Load Strength of Double 2×4 Wall?
Just when I thought we could finally break ground after 3 months of delays…. County engineers here in Colorado are balking at my double stud 2×4 wall and want me to go to 2×6 (outer) plus 2×4.
My question up front: are they correct? And if not where do I find supporting engineering support to back up the double 2×4 option? My on-line searches have been fruitless so far.
Here are the particulars:
1. Walls are double stud, 2×4 with exterior plywood sheathing. Studs between the 2 walls are aligned with 4 7/8″ gap. Cavity fill is 3″ spray closed cell plus blown Spider fiberglass. Stud spacing 24″ OC covered with 5/8″ drywall. Single sill and top plates using 11 7/8″ x 1.25″ Timberstrand LSL rim board.
2. 2-story house, slab on grade, with 9′ walls on first floor and 8′ walls on second. Parallel chord trusses for roof. All framing aligned from truss through studs to foundation.
3. Wall will mostly sit on top of 8′ thick foundation wall but will overlap by approx 4″ onto floating slab.
4. Central Colorado with county using 2003 IRC
Sure would appreciate the usual sage advice…I go to battle next Monday.
Thanks
Jim Riggins
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Replies
Read here: http://www.eng-tips.com/viewthread.cfm?qid=92407 It may be of interest. Also, I asked a similar question here a long while back, when I was still (foolishly, IMO now) thinking of using "the old standard" fiberglass insulation. Surely you'll need more digging, but these may help. FWIW, I will likely do what the engineers are telling you to do, as I don't want the interior wall to be load bearing, per se. Good luck. john https://www.greenbuildingadvisor.com/community/forum/general-questions/16346/2x6-or-2-2x4s-support-truss
Thanks John. It may not be worth the fight with the county. Moving to 2x6 outer wall increases my cost a little, and makes filling the gap a little more difficult. But, it takes my whole-wall R-value from 51 to 46, and only increases my total annual energy consumption by 295 kBTU. This won't come close to impacting our net-zero level. I may also beef up our flash & fill and go with a little more foam and a little less Spider.
Jim
I'm guessing their balking at your use of 2x4's on 24" inch centers in a 2 story load. I'm pretty sure that the OVE is only 16" on center at 2 stories...
Jim,
Are the second floor deck and roof both supported by the exterior wall? It appears that the interior of the double walls is non-loadbearing if resting on the slab. That's a poor design, since load paths on double-wall frames are often ambiguous and both walls should be fully supported to the ground.
The IRC standard is: 2x4 at 24 inch on center if supporting roof only, 2x4 at 16 inch on center if supporting one floor and roof, double top plate, 2x bottom plate, balloon framing at rake walls 2x6 framing if over 10 feet tall.
Since you will have to abide by local code and use 2x6 exterior wall framing, you can improve the whole-wall R-value a bit by offsetting the interior studs, which also makes installing interior drywall easier and less wasteful. Since the interior wall is (apparently) non-loadbearing, you could use 2x3s if you can find them long and straight enough.
You could also ask the county engineer if reducing your first floor wall height to a more sensible 8' would change their requirement. There is a big difference in resistance to buckling and lateral deflection between an 8' 2x4 and a 9' 2x4.
OVE, which was created by structural engineers, was designed for 24" (or 19.2") centers for 2-storey houses. But current building codes won't allow such resource-efficient practices.
However, I've been building 2-storey 24" oc homes for 20 years using full-dimension rough-sawn lumber. Even though this is not grade-stamped lumber, I've never had my frames questioned by a building inspector.
A full-dimension 2x4-8 has 52% more compressive strength and 78% more buckling resistance than a KD 2x4-8.
Robert:,
Thanks for the input.
Yes, the exterior wall supports both the 2nd fllor deck and the roof. From your response I conclude that my incorrect assumption was that two 2x4 walls, tied together with single sill and top plates, would exceed the load strength of single 2x4 or 2x6 wall, and would allow the 24" OC spacing.
The 9' ceiling on the first floor was an intentional tradeoff in the design. The bulk of the thermal mass in this passive solar home is the slab. It was difficult achieving the 8% glazing to floor area ratio that we wanted without going to a 9' ceiling.
Jim
James,
All you've accomplished by using Timberstrand top and bottom plates is to increase thermal bridging (which is also problematic at the second floor rim joists) and to create ambiguous load paths which may put excess stress on the slab edge.
The advantage to double wall framing is that it allows thermal isolation between inner and outer walls as well as eliminating the thermal bridging and air-barrier interruption of the floor deck. But that requires a wide enough foundation to support both walls and terminating the floor deck at the interior wall.
8% glazing-to-floor ratio is barely adequate for optimized passive solar, and it should be quite easy to accomodate up to 12% (the practical limit for passive) glazing in an 8' tall wall.
It sounds like you're not only the victim of over-zealous code enforcement but also of poor structural and solar design.
I also was forced to increase my exterior wall to 2x6 but the rationale was that the exterior foam insulation of the ICF slab edge gave me only 1 1/2" of bearing under the 2x4 wall but would give a full 3 1/2" if under the 2x6 wall. as you point out the cost differential was not worth fighting with the inspector over.
Michael,
Rationale or rationalization? A 24' wide, 2-storey house with stick-framed roof and 40 psf snow load will exert a design load of 930 plf on the slab perimeter, or 1240 pounds per 16" oc stud or 1860 pounds per 24" oc stud. A KD SPF#2 stud has a compressive strength parallel with the grain of 1000 psi, so a 1½" x 1½" bearing area can handle 2250 pounds load. And this doesn't take into account the distribution of the wall load onto the bottom plate by the box-beam structure of plywood sheathed walls.
Odd that floor joists, subjected to compression across the grain (which can resist only 1/3 as much load) are required by code to have only 1½" of bearing surface.
Robert
I certainly agree with you but the increase in cost really wasn't worth fighting the building inspectors. They had also forced me to remove the PVC sewage pipes twice from under the slab because they were "in contact with rock" - the washed gravel. The first time they had me remove all gravel from contact with the pipes and bed them in red clay but then after that was done they decided that some gravel may have gotten mixed in with the clay and so we had to remove all the packed gravel from the under slab area and tunnel under the footings, trench the pipes into the sub soil, pack with clean red clay, sleeve the pipe in sill soil to protect it from the 3/4" washed rock and then re-test it to 10" of head. All water supply piping was required to be sleeved in hard pipe. all of which cost us considerably plus they wouldn't allow me to use my plumbing license for doing a radiant floor as back up to a passive solar house with a heat pump for primary heat and forced me, after several weeks of negotiation, to pay an extra $3,000 to an HVAC specialist with a steam heat license to do the radiant on a solar water heater. At this point the additional cost for the 2x6s was looking like a battle not worth fighting. On a previous job the chief inspector for this county had turned down my sealed crawlspace on the grounds that "if the floor joists get too dry they'll dry rot" A friend of mine re-located the air handler and duct work for his house twice. (three complete duct systems for one house.)
One county over we have some of the best and most well-informed building inspectors in America but that county had a culture of hazing and intimidation. Seemed like they especially targeted "college boys."