Timber rafters - moment plate connection
Timber rafters - moment plate connection
(OP)
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
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






RE: Timber rafters - moment plate connection
BA
RE: Timber rafters - moment plate connection
Mike McCann, PE, SE (WA)
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
BA - 26 is between supports, so 13 between apex and wall.
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
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.
RE: Timber rafters - moment plate connection
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.
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
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.
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
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
RE: Timber rafters - moment plate connection
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)
RE: Timber rafters - moment plate connection
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)
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
If you can't support the ridge beam at the gable can you hide a post in an interior wall somewhere and cantilever the beam?
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
Welcome to Eng-Tips, and your baptism-by-fire of sorts on this thread! Sometimes we come across as grumpy old men (and women), but we all mean well.
Am I correct in assuming, based upon your 'handle', that you are located in Tasmania, Australia?
If so, you have some great local hardwood species that near replicate 'steel' in their hardness and durability. I especially like Tasmanian Blue Gum for its appearance.
If not, disregard this reply
RE: Timber rafters - moment plate connection
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RE: Timber rafters - moment plate connection
If you insist on using that detail, at least lengthen the two legs on your ridge moment plates considerably, to improve the mechanics of the detail. This will reduce the rafter rotation at the ridge, although it will still be considerable and will cause considerable thrust on the ext. walls. Think in terms of the amount of movement in the bolt holes as the bolts come into bearing on the wood and start to crush the wood until they pick up their full load. Look into the yield modes of bolted/doweled connections in wood. You might do well to look at timber rivets if your wood species is amenable to their use. These joints have much less play in picking up their load, although they probably won’t work in 2x members. Look at selling the owner on tie rods and turnbuckles every forth rafter. Hang a nice light fixture off of them at the center of the room.
RE: Timber rafters - moment plate connection
At least the welding heat will harden the rafters a little and perhaps increase their resistance to splitting.
... which looks like a guaranteed failure mode, given the moments and the small number of fasteners.
Mike Halloran
Pembroke Pines, FL, USA
RE: Timber rafters - moment plate connection
Thank you all for your insight and advice. This is really appreciated. Good to see so much interest in the thread. I will, at a later stage research this option, with the objective to perhaps devise (innovate!) a practically sound solution. If you do have any more ideas, please add to the thread. I can truly say this forum is relevant and very much alive, and worth keeping tuned into. Cheers
RE: Timber rafters - moment plate connection
Use an LvL ridge beam and be done with it, don't reinvent the wheel.
RE: Timber rafters - moment plate connection
BA
RE: Timber rafters - moment plate connection
Mike McCann, PE, SE (WA)
RE: Timber rafters - moment plate connection
RE: Timber rafters - moment plate connection
Even if you have a rigid cranked apex beam (and I agree with the other poster that you didn't), your walls still get pushed outwards a lot.
RE: Timber rafters - moment plate connection