Principal Rafter Tie Connection

Howdy,

Since it has been slow, I figured I would show you all one of my funky designs. Its a real doozy, but that's not the reason for showing.

I really want to get some feedback on the joint in the subject line. Now I know with most trusses you would want your compression member (principal rafter) terminating into your tension member (tie beam).

I haven't done that. Without getting too analytical with loads and such, I would like to just get some intuitive feedback. Here is a picture of the bent/truss arrangement:



The Nancy character (think that's her name) gives enough of the scale and span without providing a bunch of dimensions. One other variable would be a common rafter roof system, no purlins landing on the prafters.

I was thinking a through tenon of substantial thickness. A housing in the prafter to give additional bearing on the tie.
Here is a close up of the arrangement:



And another showing the tenon. No tenons were drawn on braces.



What do you think? Has anyone framed like this before?

RSS to the rescue?

Mo,

First off, nice looking frame. I like the rotated king.

We're doing one like that now, except it's a really low pitch (4 in 12). Looks like a similar span. Required a 1" allthread to handle the tension at the heel.

I know your're trying to avoid a can of worms, but I could be more helpful if I knew the pitch, span, and distance between king posts? Assuming 12 in 12, 24', and 16', you may be able to make the end trusses work with the heel joint you show, but the middle truss will have twice the tension -- quick mental math says 6 or 7 kips at the heel (assuming no snow). Hard to do with pegs. If the span or ridge length is more than that, it's even harder to do.

If I were building it without an engineer's advice (which I wouldn't do of course), I'd use dry wood for the tie, make sure the ends where the tenon is is perfect wood, use perfect wood for the rafters, and put a 2 1/2" thick at least dovetailed thru tenon in it with a wedge below the tie, and try to fit about four 1 1/4" pegs in there. I'd also gorilla glue the wedge.

The dovetail and wedge would be there in case the pegs broke to act as a sort of breaking system so that trouble could be spotted in plenty of time to repair it with ironwork as the joint slowly crept open.


I don't know where the frame is but if it's in snow country you can forget about it.

Also, it'll be wobbly without more bracing. I'm constantly frustrated by how "flexible" timber frames are -- even well cut b


Just my quick thoughts, thanks for posting -- this is fun.

Mo, cool design direction. Without a bunch of data its tough to determine what that joint will be doing. Even in simple span trusses up here in snow country I'm using steel for tension.

I would also consider a moment connection to the piers. You have a lot of roof surface area and not much bracing to take wind loading...

Hey Mo, hard to beat a barrel bolt and some all thread, imo.

I think Mike makes an excellent suggestion about the pier connection, too.

Hey Guys,

Thanks for the feedback. All good ideas and information. Gabel, thanks for your post, it has a lot of good information in there and I appreciate it. You are almost dead on with the dimensioning you proposed. Good eye. I hadn't thought about the effects at the central truss too and twice the load. Thanks.

As far as the connection: A question in regards to the pitch. Am I right in thinking as the pitch becomes greater the thrust is reducing as well. I guess an example (all things being equal and represented as a triangle) as your pitch goes past 12/12 the compression at the heel from the prafter becomes greater than the tension on the tie? Is it true the lesser the pitch the more thrust?

Can engineering problems on this joint come into tolerance by making a pitch greater?

Thanks for yall's contributions.

Higher pitch will reduce outward thrust - but in your design you'll be transferring outward down onto that tenon you drew - which will put it into shear. So you'd likely need some sort of bearing from the p rafters to tie, be sure your tenon is wide enough to handle shearing, and also that the area of the tie over the post can also handle shearing (likely).

Hi Mo,

You could extend the post right up to a shouldered housing cut in the underside of the pricipal rafter and then have the tie beam simply connect to the inboard side of the main post.

Regards

Ken Hume

Ken - I think the danger with that approach has similar concerns with raiding the pitch - you end up potentially shearing the top of the post off instead of the end of the tie. (but remember that I live and work in snow country...)

Whichever direction it takes I'm sure Mo will analyze it for his local conditions.

Mike,

Why would the top of the post shear in a housed joint when more than the full section of the post is available to handle shear ?

Regards

Ken Hume

Ken -

I've had to design around issues where the post above the tie beam is a lever arm, and the thrust from the rafter wants to shear the post off at the tie, as it is assumed the tie is doing its job and resisting the tension. This type of configuration can be tricky to get right - its a balance of having the tie close enough to the rafter, but still far enough to leave enough meat for joinery. I can post a sketch when I get back into the office as I can't seem to attach anything from my mobile.

Hi Mike,

I now understand what you are saying - its just that you previously used the word "top" when what you really meant was the interface between post and tie (no need for a sketch).

Regards

Ken Hume

So, I explored with Sketchup what you two are talking about I think...

In the image below, would the weak point (shear) be at the corner of the diminished housing? That's what its saying to me. If that is the point, would a bore at the corner alleviating the corner help. Sorta of like a master that bored at the birdsmouth of a rafter. Also, I think what Mike might be referring to is the fact that you have a tenon on the top of the post right above a mortise for the tie. Whole lot of post gone with that. Maybe just cut the middle of the tie out and lap on both sides around the post...




I've always wanted to take a long through tenon and make and additional joint. In the picture above the tie extends through to the prafter and is keyed with a half circle cylinder. Would this help?


P.S. this is just for fun. I like to design stuff just to file away and try and learn from you all.

P.P.S I often wonder at what point the powers that be, just look at something like this and say, "Whoa, too many variables, lets just put some metal in there!". But thats not as much fun!

Mo, you got it. You've removed material for a through mortise, just where you want there to be strength in the post, and I'm assuming that directly above the mortise you've got a tenon. Because this is a truss, the forces heading down and out (from the rafter) want to lever the top of that post off... the larger the space between the rafter and tie, the larger the forces - and the less wood you have to work with. So its a balancing act. The post can fail at the peg holes, at the bottom of the housing, or if a check develops.

Now, you may have enough meat up there to do what you want - snow loads / wind loads / etc. and size of material vs. span may allow it to work - but I like your other solution better - of resolving the truss loads within the rafter / tie connection, and then letting that resolve gravity loads down onto the post. (then we load it up with wind... and it can get interesting again...)