×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Timber rafters - moment plate connection
4

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

RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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

Mike McCann, PE, SE (WA)


RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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.

RE: Timber rafters - moment plate connection

(OP)
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.

RE: Timber rafters - moment plate connection

(OP)
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

RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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.

RE: Timber rafters - moment plate connection

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.

RE: Timber rafters - moment plate connection

(OP)
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.

RE: Timber rafters - moment plate connection

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.

RE: Timber rafters - moment plate connection

2
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.

RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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

RE: Timber rafters - moment plate connection

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)


RE: Timber rafters - moment plate connection

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)


RE: Timber rafters - moment plate connection

(OP)
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

RE: Timber rafters - moment plate connection

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.

RE: Timber rafters - moment plate connection

(OP)
We are looking at a 30' span for the ridge beam.

RE: Timber rafters - moment plate connection

If I absolutely had to make a moment connection with the timber rafters at the peak, I think I would look into using an HSS section to sleeve the timber into and then welding the two sleeves together. The timber would have to be shimmed tight into the HSS. I would image the HSS would have to be fairly long (maybe 3-5 times the depth of the timber?). This however would be expensive and a pain in the ass to build. Hopefully your ridge beam/truss works out.


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

How about using full span trusses instead of rafters with some depth at the peak but still have a lesser slanted ceiling. Gets rid of all those problems.

RE: Timber rafters - moment plate connection

tasiman,

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 winky smile



RE: Timber rafters - moment plate connection

I'm coming in late here but you will have a very hard time getting the bolts to work with all that moment cranking on each bolt group.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Timber rafters - moment plate connection

Tasiman:
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

The detail drawing shows a solid vertical line at the center of the steel plate. ... i.e., it's shown as two five-sided plates, butted together at the ridge plane, and attached to each other by means not shown. There's a fair chance that you will have to send someone up there to butt weld the plates together, and two other guys to arm the fire extinguishers.

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

(OP)
Perhaps the sketch attached earlier is not a good example - it is just indicative. The detail is actually for 4 bolt arrangement on each rafter, with beefier bolts, and the plate will be continuous with no joints. For the plate at least the maths does work, however I see that practically it will have issues. In light of the discussion here and the good and relevant points that have come out of this discussion, I have been able to incorporate the ridge beam at the top by providing an intermediate portal frame to support it. This will also help to tie the top of the walls nicely. I have remodelled the entire roof system to see the overall effects.

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

If you use the moment connection someone like me will be writing a report for why the roof is sagging, the wall is pushed out, and all the finishing is cracked. Then they see a faulty design and you get sued.


Use an LvL ridge beam and be done with it, don't reinvent the wheel.

RE: Timber rafters - moment plate connection

The easiest solution is to use a ridge beam, but if you want to get creative, you could design the roof deck as a folded plate with a tied arch at the gable end.

BA

RE: Timber rafters - moment plate connection

If you have to, use a steel ridge beam if the PSL or glulam will not work for you.

Mike McCann, PE, SE (WA)


RE: Timber rafters - moment plate connection

I have not seen a residential building that used rafters and ceiling joists for a few years . Light frame trusses are used almost exclusively here in Western Canada , Much easier to install, and cost effective. No thrust to worry about either.

RE: Timber rafters - moment plate connection

Good idea using a ridge beam.

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

With a moment connection, your rafter span is effectively 26' (not 13'). I am still wondering how you are going to do that with a 2x8.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources