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.

 

[haynewp]

See a few posts back up, he is saying he has a concrete tension tie at the base in place of the ceiling joists.

 

[kslee1000]

Also, I think the new roof by the contractor has altered the design. Without sketch, we can only guess. But I don't think the contractor simply cout off the chords and ties without any strengthening. Otherwise, they will already be in court rather than talking here.

 

[DGpe]

Attached please find roof plan with sketch of joist ceilings cut...for your perusal...

 

[DGpe]

kslee1000:
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"...

 

[COEngineeer]

is it possible that the ridge is cantilevered a little bit from a column? What size is the ridge beam? Where is the closest wall?

Never, but never question engineer's judgement

 

[BAretired]

Assuming that the upper right hip beam continues to the corner, the skeleton of four hip beams and ridge beam forms a stable structure.

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

 

[kslee1000]

Attached 3 illustrations of wood roofs to futher this discussion. Please have a look, then comment.

 

[kslee1000]

Here is the 2nd.

 

[kslee1000]

and the 3rd.

 

[DGpe]

COEngineer:
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.

 

[kslee1000]

Sorry, the 1st & the 3rd are duplicates. Here is the conventional hip roof illustration.

 

[DGpe]

kslee1000:
The framing shown would be okay it it was done with trusses. Not the case here...everything conventionally framed.

 

[DGpe]

kslee1000:
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?

 

[kslee1000]

DGpe:

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.

 

[msquared48]

I agree with Ron here, but developing the thrust of the hip at the corners is critical here, preferably due to corner plywood shear walls. With an elongated rectangular configuragtion, I have a problem with the tension ring analogy approach, although I can undserstand the logic. My guts tell me that it would be more likely to see a "snap- thru" type failure. Professionally, I use this concept only with nearly square or round structures. Just my guts here I guess.

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

 

[BAretired]

DGpe,
You asked:

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?

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

 

[haynewp]

The typical rafters are connected to the same proposed concrete tie beam as the hips are. I don't see the ridge beam taking all this bending.

 

[kslee1000]

My suggestions are (just random thoughts):

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.

 

[DGpe]

I'll try to address most issues:
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...

 

[DGpe]

Just want it to say that its been great getting everyone's input. However, there seems to be little engineering literature about this topic. All my college and later on books about wood don't address this issue.
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...