Moment Connection(?) for Timber Truss 1

[skade]

I am attempting to design a moment connection for the top-chord-to-post connection for a timber truss (diagram attached). I realize that this is a connection best avoided; however, I am trying to accommodate the architect's vision. The design is inherently unstable if all connections are pinned, so I modeled it with fixed connections at the locations mentioned above. I've done a few other timber truss designs, but none as nasty as this one. I'm thinking of trying to use steel dowels epoxied into the members, but I'm having trouble finding design values (though I doubt I'll be able to develop the loads needed).

The connections I've considered so far-- straps, bolts, plates, etc.-- I don't believe will give me the needed fixity. I also realize that there will be further design considerations if the epoxied dowels are a viable option, such as moisture content, shrinkage, etc. Also, other geometry options would ruin the desired aesthetic... it would be so easy if I could push the bottom chord up a couple of feet...

Any ideas? Tips very much appreciated

(Experience: EIT, 1.5 yrs)

 

[BridgeSmith]

So Structural dood, for a truly effective moment-resisting connection in timber, the steel connection plates would have to wrap around the side faces of the timbers? Or is there some better method of transferring moment into a timber? I haven't really looked at this much since timber design class, and we didn't spend much time on beam connections (no time left after several weeks wasted on the 'microscopic structure of wood', I suppose).

 

[dhengr]

HotRod10:
In fact, you kinda eluded to the better solution in your 3JUL18, 3:41 post. Look up shear plates or split rings, dig out your wood design textbook and do a little reading. Place split rings or shear plates 18 or 24” apart, with .25” side plates, both sides, and you can develop quite a bit of moment, with the bolt/bolt hole movement issue or cross grain shear/tension cap’y. being less of a problem. You cut a circular groove (a dap) into the wood to take the split ring or shear pl. and the bolt takes shear from stl. side pl. to shear pl. and just tightens everything together on the wood member. There is not the same kind of slippage/movement in these joints as in a multi-bolted joint, and the split ring or shear pl. have a much greater cap’y. than a couple bolts.

 

[BridgeSmith]

Thanks dhengr. We never had a textbook for my timber design class, and it was nearly 20 years ago. I was just curious about the current design and detailing practice. Split rings were one of the simplest and considered to be one of the best methods at the time, and I understood they had been used for a long time. I just wondered if they may have been supplanted by something 'new and improved'.

Edit: Thanks also for reminding me of the terminology. "Dap" - I knew there was a name for it, just couldn't remember it.

 

[dhengr]

HotRod10:
There are timber rivets driven through the side pls. which distribute the loads much more uniformly, and are quite a clean solution. There are several European systems making their way to the USA which involve CNC fab equip, which involve a centered thickness stl. pl. which is cut in to the end of the wood member with something akin to a chain saw mortising machine. Then pins are drilled and driven through the wood side members and holes in that center plate. Each of these methods eliminate the movement/rotation of the bolt and the hole tolerances in the wood and stl., etc. And, they distribute the loads more uniformly and among many fasteners. The later systems are almost akin to drift/draw boring, where driving the wooden pins in the old timber frame drew the joint tightly together. In wooden frames, many smaller fasteners distributing the load more uniformly is almost always better than a few larger bolts loosely fitted. My wood design course was 50+ years ago and I still have the textbooks and several newer ones, plus many eds. of the NDS, etc.

 

[BridgeSmith]

dhengr, you're obviously well versed in timber design. Thanks for the update. I don't deal with timber much at all in bridge design at the DOT. Pretty much all of my timber work involves replacing timbers and hardware 'in kind'. Eventually though, I may try using timber framing for the design/build of a house of my own. I anticipate using a timber or log shell with steel reinforcements and a structural steel roof system, although I'm open to considering a timber roof system, thus my interest in the advances in the structural connections. Thanks again for the info.

 

[MidwestSE]

Hey HotRod - sorry for the delay in getting back to you. Simpson came out with some post bases that are able to transfer moments - take a look at them and you'll see they do in fact wrap around the wood as you mentioned above. In my experience, I've tended to avoid using moment connections with wood beams - I've always found a way to use standard connectors and whatnot that are readily available paired with either simple span or cantilever beam systems.

dhengr - I've seen contractors going away from using timber rivets in all the projects I've worked on. Either they don't like them or don't have the right equipment...either case, do you often use them?