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caducus
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# Posted: 26 Jun 2018 05:41pm
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Something I don't understand about a shed roof that's built with no trusses... does the lack of triangulation allow the walls to flex?
The gable walls give it shear strength, so on a small shed it's no big deal. But what happens as that building gets wider? What if there is 20+ feet between the gable walls?
I can imagine that sheathing the rafters with some thick plywood would fix the problem, if there was one. But I'm having a hard time wrapping my head around the mechanics of the whole thing.
Anyone have some wisdom?
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ICC
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# Posted: 26 Jun 2018 08:05pm
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Quoting: caducus Something I don't understand about a shed roof that's built with no trusses... does the lack of triangulation allow the walls to flex?
A rafter built gable ended roof transfers the roof loads to the supporting side walls the same as a truss built gable end roof and the same as a shed roof. Sitting there with no wind the roof load is distributed with half on each side wall. The triangle shape of the rafter or truss built roof makes the roof assembly rigid, but only the roof assembly.
Quoting: caducus What if there is 20+ feet between the gable walls?
Structural OSB, plywood and other panels on the wall studs provide the anti-racking strength required to resist horizontal forces. Horizontal forces come from wind against the walls as well as being transferred from wind against the sloped pitch of a gable ended roof.
The building code contains a prescriptive method of how much shear wall bracing must be provided per length of wall. That is how wider and longer structures maintain their ability to resist horizontal forces.
Quoting: caducus I can imagine that sheathing the rafters with some thick plywood would fix the problem, if there was one.
Roof sheathing provides support for loads like the shingles and snow, as well as making the roof assembly rigid enough to resist horizontal deformation. It does not affect how the walls react to loads.
Roof and walls are connected, forces are transferred from one to the other, and all the hard work was done years ago by engineers. Today you can use the tables in the IRC along with the info from truss companies to build almost any size building from wood.
Does that help?
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caducus
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# Posted: 27 Jun 2018 10:45am
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It helps somewhat. I've got the ICC bookmarked and reference it often, but I'm not well aquatinted with it enough to know it inside and out. I haven't seen the answers I'm looking for in there.
Just to make sure we're on the same page, here's a basic drawing.
The wind is the red arrow. As I understand it, the center of wall B doesn't flex and bow in because of the shear strength and rigidity of wall A and the opposing other gable wall. I also assumed that on a traditional gable roof, with triangular trusses, the wall doesn't bow in or out (due to the wind, or due to the roof loading those walls) due to to those trusses.
Is that correct? The sheathing on wall B keeps it from racking left to right, but it does nothing (I would think) to keep the wall from flexing in or out with wind and seismic forces.
So my question, with a single pitch shed roof, as pictured, and no triangulation, how far can wall B span before its an issue?
It sounds like you're saying the triangulation of the roof trusses is 100% roof stability and nothing to do with wall stability. Which means it's just a question of ICC tables and how wide that wall can be. I that's the case, where in the ICC should I be looking?
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ICC
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# Posted: 27 Jun 2018 04:19pm - Edited by: ICC
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Quoting: caducus I also assumed that on a traditional gable roof, with triangular trusses, the wall doesn't bow in or out (due to the wind, or due to the roof loading those walls) due to to those trusses. Is that correct?
A gable roof with rafter ties or built with trusses does provide resistance to upper wall movement when the wind blows from direction B, as shown. But so do the rafters for a shed roof, or for that matter a set of ceiling joists or floor joists for a building with a second or third story floor.
One of the reasons the IRC limits unsupported wall height to 10 feet is to ensure sufficient rigidity to the wall when winds blow with strong force. Unsupported in this case means, no lateral supports as would be provided by a ceiling or floor intersecting the wall.
The lower wall plate is fastened to the floor or foundation. The upper wall plate is fastened to the ceiling/floor joists or rafter ties/bottom chord of a truss. Those connections (are supposed to) keep the lower edge and upper edge of the wall from bowing.
A structure with only the wall studs and some sheathing in place, no roof, no ceiling/floor joists, is very weak. A strong wind on a wall in that situation could bow the wall to the point of collapse as only the bottom is held in place. There may be a 2x brace here or there as a temporary support. Once the ceiling/floor joists or roof rafter or truss system is in place and sheathed the box becomes more rigid.
If you want the most rigid wooden building, build a square or a rectangle building with a hip roof.
Remember, the IRC published by the ICC, is simply a rule book. There is nothing in the way of explanations as to why something is done or how the recommended practice has been arrived at. There are papers by other organizations that get more into explaining. I like the AWC and APA information a lot. Here's a paper on lateral load resistance for walls by the AWC. Here's an APA paper on simplified wall bracing.
The APA also has an online wall bracing calculator. You need to register in order to use it. It can be used with the newest (2015) code version. They assume the user has some knowledge of wall bracing.
One thing to watch for with online documents is to note which version of the code they are referencing. The IRC has had some evolutions and later versions may list different methods or solutions. Old articles are seldom if ever, edited to note changes and most all the old stuff is still coming up in searches as people never seem to delete the older, possibly obsolete information. There were some pretty significant changes in 2015, some in 2012 IIRC.
Research "braced walls" as well as "shear walls". They are similar but different.
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ICC
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# Posted: 27 Jun 2018 04:23pm
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Quoting: caducus So my question, with a single pitch shed roof, as pictured, and no triangulation, how far can wall B span before its an issue?
A friend has a long horse barn with a shed roof. It think it is about 60 feet long. It has a 4x10 brace panel every 16 to 20 feet. The spaces between are broken up by stall doors and windows. All wood (dimensional lumber plus many sawn wood beams), it has been through some storms in something like 15+ years.
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caducus
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# Posted: 28 Jun 2018 08:27pm
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Quoting: ICC A friend has a long horse barn with a shed roof. It think it is about 60 feet long. It has a 4x10 brace panel every 16 to 20 feet. The spaces between are broken up by stall doors and windows. All wood (dimensional lumber plus many sawn wood beams), it has been through some storms in something like 15+ years.
Hard to imagine a barn like that not having some sort of diagonal bracing. I'll have to take your word for it!
Those were really good articles, thank you.
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