This sounds a bit like a no-brainer, but somebody asked me to check if a collar beam affects the length of span of the rafter for code-compliance purposes.
I said, no, it doesn’t. Span is from support to support, and a collar beam doesn’t support anything; it just holds the two rafters in the set together and prevents spreading. Of course, it incidentally also helps prevent some bowing….
Hmmm. Now I’ve got me wondering. And since I traded my full copy of the code for another framing nailer years ago, all I’ve got is photocopies of the critical tables, no text pages to look up the answers, duh.
So, stupid question no. 1: Figuring a hypothetical rafter with a 16-foot span and a collar beam at 8 feet, doesn’t that collar beam have some effect on rafter deflection at the mid-point? Stupid question No. 2: Does using a collar beam allow use of a smaller rafter scantlling for the same span?
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice….
Replies
1. Yes
2. No
Does using a collar beam allow use of a smaller rafter scantlling for the same span?
What do you mean by "scantling"? All I can find are some 17th and 18th century definitions that seem to equate it to a stud ("an upright in house framing").
Scantling is a term used a lot in boatbuilding circles to describe the sizing of component elements. Based on his vast knowledge of ropes, I'll bet Dino is well acquainted with that world.Bill
The answers are yes, yes.
The problem you have is that prescriptive code does not consider this so you will need to do engineering.
The problem you have is that prescriptive code does not consider this so you will need to do engineering.
Heh, heh.
Prezackly.
The client's roof framing is all 2x6 rafters, 24" OC...but the span is ~16' (17' long pans). I'm looking in my tables and find that for that span, she's gotta have at least 2x10s on 16" centers or 2x12s at 24"OC. It thus seems obvious there's no way that framing could have held up the roof in a 2.5kpa snow-load area for all those years...but it has, and the framing shows no signs of stress or damage.
For the benefit of any engineers who might just possibly be bored this evening, here's a rough sketch of the roof structure as it currently exists. (The current roofing is screw-down steel on 2x3 strapping 24"oc across the rafters.)
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Edited 8/29/2007 4:59 pm ET by Dinosaur
Edited 8/29/2007 4:59 pm ET by Dinosaur
Sorry, I screwed up sizing the drawing so I deleted it above. Here it is:
View Image
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Ba-dump- Bump!
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Holy tabernac! Your question is going to open up quite a debate.
According to yours and my code, or at least the old one, section 9.23.13.7 refers to intermediate support for rafters and joists. Without typing out the whole thing it talks about collar ties "are permitted" so that means "if" you use them they are to be used and installed in a particular, albeit, vague way. It doesn't say you "must"use them or "shall" use them.
In summation, I agree with your original statement:they are not needed.
I started in construction as a 16 year old kid as a structural steel draftsman and had to learn all about structures and if components of a structure were necessary there would be very detailed formulars and material use like in beams and trusses etc.
Anyways, that's what I believe and I'm sticking to it;)
roger
I'm not quite following the sketch -- don't the 2x4s define a common plane with the 2x6s on the room interior? I'm assuming the 2x4s are at 16" centers and were just infilled for lath.
I'm assuming the house is balloon framed, too, right?
I think you already have your answer about the fact that the collar ties acted to increase the effective span of the rafters. Without a stamp from a PE, the city will never believe it, of course. Real world performance means absolutely nothing to them (at least it doesn't to the city office where I live).
By the way, when was the house built?
Edited 8/31/2007 2:06 am ET by Ragnar17
The 2x4's are acting solely as ceiling joists to hold up the gyprock. They are not connected to the new roof frame (the 2x6's) at all, but are nailed to the original rafters...which are unsawn, round-log stock (about 4-5" dia) set on 3½' centers (+/- 4"). These original rafters were all sawzalled off just above the ceiling joists when the roof was rebuilt about 15-20 years ago. The sloped part of the ceilings on the second storey is attached to what remains of those original rafters; the flat part is attached to those 2x4s.
The collar beams are part of the new roof structure, and are not connected to anything but the new rafter sets. They 'float' about 2" above the tops of the ceiling joists.
I don't have to get the roof frame approved; it already exists and until somebody with a badge and a gun comes by and says tear it out it's gonna stay there. The question about its strength arose when I suggested to the owner that we deck the roof with 1x6 rough hemlock instead of the crapola ½" beaver barf the local quickie-roofing guy proposed. There is no decking on it at present; just 2x3 strapping on 2' centers with screw-down steel on top of that.
She likes the idea of the hemlock--it doesn't rot, is very strong, and is less than 50 cents a board foot--but wondered if the wood might be too heavy, especially as it will be green when we put it up (nobody planes or KDs hemlock around here).
I'm not really worried; but my conservative nature makes me wanna get a few other opinions.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Thanks for the additional information. I was surprised to see the 2x6 rafters on what seemed to be an old home. With small logs as rafters, I guess the house must be a lot older than I intially thought -- any guesses on the approximate age?
I'm surprised to hear you characterize hemlock as rot-resistant. Out here in the PNW, it does not enjoy a good reputation. Of course, it doesn't seem like you'll be installing it in a location that will see weather, so it may be a moot point.
Here's a question from a non-expert on framing: if you're going to do a tear down on the roof, why not just sheath with ply? Wouldn't panels do a whole lot more to stiffen the structure? Are you going to be installing skip sheathing for wood shingles or something like that?
The house is over a hundred years old but I don't have more precise info than that.
I am surprised to hear you say hemlock isn't well regarded out in the great north wet, but maybe we're not thinking of the same specie. The stuff I'm talking about is called 'pruche' in French; it's the tallest of the eastern conifers and has short, flatish needles and acorn-sized cones. It is very strong, and is rated higher structurally than spruce in my tables.
Around here it is generally only cut for posts (6x6 and up) but these can be used for some of the worst kinds of service--earth retaining walls, dock posts, fully-exposed decks--and will last almost as long as red cedar. I know a few mills which saw hemlock into 1x and 2x stock; the price is about one third or one half what I'd pay for spruce.
Sheathing the roof with 5/8" plywood would be okay but in addition to being twice the cost of the hemlock, 4x8 sheets are a pain to handle up on a windy roof, especially a steep one that isn't square, LOL. This job will be done by a very small crew (3) so planking it will be a lot easier. If racking becomes an issue, we can run some of the 2x3 strapping we'll be tearing off as 45º bracing on the insides of the rafters. (Remember, there's no sheathing at all on the roof right now, either. Only steel over strapping.)
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I am surprised to hear you say hemlock isn't well regarded out in the great north wet, but maybe we're not thinking of the same species.
I did some reading, and the type of Hemlock out here is "Western Hemlock." Out here, it's commonly used for framing, and I think it's also used in manufacturing PT lumber. Doug fir is typically specified for critical framing members or whenever a higher allowable bending strength is required.
http://en.wikipedia.org/wiki/Tsuga_heterophylla
The articles at Wikipedia seem to suggest that Doug Fir is faster growing than Hemlock; if that is true, I can't see what advantage Hemlock has to growers.
The other major difference in performance is regional climate, of course. The moderate but continually damp "northwet" climate can be tough on species that perform great in other locations.
Are you in Quebec, then?
Edited 8/31/2007 9:11 pm ET by Ragnar17
Yup he be in Quebec!
North of Montreal.
I also know Hemlock as a rot resistant wood. Also known as Barn wood here!
Turns gray over time and is bullet proof to rot."No doubt exists that all women are crazy; it's only a question of degree." - W.C. Fields
Too bad Ian and Robert have gone back to school; I'm probably starting this job on the 10th and could have used them as helpers. I gotta get this roof torn off and blacked back in again on the same day (so, one half at a time max) because there's renters living in the place.
Oh well; I've got two guys lined up. The main roof is only 10 square so we oughta be okay.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
The NBCC lists Hem-Fir in category 2 of the tables, just below DFir-Larch and above SPF, and mentions that Hem-Fir includes Western Hemlock and Amabilis Fir. Douglas Fir (rather chauvinistically referred to as 'B.C. Fir' up here, LOL) is still the overall champ for strength in framing wood, but Hemlock isn't far behind. In #1&2 grade, the span numbers are identical for the two species groups, and for No. 3 (not that I would use it for framing) Hemlock is actually rated for longer spans that D-Fir.
I'd like to see it more generally available as a framing lumber around here--especially given its low cost--but this is SPF territory; if you want anything else you gotta look around for a mill unaffiliated with one of the two big chains in this province (RONA or BMR).
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Edited 9/1/2007 8:10 am ET by Dinosaur
I don't have enough engineering backround to aswer the questions but I would guess that the collar ties would have some benifit to the strength of the rafters. I could see them working with the rafters in such a way that would be similar to how trusses are designed. I'm sure there are load pathways that allow for distribution and cancelation of other acting forces. This is where I stop however. Talk to an engineer.
Jason
"it aint the work I mind,
It's the feeling of falling further behind."
Bozini Latini