it is in an 'efficient' shape.
In my response to Mtn__Bob I was being sarcastic and funny (apparently unsuccessfully). With me you have to look at my posts firstly as light hearted then as serious, and never as malicious. I believe Mtn__Bob had only been responding to the thread in general and had clicked on one of my posts to enter in the discussion or when in and tried to pick the "ALL" choice and got me without realizing it. Poor guy. I didn't mean any harm to him and in a long winded way was pointing out that there were two ways to interpret his test. The original poster wanted to understand how a round section was stronger than a square section of the same cross sectional area. It didn't take to long before the thread became unraveled with many of the posts bringing up different aspects or conditions (but not clearly) of determining the strength of wood.( Tensile, compression, bending, impact/shock resistance, twisting, solid verses hollow, forms and shapes. And we didn't even get into moisture content, air verses kiln dried, natural verses reconstituted or temperature! ) There isn't a simple answer to that question without factoring in all the variables. Since we don't have too many, if any, wood technologists / engineers on the forum there were no definitive answers. We did have lots of vaguely recalled bits from a class taken years ago mixed with something read in an article or two coming out as "facts". There was to my minds eye a bit of absurdity in all of this and that was why my post manifested itself in the fashion that it did as I typed it. If PF is still following this thread and still wants an answer to the round / square question I suggest getting your hands on some books or if someone has a link to the information, be kind enough to post it.This book is one that should be in everyone's wood library and will start to answer the big question.http://www.leevalley.com/wood/page.aspx?c=2&p=20150&cat=1,46096,46127&ap=1And this book will add more to your knowledge base if you can follow it all.http://www.leevalley.com/wood/page.aspx?c=2&p=45997&cat=1,46096,46127&ap=1If you want more than these two offer I think it's off to college with you. ;-)
I was pretty upset with your post before I realized who the poster was. Isn't there some way to say it without having to say 'just joking or ....' That ruins the whole saying. I've tried to be funny and it just doesn't go over very well. Appreciate your posts.
Hi QCInspector ,
Very well said . We all can get carried away from time to time and sometimes miss the point or travel off the topic .
Oh , today I dropped my neighbors round mast off off my shop roof and an old 6X6 I had laying A ROUND , guess what happened ??????
NOT
dusty
Since I don't have the knack for joking with people here without over stepping into offending now and then. I'll drop back to lurking, unless I have a contribution that's completely factual and on topic.
Inspector, just keep throwin' 'em out there. As far as I know, nobody in here has been shot or arrested for their posts, yet. Be yourself. Let's all have some fun, and learn in the process.
Before I answer, which way was the wind blowing? And, what was the time of day?
Sarcasm can easily be lost in the post. But I did detect a hint... The only reason I'm reading this topic is to actually find out why the mast of a sailboat is round!
Buster ,
Perhaps masts are round for the same reasons that boats are not square .
dusty
"The only reason I'm reading this topic is to actually find out why the mast of a sailboat is round!"Masts are not all round. There are square masts built of wood both in solid (google Thunderbird or T-bird sailboats and you'll probably get an example)) and hollow. There was a mast builder in the shop next to my father's business that only made hollow built up square wooden ones, many in 2 parts to fit on the Semi's that delivered them. In addition to round and square, there are rectangular, oval, "D" shaped, teardrop, and moving all the way to airfoil shaped wing sails.The reasons for round being the first types to evolve were that it was easiest to get a tree the right size, (and as an earlier poster asked "Do the trees grow square where you come from?") and put it on the boat.Another has to do with holding the sail to the mast before the advent of metal sail tracks. The rope lashings will allow the sail to move into the different positions needed without binding and minimal chaffing. Larger boats had hoops made of wood to take the forces in addition to wear and tear. The Booms along the bottom of the sail and the Gaff at the top were often forked to fit around the mast with a rope over the open end to keep them on. They don't spin to well on anything other than a round mast as stated in the 6th post. Lastly (without trying to start the thread over again). A round column resists bending from all directions the best.Hope that satisfies your curiosity, otherwise the Wooden Boat forum has more qualified people that could help.http://www.woodenboat-ubb.com/vbulletin/upload/
A round column resists bending from all directions the best.
Wait a second! Are you saying round stock is stonger than square? I once through a dowel off my roof...
Square masts really don't flow throught the breeze to smoothly.
I can think of two reasons a mast might be round. First, perhaps the cylinder is strongest at resisting bending, per pound. Sort of like a sphere holds the greatest volume per square inch of surface area. Second, the booms are swung 'round a mast, hard to do around a square mast.
I think if we're going to try to be exacting, we should say what we mean by "strongest." Do we mean, resistant to bending across a weighted span? I submit an I-beam is far stronger per pound than either a cylinder or rectangular beam; with wood, a torsion box. If we mean in an application like a mast, where bending force can be applied from any direction, I would think a coopered hollow, glued and banded, might be strongest per pound, but a simple cylinder competitive and much cheaper.
I suspect a cylinder (dowel) is the stronger than the same weight of wood in a rectangle. I agree with Lee that more generally stronger; if you had a dowel and cut it down to a square, I think it's weaker. If you had a square and cut it down to a dowel, I think it's weaker again.My goal is for my work to outlast me. Expect my joinery to get simpler as time goes by.
HMMMMM! Well stated.
A bad day woodworking is better than a good day working -- yes, I'm retired!
The analogy of the bat also fails do to the type of impact. I don't think structural engineers refer to strength as how well something holds up to impact. See chair dropping comments.
First I think when we are refering to wood, we should clarify we are looking at the same orientation of grain. That's obvious. Obviously wood is 'isotropic' in that it behaves differently to forces applied in different directions.
There is some strength gained due to shape when we refer to hollow objects. The egg is the classic example. The square and the triangle are another (the triangle is stronger). This all depends on the ditribution of forces over the shape. I can see how this may extend to solid objects. As a note I did a quick search and found a reference to cement: a cylinder with a diameter 0.8 that of an equivalent square has the same strength. It was in a forum, so take that as you will...
I can think of an example where removing a large amount of material would not weaken the wood dramatically. If you 'cut' the middle out of the board and made a torsion box...
It's all how the forces are distributed...
From your post, Buster, "I can think of an example where removing a large amount of material would not weaken the wood dramatically. If you 'cut' the middle out of the board and made a torsion box..."A torsion box is not simply hollow wood. It's comprised of multiple ribs glued on both sides to a skin. The shear strength of the glue line is where the torsion box gets it's particular type of strength. Simply hollowing out a board will weaken it considerably in every way.Lee
A torsion box is not simply hollow wood. It's comprised of multiple ribs glued on both sides to a skin. The shear strength of the glue line is where the torsion box gets it's particular type of strength. Simply hollowing out a board will weaken it considerably in every way.
Understood, my idea was cut short due to a call for dinner... Fully agree if you just hollowed out the inside it would weaken the stucture.
The thought experiment I had in mind would be to cut a layer of 'skin' from each side (essentially removing the middle). Then use the middle to create the inner ribs. Since you'd only be making a rib structure, you'd be leaving a considerable amount of wood out.
The square stock is stronger. That is certainly my belief so much so that I wouldn't give much credence to anyone who would say difference. You must say, though, that round gives a lot of efficiency and bracing effect.
It may be that the stiffness:weight ratio is what was originally being referred to, clumsily and even that will change with different wood species. Sawn wood that is shaped into a round profile is weaker than split wood that has has the high spots and corners removed, right? Objective tests for strength are easy to do. Why doesn't someone do them? Because they have! There are building material testing facilities at the U of Wisconsin- Stevens Point (or it may be Stout State), among other places and if someone looked up wood or lumber testing, it's all out there. Grain orientation makes a big difference in the amount of force needed for failure. If the grain of the square stock is perpendicular to the bending force, it will fail sooner than a dowel with the same diameter having the grain in the direction of the force. Dropping a chair off of a building? Now, THAT'S a useful test! I don't know how many times I have accidentally launched my chairs from the roof and they just don't hold up.The belief that one shape is stronger makes no difference at all. Objective testing is the only way to find out if one is going to fail sooner. If the square one is .000000000001% stronger, it's still stronger but using a dowel may look better for the particular piece being made and it won't make a difference in strength at that point. "I cut this piece four times and it's still too short."
Edited 4/18/2006 9:31 am by highfigh
The strength of a piece of wood is directly related to the orientation of the grain and the applied load. A quartersawn piece will have a different strength than flatsawn.
Here is another way to lookat this problem, a piece of wood supported on two blocks with a weight in the center has a max stress at the extreme outer fiber per this equation S = Mc/I S= stress, M= bending moment, c = distance from neutral center to extreme fiber and I is the moment of inertia. The I for a square is .0833hto the 4th power and for a round section it is .041d to the 4th power so the stress will be less for the square than for the round section assuming the square dimension is the same as the circular dia.
Depends on how you define "stronger". If you mean bending strength or stiffness, it depends on the "area moment of intertia". If I remember my engineering correctly (it's been a while), the moment of inertia of a 1" square rod is about 40% greater than that of a 1" diamater circular rod. Of course there are practical issues like grain orientation that come into play as well.
Yeah, the last sentence is what I originally thought could be the crux of the issue here. If you look at my original post, I am talking more about a tapered/trapezoidal stock vs. square (but I think round works for the sake of discussion). If you take a plainsawn square piece of stock and "cut out" the plainsawn so that you essentially have a quartersawn piece - is this new smaller quartersawn piece "stronger" than the first plainsawn piece...that's all I was wondering...
OK all,
Since square masts are strong. Since round masts are strong. I have finally figured out now why bass boats don't have masts -- just 1000 hp outboards so they can go 100 MPH to get to all those fish!
Right?
A bad day woodworking is better than a good day working -- yes, I'm retired!
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