Design literature on hip roofs
Design literature on hip roofs
(OP)
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.
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.






RE: Design literature on hip roofs
Never, but never question engineer's judgement
RE: Design literature on hip roofs
RE: Design literature on hip roofs
The thrust has to be resolved by some means. Just follow the load path.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
Once you have came up your own solution, then you can back check, and point out the problems with the non-engineered works with confidence. Two specific areas I would look into - the roof geometry and member continuity, with the emphasis on the latter.
RE: Design literature on hip roofs
Does any one have any engineering literature in design of these type of roofs or just follow basic principles (i.e., hip will behave as beam column, etc)
RE: Design literature on hip roofs
RE: Design literature on hip roofs
I think that's the key. In a beam-column (rigid bent), as long as the joint is capable of handling the moment carried over by the members, the horizontal load is transferred from the joint to the column base below. Therefore, contunity should be the point of focus.
RE: Design literature on hip roofs
Mike McCann
MMC Engineering
RE: Design literature on hip roofs
A gable roof can be a little more forgiving because the ridge can act [somewhat] as a beam. But with a hip roof, you should be able to see movement at the top of the walls pretty quickly - especially near the center of the wide portion of the house.
I agree with hokie that both the contractor and the owner are morons if they don't think there is a huge potential for a problem.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
If a cathedral ceiling is intended, then you may have to introduce a horizontal beam with light cable struts providing the reaction; check for compression because it may get trickier for unbalanced loading.
Dik
RE: Design literature on hip roofs
Once the first rafter off the hip in the primary structure is stable, the hip can be framed into it with less of a problem, but you will need to use succeeding rafters to accommodate the lateral thrust component. That will yield a tension diaphragm in the sheathing.
As hokie66 said, follow the load path...which becomes a bit complicated with an unbalanced condition, but resolvable.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
Also, I'm trying to deal with pin connections...very difficult to do moment connections in something like this.
Thanks for the comments anyway.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
The cutting of the main collar tie at the hip, or ceiling joists if the case here, WILL affect the hip connection as serious structural instability is created.
Mike McCann
MMC Engineering
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
I think the matter is currently stable because its under gravity only and, as mentioned previously, the perimeter concrete beam is acting as a tension ring...but I still don't like it. And when I say the contractor cut off the ceiling joists that's exactly what happened. Now its a cathedral ceiling and the owner "loves it"...
RE: Design literature on hip roofs
Never, but never question engineer's judgement
RE: Design literature on hip roofs
One problem is likely to be the adequacy of all six of the connections for wind uplift and gravity load. Another issue may be the capacity of the ridge beam as a flexural member and each hip beam as a beam-column.
It is probably behaving as a folded plate structure at the moment, but that cannot be relied upon over the long haul because the roof deck was not built with that behavior in mind.
BA
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
RE: Design literature on hip roofs
See revised sketch. Only 1 partition and a flitch beam (3- 2"x12" w/steel plate) about 4-ft from the ridge. Other than that, nothing; place is gutted.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
The framing shown would be okay it it was done with trusses. Not the case here...everything conventionally framed.
RE: Design literature on hip roofs
Situation most resembles your case 2 without cross ties or ceiling joists.
I agree that current behavior is similar to that of a folded plate. Question is: what to do to make safe in the long run and somehow maintain the cathedral look?
RE: Design literature on hip roofs
How long this modification/change has been up? You may want to check if the contractor had beefed up the rafters, and strengthened the connections along the ridge. I am not saying it will work for sure, but it can be done, and may have already been done. Also, keep in mind that effects of snow and wind on pitched roofs are highly arguable, and problems are usually in the valley rather than at the ridges, or end hips. Again, without further look into the details, every argument/comment is merely based on guesses, and could be way off the mark. My intention was not meant to brush this event aside, rather I think this is a good opportunity for all of us to learn something outside of the conventional ways in handling this type of situation, if you can keep feeding us required information.
RE: Design literature on hip roofs
I'm still having a problem here with the location of the existing concrete beam that acts like a collar tie. Where is it on the diagram of the structure first posted above?
Mike McCann
MMC Engineering
RE: Design literature on hip roofs
You asked:
1. Convert it into a proper folded plate with a plywood skin on the bottom of the rafters.
2. Truss the underside of rafters to achieve the same as item 1. above.
3. Add a beam on the long walls designed to carry the lateral load from end wall to interior partition (if you feel it is adequate). This beam could be structural steel and could be connected to the concrete tie beam. It could be inside or outside the wall. If outside, you may have to lower the eave soffit to accommodate it.
4. Beef up the five beams so they can take all loads. This would still rely on the concrete tie beams to be adequate at the corners.
BA
RE: Design literature on hip roofs
RE: Design literature on hip roofs
1. Add short horizontal pieces to make the rafters a A-frame (stabilize the plane in direction of rafter, and not very bad looking from below).
2. Make sure the lower end connections are capable of transfer the loads.
3. Beef up the hip beams, and theirs lower end connections to resist the load carried over by the ridge beam, hip rafters, and longitudinal thrust.
4. Watch out ventilation requirements.
RE: Design literature on hip roofs
1. Alterations were done in the last 3 weeks.
2. Concrete beam is an 8"x12" perimeter beam all around. Its called a tie beam around here because it caps the masonry walls below all around. Which explains why I'm not too concerned about the thrust as the 4#5 rebars in the tie beam should take all the tension (i.e. tension ring).
3. The ridge beam, although only a 2"x10", i think it makes no difference because the rafters in the remaining "A" frame (without ceiling joists) can be designed to act as a 3 hinged arch which will only produce a reaction (vertical + horizontal) at the concrete tie beam (exterior supports, not at the ridge).
4. I like the idea of the folded plate behavior and will have contractor put plywood sheathing at the bottom of the rafters.
5. Finally, I think that if I can design a connection that will "grab" (for a lack of a better word) the ridge beam and both hips and, of course, assuming the tie beam below can act as a tension ring, then the design should be safe for gravity loads. For reversal (i.e. wind), if the connection at the tie beam below is adequate for both thrust and pull, the the connection at the hip (previously mentioned)should also hold its place.
What do you guys think?
What a royal pain in the ass for a residence...wait until the owner gets my bill...then he's gonna have a cow and I'll probably not get paid...
RE: Design literature on hip roofs
Thanks a million any way...I just need to be able to sleep at night and I guess that's why we chose to be what we are...
RE: Design literature on hip roofs
RE: Design literature on hip roofs
BA
RE: Design literature on hip roofs
RE: Design literature on hip roofs
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.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
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.
RE: Design literature on hip roofs
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.
RE: Design literature on hip roofs
It is not a trivial problem.
BA
RE: Design literature on hip roofs
RE: Design literature on hip roofs
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.
RE: Design literature on hip roofs
RE: Design literature on hip roofs
Never, but never question engineer's judgement