Stress question
I want to use a 2×4 wall as my load bearing wall (parallel to the ridge line) supporting trusses. Because I’m using cantilevered truss s to cover front and back porches I have unusually long truss s (56′) and am building where the code requires design for 75#/sq ft snow load , I’m investigating loading and stress. The load bearing wall is the inner of a double stud wall and is sheathed on it’s outer face with 1/2″ plywood sealed as my primary air barrier. If I frame the wall 16″ OC it can easily support the snow load + dead loads. I’d like to use truss s 24″ OC but the concentrated load at the bearing points, under maximum snow load+dead load results in excessive cross grain stress if the bearing area is only 3 1/2″ x 1 1/2″. Can I use a 2×6 top plate on a 2×4 wall to solve the problem? The 2×6 top plate will work because I’ll be sheathing the bottom of the trusses and adding furring to form a ceiling “service cavity”. I would center the top plate over the 4″ (sheathed) wall below so there would be 3/4″ overhang on both edges and glue it in place with construction adhesive for air sealing and added strength. If it will work, the 2×6 top plate is a lot more economical than resorting to 16″ OC truss s which also solves the problem.
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A 2x4 wall will carry 75 psf snow, plus dead load on a 56' span? Are you an engineer, or did one OK that? It just seems long from my limited experience. Why not just use 2x6 studs on 24" centers and sleep well? Simple. Forget the extra cost. In the grand scheme of things, it is trivial. Best to you.
Yes, I am an electrical engineer and expect to get a registered civil engineer,s approval if needed.. The desire to use 2x4 16 OC rather than 2x6 24" OC is motivated by by the fact that it puts 2/3 of the insulation on the outside of the OSB primary air barrier. With 2x6 inner wall it would instead be 1/2 which would result in condensation on the OSB at far higher temperatures. For example if the indoor air were 70 degrees with a 40 degree dew point the 2x6 wall would experience condensation at outside temperatures below +10 f while the 2x4 wall would need -20F Double the indoor to outdoor temperature difference! FWIW the load capacity for truely uniform loading is proportional to the stud cross section which for a 4 foot section of wall is 10.5 sq in for 2x4 24OC, 15.75 for 2x4 16 OC and 16.5 for 2x6 24 OC True the 2x6 wall can carry 5% more load. Looking at the problem slightly differently I ask the question: with a 56 foot truss what is the maximum load considering only the roof. First assuming that wall & truss are aligned. The answer for the 2x6 24" OC is 110 PSF. the 2x4 16" OC is 105 PSF and 2x4 24" OC is 70 PSF or the allowable dead load with 75 PSF snow load is -5PSF for 2x4 24" OC 35 PSF with 2x4 16" OC or 40 PSF with 2x6 24" OC.
Jerry, I'm sure there are regional differences, but my experience here is that trusses over 45 feet are invariably designed with some intermediate bearing. It might be worth discussing your roof framing with a truss manufacturer.
28' tributary width (I don't think the "length" of the truss is calc'd along the top cord, but rather horizontal distance, for some reason) times 75 psf gives 2100 lbs/ft. I don't think a 2 x 4 x 8' will carry that. A 9 footer carries something like 1700 lbs, while the 2x6 is around 7K. I think you'll need 2x4's on more than 12" centers. But, I am not an engineer. Can you do more insulation outside?
As the need for structural integrity trumps pretty much every other aspect of green building (yes, even the necessity for 'granite countertops to enhance thermal mass' as I recently read) I am pretty sure your cantilevered truss will require more than the generic opinions of GBA commenters, wise and all-knowing as we are. In fact I am pretty sure your building inspector will require sealed engineering on your proposals too, so take the 'if needed' out of your question and get this fundamental issue resolved by a qualified engineer as soon as possible, while you are still in the planning stage. The good news is there's a good chance your truss supplier will provide this service for free, including the requirements for the bearing wall. 56' is a very long truss by ordinary residential standards and they should also be made aware of your site access conditions for transport and installation.
James,
Yes the truss supplier will provide the engineered truss plans. I've built a pole barn which had 62 foot trusses so I've been thuough the drill, so to speak. One must give the truss supplier a starting point, that is what I'm trying to arrive at. I now believe the best answer to the loading issue is to arrange for both the inner AND outer wall to be load bearing. This means real headers in both, of course The problem then becomes assuring that the load is really shared, I think I have an answer. Here it is: I'll place a 2" thick strip of 2 PCF EPS under the outer wall's bottom plate to serve as a crush able spring. The EPS has a compressive strength 30PSI +/-15 % , between 26 and 34 PSI. Under a 2x4 wall, this limits load to between 1092 pounds per foot and 1428 pounds per foot. When the roof loading exceeds these values the remainder will be carried by the inner wall which has no foam. During maximum snow load conditions assuming 15 #/ sq ft dead load, the 28 foot tributary load is 2520# / ft with 1092 pounds on one wall and 1428 on the other at either extreme of foam compressive strength and 1260 on each at nominal foam strength. The foam will also allow for greater shrinkage likely to occur in the inner wall as it has more cross grain wood . So to the truss supplier I'll show both walls as load bearing.
I now think the best answer is to make the inner wall bear the full load, do it with 24" OC by doubling every stud. and use a 2x6 top plate on the inner wall.