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caducus
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# Posted: 16 May 2018 03:19am
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Can anyone chime in here. I'm trying to work out the framing and design of a cabin. I'm not doing it to code, but I am looking to the ICC code as a guideline.
The front of the cabin is not a load bearing wall, if I understand that correctly. The sides are what bear the weight of the rafters. I have a very large door on the front. 9' opening.
The ICC says "Load-bearing headers are not required in interior or exterior nonbearing walls. A
single, flat 2-inch-by-4-inch (51 mm by 102 mm) member may be used as a header in interior or
exterior nonbearing walls for openings up to 8 feet (2438 mm) in width if the vertical distance to the
parallel nailing surface above is not more than 24 inches (610 mm). For such nonbearing headers,
no cripples or blocking is required above the header."
If I'm reading that right, I can basically get away with a 2x4 for a header on that giant door. That doesn't seem right to me though, with span that large.
So can anyone offer some insight?
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toyota_mdt_tech
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# Posted: 16 May 2018 08:52am
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I would want something more stout just to stabilize that large of an opening so its not flimsy and what about shear with that big of a hole. We have a couple engineers who will chime in. If it was me, I'd do a nice sized header just for the strength alone for the door.
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caducus
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# Posted: 16 May 2018 09:08am - Edited by: caducus
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Yeah, stabilizing the wall was the other concern. I’ve been looking at other bracing and how to design the other windows and sheathing for that reason. A larger header seems smart. I’ve got a couple of nice 1.75x12 lvl beams I picked up from the habitat for humanity ReStore that I’d originally planned to use.
Mostly just want to clear up my confusion about how im reading that code.
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ICC
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# Posted: 16 May 2018 06:18pm - Edited by: ICC
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The first caveat is the opening as drawn is 9 feet and the code states "openings up to 8 feet". That section has to be read and used along with the other sections of the code that specifies how to make a wall resist forces down the wall length, as well as wind forces against the face of the wall.
You do not need a vertical load bearing header, but the wall is deficient in other structural areas.
What you basically have there is a garage door in a wall. Those virtually never have the wide door opening come so close to a corner. Reason being they usually need 24" on each side of the opening to build a proper shear wall. That is a well nailed, solid paneled piece of wall usually 24" minimum measured horizontally.
Common practice for a shear wall is one 4 foot panel per 25 feet of wall length. Shorter lengths should also have a full 4 foot length without and perforations (no windows, no doors). Shorter lengths are used but usually never less than 24" without engineering. Search with google. This site has a pretty good slideshow on shearwalls.
Also search "portal frame". Garage doors usually incorporate portal framing in recent editions of the IRC. A portal frame joins a shear wall section on at least one side of the door opening, usually both sides, with a header above the opening. Those three pieces are solidly connected, very rigid, and make an upsidedown U. They are also tied solidly to the foundation. JLC has a pretty good portal frame primer.
As drawn that end wall has nothing to prevent a strong wind from pushing the walls out of squareness. I would recommend deleting the ground level window to the right and having only a single upper window and smaller at that. That will give space enough to construct a solid portal frame. I know that probably restricts the view but you need something to keep the cabin box square and safely in place.
As drawn there is not a single stick that runs continuously from bottom plate to top plate. Strong winds against that wall will cause flex, not desirable in a glass wall. When you see well-done glass ends to houses there will be an engineer behind the design. Note I said a "well-done" wall; not everything we see reveals the thought or lack of thought behind the design.
I'm not a licensed engineer but know a couple and have built some of their designs.
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Borrego
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# Posted: 16 May 2018 07:57pm
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What they said.....
Have you thought about getting rid of the window to the right of the door and running a header all the way across? This would enable you to build a shearwall on each side of this large opening...As ICC said, 2 ft is the minimum....If you absolutely cannot, you can buy all steel shearwalls in narrower widths. You need the right anchors/holdowns in the footing for any shearwall, too....
I do a lot of custom home remodels and the big rage are the door wall systems. Although your opening is not as large as most of these, I would still be concerned about the rigidity of that wall (as ICC is talking about). I note that these systems I'm doing have a LOT of connection with the foundation and footings. we usually have to create a 'grade beam' etc.
What you have looks really flimsy to me, maybe run it by a local engineer? Might only cost $800 or so.....
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caducus
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# Posted: 16 May 2018 10:01pm - Edited by: caducus
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Interesting, thanks. I'll read up on portal frames. I could scrap a window and/or incorporate some steel.
"You do not need a vertical load bearing header, but the wall is deficient in other structural areas."
ICC, I realize what you're talking about on the latter point. But on the former I'm still somewhat surprised by the code. Lets say that opening wasn't 9, but 8 feet. That wall isn't load bearing so a single flat 2x4 would suffice? Does that change with windows above it?
That question isn't applicable to this project. Just trying to understand the code, as an opening that wide, topped with a 2x4, took me by surprise.
EDIT: That JLC article was very informative. Thank you.
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ICC
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# Posted: 16 May 2018 11:17pm
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Quoting: caducus --- if the vertical distance to the parallel nailing surface above is not more than 24 inches---
Not sure, but I interpret that to mean that your wall does not comply as there appears to be much more than 24" above the top of the lower wide window. If the distance between the header and the next horizontal piece above is greater than 24" that rule does not apply. The next piece up the wall would usually be a wall top plate.
Also, I believe the fact of having windows above that 2x4 header disqualifies the 2x4 for use as a header as that does introduce different loads. I think if you can find an illustration in the IRC or the energy efficiency guide, it will show an unbroken span above the header. The energy efficiency guide is involved, as increasing the thermal efficiency of the wall is the whole idea of removing the traditional load bearing header.
IF there is a loft floor that joins into that end wall above the 9 foot wide opening than that may, depending on design, act to stiffen the end wall against wind forces on the wall. I kinda skipped over the loft in my previous comment.
Sort of off the topic, but maybe only sideways...... I have a friend in OR who just finished the lower level of what will be a three level home built into a slope. The garage is in the lower level. The engineer designed a steel reinforced, concrete portal frame wall. Rebar n concrete header and side supports that are poured as a monolith concrete box foundation. All because of the seismic potential for damage. There is an amazing amount of steel in the portal frame and the footings it is tied into.
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ICC
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# Posted: 16 May 2018 11:29pm - Edited by: ICC
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A further comment or question. How accurate does that drawing (sketchup?) detail the construction? Are the two roof supporting beams sitting directly on top of the posts or columns? It'shard to tell if they do or if they are attached to the interior faces of the columns. They should be on top. Mechanical attachments on the side of a column need engineering proof.
Also, if those beams do protrude out from the wall they would be a potential 'fail' point. If, or more likely, when water insinuates it's way in there you may be looking at rot in a place that makes for a difficult repair.
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caducus
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# Posted: 16 May 2018 11:45pm - Edited by: caducus
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That drawing isn't totally accurate. I have another one for framing. But it's close.
In any case, those protruding beams are sitting on top of columns inside the wall. They don't extend into the cabin.
You bring up a good point though. I had them going into the framing and while I planned to do what I could with trim, flashing and water shedding, there is still a high risk in having that break in the barrier that sheaths the cabin.
Does Simpson make something like a header or joist hanger that holds up a 6x8? Those posts and roof support beams don't take much weight. Just an 8 foot (light) roof extension for the porch. It would probably be better to sheath, wrap and seal the cabin completely and mount those beams externally, if such a structural bracket exists.
Edit: It seems they do. I could see using it, along with some strapping, like hurricane ties, to provide a little more uplift resistance.
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