# stacked vs solid wood for beam



## captonager (Nov 15, 2009)

I have a question about the relative strength of a stacked beam (made from multiple boards.laid on end) vs a solid beam made form a tree trunk. This beam is for the throwing arm of a catapult. This means all the load stresses are in one direction. I have had some discussions with various other catapult builders, with varying opinions. Can anyone here help me?

Thanks in advance.


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## mics_54 (Oct 28, 2008)

Are you refering to glue laminated timbers? If not, I certainly couldn't make any prediction but would think it would have many variables. 
If you are refering to glue-lam beams I would refer you to http://www.aitc-glulam.org/faq.asp


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## BHOFM (Oct 14, 2008)

In boat building we use laminated pieces for stiffness
and strength. A piece made from three or four layers
is about five times stiffer and twice as strong as a
single piece of the same wood of the same dimension.

How ever, the stiffness can cause problems as it does
not move the same as the rest of the structure.

On some of the older wooden boats the frames are
laminated and look tiny for the loads they need to 
carry.


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## GeorgeC (Jul 30, 2008)

that a laminated beam in stronger than a solid beam is an understood fact. However, what is not clear is whether or not this is good in your specific application. That may be why you are getting various answers from other catapult builders. The flexibility of the solid beam may add to the function as long as the solid beam is within the strength needed.

I would think that you are most likely going to have a problem getting an educated answer on this question.

G


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## Tony B (Jul 30, 2008)

I suggest that you get in touch with your local SCA (Society for Creative Anacronisms). They can be found on the web. They are a very large organization into history and actually living it in their own kind of way. A very unique and diverse group and incredibly knowledgeable on ancient weapons of mass destruction. They have built all sorts of ancient seige weapons such as catapults, trebuchets and ballistas.


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## captonager (Nov 15, 2009)

Thanks for all your comments. The throwing arm is not glued, but the laminated boards are held together using nylon rope at 1 foot intervals - please see the attached picture. I have been making these arms for some time as I have been competing in Punkin Chunkin for many years (see Science Channel). 

It sounds like a solid arm made from one piece of wood is more flexible, but not as strong as the laminated arm.

How do they make ships masts on large boats? are these laminated also?


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## mics_54 (Oct 28, 2008)

> It sounds like a solid arm made from one piece of wood is more flexible, but not as strong as the laminated arm.


 
..maybe


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## BHOFM (Oct 14, 2008)

Not sure if bound beams are of any advantage.
The mating surfaces would need to be very rough
to prevent movement between that layers.

Boat mast are made in several ways. Built up,
boxed, solid. Very few are just laminated. They
would be a bit stiff. The mast needs a certain
amount of flex.

The built up are normally four sides with a void
in the center, then shaped to suit. The boxed
had a frame and are covered with a skin. And
of course the solid are just wood.
Spruce is the wood of choice for the most part.

Here is the mast for my twelve footer. The mast
is just over twelve feet and appears heavy, but
it is a box and only weighs about seven pounds.










Here it is before the 3/16th skin was put on.










I know most of this has not relation to what you
are doing, I just like to talk!:laughing::laughing::laughing:

I'm gone!
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^:boat:^^/\^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^


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## captonager (Nov 15, 2009)

nice looking mast - interesting construction - light weigh but strong, makes sense since it is the outside surfaces that carry the load and do the work. 

How did you put it together? use glue? screws? or both?


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## woodnthings (Jan 24, 2009)

*I don't know nothin' about your punkin chucker*

But it seems like there are both static and dynamic loads on the beam. Obviously in a dead vertical load or a bending load a laminated beam will have greater "strength" or resistance to failure. In the case of your throwing machine, it would appear that the types of loads on a laminated beam would tend to shear the glue joints as the forces move along the bean, whip in other words. There are engineers here that should be able to give you a better feel for this. The other way is "seat of the pants" engineering which I practice. Make it, try it, if it breaks make it bigger or change it. You gotta start somewhere. You may have hit upon somthing that others have either not thought of or tried and did not work. The idea of binding layers of wood with rope or suitable cord/wire/ possibly even a graphite or polyester reinforced cloth to add to the adhesive in the laminations may prove an advantage. I don't know if flexing is an advantage or stiffness is what would work better. The solid beam of one material may flex, whereas the stiffer section of a laminate may not. I donno :blink: Just some additional thoughts on the subject from an art student.:laughing: bill


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## BHOFM (Oct 14, 2008)

captonager said:


> nice looking mast - interesting construction - light weigh but strong, makes sense since it is the outside surfaces that carry the load and do the work.
> 
> How did you put it together? use glue? screws? or both?



TB III only!


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## frankp (Oct 29, 2007)

The question, as woodnthings mentioned is more a factor of whether you're using a "flexing" arm catapult or more like a trebuchet. As far as I can tell from your picture it looks like you're using a traditional catapult. In that case, you want more flexibility and the added strength of the lamination may be a hindrance. It would be interesting to see if you made the lamination parallel to the flexing axis whether you would retain a certain amount of flexibility while still maintaining a high strength. As woodnthings mentioned, a pure static load (and mild dynamic loads) the laminated beam is the better choice, but in something that needs to essentially "spring" with a massive dynamic force, it may be better to use a single beam. I'm still inclinded to think your method of "lamination" using rope as a binding rather than a glue adhesive, may give you the best option as you can repair any broken segments more easily than if you had to glue up a whole beam, but you have less likelihood of the hidden weaknesses than you would with a single solid beam.


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## ron9876 (Jun 1, 2009)

*Glulam Beam vs. Solid Beam*

I would question that a glue laminated beam would be stronger that a solid beam assuming that there are no knots or checks in either one. The reason that design codes allow larger values for glulams is because there is a much smaller chance for knots, checks, etc. which weaken wooden beams.


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## TS3660 (Mar 4, 2008)

My two cents is that a slightly flexible beam would throw an object further. Much like a flexible golf club launches a ball further than if it were stiff. But this may have more to do with the fact that a golfer can flick his wrists at the perfect moment. A catapult can't.


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## frankp (Oct 29, 2007)

ron9876, statistically speaking, a gluelam is significantly stronger than a solid beam (even a clear beam with no defects), not just because of knots but because the individual pieces stress less and are therefore subject to less chance of breaking. Think of it as a blade of grass versus a small tree... you can bend a blade of grass in half and it will "spring back" to its original shape. A small tree will simply snap (in many cases) or will not quite return to its original shape. 

Test after test has shown this to be the case whether the individual beam is a "stronger" wood, whether it was clear or had defects, and several other factors.

Once again, this all depends upon how you choose to define "strong", which I think was the point of the original question.


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## ron9876 (Jun 1, 2009)

*Glulam Beam vs. Solid Beam*

If the section modulus and the moment of inertia are the same and if the wood is the same don't see how there can possibly be a difference.

The flexural stresses would be the same at the extreme fiber and the cross sectional area would be the same for shear stresses. 

Again the reason that the allowable stresses are higher in a gluman is because of the confidence in the quality of the wood. Gluing them together just makes them act like a solid piece.


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## captonager (Nov 15, 2009)

In my application with my catapult, a stiffer beam is perferred to one that flexes. I want to utilize the power in the torsion spring, not the spring in the flexing throwing arm.

The way I have stacked/laminated the pieces of wood is on edge, to minimize flex and maximize strength.


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## woodnthings (Jan 24, 2009)

*There are 2 ways to use a glue lam beam*

My assumption initially was with the lamination horizontal or parallel with the ground. This may allow the beam to flex. The other way is to orient the laminations vertically. This may create a stiffer beam. 
I donno, I studied Art in school, not calculus or engineering! :thumbsup: bill
BTW what is moment of inertia? I have had senior moments, possibly one of inertia, I donno.


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## frankp (Oct 29, 2007)

ron9876, it's not just about area, it's about "reinforcement" of each individual pieces. If you put board against an edge and flex it, the amount of flex changes based on how much board sticks out over the edge, right? The gluelams act as reinforcement of each piece in the lamination. As I've said, plenty of tests prove that whether the lamination is done with "good quality" boards or not, they are stronger than an individual beam.

I'll not bother any more since the original question has been answered and this is just a lesson in tactful and polite argument at this point.


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## mics_54 (Oct 28, 2008)

> Again the reason that the allowable stresses are higher in a gluman is because of the confidence in the quality of the wood. Gluing them together just makes them act like a solid piece.


Inspite of any arguement to the contrary...ron would be correct.


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## woodnthings (Jan 24, 2009)

*Piling on*

What about laminated bows? Aren't they stronger, less prone to breaking, and have more "push" than solid wood bow and if not why are they so popular? I donno? :blink: bill


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## mdlbldrmatt135 (Dec 6, 2006)

woodnthings said:


> What about laminated bows? Aren't they stronger, less prone to breaking, and have more "push" than solid wood bow and if not why are they so popular? I donno? :blink: bill


 
I think some of that has to do with keeping the wood grain..... and forming it to a new shape... (with the adhesives holdign it in that shape) to retain the strenght and flexibility of the wood used.


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## Tony B (Jul 30, 2008)

*If wood was man made...*

it's strength would be equal along the entire length of the beam. Since wood is not man made it strength varies through-out its length. So, if we take that beam, somewhere along its length there will be a weaker point caused by any number of factors including internal stresses. If we took that same beam and sliced it up and then mixed up the slices and picked up the pieces at random and reglued the boards together to make a beam, the weak points will be scattered throughout the beam at randon resulting in a stronger beam. The theory being that the original beam with the one weaker area is only as strong as that spot. Same as a chain is only as strong as it's weakest link. If we scattered the weak spot throughout the beam, we can argue that the overall strength of the entire length of the new glued up beam would average out to be higher than the the original beam's 'weakest link'.
Another thought to apply is that a good glue joint is stronger than the original wood. So why wouldn't several glue joints be stronger than the original wood. 
Another thought on stiffness......If we glue 2 boards together - flat to flat - they are in tension with eachother if something tries to bend them. Same rules apply to a torsion box or an I-Beam. If there are any doubts on this ask anyone that uses bent laminations. I used to make curved combat shields for a renaissance group. You would not believe how strong 3 layers of 1/8" plywood can be.


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## ron9876 (Jun 1, 2009)

*Stack vs laminated*

The concept of not having a fault full depth of a member is what I was trying to say before. Glue may be stronger in resisting horizontal stresses but if the natural wood is strong enough it won't really matter.

All that I can say for a fact is that the NDS, which is the structural engineering reference that governs wood (and glulam) design, assigns the same stiffness properties to solid members as it does to glue laminated members assuming they have the same width and depth.

The actual strength of an individual piece of wood is determined by the species of the wood and how many knots and checks it has in it. When the codes that determine the allowable stress of wood members are developed they have to account for the risk of faults in the wood. That is why a select structural member will typically have higher allowable stresses than a No.2 of the same species. That is also why a mechanically tested member is stronger than a visually inspected member.


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