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Removing existing wall, What joint should I use?

Started by Jhoff, October 14, 2020, 08:55:03 PM

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Jhoff

Hi guys, I am removing a load bearing wall between my kitchen and living room. I have a cape cod style home. I will be clear spanning 12' with an 8x10 piece of clear red oak I milled. It will bare on 2 8x8 read oak posts at either end. My question is what joint can i use to tie the beam to the post. I don't think I can use a simple Mortise and tenon due to the ceiling and floor restricting vertical clearance to fit the joint. I was thinking maybe an open mortise Joint may work. I would be able to set a post, slide The beam horizontally over the tenon and then slide the other post and tenon Horizontally in to the other end of the beam. Is an open mortise going to be strong enough? I really don't want to just stack the beam on the posts and use a plate, however I will if I can't find a joint to work for this application. Any input would help. Thanks guys.  

Don P

That might work all depending on whether there is room to slip it in to the end. Retrofitting stuff in to buildings with existing finishes is a bear. If there is a break in the ceiling finish (sheetrock?) where the existing wall is you might need to put in a filler piece to get your beam into bearing with the framing above. As far as strong enough, the perimeter walls of a building are usually considered to be the lateral force resisting system, the internal members usually just take care of gravity loads. If that is the case then the joinery simply needs to keep the members in alignment, hold the beams on top of the posts.

The bigger consideration in my mind is, have you looked under those posts. The post and beam assembly has collected what was a uniformly distributed load and concentrated it onto the base of 2 posts, what is under them for support? The load path needs to be looked at from there all the way to adequate ground support. I've been called in to buildings with posts bearing on nothing more that floorboards and unsurprisingly the posts were driving down through the floor. Basement support posts placed on slabs rather than over footings cracking the slab and beginning to punch through, etc. Follow the load path and confirm adequate support the whole way down.

On the job from the past couple of weeks, it is a gristmill built in 1875, 3 stories of milling equipment over a "basement". The basement posts supporting the main floor beams are completely offset from the support posts above that carry the upper floors. The upper building posts bear at roughly midspan on the main floor beams. Those beams have not failed but they certainly are not straight anymore and I've found shims between that beam and floor joists that were an attempt at releveling the floor at some point. The discussions around things like that are interesting "What were they thinking?", "It hasn't failed in all this time", "I see age related damage from insects and decay- past performance is not a predictor of future performance". Mainly what I'm saying in all that is to step back and look at the whole structure.

Jhoff

Thanks Don, the 8x8 posts will bear on the floor deck, blocking under the floor to the main carrier beam in the basement and lally columns under the carrier beam to new footings in the slab. Is an open mortise joint in the beam just as strong in shear as just stacking the beam on the post with full 8x8 bearing? I would think it is, especially if I left a 1.5" shoulder in the post in front of the tenon. What do you think? Thanks again for the input. 

Don P

Building code says yes, minimum of 1.5" of bearing wood to wood members, 3" wood to masonry. But the real thing to check is the load on the post divided by the square inches of bearing surface and check that against the allowable compression perpendicular to grain for that species. You're making sure the post end grain doesn't crush the side grain of the beam. Those allowable design values are in the Supplement to the NDS under the publications tab on the awc.org website.

Easier... let me roll backwards, heavy timber beams and stringers red oak 820 psi

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