Side wall bracing

[gitnoob]

Originally Posted by Howard Klepper

Don't butt your side braces to the top and back. It doesn't have the problem of creating a stress riser like you get when butting them to the liners. But they will tend to telegraph through to the top when the side shrinks in lower humidity, and in extreme cases can make the top or back separate from the sides. If you have already installed them this way, taper them down to near nothing at their ends.

Wouldn't the tail block have the same problem? In general, all cross-grain bracing is going to cause some stress when the humidity changes.

[KingCavalier]

Originally Posted by KingCavalier

Howard, can you tell me more about this. I might have to change what I just did.

Okay, I just spent the last hour reading about the dreaded "stress riser"
There's tons of technical info. available describing various scenario's that lead to a point of failure.

All that just to describe why a butt joint isn't the best idea. Okay, I'm not an engineer just a carpenter but I think I get the idea. Better to mortise/inlet or scarf join. Looking at the braces I went with I'm calling them scarf joined.

I didn't know about the term stress riser but I do know enough not to butt join anything made from wood.

SA

[gitnoob]

Originally Posted by KingCavalier

I didn't know about the term stress riser but I do know enough not to butt join anything made from wood.

That's why you usually see the brace tapered to nothing at the joint. I think it's mostly an issue for cross-grain braces, but people seem to taper even for the braces mostly parallel to the grain (like the sound hole reinforcement).

[John Arnold]

All that just to describe why a butt joint isn't the best idea.

It's not the butt joint per se, it is the fact that the joint line is parallel with the grain in the sides. The edge of the kerfing is a stress riser...that is why the best kerfing is tapered to minimum thickness on the inside edge.

[Howard Klepper]

Originally Posted by gitnoob

Wouldn't the tail block have the same problem? In general, all cross-grain bracing is going to cause some stress when the humidity changes.

Tail block grain is running in the same direction as the grain of the sides (or should be--I don't get the recent trend to using plywood). It's running them 90º top that and butting the end grain up against the top- and back that can cause problems because wood hardly moves at all along its length. This is a separate issue from the stress riser when you butt the side brace to the liner. Elsewhere braces get tapered because it keeps them from separating at their ends if there is a blow to the top or back, as well as to avoid stress risers where the brace runs across the panel (and also to give more flexibility around the edge for tonal reasons). The head and tail blocks are short enough in proportion to the width of the top and back that they don't introduce enough stress on their sides to be a problem.

A lot of builders do butt the side braces to the liners and seem to get away with it--most of the time. If they are already in (Pickinbob), I'd suggest tapering them down at their ends.

Cavalier: hey, cool looking side braces! I'm wondering if lapping them over the liners the way you did will avoid the stress riser issue. If you have a close fitting joint where they overlap, I think you might avoid the problem. I'm not sure.

[John Arnold]

A lot of builders do butt the side braces to the liners and seem to get away with it--most of the time.

My primary business is repair, and I have seen too many cracked sides to count. If the side braces are butted to the kerfing, the cracks alomost always occur there.
I have lapped the braces over the kerfing like KingCavalier, but I ran the braces much further. I made them the same thickness as the kerfing, and stopped them just short of the top or back. I use triangular kerfing, so the lapped braces automatically taper to nothing at the ends.

[Szk]

Originally Posted by Howard Klepper

Tail block grain is running in the same direction as the grain of the sides (or should be--I don't get the recent trend to using plywood). It's running them 90º top that and butting the end grain up against the top- and back that can cause problems because wood hardly moves at all along its length.

So Howard, I wonder if I understand correctly. Do you suggest the tail block grain should be oriented the same as the side grain?

I have to say it's never crossed my mind to install like this. I'll definately give it some consideration. My tail and neck blocks have been installed grain running up/down.

[KingCavalier]

Thanks guys, I'm going to leave this one the way it is but going forward I think I'll make a change. I might even start to tuck my tone bars and finger braces as well.

[Ben-Had]

OK, so now I'm getting a little concerned. I was lead to believe in my training that beveling the ends of my side reinforcement strips where they meet the kerfing reduced any stress riser. Is that not correct and if not what is the consensus for the best side reinforcement strips configuration?

[jeff crisp]

Originally Posted by Ben-Had

OK, so now I'm getting a little concerned. I was lead to believe in my training that beveling the ends of my side reinforcement strips where they meet the kerfing reduced any stress riser. Is that not correct and if not what is the consensus for the best side reinforcement strips configuration?

Ben take this with a grain of salt if you like because I know there are others here with more insight and knowledge on this subject but I think there are alot of ifs along the way here. Species of timber - is it prone to splitting/cracking and/or does it suffer a lot from expanding and contracting across the grain when subjected to different levels of humidity? Was it properly seasoned timber to begin with? Was it built with the relative humidity being observed or in a controlled environment? Is it for sale and if it is where will it be going and will they know how to look after it? Tapering the ends as you have will probably help stop them from popping off under compression though a shallower taper would help more and I think its better then a butt join that can potentially separate and may lead to an unwanted buzz. I myself have been aware for sometime now about this problem and have not yet done anything to resolve it with my own builds though I would like to. I use solid bindings and that adds an extra degree of difficulty to the problem...I think. Though if anybodys got any ideas I love to hear them. Many boutique guitars have been built like this and I would suspect that most of the problems that arise with not inletting side braces into the linings are from more than this one reason. Though as I said at the start, take this with a grain of salt because I may well be wrong. Hopefully we will both find out all the facts.

Jeff.