How useful is the CF under the brace?

[seeaxe]

Going through my composite section spreadsheet looking at bracing sizes to get my EI value for the next classical guitar down to a better value, I was struck by the fact that the CF between the brace and the top is so close to the neutral axis as to have very little, I would say insignificant, effect on the total EI. For a brace design of 2 x 7mm high primaries, 2 x 4 mm high secondaries each 4mm wide on a 2.1mm top, these tow strands contribute less than 0.2% of the EI.

In this location, will they provide any effective resistance to long term creep of the top?

[North]

One single carbon fiber tow doesn't do much, regardless of which side of the brace you glue it along. What is important is the combination of the two rows, one of each side of the brace.

[North]

What is important is the combination of the two rows, one of each side of the brace.

What is important is the combination of the two tows, one of each side of the brace.

Sorry for typo.

[Steve.Toscano]

I have 2x falcate braced steel string OMs in my workshop where there is no CF under the falcate braces, but CF on top only. Braces were glued to soundboard with titebond. Everything else on these guitars is the same.
After 2 years there is no discernible difference on these instruments compared to ones with CF tow on both sides.
However too small a sample size to know for sure.
I still put CF on both sides

[kiwigeo]

Isn't there an analogy with an I girder?

I have 2x falcate braced steel string OMs in my workshop where there is no CF under the falcate braces, but CF on top only. Braces were glued to soundboard with titebond. Everything else on these guitars is the same.
After 2 years there is no discernible difference on these instruments compared to ones with CF tow on both sides.
However too small a sample size to know for sure.
I still put CF on both sides

[seeaxe]

Isn't there an analogy with an I girder?

I have 2x falcate braced steel string OMs in my workshop where there is no CF under the falcate braces, but CF on top only. Braces were glued to soundboard with titebond. Everything else on these guitars is the same.
After 2 years there is no discernible difference on these instruments compared to ones with CF tow on both sides.
However too small a sample size to know for sure.
I still put CF on both sides

Martin, If we were building I girders there would be but we are not, we are building Tee beams. One flange of the "girder" is massive - the soundboard and the other flange, well there isn't one, just the brace with the CF on the end. This geometry drags the neutral axis from the centre of the brace (where it would be if it was a regular I beam) right up to the soundboard, or very close to it. By definition, there is no bending stress at the neutral axis and it increases linearly from the axis to the extreme fibre in both directions. Creep is a strain driven by stress. There's lots of stress at the end of the brace as its furthest from the neutral axis so the CF there is doing the job, but there's next to no stress and therefore next to no strain where the inner strand is. So its not going to prevent creep. I haven't done the numbers but the tensile stresses in the top of soundboard behind the bridge (where its trying to belly upwards under string tension) should be pretty low. Hence I would have thought our main creep concern is the edge of the brace furthest from the soundboard.

Steve, I'm not surprised there's no difference but given the time and expertise that has gone into Trevor's design, I'm guessing there must be a reason for it and I'm missing something.

[Steve.Toscano]

Steve, I'm not surprised there's no difference but given the time and expertise that has gone into Trevor's design, I'm guessing there must be a reason for it and I'm missing something.

I wholeheartedly agree, hence why i still put the CF on both sides

[kiwigeo]

Martin, If we were building I girders there would be but we are not, we are building Tee beams. One flange of the "girder" is massive - the soundboard and the other flange, well there isn't one, just the brace with the CF on the end. This geometry drags the neutral axis from the centre of the brace (where it would be if it was a regular I beam) right up to the soundboard, or very close to it. By definition, there is no bending stress at the neutral axis and it increases linearly from the axis to the extreme fibre in both directions. Creep is a strain driven by stress. There's lots of stress at the end of the brace as its furthest from the neutral axis so the CF there is doing the job, but there's next to no stress and therefore next to no strain where the inner strand is. So its not going to prevent creep. I haven't done the numbers but the tensile stresses in the top of soundboard behind the bridge (where its trying to belly upwards under string tension) should be pretty low. Hence I would have thought our main creep concern is the edge of the brace furthest from the soundboard.

I haven't looked at The Books for some time but I remember an I beam analogy being discussed by Trevor. My understanding is that the CF's main function is bolstering up the stiffness of a brace while keeping the brace light.....the mission statement as far as bracing goes. My knowledge of girder engineering is limited but I believe an I girder would be stiffer than a T beam (eg a lintel).

[seeaxe]

Martin, you are probably thinking of section 4.4.6.1, where Trevor discusses the efficacy of putting the CF at the top and bottom of the brace - like an I beam. To quote the opening paragraph, the stiff material should be placed as far from the neutral axis as possible... and uses the example of reinforced concrete, the reinforcing steel being the stiff material. All of which I understand.

4.4.6 which precedes that talks in general about CF reinforced braces, but seems to talk about them in isolation, rather than as part of a composite soundboard and bracing structure. My point is that once its glued onto the sound board the CF in the joint is very close to the neutral axis, so not doing much. Fig 4.4-19 shows this pretty well.

For a classical with much smaller braces, the neutral axis is even closer the soundboard brace joint.

I guess one answer is if I don't think its doing much good I can leave it out and see what happens

Moving on.....

[Trevor Gore]

One of the main reasons for adding CF to the structure is to control cold creep, so it allows you to remove a lot of wood that would otherwise have to be there purely to keep the stress levels down so there is less cold creep. Over time, as wood creeps, only having one layer of CF will likely come back to haunt you. I'm sure you can figure out how the neutral axis will move in that time.

My original falcate prototype is now over 15 years old, has been at full string tension all that time and shows no signs of needing a neck reset. The bracing sizing is as per the book, i.e tiny compared to typical X bracing. I doubt it would be in such good shape with just one layer of CF, but feel free to leave out the middle layer to see what happens. Will probably take 5-10 years to find out though!

[seeaxe]

Thanks Trevor, much appreciated.

[Andos]

It has been a while since I've read the books, but I thought the purpose of the CF was to prevent the static string forces for collapsing the guitar.
CF only has tensile strength, no compressive strength. So if you only put CF on one side of the the brace you only add stifness in one direction.
In the I beam analogy, you move the bending axis toward the CF under tension
And the T beam, when the top vibrates down the bending axis is toward the CF, and on the up stroke it will be toward the top plate.
Might be worth trying a wee experiment.
Make up 4 braces glued to say a 40mm wide off cut of sound board. 2 as per the book and 2 with a single strip of CF. Put them in a bending jig under the same load, one top up and one top down and measure the deflection. Then load them up til failure and which face fails.
I'd be interested in the results.

[Trevor Gore]

...CF only has tensile strength, no compressive strength...

I think you need to re-check the theory of composite structures... Hint: the CF is held in a matrix...

[KreherGuitars]

This was a very interesting topic to read. I hope to build my first falcate some day in the future and it’s very good to know that under the bridge plate is “proper” but that people have had much with over it as Well. It sounds like alot less of a headache going over to be honest. I know I’m gonna make a big ol’ mess with the epoxy

[seeaxe]

I havent built any falcate steel strings but if i did, I would put the tow under the bridge plate not over it.

What Trevor pointed out if I understood him correctly is that while the tow on the sound board is very close to the neutral axis at first, the effect of long term creep will move ghe N.A. further away into the brace and so the tow will be under tension where any upward deflection of the soundboard happens, typically just behind the bridge. If you put a kink in the tow right there, then when the tension arises it will try to straighten out and wont be as effective as a straight tow.

For me the analogy is a reinforced concrete beam, the rebar must be straight. The only time you allow a crank ( kink) is when you lap the bars and you make sure those laps are staggered and not at the location of maximum stress.

Happily for me falcate classicals dont have bridge plates so problem doesnt occur.

[WilliamDavidReynolds]

I havent built any falcate steel strings but if i did, I would put the tow under the bridge plate not over it.

What Trevor pointed out if I understood him correctly is that while the tow on the sound board is very close to the neutral axis at first, the effect of long term creep will move ghe N.A. further away into the brace and so the tow will be under tension where any upward deflection of the soundboard happens, typically just behind the bridge. If you put a kink in the tow right there, then when the tension arises it will try to straighten out and wont be as effective as a straight tow.

For me the analogy is a reinforced concrete beam, the rebar must be straight. The only time you allow a crank ( kink) is when you lap the bars and you make sure those laps are staggered and not at the location of maximum stress.

Happily for me falcate classicals dont have bridge plates so problem doesnt occur.

Concrete shear stresses are curves and 45 degrees at the point of support. The curve deflection shear is from the top of the beam or slab at the support and curves to bottom roughly around a 1/4-1/3 of the span. Punch shear is inside the 45 degrees running upwards from the bottom of beam or slab at support.

The crank bar in a beam is located at a 1/4 span plus the depth and usually the beam design places this at a 1/3 span, in slabs the crank bars vary based on loading which I’ve never calculated but probably can now after practicing the equations in the Gore/Gilet book.
I’ve build some psycho suspense concrete and the stress of building guitars is a breeze, but I endeavor guitar building which same life and death approach in my quality control. But I’ve never worried above a guitar killing people either!! Haha so it’s refreshing beyond belief for me.