Fitting a wind brace to an existing timber frame

[AJB Temple]

Does anyone have a jointing method (other than letting in from the face) for letting in a pair of wind braces to strengthen an existing timber frame.

Normally I cut a tenon in the brace, (the braces are usually made from 2.5" thick oak and are about 26" long, depending on how I want them) and a stopped, chamfered mortice in the post and in the beam. The brace goes in as the beam and post are mated together.

However, I want to do a repair to part of a converted barn where there are existing mortice slots in the structural timbers, but someone for reasons best known to themselves, has sawn out the angled wind braces.

I can't lift or otherwise move the frame and I would like to get braces back in that serve a modicum of structural purpose (obviously I could fake it up).

One way is to cut the mortice slots out over long by the depth of a tenon, then put a decorative wedge in to fill the gap when the brace is in. I am just wondering if there is some other solution that I have not thought of. Any ideas much appreciated.

 

[AJB Temple]

Guys - please don't tell me you are stumped! 65 views and not a single solution?

 

[meccarroll]

You may consider a loose tenon. Mortice post and tie beam as usual form a fixed tenon on one end of brace as normal (may need some tweaking prior to fitting stage) use loose tenon in the other end of brace. When you form the mortice (slot) in the end of the brace to receive the loose tenon make it open on the top edge so the tenon can slide into the slot as you push the brace into position but do not continue it right the way through to the underside. Stop the mortice slot just short of the underside of the brace so when the tenon is slid into position it is all fit together and you don't see the through tenon on the underside of the brace.

Hope you can understand.

Mark

 

[Student]

Mike G over on WH 2 is the timber framing guru; he has a whole thread on rebuilding his timber framed house. From what I remember, his technique for inserting new pieces of timber in an existing frame is to do what you suggest i.e cut an overlong mortise and then wedge it after you've inserted the new piece.

Of course, whilst writing, Mark has come up with a solution!

 

[katellwood]

something like this?

The 45 degree segment in the mortice could be filled with an infill piece glued into place prior to fitting the main brace. Then dowels through the meat of the remaining tenon

 

[AJB Temple]

Thanks. Mark, I understand what you mean but wonder whether the brace would have the strength necessary with a loose tenon at one end. Shall have to think about that. Seems like it could work.

katellwood. Thank you, clearly works as a fitting solution. I need to think about whether it works structurally. Wind force on the structure (actin longitudinally along the top beam) would, I think, tend to try to push the bottom tenon down and out, rather than further into the mortise socket. If so, then a lot of reliance is being placed on the securing dowels. That said the building is pretty rigid anyway and these braces are unlikely to take much load.

AJ

 

[PAC1]

You could get some stainless steel angle bracket and bolt the frame and braces to it. not pretty but would be functional. You could mortice the angle into the ends so it is hidden

 

[AJB Temple]

Generally people avoid mixing steel and oak. The building moves and the oak moves as the green oak dries out. I would be a little bit worried that the structural integrity of the hidden steel is compromised. It is a logical solution though and I must admit it had not occurred to me as I have been a bit fixated on the timber joints.

I think on balance, given that the original mortise holes are already in place, I will adjust them to allow the braces to fit and put a sizeable block in which I will make into an architectural feature and peg it in place. Before making a final decision I will measure up and see if the angled idea drawn above works in situ as that is the neatest.

 

[will1983]

How about making the brace with joints detailed in the traditional manner though make it in two parts.
This could then have a wedged splice joint in the centre to join the two parts together.

 

[hanser]

Just my thoughts
Hot dipped galvanised, copper and silicone bronze metal/fixings are fine with oak.

I appreciate you may wish to stick with a timber solution and have confidence in your construction. I'd cut 3 lengths of oak to the dimension of the brace. Cut a scarf joint in the middle piece and then trim/ fit to the two mortices. Cut the two outer leaves to form the cheeks and then foaming glue/clamp/peg/bolt etc. Another fixing method could be copper nails and roves. I suppose it's a variation of the 'loose tenon' proposal except that the loose tenons are the length of the brace and consequently the 'laminated' brace is a lot stronger in compression and stretch.

 

[AJB Temple]

hanser. Good idea. Thank you. I will be doing the job on Friday as I have the day off. It is a bit of an educational experience for me so this is a useful challenge. To be fair the main building was converted from a perfectly good 16th C oak timber frame in 1980 and bodged up beyond belief so anything I do is an improvement.

 

[PAC1]

AJB stainless steel is quite common in oak frames to strengthen difficult joints. Being stainless steel it does not react with the oak. http://carpenteroak.com/inspiration-and ... ase-study/

 

[Droogs]

Why not mix all that's been suggested already? Use a loose tennon but made from S/S or thick aluminium, bolted and epoxied in place and then just plug the bolt holes. That way you can use the existing mortices in the posts without modification and just make the through holes in the metal elongated to allow for movement

 

[AJB Temple]

OK! Still thinking. THis thread has helped me a lot. What I didn't say originally is I will end up with at least 24 of these to do when I run through the refurb, as every single brace has been stripped out of the building on teh ground and first floors when the (awful) plasterwork was done. This seems to have had cosmetic justification coupled with structural ignorance. THis has been addressed to a degree with some brick clad steels.

AJ

 

[katellwood]

Another suggestion which may address your concerns above. Rotate it in

However it makes the top tenon slightly smaller.

Food for thought

 

[Racers]

How about a birdsmouth cut into the beam and post then slide it in from the side and peg.

Pete

 

[PAC1]

The other option if you are only going to use timber is to extend one of the mortices by a few inches so that a full tenon can be fitted and then fill the extended mortice with a patch to make it good after you have inserted the brace.

 

[AJB Temple]

Yes. I had thought about that originally. I don't have your graphics skills so had not drawn it out but did think it should be possible to make a conventional brace bottom tenon and a curved top one (or vice versa depending on where the force seeme teh biggest risk).

 

[AJB Temple]

Thanks Pete, yes I could do that, I think I mentioned it in my original post. according to my timber framing books that technique was used a lot anyway for braces in the fifteenth century. It is obviously not as strong but is much easier to assemble. In this case it would not work aesthetically. I will probably do this job tomorrow on the two exposed areas.

Oddly enough another part of the building has a new oak frame attached, not made by me but by one of these places that advertise in the magazines. (Already here when we arrived). Some of their braces are fixed to the posts solely by drilling through the outside edge and sticking a 1" dowel through the brace and into the post (and beam) at an angle. This is structurally almost pointless.

 

[AJB Temple]

OK. I have done 4. I found in the end that the most practical way was to elongate the mortise slots, fit the brace, then drive in a glue coated wedge very tight to refill the excess slot, then peg them. The oak was iron hard, and I used a combination of multi tool (to get clean edges easily), auger bit (hogging waste out) and hand chiseling plus 3 lb hammer to deal with the work. It is much harder cutting mortise slots above head height in an already erected frame. I am quite pleased with how they have turned out. My main issue was lack of access and I didn't want to end up with very small tenons.