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Design literature on hip roofs

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.

RE: Design literature on hip roofs

on my previous job I specified bent steel beam (not really bent, just welded in an angle).   

Never, but never question engineer's judgement

RE: Design literature on hip roofs

(OP)
This project is an old residence (1956) with a hand framed hip roof. Problem is the new owner wanted a cathedral look and during remodeling the contractor saw no problem in removing all the ceiling joists (that basically act as tension ties)and gave the owner what he wanted. I had done the structural engineering for a 2-story addition. Of course, during a site visit I saw what had been done and almost had a cow.But the contractor (not an engineer) keeps reassuring the owner that I'm exaggerating and is not a big deal...Any thoughts?

RE: Design literature on hip roofs

I think the contractor and owner are both morons, and they may well come to that realisation in the next big wind storm or snow storm.

The thrust has to be resolved by some means.  Just follow the load path.

RE: Design literature on hip roofs

DGpe....if the walls can't handle the thrust, you will have to have collar ties/beams.  If I can dig one up, I have photos of a similar failure...this one due to excess collar tie spacing and no ceiling joists.

RE: Design literature on hip roofs

Before jump into conclusion, I suggest to look into this matter from another angle. If you are asked by the client to come up a solution to make the existing roof the cathedral type, what would you do? I think you will be glad to take on this challenge rather than an outright answer "No".

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

(OP)
I have a pretty good idea of how to design it (if from scratch). but on an existing roof, how can you handle the reaction from the hip rafter at the top of the ridge without ripping whats remaining of the roof to take care of this load? Other than that, it's  a matter of detailing the proper connections between members and using the proper connector to transfer the thrust at the concrete tie beam which should act as a tension ring okay.
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

I'm not sure how the removal of the ceiling joists has affected the hip to ridge connection.  Wouldn't that connection have always been subject to thrust from the hip?

RE: Design literature on hip roofs

"it's a matter of detailing the proper connections between members and using the proper connector to transfer the thrust ..."

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

The concept works well for a simple rectangular hip structure, but can get real complicated real fast when the rectangularity is lost, as is with most Architectural designs.   

Mike McCann
MMC Engineering

RE: Design literature on hip roofs

If there is a really bad problem, you will be able to see it almost immediately in the walls.  Put a level on the walls to see if they are still plumb.  I have seen this many times when people try to add a loft even on a gable roof.  

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

I'm not sure how the removal of the ceiling joists has affected the hip to ridge connection either.

RE: Design literature on hip roofs

YOu could provide diagonal struts from the top plate to grade and grow grapes or vines on the struts to improve their appearance.

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

Removal of the ceiling joists allows rotation at the ridge connection, for which it was not designed. In order to counteract that, you will need to place a collar tie or create a moment connection at the ridge.

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

He said there is a concrete tension tie beam at the base of the hip. This could take the thrust in theory. If the ridge rotates some, as long as the thrust is resolved at the base I guess (and as hokie said) I am not seeing it necessarily as an issue at the pinned ridge. Unless the concrete tension tie is very flexible compared to the old ceiling joists.

RE: Design literature on hip roofs

(OP)
I'm fairly comfortable that the perimeter concrete beam can serve as a tension ring to handle the thrust. My concern is the hip beam. If you can picture it the difference in height between supports (low tie beam and ridge) is approximately 48"; the loading (DL+LL) is triangular (max at ridge and almost zero at tie beam). Obviously because of the slope, there will be an axial load in the hip bean (thus the thrust)but there will also be a vertical reaction at both the tie beam and the at the ridge. Question is: How to handle the upper reaction at the ridge? Of course, everything would be reversed under wind uplift, which is the norm in this part of the country...so, potentially there can be a gravity concentrated load at the ridge where the hi begins and a net uplift load at the same location under wind...How are those loads handled without cross-ties or ceiling joists? Sorry if over extended myself.
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

Make a simple sketch on the contractor's modification and show the original roof by dashed lines. This way will be much easier for people to locate the problemic areas.  

RE: Design literature on hip roofs

The vertical loads get down to the base of the hip and tie beam like in an arch, if there is something resisting the thrust at the base then a vertical support at the ridge is not needed. I guess I am missing something about this particular case.

 

RE: Design literature on hip roofs

Same here.

RE: Design literature on hip roofs

The top of the hip at the ridge is usually supported by a ridge beam that is supported by either a nearby column, or a collar tie arrangement.  

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

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

RE: Design literature on hip roofs

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.

RE: Design literature on hip roofs

(OP)
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"...

RE: Design literature on hip roofs

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

RE: Design literature on hip roofs

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

RE: Design literature on hip roofs

(OP)
kslee1000:
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

(OP)
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?

RE: Design literature on hip roofs

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.  

RE: Design literature on hip roofs

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

RE: Design literature on hip roofs

DGpe,
You asked:

Quote:

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

RE: Design literature on hip roofs

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.

RE: Design literature on hip roofs

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.

 

RE: Design literature on hip roofs

(OP)
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...

RE: Design literature on hip roofs

(OP)
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...

RE: Design literature on hip roofs

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.  

RE: Design literature on hip roofs

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

RE: Design literature on hip roofs

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...

RE: Design literature on hip roofs

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.

RE: Design literature on hip roofs

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?

RE: Design literature on hip roofs

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.  

RE: Design literature on hip roofs

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.

RE: Design literature on hip roofs

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

RE: Design literature on hip roofs

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.   

RE: Design literature on hip roofs

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.  

RE: Design literature on hip roofs

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.

RE: Design literature on hip roofs

Doesnt look good.  You need to fix that asap.   

Never, but never question engineer's judgement

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