Wood Knee Brace Joint - Screwed Connection

[RFreund]

I'm looking at a connection design for what I would call is a faux- mortise and tenon knee brace connection. The knee brace will be subject tension and compression (small loads). I'm mostly focusing on the tension brace for now.

I have attached two possible screw orientations and I have a few questions:

1. What brace seems intuitively "stronger" or which would you prefer?
2. How would you calculate the connection capacity? In KB1 the capacity of the screw in the "main member" is fairly straight forward. However, figuring out how to calculate the capacity of the side member seems fairly complex. The situation is reversed in KB2. But even in KB2 the end distance and thickness of the side member become complex questions.

Kicker Brace Joint Detail

I will share some additional thoughts and clarify my questions in a follow up post.

Thanks!

 

[jayrod12]

For tension loading I'd personally be more partial to detail 1, less bending in the screws.

 

[dhengr]

RFreund:
I don’t know that I’ve ever counted on knee braces, the way you show them, for any tension, they just tend to pull apart. They work fine in compression. I think I prefer the screws going in perpendicular to the beam and the col., that is Det.1. That pulls the joint tight together, while Det.2 might tend to lift the tendon off its horiz. seat which isn’t good. I would delete the “(oversize mortise ¼” )”, and dimension the tendon 4” wide (that is 2-.75” shoulders), 2” deep, horiz. seat on the bot. as you show, and sloped on the top to match the knee slope. Then I would put a bead of caulking in the crotch btwn the knee brace and the col. Alternatively, make the knee brace 10x10, since you are already buying that size material. Keep the tendions and use 3/16” or 1/4” side plates btwn. the knee brace and the col. and the beam. You also need side plates up at the col. to beam connection, you can’t just let that flap in the breeze. You predrill the side plates and they are your screw driving templates. I would run a simple frame analysis program on the wood frame to determine what the member forces are and then design the joints accordingly.

 

[RFreund]

Thanks for the suggestions.

DH - you make some good point, but especially the point about relying on them in tension. I suppose I could just try to rely on the compression side and see if that will work. I would have a lot more faith in a real mortice and tenon joint with a through bolt (if it needs to take tension). However, this is for a pergola and the contractor seems convinced that it is overkill. He is partially right, but there is still some load on it, so we need something.

 

[dik]

I've encountered numerous century timber framed barns where there are no pins at the mortice.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Mike Mike]

My gut says detail 1 is better, and will probably work just fine for a pergola. One concern I typically have with wood lateral frames is deflection, particularly from joint slip due to differential shrinkage/swell between wood and steel connectors, but if it's just a pergola deflection is probably not crucial to it's serviceability. That being said, I typically design pergolas with moment column bases and pinned beam to column joints, instead of vise versa like you're doing here.