Knee brace connections for timber structures of picnic pavilion 1

[dcceecy]

We are designing the timber framing of a structure like picnic pavolin. we use post 8x8, beam 6x12 and knee brace 4x6. column spacing is about 15.5'. the knee brace connects column and beam.
The problem is that there is about 7 kip axial load in the knee brace (Dead load+snow load). What kind of connection can transfer 7 kip?
I was told not to use the mortise and tendon with pegs because of the cost. Nails, lag screws and bolts do not have much capacity.

Do we have some metal connectors (like joist hanger)?
I never design new timber structures before.
Any suggestions? thanks.

 

[Ron]

Knee braces are most commonly fastened with lag bolts or through-bolts. If you have a 7kip axial load in the knee brace, you should be able to handle with 3/4" bolts, two per end.

 

[BAretired]

Under dead load plus snow load, the knee braces are in compression, so the connection needs to handle vertical shear at the column and horizontal shear at the underside of beam. The normal components are taken by wood bearing.

Under wind load plus dead load, you may get tension in one of the knee braces. In that case, the entire force has to be resisted by the hardware.

BA

 

[msquared48]

Use a combination of endgrain notches and thru-bolts. You will get there.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

This article shows another alternative for knee braces.

http://www.structuremag.org/article.aspx?articleID=334

BA

 

[ToadJones]

being a former carpenter (kinda still am), the heavy hardware turns me off a bit.
These wood pavilions are nothing new, I'd try to make it work with classic methods first.

 

[dcceecy]

Thank you for your replies.

Actually, I can embed the timber post in concrete footing so the base can be modelled as fixed connection. in that case I do not even use the knee brace to make a lateral resistance system. but the knee brace make the frame looks good.
but if I did not put a strong enough connection, the knee brace may be pulled apart from beam during a big wind.

 

[ToadJones]

I'm not sure I would want to embed beautiful wood timbers in concrete ...they will be subject to rot.
If you are going to embed them, add the knee braces for additional stiffness and looks

 

[BAretired]

I don't like embedment either because of rot. Any water finding its way into the post flows down and is confined by the concrete.

BA

 

[ToadJones]

if pressure treated, warranties are voided by direct contact with concrete = no embedment.

 

[Ron]

If you want to embed, put a pipe sleeve into the concrete, then set your post in epoxy. Takes about a gallon of epoxy for each post...expensive, but works fine.

 

[tclat]

If you embed in concrete, and eliminate the knee brace are you designing the post like a flag pole? I wouldn't count on a moment connection between the beam and post.

Why not fabricate a steel bracket for the knee brace.

 

[demayeng]

Ron,

Have you got any more information on embedding using epoxy?

Wouldn't the timber soak water in that would run down and trap behind the epoxy?

We've recently just had to design repair solutions for two pole houses that had the poles embedded in concrete footings. One was only 5 years old and the 300mm diameter posts were around 70% rotted away. It would be hard to recommend even using your epoxy method in our region..

thanks

 

[Ron]

Concrete will maintain moisture, which promotes wood rot. Epoxy does not.

 

[BAretired]

How does the water get out when you use epoxy?

BA

 

[Ron]

You should prevent the water from getting in, as much as possible. Use pressure treated wood and coat it a couple of feet above the embedment, preferably with an asphaltic mastic, but to prevent getting that on everything, other coatings are acceptable (trowel grade urethane sealant for instance).

 

[vandede427]

what's the expected life of a wood framed pavilion? 15-20 years?

 

[BAretired]

I would expect it to be a lot more than that. Maybe 50 to 75 years.

BA

 

[Splitrings]

Dcceecy,
A 3/4" bolt and one 4" split ring will likely have sufficient capacity. It would be a much cleaner connection and is also fairly inexpensive compared to custom steel brackets.

 

[ToadJones]

You might consider trying to design the bases as pins and using the frame action of the knee braces for stability.
I have seen these pavilions set on concrete piers that had T's bolted/embedded to the pier. There was a slot cut into the bottom of post and it was slid over the stem of 'T'. A hole was cross-drilled through the post and a pin inserted through those holes and a hole in the 'T'. It was quite clever.
Also, I think a proper construction sequence could eliminate some dead load in the knee braces. If the 6x12 beams can handle the dead load without large deflections the knee braces could added last, thereby reducing the dead load in the bracing....of course over time the knee braces may see more dead load as the structure settles/moves.