Value of chair corner braces?

[Jay Aubuchon]

How important are the corner braces typically seen under the seat of a chair? Do they contribute much strength?

[Mike Henderson]

How important are the corner braces typically seen under the seat of a chair? Do they contribute much strength?

I think they do. But perhaps a more important role is that they hold the chair together when the joint fails. You'll see chairs that you can rock the back back-and-forth, and people are still using the chairs. That's because the joints failed but the corner blocks (which are screwed and glued) are holding things together. So think of them as safety devices.

If they didn't provide value, the commercial chairs would have eliminated them for cost reasons - but even on really cheap chairs you'll generally see corner blocks.

Mike

[Derek Cohen]

Glue may fail (owing to movement, which is high in chairs), but the corner braces will keep the joint together ... simply because it wedges the chair leg joints in and will not allow them to release.

Regards from Perth

Derek

[Mike Henderson]

I'll make a follow up to Derek's comment about glue failure. I've repaired a lot of chairs and the failure mechanism that I usually see is wood failure, not glue failure. That is, the glue is still good and is stronger than the wood but the wood that the glue is attached to fails.

The reason for this is generally insufficient glued surface area and is often because dowels are used to hold the chair together. Many chairs have two dowels in the rear joint between the chair seat and the back and that just does not provide sufficient glue surface area. A tenon, even a loose tenon, provides greater glue surface area in the same space as two dowels and will give you a stronger joint.

This is not to say that a tenon joint will never fail - if it does, it fails the same way, as a wood failure and not as a glue failure. Modern glues - back to UF glues - are very strong and long lasting - but the joint can be stressed beyond the wood's limit.

So if you had a doweled chair and wanted to glue fresh dowels in the joint, you'll have to use larger dowels because the old dowel holes will be larger due to the wood failure - you'll see wood attached to the old dowel. But put a loose tenon in - you'll have a stronger and longer lasting chair.

Mike

[Tom Bender]

I think Mike will agree that the most common failure of side chairs (simple chairs without arms) is in the seat to back joint. Chairs are not stools. People lean back in chairs which puts great strain on this joint. I think it is unfair to blame the sitter, the designer should build a stronger joint and corner blocks and tenons are part of the solution. A little more wood in this area would be most helpful.

[Wayne Lomman]

Thanks to traditions originating in centuries past, chairs actually evolved as more elaborate versions of stools. The back was for decoration, not for leaning against. Today we are much more relaxed but the traditional chair is largely unchanged and suffers accordingly.

The glue blocks provide additional glued surface area for each joint. This braces the joint and extends its life. Any metal fasteners are meant to hold the block in place while the glue dries and not much more. Cheers

[Carl Beckett]

I always thought the older chairs loosened up due to wood shrinkage. So assumed no amount of surface area was ever really enough. (perhaps false assumption!)

Certainly the seat to back joint takes a lot of stress, and with the seat front acting as a lever with full weight of the person cranking on it when they lean back.

The last chairs I made I went all the way through with this joint. Then pinned it. Really dont know how it will do 50 yrs from now, but is about as strong a design as I could think of. I do see a lot of chairs made with a couple dowels for the joint, which seems inadequate imo. I did not use corner blocks.

On another chair I had blind tenons which were not as much depth/surface area. On these I did use corner blocks. Figured if the joint loosened for whatever reason it would still hold the pieces together - but to be honest if it did loosen I would take it out of commission for fear it would fail with someone sitting on it and cause an injury. So I added the corner block, but really cannot justify 'why' from a technical perspective. They might help prevent onset of movement by holding everything a little tighter. Once the joint starts moving, it is only a matter of time before it comes apart.

[lowell holmes]

I would not be adverse to reinforcing chair joints by any means, dowel peg or wood screw, if the chair is failing.
I have done it some years ago.

[Mel Fulks]

I think it is important to blame the sitter if it is a fine chair. But if a guest looks like they just left a mining camp you have to entertain them in a room furnished with cheaper stuff. Stretchers add strength but the very finest Hepplewhite chairs
usually don't have stretchers.

[Jay Aubuchon]

"Thanks for your perspectives! This came up because I have been repairing an old friend's old chair. This photo shows the condition when I picked it up.

IMG_1782.jpg

Most of the joints had failed, two corner braces were missing, and the other two were barely attached by small nails. The braces never had screws running diagonally into the legs, as seen in the example below. Isn't it that screw that "wedges the chair leg joints in," as Derek puts it?

IMG_1829.jpg

Yes, my friend was still using this chair.

[Gary Ragatz]

IMG_1782.jpg

Most of the joints had failed, two corner braces were missing, and the other two were barely attached by small nails. The braces never had screws running diagonally into the legs, as seen in the example below. Isn't it that screw that "wedges the chair leg joints in," as Derek puts it?

Even without the screw into the leg, the corner braces would still help to keep the seat frame from racking (which would also help protect the seat frame-leg joints). This would be particularly important if the chair didn't have a solid wood seat.

[Mike Henderson]

"Thanks for your perspectives! This came up because I have been repairing an old friend's old chair. This photo shows the condition when I picked it up.

IMG_1782.jpg

Most of the joints had failed, two corner braces were missing, and the other two were barely attached by small nails. The braces never had screws running diagonally into the legs, as seen in the example below. Isn't it that screw that "wedges the chair leg joints in," as Derek puts it?

IMG_1829.jpg

Yes, my friend was still using this chair.

When repairing a chair you really need to disassemble it, fix the joints (maybe put loose tenons in the back) and re-glue. The good thing about that chair is you won't have too much trouble getting it apart

Mike

[Jay Aubuchon]

When repairing a chair you really need to disassemble it, fix the joints (maybe put loose tenons in the back) and re-glue. The good thing about that chair is you won't have too much trouble getting it apart

Mike

Thanks, Mike. I've already done that. It was quite the job removing remnants of previous attempted repairs by others. The last step would be to add the braces, and I was wondering whether they're worthwhile. The feedback here indicates that they are.

[Brian Holcombe]

They keep the apron from racking, if the aprons are thin. Chairs with components designed to aid against racking don’t need them.

[Mel Fulks]

Chairs carved by Samuel McIntire are widely regarded as "very good". Entertaining today is much less formal . People visit
for the super bowl and stay all day . And declare "dibs" on your only BarcaLounger. Formal furniture is a different deal.