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.

 

[COEngineeer]

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

Never, but never question engineer's judgement

 

[DGpe]

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?

 

[hokie66]

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.

 

[Ron]

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.

 

[kslee1000]

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.

 

[DGpe]

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)

 

[hokie66]

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?

 

[kslee1000]

"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.

 

[msquared48]

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

 

[enginerding]

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.

 

[haynewp]

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

 

[dik]

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

 

[Ron]

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.

 

[haynewp]

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.

 

[DGpe]

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.

 

[kslee1000]

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.

 

[haynewp]

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.

 

[kslee1000]

Same here.

 

[msquared48]

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