Timber tenon shear stress

[sbw]

I am checking the capacity of an existing timber bent frame with mortise and tenon connections. I would think you would get some contribution from the primary member (just away from the reduced section at the tenon) to resist the shear...thoughts/input would be appreciated. I have one tenon that's overstressed and trying to decide if I should require retrofitting.

Thanks

 

[JAE]

The shear would be taken by the tenon wouldn't it? All of it? Because that is the only thing truly connecting the one member to the other.

Or do you have a ledge type connection (per the attached drawing)?

 

[sbw]

Agreed, but the shear develops on the horizontal plane and much like the picture you provided the primary member is much wider than the tenon. So the question is, can you develop shear failure in the relatively short tenon without gaining some benefit from the primary member. I can visualize this failure mechanism, but my sense is that the typical allowable shear values would be conservative. That being said, I've re-supported the connection with a more conventional bracket.

No ledge.

Thanks

 

[Ron]

It looks like a shared tenon/ledge bearing condition. If so, you have shear parallel to the grain, but in the plane of the grain (ledge) and another plane crossing multiple grain rings (tenon) (or opposite that if the cut is oriented the other way). That might make a difference for long term creep, but not for the short term....in any case, the shear is shared by the tenon and ledge.

In a microstructure sense, the bearing compression under the tenon and the ledge can be different, but probably not worth trying to figure out.

I would compute the shear at the perimeter of the combined ledge and tenon.

You could also consider that all the shear it taken at the ledge, but if the ledge is oriented as I noted, then you could get intergrain splitting.

 

[msquared48]

Ron:

All this is assuming a tight fit, with very good craftsmanship to get full bearing on both the ledge and dowels equally. I would have top back away from the optimal condition to something less, and that's a judgement call based on the level of craftsmanship seen, or expected. I realize that the code values have their reductions based on craftsmanship, but this is a combination bearing situation, different than those included code reductions in my mind here. Kind of like firing two guns at the same time here. Rather hard to do.

Also, in his last post, sbw said no ledge condition. The sketch was JAE's.

How things going?

Mike McCann
MMC Engineering

 

[kslee1000]

Not a wood person, just a picture in my head:

Without ledge, the tenon takes the full load from bearing, the highest shear occurs at the tenon-beam interface, then reduces smoothly into the wider beam section that does contribute some shear strength. However, due to shear lag (?), the wood grains could split as a result, thus offset the advantage provided by the wider section. The code is justified to be very conservative in this case with large amount of experiences.

 

[JAE]

kslee1000 - that's just about what I was thinking. The ledge was indeed just included in the picture I found to post. Without the ledge - the "weak link" in the chain here is the tenon only.

 

[Ron]

Mike...you're right (and so is kslee1000). I looked at JAE's sketch and it stuck in my wee little brain as sbw's.

Workmanship is a key issue, as would be the tenon size, since as kslee1000 and JAE pointed out, the tenon takes it all.

Things are going well. Workload starting to pick up.

Will email you about the other.

 

[kslee1000]

JAE:

Sorry to hijack your idea, just wanted to say something in a slow, lazy Saturday morning. You got there first.

 

[JAE]

No problem - you stated it very well. It is so beautiful outdoors right now - what in the heck are we doing here?

;-)

 

[kslee1000]

Chilly here in Chicago, still in hidernation mode.

 

[msquared48]

I just caught three squirrels in my squirrel traps. That's over 35 since last August. I am practicing for the famine that is coming...

Don't worry, we let them go in the parks in Snohomish across the river 6 miles away. We had an infestation last summer with them nesting in my attic and we had to do something about it. The trapping program seems to manage the problem OK.

Mike McCann
MMC Engineering

 

[hokie66]

Mike,

They make a good Brunswick stew. Use also some chicken and beef, lots of vegetables, loaves of bread for thickening, salt and pepper, cook all day in a big iron pot until none of the ingredient are recognizable.