Design literature on hip roofs

[DGpe]

Can someone point me to where I can find design techniques for wood framed hip roofs? I refer to hip roofs that have no ceiling joists or cross ties, providing a cathedral type of look.
I'm sure the hips will produce a thrust at the lower support corners. But I'm unsure to what happens to the reactions from the hip at the top of the ridge.
Also if there's a continuous ridge, can the rafters at either side be designed as a 3 hinged arch? Of course, this will also exert thrusts on the tie beam below.
Any suggestions on finding calculated examples will be welcomed.

 

[kslee1000]

Sounds a plan. But if the owner likes the exposed wood works, you may have more head aches ahead to convence him to pay for cover it up. Good luck.

 

[BAretired]

The roof has to be fixed in accordance with your best judgment. If the owner will not accept that, explain your position in writing and escape. Removal of the ceiling joists was not your decision in the first place, but if you allow your judgment to be overruled by others, it won't matter because you will be responsible for whatever happens.

BA

 

[enginerding]

You could also try to convince him to add decorative rafter ties at the ceiling level at 4' on center (ref IRC). These will prevent the thrust and can keep the roof stable. From the sketch it seems like a really low pitch which means high thrusts on that ring beam...

 

[hokie66]

I would not be as confident as you that the 8" x 12" concrete beam will take out the thrust. It will not be a tension ring, it will be a bending element, spanning between transverse shear resisting walls. It has to take the same loading as the ceiling joist did previously.

Back to my comment about the hip to ridge connection force not being changed from the original...the hips just take the jack rafter reactions, and those reactions are unchanged if the bottom horizontal force at the concrete beam replicates the ceiling joist reaction. The connection may be not good, but the force hasn't changed.

 

[hokie66]

One other thing... all of us here have only considered the rafters and ceiling joists. Are you sure the roof wasn't originally supported, at least partly, off the ceiling joists; or conversely, that the ceiling joists weren't hung from the roof?

 

[oxbridge]

the concrete ring beam might be the reason the roof is still standing! so i think it would be worth checking this. can you imagine if you specify lots of additional strengthening works and then the client gets an independent opinion from another engineer who's able to prove the ring beam is fine. someone suggested the thrust will be high due to the shallow pitch and they're dead right so you'll have to check this.

you need to also check span/effective depth ratios of the concrete ring beam (laterally) to ensure the deflections are ok as if it moves too much in the middle it could push the wall out and you'll get cracking.

the other thing to bear in mind is with the roof being so flat, you'll get more snow build up and by the sounds of it the roof hasn't seen this load case yet which could fail the roof. So I'm wondering whether you should be telling them to laterally prop the walls until you've done the calcs.

might be a good way out the project as if they don't follow your advice you could suggest terminating services. if there not interested in temporary propping whilst you do the numbers you can bet your life they won't want to do permanent works.

 

[haynewp]

I had a long response but it got lost when the I lost the connection.

Direct tension concrete members are required to meet certain splicing requirements by ACI. It may not qualify anyway.

The concrete beam plus part of the cmu wall below may be bending between transverse walls as a unit, if the concrete beam is adequately anchored to the wall below.

 

[BAretired]

There is more to the problem than structural issues. The building envelope also has to be considered. If the insulation was previously resting on the ceiling joists, where will it go now? How will the space between rafters be vented? How will the roof be fire rated?

It is not a trivial problem.



BA

 

[miecz]

Judging by the sketch, the roof pitch is around 3 on 12, so the lateral thrust is high. Looks like the concrete beam is 8 inches wide and 12 inches deep. So it may even be subject to torsion if it's loaded at the top and supported from below. I agree with hokie66. I'd double check that beam and it's connection to the resisting element.

 

[kslee1000]

DGpe:

Just to remind you one thing about the "folded plate" theory you were thinking. In my mind, it is a whole piece of material folded to shape, in contrast to your case that has two elements formed to shape through mechanical means. The stress flow of thes two cases should differ.

No matter what, I don't see stop sign in making this system work, though the checking and details could be challenging. Look out for the latest concerns from BA, which in need of clearing before proceed.

Again, good luck.

 

[oxbridge]

Yes i think that BA's points on insulation etc should be looked at 1st. I'm suprised the contractor/client haven't considered the insulation unless i'm missing something. Obviously any insulation and plasterboard added at rafter level will stress the the rafters more.

 

[COEngineeer]

Doesnt look good. You need to fix that asap.

Never, but never question engineer's judgement