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Which joint would be best

Started by TW, July 19, 2007, 01:32:26 AM

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TW

I am well and working again.

I have to join a 6x8 inch beam to a 5 1/2x6 inch post in each end. The posts will be bolted to the inside of a loghouse to stabilize the walls. I will put the bolts in oval holes to allow the logwalls to settle. The beam would take tensile forces as it keeps the logwalls from spreading. There will be some vertical load(bending) on the beam too. The beam would be about 30 cm(1foot) below the top of the posts. As the posts will prevent the walls from buckling there will be bending force on them, so I want the cross section as little reduced as possible.

I think about a housed tennon joint and an iron strap nailed on to take the tension, but is there something better?

Jim_Rogers

If this joint is going to have some tension, then I'd use a through tenon with a half dovetail and wedge it.
Due to the size of the timbers, I'd make the tenon 1 1/2" wide, and center it on the post and possibly on the beam depending on which side you need flush.
If you need a drawing let me know and I'll create one for you....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

TW

Thanks Jim
That drawing would surely help. I have never made any timberframing joints before.
I intend to use the posts with the flat side towards the logwall. Then I get the same 6" with on both beam and post. The posts are spruce and the beam is pine. The strenght properties for the both species are almost similar.

Jim_Rogers

Here is a shot of the post, tie beam and wedge:



Here is the same joint, see through:



Here is the joint exploded so you can see the mortise and shoulder on the post:



Here is a detailed drawing with dimensions:



Wedge is usually hard wood, such as oak or equal strength.

If you have any questions or need to see these in larger sizes, send me an email and I'll send them out to you.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

TW

Thank you Jim. You solved the problem for me, and saved me from making a number of misstakes.

E-mail sent for a larger size of the drawing.


Don P

Can you quantify the tensile load and check it against the shear value of the wood in the half dovetail?

Jim_Rogers

Don:
Who are you asking to do this?

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

TW

The tensile load is unknown. This is a renovation work on a house built in the 1860-ies, with no structural drawings at all. In fact theese posts and beam are my addition to the original renovation plan. I told the owner that i will not work for him unless i am allowed to stabilize the side walls, because they buckle badly, and I do not want to be feel guilty if they collapse. I have seen similar reinforcements elsewhere made with smaller dimensions, but i wanted to stay on the safe side and he had theese bigger timbers on hand. It is a small house, less than 5 metres wide. The tensile load comes only from the outward slope of the walls. The walls slope outwards about 5 cm on 270 cm height. The vertical load will be greater.
In Finland it is still legal to build one family houses by eye.

If it collapses neither I nor Finnish law will not hold Jim guilty in any way.

maineframer

Nice joint Jim. I don't believe this joint  will fail in the next 300-400 years if properly cut. It is unkown what the shear strenght of the half dovetail is but the common sense quotient is high.
David

Don P

I was really just throwing it out there to remind us that joinery strength needs to be thought about as well as the timber's strength. By eyeball I'd guess 5.5"x 1.5"x 150 psi for Finnish scots pine in shear= 1238 lbs of allowable tension.

I've done quite a few eyeballed posts on long or tall walls to stiffen an already aproved design. Several log home companies specify 5.5"x 5.5" stiffeners.

Our clients are in Finland now, their daughter is getting married tomorrow. They sent us an invitation, but alas, no airplane tickets  :D.

Jim_Rogers

Thanks David (maineframer).
If you didn't know we have a smart tag for this joint, but the computer doesn't actually cut it totally correctly so I had to modify it a bit myself.
Then I used 2-7-1 to cut the post.
Jim Rogers
(David and his wife use the same CAD drawing program that I use, to draw the frames the build).
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

TW

The joints are cut.

I left out the dovetail and vedge and instead I will drill a couple of holes through and insert pins made from round iron. They will be sheared in two places so the will be strong. The reason is that the joint ended up much closer to the top of the post than I had thought and I do not trust the short fibres in the post top.
Othervice I followed the drawing, but I cut a housed straight tennon and mortice.

I may post a picture later, if daddy lends out his digital camera.

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