Okay, here’s another stupid question (the longer I live it seems like the less I know!):
We are replacing four large (52″ wide) windows in a gable end wall with three narrower (36″ wide) windows. (Actually (oh, no, the dreaded word) they will be paladian windows–there is a half-circle window going in over the middle window.)
The roof is scissors trusses and they bear on the walls that are perpendicular to the window wall. My feeling is that this wall is a curtain wall and only needs to be able to support the weight of the materials above the windows and so massive headers are not needed. But right now there is a continuous double 2×10 above the windows. It is supported at each end of the wall and by a double king stud between the two pairs of existing windows.
I think this is overkill, but the guy I work with has a plan where he will support this header with new studs as he gradually removes the old windows, then he will cut the header to fit in the round window. He also plans on some sort of header over the round window. I’m thinking more of something on the order of doubled 2×4’s and cripples up to the bottom chord of the last truss. He insists that we need big headers on this because the depth of headers must be about 1.5 times the length, just converting to inches instead of feet–so a four foot header should be 6″ deep. I argued that that might be true for the side walls because the roof trusses are bearing on them, but not on the gable end. Do we need such big headers?
Replies
Danno,
I have a non-answer for you ;-)
Do you know whether there is actually a truss directly above the wall?
Often times the gable wall is stick framed to the roof sheathing and actually then supports a small portion of the roof load and the framing needs to reflect that .
Good point, but I did see what I thought was the bottom chord where they didn't put blocking. I guess it doesn't hurt to be safe instead of sorry though! The only reason I say there are trusses is that's what the guy I work with said--I don't know how he knows.
But that did bring to mind another question--with scissors trusses, the load is taken care of so it doesn't spread the walls, but what about in cathedral ceilings? I rarely see anything going across (like ceiling joists do) to keep the side walls in place.
Edited part in italics.
Edited 4/24/2007 7:32 am ET by Danno
Danno, Every Cathedral Ceiling I have ever worked on had joists, collar ties or a structural ridge.As Boss pointed out , even what "looks" like a truss at a gable end is quite often not a load bearing piece. I know I have to tell my truss plant I want a structural gable end unless I want to have a non-structural one delivered.The wind load affecting the header size is a very valid point. It certainly sounds to me as if you are dealing with a structural shear wall. Given that I would point out that any changes it will affect the calculations that were done in the first place. Might want to mention the need for engineering the new scenario.....
"Poor is not the person who has too little, but the person who craves more."...Seneca
>> As Boss pointed out , even what "looks" like a truss at a gable end is quite often not a load bearing piece. I know I have to tell my truss plant I want a structural gable end unless I want to have a non-structural one delivered. <<
Would it be safe to say that if the webs in the gable end truss are verticle and more like studs then the truss is designed to transfer load straight down (be it minimal) and that if the webs are at angles forming a 'W' or other shapes containing triangles then it is designed to transfers the loads to the walls at either end of the truss?
From what I understand there is really no structural difference in a W truss compared to a gable truss.
The gable truss has the bracing in vertically for sheathing to be nailed to.
Some aeas you have to use a regular truss instead of a gable. Then you have to add blocking in between the chords and webs for the sheeting to land on. If you didn't add lumber you might end up with very large areas where the sheets will be floating.
When you look at truss layout sheets - or at least the ones from the truss vendors I use - there is a layout sheet for each individual truss. Below the picture of the truss some manufactures show the bearing points. If the bearing points are shown, most often - maybe always - again - in my experience, the gable end truss with vertical "studs" (or webs) is shown with solid bearing below for the entire length of the truss, whereas a common truss (W truss, fink truss, howe truss or whatever) is shown with bearing points at the ends, or perhaps 1 or 2 midpoints, so although the gable end vertical webs are there for nailing of sheathing, there is a significant structural difference between the 2 types of trusses.
See attached pics. Not a great example, but just what I happened to have pics of....
Also, if you look at the first and last truss on this page and check out the pictures, you will see another graphic example.
"From what I understand there is really no structural difference in a W truss compared to a gable truss. "
That's not right.
A typical truss is made to span from exterior wall to exterior wall with no interior support.
A gable truss only has verticals, and is not structural - It requires bearing accross the entire bottom chord.
But - There are some odd combinations of the two types - Trusses that are part gable and part truss. Some of them can get pretty funky.
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Boss:
Look at the attached pic (2 copies of same pic) of a gable end truss.
Why do they show the bracing like that? I'm pretty sure I know what they want, but the graphic just doesn't make much sense....
Did you see my thread on CLBs?http://forums.taunton.com/tp-breaktime/messages?msg=87378.1Basically what that gable is calling fior is CLBs, or web braces to prevent buckling of the verticals under load. You'll notice that the CLBs are only called for on the longer webs. All the verticals are in compression, but only the longer ones need braces.In reality, you can't easily put CLBs on gable webs. But for some reason the truss design program does that by default instead of "L" braces. The truss designer can change that, but I don't think most do. I know I don't - Most guys never look at the drawings anyway.(-:
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I had seen your thread on CLBs but hadn't really read it through since I thought you were mainly talking about common trusses (fink, mono, or whatever) and maybe floor trusses, but not gable trusses. The truss diagrams we get are quite clear about the bracing for the common trusses.
Your answer is pretty much what I suspected about the gable end bracing - as you say we just strong back 'em and then put in some diagonal bracing that ties back into other parts of the house framing. I had asked my truss guy about it but just got a round about answer instead of your straight forward "In reality, you can't easily put CLBs on gable webs" (as shown on the pic I posted). Ie - it can't really be done like it asks for on the diagram.
Don't think I'm anything but appreciative for you direct answer but let me address "Most guys never look at the drawings anyway".... I feel I have no choice but to carefully read the the truss drawings. The bracing HAS to go in as specified, and there is no other way to get that info. When I have a framing inspection the inspector ALWAYS flips through the truss diagrams and verifies truss bracing - and that is for the multiple building jurisdictions that I have built in within the last 10 years or so. Even the non-english speaking (and obviously non-english reading) Latino framers know how to look at the diagrams for the bracing.
For a somewhat extreme example, the other day I was filling in for the super at a condo building my company is building. I was working with the framers and we were going over the bracing for the roof. The roof truss drawing packet was the size of a phone book for a medium sized town. To think that roof could be braced without going through the diagrams would have been about smart as pi$$ing on a high voltage wire ;-).
I'm gonna guess that you might feel like nobody reads them since your situation is that you only hear back about the problems. If it went smooth, you never hear about it...
BTW - as long as we are talking about truss diagrams it would seem like they could be made a little more user friendly. The "user" being the guy who actually installs them, and the builders who are the truss customers - not the guys at the truss company.... For example I guess the industry standard is to represent length measurements in a somewhat odd way, specifically the fractions. Ie: 33-2-8 which means 33' 2 1/2" (the 8 means 8/16") which obviously you know but some people don't - or at least I guess you use that convention? Or how about making the end user information in a larger font (or bold) and then all that engineering stuff about reaction loads, etc that the end user doesn't give a sheet about in a smaller font. Granted the mfg guys who do it all day everyday may use some of it, but probably don't hardly look at the diagrams anyway since that is all they do. Not that you have any control over this kind of thing... What I'm saying though is if they want people to read something they need to make it readable - not all diluted with a bunch of extra BS.
Edited 4/25/2007 7:23 pm ET by Matt
" I feel I have no choice but to carefully read the the truss drawings............When I have a framing inspection the inspector ALWAYS flips through the truss diagrams and verifies truss bracing"
I've never worked anywhere that had framing inspections. So while we send out drawings of every truss on every job, they're quickly thrown in the trash. Some builders even get hostile with the drivers about it: "Don't leave that crap here - Take it back with you".
I'd venture a wild guess that half of all CLBs never get put on.
"as long as we are talking about truss diagrams it would seem like they could be made a little more user friendly. "
The FT-IN-SX format has been a pet peeve of mine too. It comes naturally to me, but not to anyone who doesn't work with that format every day.
The people who write the software use the FT-IN-SX format every day and don't actually have any contact with the customers, it doesn't bother them. They see no reason to change it.
I think it's slowly changing anyway due to pressure from our customers trickling down. But it will take time.
Regarding leaving the engineering info off the drawings - I don't think that's gonna happen, for liability reasons. The truss companies don't want to be accused of leaving off any info that someone might need. That's the reason for all the BS and disclaimers.
An optimist is a guy who has never had much experience. [Don Marquis]
I understand the engineering info can't be left out, I was just thinking it could be in a smaller font than the end user information.
Boss, have you ever thought of writing an article for the magazine? Take it from the drawings (with explanations) that accompany the trusses and the design criteria that are used to come up with the final design and then applying it to the the real installation of the trusses. I don't think I'd be the only one rereading it and using it for a reference.
Maybe it's already been done & I missed it?
"have you ever thought of writing an article for the magazine?"
Yup. Tried it like 3 times. Spent some time writing outlines and conversing via email with editors, only to have them drop the idea without any explanation.
Given what they pay for an article, I don't think I'll be putting a lot of effort into it again.
College football is a game which would be much more interesting if the faculty played instead of the students, and even more interesting if the trustees played. There would be a great increase in broken arms, legs, and necks, and simultaneously an appreciable diminution in the loss to humanity. [H.L. Mencken]
That figures. I guess the detail, depth and length of the article isn't conducive to magazine based on articels no longer than 5 or 6 pages (kind of normal for mags). I get more out of what I read here @ BT than I do from the magazine anyway.
I can SORTA understand their perspective. They want an article that appeals to a wide audience. It's hard to do that with trusses. People who stick frame + HOs would have little interest. Still - I don't like putting effort into something, then having it dropped without any explanation. So they've pretty much burned the bridge with me.
Those who beat their swords into plowshares will plow for those who don't
I get scissor gables from my local supplier, these don't require full bearing on the bottom chords. Thats what I was talking about in Danno's case.
But I understand yours and Matt's point on regular trusses. I pretty much grouped them together, which makes my post misleading.
O.K. - I understand what you were getting at.BTW - Not all scissor gables are structural. I don't know if you meant to imply that or not, but it kinda sounded like you did.
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You deal with these day in and day out, if they make non structural ones you would know. I am just a guy that puts them together.
I agree that the CLB's on gable trusses are a pain, you go from the web's of a normal truss to the verticals on a gable.
Scissor gables that are NOT structural have verticals only. Scissor gables that ARE structural have diagonal webs, with the verticals cut in around them. One exception to this is small scissor gables that only require a vertical at the center in order to be structural.On gables - I personally wouldn't bother with CLBs or "L" braces. I think it's bettter to put a strongback across the center of the gable and brace it to something. That should keep the surface of the gable flatter, and is likely stronger than other types of bracing.
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THanks for the clarification. I thought that all scissors were structural for the most part. Vertical members or not. I haven't seen too many with the vertical member now that I think of it.
I balloon frame most gables with scissors and nail the gable to the inside for a drywall nailer. Never really researched the structural aspects of them. Thats probably a typical response you hear out in the field.
I can't remember reading in any of the site delivered packages that the bottom chords need to be fully supported in scissor situations. Not really sure why they are made if they aren't sturctural. Why even bother with them?
Every framer is different. Some want a flat bottom chord gable, and they frame a wall inside it to match the ceiling. Others wasn a scissor gable, and they balloon frame the wall to match underneath it. Not many around here do it like you do - Balloon frame the whole wall and put a scissor truss niside it. But I've seen it done.
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I like to balloon the entire wall to keep it stiff, otherwise it has too many hinge points in a truss situation. Maybe I am overthinking it.
IMHO you're not over-thinking it. What you do is just the way you like to do it. There's certainly nothing wrong with it. I've heard a lot of talk and theories about scissor trusses and "hinge points". But I never have heard of any structural problems resulting from the existence of the hinge points. It could be more of an issue in hurricane or earthquake country. But around here no one has raised any fuss over it. The "hinge point" would likely be a bigger problem on large scissors + walls. Most of the situations where we do this is smaller gables on the front of a house, where the trusses are maybe 14' to 16' wide.
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>> I've heard a lot of talk and theories about scissors trusses and "hinge points". But I never have heard of any structural problems resulting from the existence of the hinge points.
It could be more of an issue in hurricane or earthquake country. But around here no one has raised any fuss over it. <<
I guess when a tornado hits a house's hinge points don't matter since the house is just flattened anyway :-)
Here our inspectors make us strap any hinge points framed in exterior walls (ie vaulted ceiling gable walls) with metal strapping. I also always use hurricane clips at all roof truss - exterior wall bearing points just for good measure.
Talking about hinge points reminds me of my first (and only one I ever designed) house I designed for some people--had a huge roof with an 8' wide overhanging gable that was supported by glu-lams. The GL's were supported at one end by the fireplace and at the other by steel posts. The gable wall consisted mainly of a wide sliding glass door and windows above. The owners insisted on using brick on that wall, even over the slider.
The contractor built two walls one above the other. The double plate where the two walls met was just above the slider. Anyway, when I came onto the job site, that plate was acting like a hinge and the bottom wall was leaning out and the upper wall was leaning in. They finally put steel in the wall going from floor to ceiling. If they had just balloon framed it to begin with, I don't think steel would have been necessary (maybe need something beefier than 2x4 studs though). In retrospect, I should have included a detail of what I expected to be done there.
Here is a pic of a scissors truss with vertical webs. It requires continuous support from below even though this particular mfg does not state it on the drawing - which, BTW they show no bearing points at all.
The live and dead roof load is transferred down through the vertical "studs" directly to the wall below.
Maybe your builders are having their truss packages designed so that the vaulted ceiling just gets a regular scissors truss at the gable end? I'm not sure if this would save any money or time but I can see that your method would be stronger assuming the truss nailed to the full height balloon framed wall is structural - although I have never seen it done that way. Further come to think of it, regarding the economics of it, if your balloon framed wall is full height, it would seem that that last truss is just redundant and a waste, and that it could be replaced with 2 2x4 nailers at the ceiling plane levels. - If I understand you correctly...
Edited 4/25/2007 8:13 pm ET by Matt
I haven't had any scissor gables delivered before, now that I have thought about it most of the day. I have used some huge scissors as well, some as large as 42' long.
This has been a good thread, I have learned something I should have known a long time ago. I forget that my way of framing houses isn't the only way of doing things.
When I started framing with another crew other than my family I saw them framing the walls with pre cuts, setting the scissor trusses, then coming back and framing in between the truss and the walls, then adding drywall nailers. Alot of wasted steps and time in that process.
My grandpa and uncles showed me the balloon framing method and I never looked back. Once you frame it it's done, no coming back.
What do you think about the idea of just leaving out that last truss?
I don't leave it out, the last truss gets nailed to the inside of the wall for a drywall nailer.
If customers saw trusses laying on the ground when the roof is being sheeted they would panic. Thinking I left something important out.
>> I don't leave it out, the last truss gets nailed to the inside of the wall for a drywall nailer.
If customers saw trusses laying on the ground when the roof is being sheeted they would panic. Thinking I left something important out. <<
I guess the idea would be to tell the builder ahead of time that it's not needed so it could be removed from the truss order, thereby saving some money. Roughly $100 for a drywall nailer is a little steep.
Edited 4/26/2007 7:21 am ET by Matt
I have told them that. They tell me that the computer draws it that way, so thats the way they have it delivered.
Some of the companies they use are lacking knowledge to be PC. Tell them to leave one truss off the order would result in a whole set of them being left off the order.
Matt,
That is correct. All that piece is is a flat studded gable end wall. Pretty simple actually . Imagine the "Truss" with only verticle webs placed inside the building from the gable end. Now place imagine a load being placed on it.Without the triangulation and breaking of the spans provided for in trusses the load carrying member is only the bottom chord. Try a 2x4 spanning 30' and then placing a roof load on that. The verticle "webs" really don't even make that piece a "truss" even if most people call it one.Another point about those verticle "web" pieces is that they have much less ability to withstand wind loads because the verticle members are always placed narrow dimension to the load. Here we have had to add reinforcing studs that run from plates to top chord to meet code if the truss "web" height exceeds given dimension.
"Poor is not the person who has too little, but the person who craves more."...Seneca
Thanks to all who responded. The question is moot now because we spent the day taking the old windows out and putting the new ones in. The old header was left in and runs from each corner to within about 18" of the center (center window being about 36" wide). The header is supported on each side of that center window by two 2x4 studs 12" apart. A "king" stud flanks the center window and goes up to the bottom chord (I'm calling it a chord, assuming it is a scissors truss and it certainly looks that way) of the truss. There is another 26" wide window on each side of the center window, with a 12" space between them and the center window. Then there is just a double 2x4 plate toe nailed into the king studs that runs horizontally above the center window. The half round window sits on the that horizontal "plate".
I started this thread saying the old header and so on was overkill, but I think it was okay. The wall now lacks unifying features horizontally. It would have been better if we could have maintained a continuous header, or horizontal plate. You sort of have to see it in person--even photos wouldn't tell much and I didn't bring my camera, and I don't think either the HO or the guy I work with would like my posting photos here. What bothers me a little now is that we had to cut the window sill plates, which used to be as long as the header--running the full length of the gable end wall under the windows. We cut them because after we got the windows in, the two that flanked the middle one were too high and it would have been very difficult to raise the middle window (which was a different type than the other two), so we had to lower the other two. To get room to lower it (1-1/2") we had to cut the window sills and shorten the jacks under them by 1-1/2". I even encouraged that, because otherwise the windows not being the same height across the bottom looked bad. Maybe when we nail the horizontal redwoor siding back up, it'll stiffen the wall.
Another thing I don't likeis that the center of the center window, which is very apparent because there are face divided lights even in the round one, are not lined up with the peak of the cathedral ceiling--misses by about an inch. I would be inclined to play with the tape joint to move the "ridge line" over slightly, but the ceiling is not scheduled to be touched.
Another thing that really bothered me is that the center window has one nailing flange (on the right side) missing! It was cut away (at the factory, I guess!)--I have no idea why! Maybe they thought they were prepping it to go into a brick wall and then stopped. I could not pursuade the guy I work with to use flashing (like water and ice shield--sticky-backed bituthene). He says that's what brick mold is for.
Yeah, I'm still working for him. I'm thinking maybe this is not the line of work for me and not to fight it (and not go ahead and get my license). Maybe I should be an accountant or a porpoise trainer or run a hot dog cart.
>> Here we have had to add reinforcing studs that run from plates to top chord to meet code if the truss "web" height exceeds given dimension. << I have seen that but most often, rather, we have to add a horozontal strongback or 2 (depending on the truss height) across the "flat" studs. This strongback is then tied back to the other roof trusses, and other places like the roof plane or the top of an interior wall with diagional 2x4s. This is shown on the pic I posted in the above post. Again, not a great pic, but the one sheet does show bracing requirements for a gable end truss to meet wind load requirements. It refers to a "stiffback". We normally do it a little different in that we nail a 2x4 and a 2x6 together lengthwise forming an 'L' into what we call a strongback.
Minor point of clarification: You said >> But right now there is a continuous double 2x10 above the windows. It is supported at each end of the wall and by a double king stud between the two pairs of existing windows. << did you mean "king" or jack?
I meant a jack--it supports the header (header rests on jack end). King would run past end of a header.
Anyone calculate the wind load on this gable end wall?
That may require a more substantial header.
No calculation of wind load--I never saw that done even when I was helping build houses on a lake front where the winds were strong and sometimes the gable end was full of windows facing the lake.
This wall is like a bump-out--on the left side, the bearing wall continues into this "bump-out," but the right wall is only about 4' long, then it meets a perpendicular wall that runs 6' or so. The roof above this area is smaller (and therefore lower) too, and butts into the main roof. I suppose though, since originally, the strip of 52" tall windows went the full length of the wall, they felt they needed something to resist wind load. Good point. Since the big header's already in, keeping it may be easier than cutting new studs that go all the way to the bottom of the truss. Less folling with the siding and everything too.
The interior walls were plywood nailed as if it were a shear wall (nails every 2" at the edges and more like 8" in the feild) and then tongue and groove, vertical boards nailed to that. We tried to save those, but they were very brittle and glued as well as nailed and they were only about a half inch thick. We ended up yanking down the plywood with the T&G still attached. It also has plywood sheathing on the outside and then wood lap siding. Strong construction! Well, I'd better get going, or I'll be late to get back to this job!
pictures would help
When we do scissor trusses, sometimes the framer wants a structural scissor truss on the end wall. But often is't a non structural scissor gable or flat bottom chord gable.
There won't likely be a LOT of weight there, but there could well be enough that a header is needed.
BTW - The length/depth rules of thumb are ONLY a starting point when you're talking about headers. You still need to consider how much weight the header is carrying.
The poor fellah is trying to use a generic rule of thumb and apply it universally.
The only live load on this will be wind load
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