Timber rafters - moment plate connection 4

[tasiman]

Hi There
I have a design where the owners require a slanted ceiling (rather than a flat ceiling). The span between walls is 26' (rafter span will be 13'). I have designed the rafters to be 2x8 @ 50", connected at the apex using moment plates either side of the rafters (to prevent sag/creep). By providing a birdsmouth cut to the rafter at the wall end, I am assuming there is no horizontal thrust exerted on the walls, or the horizontal component (in the rafter, due to deflections) is resolved within the moment plate. I have checked the moment plate for this load, the moment due to the udl along the rafter, and the eccentric moment due to a vertical point load at the apex (from hip rafters). I have attached the moment plate design (ignore the M12 bolts - this has been changed to M16).

I do acknowledge that although the moment plates will restrict vertical displacement, there may be long term creep in the timber rafter, which induces horizontal thrust to be resisted by the wall, or the moment plate.

Do you agree with this approach?

All replies appreciated.

Cheers

 

[BAretired]

No, I do not agree with this approach! Are you serious? The moment at the apex will be wL²/8 where L is 26'. There is no possibility of the detail working in the absence of horizontal reactions at each end.

BA

 

[msquared48]

Agree totally with BA here. You need a ridge beam or collar ties.

Mike McCann, PE, SE (WA)

 

[tasiman]

Sorry the detail is to have a 8 bolt arrangement - 2 top and 2 bottom, each side.

 

[dhengr]

It just staggers the imagination, but the basic concepts and fundamentals of Statics, Strength of Materials and Design of Timber Structures have completely been obliterated from the consciousness of those pretending to be engineers. Do they teach anything about engineering in engineering colleges these days? In one more generation, owners will want roof systems designed out of wet noddles because they are lower in protein and slightly less expensive than timber in a desert.

 

[tasiman]

Mike, so with collar ties, my understanding is that its purpose is to prevent the rafters from pulling away from the ridge beam/board (if one was installed. That is what I am trying to (partially) achieve with the moment plate. Correct me if I'm wrong.

BA - 26 is between supports, so 13 between apex and wall.

 

[tasiman]

dhengr - Perhaps rather then being so negative and demoralising, you can be a bit more of a help. This design is just a thought at this stage, in relation to what I have seen out there (yes I have seen these connections at work, and the roof is not collapsing!) It is an opportunity to be innovative, rather sticking to the basic solutions. If you have any solutions or suggestions then please do reply, else please do not waste time. Cheers

 

[tasiman]

Getting my metric to imperial conversion right, the rafter spacing is 24" while the rafter span is 27.88'.

 

[tasiman]

Wall to wall span is 27.88', rafter span 13.94'. There.

 

[dhengr]

Promote it as the structure with the leaning/living walls. Lean the walls in by 5̊ when you build them and cut the birds mouths , within short order they will be plumb, and after the first real roof loading they will be leaning out by 5̊. This all happens when the ridge moment connection starts to settle into really picking up some of its loads. Imagine the movement in that connection before the bolts really start bearing on the wood holes. If you can live with that, go for it. You may want to place some guy wires from the top of the wall to the floor system in the middle of the room so as to limit the outward movement of the walls, but the sloped ceiling sure is sexy.
My positivity or negativity will not eliminate the lateral thrust at the exterior bearing walls, so you better figure out how to deal with that, if you care. Otherwise, go for it.

 

[KootK]

I mostly agree with your statics. Two issues for me:

1) It's an error to assume that the bird's mouth cut affects whether or not the walls will attract horizontal thrust. The walls generally do no attract horizontal thrust because they are relatively free to rock back and forth laterally.

2) As others have intimated, the capacity of your moment connection is dubious.

I recommending posting a few free body diagrams: one of the overall frame and the loads imposed upon it, one of a single rafter, and one of your moment splice plate. With that information in hand, we can ensure that you're on the right track with your analysis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tasiman]

Thanks for the input dhengr. Appreciated. There is always a better way to put your opinion across. For your information, I have also thought about this thrust and how to address it earlier. Strengthen the top plate in the horizontal direction, and tie it back to the transverse end walls (or intermediate walls). This would limit/restrict the bending of the top plate in the horizontal direction. Just a thought at this stage.

 

[woodman88]

tasiman
Do some numbers so we can tell you where your mistakes are.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Lion06]

The statics work, but practically speaking, the detail is not a good one. Stay away from moment connections in wood, especially where rotational stiffness is important for limiting serviceability concerns. Why would you NOT use a ridge beam? It's a much more reliable way to achieve the same result. The fact of the matter is that connection will allow rotation of the joint, regardless of strength of the joint. The ridge will move vertically as a result and the drywall will crack. AND this detail is sure to be more expensive than a ridge beam with joist hangers for the rafters.

 

[XR250]

Even with a perfect moment connection, your rafters are undersized and will splay outwards significantly.

 

[manstrom]

I have used this detail before, but with a bigger plate and stronger connection. The wood bolted connection is a bear to design since the loads are perpendicular to grain with low edge distance.

Yes, the rafters will thrust out on the wall. You would have to take this into consideration.

If you can get a beam up top, I would recommend it over this type of frame. The plates will be more expensive than the beam you will need. If you use a girder truss, you can get better spans. Consider framing down a false ceiling with decorative rafters (say, 3:12 pitch) and a real roof with a 5:12 pitch. You can hide the beam in the gap between the two. You will get the look and a real structure as well.

Alternatively, you can use tie rods or cables to resist the thrust.




When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[msquared48]

Tasiman:

The purpose of collar ties is to limit, not prevent, the lateral spreading of the walls, and to decrease the size if the ridge beam needed.

Mike McCann, PE, SE (WA)

 

[msquared48]

If you absolutely must have a moment connection at the ridge, consider embedding the rafter (4X) in steel tube sections welded together and thru-bolted. You will get extra capacity in the bearing of the wood on the top and bottom of the tubes.

This design was done by a former employer of mine in the early 80's for a local contractor for various loads and spans, and does work if properly engineered. However, if your experience is limited in wood/steel moment connections, I would suggest passing off the design to someone who is.

Mike McCann, PE, SE (WA)

 

[tasiman]

Thanks for all the comments guys. Really appreciated. I will take these points on board. Few things that I was not clear about. The design is to cater for dead, live and low wind loads only, no snow loads. The moment plate has a 8 bolt arrangement, not the 6 shown. My first option was to put a ridge beam, however the gable ends have a continuous glass opening, and a post supporting the ridge beams was not ideal for the look. Hence looking at this option. Cheers

 

[Lion06]

If the gable end has a wall below the eave line and is glass above the eave line, you can use a beefed up rafter and tie arrangement to support the ridge beam. There is no reason the support has to be a vertical post.