Collar Tie Design Residential Wood Framing

[trackman417]

Came across something I did not find in any of the forum posts. Thought this would be interesting to discuss

Note: Collar ties referenced here-in are for ties that are in the upper third of the roof system.

All of my analyses are performed by skyciv.
For replication, my nodes are(x,y,z)(in feet):
Main Members/rafters(2X12's):
-(0,0,0)
-(12,9.33333,0)
-(24,0,0)
Collar Ties:
-(9,7,0)
-(15,7,0)

This is a setup for a house with a 8:12 gable roof that is 24' wide and 9'-4" tall. Collar ties 3 feet below the top of ridge.

Setup One:

The rafters sit on the top plates of a structural load bearing wall that is about 4 feet above a toe plate.
I used one pin(rotation free, x & y fixed) at coordinate (0,0,0).
The other end received a quasi pin(rotation free, y fixed, x free).
This will allow the whole system to try and relax as the roof gets more load thrown at it.​

Setup One Results:

Collar ties take all tension(approaching 1900lb) as the wall is allowed to deflect
X-direction deflection is 1.357 inches
Assuming both walls deflect the same amount, we have .67inches on each side of the wall​

Setup Two:

Same as setup one, with the exception we have two "typical" pins holding both sets of rafters into place.​

Setup Two Results:

Collar ties take only compression (550lb) and the walls soak up the resulting thrust from the setup(460lb)​

Every post I have read about collar ties and their inability to soak up thrust from this setup seem to stem from models having both rafters fully pinned rafters at the top plates. This does not seem like a very realistic model as the walls themselves CAN and WILL deflect in the x-direction.

Setup 1 seems to be a very realistic case where the walls can and will move to allow the collar ties to soak up the thrust generated from loading. Although, I do not think IBC takes into account these loads and a the collar tie connection to rafter must be designed.

Pictures are:

Loading
-Setup 1 deflection, axial loading, moment
-Setup 2 deflection, axial loading​

Thoughts?

 

[North2South]

Set your x restraint at the ridge. Neither wall is technically a pinned support. Using the ridge simulates the diaphragm as being the x restraint. This should provide similar results to your setup one.

 

[Homeengineer]

Hi Trackman417,
I am guessing that you want to remove ceiling joists to create a "cathedral" ceiling in the house...so you are trying to determine if you can use a collar tie to essentially form an "A" shaped truss, with the horizontal being a tension member.
With your tie member so high up, the tensions forces that you will get will be very high, the deflections at the lower ends of the rafters also very high as well, and you won't be able to come up with a reasonable way to fasten the collar tie to the rafter...you will also have very high bending forces in the rafters at the tie-to-rafter connection node....in my experience, the only way that this will work is if the tie is situated much lower in the roof...maybe only a foot or so above the original ceiling height.
So, you may need to abandon this concept and go with a ridge beam instead, and then the rafters become roof joists...
But forgive me if I have misinterpreted where you are going with this.
I am in Toronto, Canada, and our snow load is around 22psf...we always revert to a ridge beam arrangement for something like this.
The Ontario Building Code does not really address this sort of thing at all...it falls into engineered design....and collar ties are viewed as compression members in a non-engineered roof...they serve to shorten the effective span of a rafter. But ceiling joists are expected to act as a tie, to prevent the outward spreading of the rafters where they bear on the frame walls.

 

[Ron247]

We have had part of this discussion before I think. Be careful about confusing Collar Ties with Rafter Ties. Depending on the code your are using, these 2 terms can be totally different items. In the IRC or IBC, the Collar Tie in the upper third is only to keep the 2 planes of the roof connected to each other under a high wind. They are also called "wind braces". The ones located in the lower third are called Rafter Ties and that is what you use when you want to remove the ceiling joist that typically sets on the wall.

While many of us engineers call Rafter Ties, Collar Ties, in some building codes, they are 2 totally different components that serve totally different purposes.

 

[Ron247]

Look for a post in April of 2019 called "Collar Tied Roof Fail".

https://www.eng-tips.com/viewthread.cfm?qid=451285

 

[trackman417]

Thanks for the thoughts. This is by no means something I am trying to design. Just something I thought I would look into more on the 2D frame analysis realm.

It appears to me that the only time we see compression in a collar tie is if we assume the walls bracing the rafters do not deflect or move. This does not seem to be very realistic since the walls will move to accommodate and allow the collar ties to take tension. Although, as previously stated they will require quite a beefy connection to keep them from yanking out of the rafter.

I do see your guy's points that the bending moment in the rafters becomes absolutely huge and really only 2X12's would work in this sort of arrangement.

Look for a post in April of 2019 called "Collar Tied Roof Fail".

https://www.eng-tips.com/viewthread.cfm?qid=451285

After looking at the posts, it looks like the rafters never made it past the paperwork and into some sort of modeling software.