Collar Tie Design
Collar Tie Design
(OP)
Can anyone provide a design resource for collar tie design for rafters. I have a contractor that has deviated from the requirements of the building code. Thanks
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RE: Collar Tie Design
RE: Collar Tie Design
1. Solve for reaction of the rafter at the wall.
2. Solve for horizontal force in collar tie by method of sections. Create free body diagragm, of half the roof and solve about the ridge. Known values are the roof load and the reaction. Solve for horizontal force in collar tie.
Design connection to resist horizontal force.
RE: Collar Tie Design
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www.SlideRuleEra.net
RE: Collar Tie Design
What do you suppose a collar beam of the sort described in the note, actually does?
Surely it has extremely limited structural function.
Your thoughts?
RE: Collar Tie Design
Here is the link to the paperhttp://www.pubs.asce.org/WWWdisplay.cgi?9700358
I assume that the Forest Products Lab determined recommended size and placement years ago since a summary of the use of collar beams in in their classic book "Wood Frame House Construction, Handbook No. 73". You can get a .pdf copy of this document from my website, see pages 78, 89, 223, 232, 242 & 248.
www.SlideRuleEra.net
RE: Collar Tie Design
They work the same way, but serve different purposes. Collar ties aren't going to prenvent much rafter/wall spread.
RE: Collar Tie Design
Often collar beam/ties actually perform very little structural function or at least there are multiple ways to accomplish the same thing.
How has the contractor deviated and what specific requirements?
It has been my experience that collar beams and ties are often misunderstood or their function is misstaded or stated in language that is easily misunderstood. If you could describe the conditions (or provide a sketch or link to illustrate things) perhaps that would be helpful.
SlideRuleEra;
Great post. Thanks
RE: Collar Tie Design
I just had a conversation with a client for this very reason. They are trying to elevate the ceiling by using an LVL to span between rafters. The roof rafters are in place with a 2x10 (non-structural) ridge beam and there exists no tie between the top plates of the wall. I explained that the top of the wall has to be braced or the wall must be able to withstand the thrust force from the rafters. The other option would be to install a structural ridge beam.
Unfortunately, this isn't the first time this discussion has occurred with this client, but I believe he is finally starting to understand.
As BrainRost, stated, "no more than 1/3 up from rafter bearing at wall". Although, you should still consider deflection. Even if the rafter tie reduces the thrust it can still produce undesirable wall deflection. Thus leading to calls from the owner about drywall separation at the ceiling/wall corner.
Just food for thought.
RE: Collar Tie Design
The "rule of thumb" may function as a starting point (to begin to investigate the viability) but the tension connection in the tie, bending in the rafter member and deflection (actually both vertical deflection and horizontal spread) are often critical in such cases especially as the tie location gets closser to the 1/3 point.
It's not unusual for the rule of thumb to not "work".
Sometimes I can get adjacent floors to resist the thrust. Thsi may work unless the room projects out from the main body of the house. Also, if one side is not braced by a floor, all of the thrust will go out to the "open" side wall.
RE: Collar Tie Design
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Dik
RE: Collar Tie Design
Good Luck!!
RE: Collar Tie Design
ctcry is also dead on...there is a distinct difference between collar ties and rafters ties.They are interchanged to many times in the wrong situation.
Sum moments about the ridge……
RE: Collar Tie Design
You indicated that the builder has deviated from the requirements of the building code. Based on that statement I am assuming that in addition to the sloping rafters you also have ceiling joists at the rafter bottom spaning from support wall to support wall.
In a system where you have rafters sitting on the wall top plates with ceiling joists running from one wall to the other, collar ties are of dubious value. In the past some model building codes required them in their wood frame conventional construction provisions.
My understanding is that the use of collar ties was based on common framing practice in some parts of the country in the past. As a result that practice was included in some building codes with out a specific engineering reason to do so.
When you have rafters and ceiling joists collar ties generally are of little or no value. As others above have indicated to make the ties work as part of the system requires large connection forces which are probably not achieved through the tie nailing common in the field.
Note if you don't have ceiling joist at the bottom of the rafters then you have rafters with a raised tie member. In that case all the members and connections must be designed to carry the applied loads.
RE: Collar Tie Design
RE: Collar Tie Design
You need to think long and hard about this design. If you only have rafters and collar ties (up at the ridge) then you have nothing that will resist the thrust load or deflection at the top of the wall. If the contractor doesn't want ceiling joists, the only option you have is to install a structural ridge beam or installing something at the top of the wall that can span corner to corner that will handle the thrust force and associated deflection.
Good Luck. You have a battle of you.
RE: Collar Tie Design
easleyml: I'd be very surprised if omitting the rafter ties (ceiling joists) at the plate met code without the provision of a load bearing live ridge... .02