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Moment connection between two wood joists

Moment connection between two wood joists

Moment connection between two wood joists

(OP)
I have an existing condition where we are taking out an existing chase and infilling the opening. The chase was next to an existing masonry wall.

The existing 2x12 roof rafters which are bearing on the chase now need to be sistered to others to extend and bear on the wall. The required additional span is 4' with overall span of 23'. I am trying to design a moment connection between the two parts using thru bolts but with a 4.5 k-ft moment, it appears that I'll need a lot of thru blots to connect the two parts. Is there some kind of Simpson device or anything else to create a moment connection in wood members?

RE: Moment connection between two wood joists

I did something similar once.  It's not easy to get the connection to work out.  I don't think my moment was on the order of 4.5k-ft, though.  Is that the midspan moment or the moment you need to design the connection for?

RE: Moment connection between two wood joists

Joe- the only moment connections in wood that I have seen are in very heavy timber construction and involve steel plates, saddles, buckets, etc. They are in general to be avoided, especially in light framing. Now we all know you can take two 2x12s and with enough overlap and nailing basically form a moment connection, but this is usually an engineering judgment, case-by-case method, not something you will find a textbook method for. But maybe there is and I have not seen it.

The problem is similar in theory to a group of bolts in a steel connection in rotation, you have to calc the center of the bolt group and distribute shears amongst the bolts..

If I read that right you want to extend a 19' span to 23'? My gut feeling without any calcs is that if you conservatively take a 12' long 2x12 and stagger nail with 16d that you could cantilever out 4' no problem with regular 20/20 psf type roof loading... Shame there isn't a table somewhere for this situation. Have them build up a mock on site with (2) 2x12s and then drive one of their trucks over it, see "if that'll hold her!"...

Guess you also have the options of full length LVLs to span the entire length.

RE: Moment connection between two wood joists

Why not put a header across at 4' where the chase bearing wall was.  Then sister onto the full length jsts. on either end of the chase to pick up the new header loads.  Then infill the 4' span with whatever works?

RE: Moment connection between two wood joists

Its a bitch and very hard to do in wood!!

Try spacing your bolts way out - you will get some help that way..

Don't know your "overlap" length - but you might consider some new joists or sister joists.

And NO - no one makes a moment connecting device.  The closest would be a truss plate but you would need a 20,000 lb machine to install them.

RE: Moment connection between two wood joists

(OP)
Thanks guys.

RE: Moment connection between two wood joists

(OP)
Actually I guess I have one more question. One the ends of the existing rafters I saw that they were simply bearing on existing brick with mortar clearly touching the rafters. And they seemed to be in good condition still. I thought you couldn't grout around an rafter end, because if that's acceptable I would just ask the contractor to match the bearing condition for the news to the existing.

RE: Moment connection between two wood joists

I would use fasteners (screws or nails) less than 1/4" in diameter because of the group action reduction factor involved with bolts and the considerable section being removed from a 2x12 with the bolt holes.  The lap length and the number of nails will probably be higher than you might imagine.

RE: Moment connection between two wood joists

This doesn't really apply hear but its something that I but heads with my father (4th generation homebuilder) all the time - in any substantial connection I design such as reinforcing a truss plate with plywood side plates or a moment connection I always prefer to use thru bolts spaced adequately as mike suggested however he swears that gluing and stapling the piss out of it is the way to go.  He argues that nails/bolts/screws will spit as the wood dries however the staples will not. However I don't know of any design values for staples.

EIT

RE: Moment connection between two wood joists

There are plenty of tables for staple design values in shear, ICBO and otherwise.  However, quantifying the connection to a building department will be a bitch.

Just sister the joist and call it good.  Assuming there are few elecytrical;, mechanical and plumbing lines to temporarily dismantle, it's the simplest, cheapest, quickest, and most reliable fix.

Mike McCann
MMC Engineering

 

RE: Moment connection between two wood joists

With this type of thing it is best to work backwards.

Find out what the strongest bolted connection is for shear capacity in the given configuration.

Then calculate how far apart they will need to be to take the required shear and moment. i.e V/2 + M/spacing<bolt shear capacity.
 

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