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Knee Brace Tenon and Mortise 1 1/2″ Layout

Knee Brace Tenon and MortiseThis is an exploded view of a 3″x5″ knee brace tenon and mortise,  with tenon layout of 1 1/2″ off the face and 1 1/2″ thick, and its mortise in a tie beam.
The peg is 3/4″ in diameter and it’s placement is 1 1/2″ off the shoulder and 2″ off the bearing end.
The tie beam has been rotated 45° so you can see inside the mortise.

10 thoughts on “Knee Brace Tenon and Mortise 1 1/2″ Layout”

  1. Is it necessary to have the housing cut out to fit the full size of the brace into the beam? Or could you get away with just mortissing out for the tenon?

    1. You can omit the full housing but I would not recommend it as a general rule. The housing adds a lot of strength to the joint and it hides the gaps in the mortise as the timber dry.

  2. William H. Cary

    Do you offset the holes for the pegs in the mortise and the tennon and by what amount ? 1/8 inch?

    1. That is called draw boring and it is not always recommended to do. If you are using oak or equiv. you can do it but if you are using a softwood you have a good chance of blowing out the mortise rendering the joint useless.

  3. Braces have been found in old barns without any pegs at all. Braces work in compression. It is very unlikely, in my opinion, that you’d blow out the tenon with a 1/8″ draw bore in softwoods.
    I do draw bore brace peg holes. But I do it in two directions to help pull the brace to the housing and to the bearing end.

    1. Mario R Kodrzynski

      Under wind load one brace of the portal frame is under tension and the other is in compression. My finite element analysis models indicate that for a moderately windy inland area near Ottawa, Ontario province, Canada; an 18′ x 18′ single story 4 bent hammer beam frame with only a roof and no walls shows 5000lbs tension per windward brace on the middle two bents. No single 1 inch hardwood peg can resist this with-drawl force. It would require at least 5 x 1/2″ steel bolts per wind brace tenon which is likely not possible given the typical wind brace joint geometry (4″ x 1 1/2″ tenon). In order for the frame to stand on its own one possibility is to use ASTM A307 grade A steel rod with 3″ x 3″ x 1/4″ square washers and stout nuts from angled pockets in the hammer beam into the end of the wind brace to another perpendicular pocket in the brace.

  4. When you mortice out for a knee brace is the mortice square or is it cut to follow the angle of the tenon on the brace? Thank you..

  5. I understand there are 2 schools of thought on pegging braces, no pegs or pegged. My frame is pre assembled and thanks to my buddy who cut all the braces they are 100% to spec. The frame doesn’t move, almost too tight if that’s possible, I plan to disassemble the frame and have it up next summer. What’s your thoughts on this?

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