Collar Tied Roof Fail 3

[XR250]

So this is a job one of my competitors did a few years ago. 60 ft. span trusses converted to a vaulted ceiling by sistering with 9 1/4" LVL's and adding LVL collar ties. 6" of measured sag. The whole roof is getting ripped off and replaced with trusses again. I guess he used the "1/3 from ridge" collar tie rule without checking in 2D software. Ironically, the owner is not seeking legal action.

 

[MIStructE_IRE]

Wow. That even LOOKS wrong! Its way too shallow.

 

[Ron247]

Guess they though a collar tie and a tension tie are the same thing. I though collar ties only keep the ridge from separating under high wind uplift. I never pictured them as tension ties. We used to put them right under the ridge for stick-built roofs.

 

[txeng91]

Wait is this a contractor going rogue or an engineer actually signed off on this? Even if the ties were low enough for the rafters to be able to take the bending moments you could never get the connections to work. Also nice 60’ hinge they have going across the end wall at the plate break.

 

[XR250]

Wait is this a contractor going rogue or an engineer actually signed off on this? Even if the ties were low enough for the rafters to be able to take the bending moments you could never get the connections to work. Also nice 60’ hinge they have going across the end wall at the plate break.

Yes, designed by a PE (believe it or not). Gotta love the 60 ft. hinge. That wall will be re-framed.

 

[Ron247]

Put some really big speakers near the endwall and watch the wall move to the music. Disco House.

 

[txeng91]

Oh wow, bad look for us... Physics: 1,
Structural engineers: 0

 

[CANPRO]

yikes, thats ugly. I always understood the 1/3 rule of thumb to be 1/3 up from the top of wall, not down from the ridge.

A question for the fine folks of E-Tips - is there an obligation to report the substandard work of this engineer? You could argue this wasn't really a life-safety issue (could have been, but realistically probably not) - but if the engineer is producing this kind of crap work, maybe the next mistake is a dangerous one.

 

[MIStructE_IRE]

Canpro - that was always my understanding of the 2 thirds - 1 third rule also! I guess this guy got it backwards!

 

[XR250]

There is no rule of thumb for "rafter ties" Each case needs to be analyzed. Collar ties are required to be in the upper third but are not intended to be ties to prevent spread at the walls.

 

[Ron247]

For collar ties, the code we use says located in the "upper third". To my knowledge they are only intended to keep the ridge from separating in a high wind. Watch some wind disaster video and you see an entire side of a roof lift up and disconnect from the other side. To my knowledge, collar ties are not intended to serve as tension ties at all as related to what a ceiling joist provides.

when I was a carpenter years ago, in addition to upper third, we also made sure we placed them at least above head level and preferred them close to the ridge from a material usage standpoint. Real aggravating to walk thru an attic and have to duck down every 4' while crossing over ceiling joists. The most comfortable place for nailing was about shoulder level but above the head was not that much harder. Once you nail them too low, anyone in the attic has to deal with them for the life of the house. For those of you putting them in the lower third, I am sure someone has cussed about that. Worse than having to duck down a little, is having to get down on your kneels to crawl under or attempt to step over the low ones. When I do have to do roof framing as an engineer, I actually state they must be at a specific location or range that suits me.

This is a definition from the internet: Collar ties and rafter ties are examples of tension members. ... A collar tie is a tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge beam during periods of unbalanced loads, such as that caused by wind uplift, or unbalanced roof loads from snow.

The snow part of that definition I have never heard, but I am in a 5 psf snow area.

 

[Ron247]

CANPRO, as far as reporting another engineer, I bet all of us have wrestled with that. I am yet to report one but have run into work I honestly felt was substandard but fortunately for me, no one was paying my time to investigate enough to make a valid accusation. If I do not do enough investigation before I report someone, I feel like I am treating them unfairly and would not want to be treated that way myself. I might call them though as a courtesy. I have done that and I have documented I made the call. In those cases, I had a "suspicion" but it was not verified. If I saw an absolute train wreck in the making, I would be more aggressive, but have never been faced with that.

I have done designs but not been paid to review the final product. On those projects, I have no idea if it was installed per my instructions. So, you could look at one my projects, see crap, but it may be because the contractor did not do as instructed. The track record on the ones I have been asked to go back and review, easily 60% had something wrong that had to be corrected. Of that 60%, on half of them the contractor "did not see a need for that". My favorite was "my boss said that was optional". I asked him to show me where in my drawings it said detail C is optional. I told him to correct it, but whether he did it or he made his boss do it "was optional".

I also know from actually being in construction for several years, most contractors I met disliked us and for some reason, almost insist on doing something not per our drawing for some really strange reason. They just had to change something.

 

[OldBldgGuy]

Sorry Ron247 & Canpro but I'm with OP XR250: There is no rule of thumb for "rafter ties". Each case needs to be analyzed. "Collar ties are required to be in the upper third but are not intended to be ties to prevent spread at the walls." That's not necessarily right. This whole thing looks like a disaster, but nothing can be assumed without analyzing it, and many smaller buildings successfully use collar ties as TIES for the walls and have for hundreds of years.
Collar ties, as the name says, are frequently in tension, but also as building codes (OBC & NBC) recognize are often in compression. I might shudder to see this one, but I also wouldn't make the case that there is only one case.

 

[CANPRO]

To clarify - I wasn’t suggesting a rule of thumb be used to replace analysis, especially for the spans shown on the OP. But it is a good starting point for your design.

 

[Ron247]

Guess we will just have to agree to disagree OldBldgGuy. But what we are actually disagreeing about is "terminology". While there are a lot of components we could correctly call a collar tie, the "collar ties" referred to in most roof framing I have seen are the ones not intended to keep walls from separating but intended to keep roofs from separating at the ridge. The ones in the code stated as upper third are not intended at all to keep walls from separating. Put them right at the ridge and they will prevent no spread of the walls regardless of how big or how many fasteners unless you make the rafter A LOT bigger. A minimum 1x4 at 4' on center with four 10d Box nails is not that much resistance to wall thrust. I think the Box Nail is the lowest of the framing nails (Box, common, sinker).

Now when you use a tie at the lower area of a rafter because you have moved up or removed a CJ or your ceiling joist frames parallel to the rafters, you can call it a collar tie or call it a "rafter tie or a rafter collar tie" but it is not the same tie as the upper third of the rafter one. IRC 205, Section R802.3.1 calls the ones used when you place the CJ higher than the wall or the CJ perpendicular to the rafters a "Rafter tie". Collar ties are later mentioned in that same section of the code and directly state "to prevent wind uplift". Also, Rafter ties are minimum 2x4 while collar ties are minimum 1x4. I agree that given a big enough rafter, I could place a tension tie in the upper third that would hold the walls together and I could call it anything I want. But this discussion came as a result of the upper third comments. We both probably feel equally strong about our viewpoint. But that is why discussion is good for all of us.

Again, we are having a terminology clash. If in a document, I state a collar tie holds my shirt closed at the top, then for that document that is what a collar tie is. IRC, states what a collar tie is when you are referring to a roof. A rafter tie is yet another term they define.

 

[kipfoot]

I suspect that the analytical model for this had pinned connections at each support....and unfortunately the building does not.

 

[XR250]

From what I know of this engineer, no analytical model was ever created....

 

[GC_Hopi]

Here is a decent article on collar ties vs rafter ties. It may help with the terminology: Link

 

[Ron247]

Thanks GC for the link. That does give a good outline in one document of what we are discussing. The statement in the article that collar ties may not be required in the code I disagree with. I did not find anything in the code that says you do not have to have them. The article may mean that if you do not see them, they may be done as the metal strap over the ridge as depicted in the 2015 IRC that you might not be able to see. So reporting them as not present may be erroneous. Installing collar ties (in my area carpenters may call them "wind braces'), is a lot easier than the metal strap over the ridge. We always used to install them the first time it was raining after we decked the roof. Deadwood, rafter bracing and collar ties was done after decking but before shingles. Gave us rainy day work. We could also used the rafter bracing to help level out the roof surface after decking.

In my area, houses 60+ years old not tend to have collar ties.

 

[BAretired]

The following is from the Link supplied by GC Hopi:
Tension

The roof framing mock-up below shows a standard collar tie. As the load is applied downward, tension in the collar tie is increased.

The first comment about load applied downward is not clear to me. It seems to me that when load is applied downward, the load in the rafter tie will be tension but the load in the collar tie is more likely to be compression due to the bending of the rafter.

BA