Hello,
I have an engineering question: if a point load bears on the top plate of a stud wall, does the stud wall spread out the load by the time it reaches the bottom plate? My WAG is that it is spread over a width equal to the height of the stud wall. In my application, the stud wall is on a 4" concrete slab that (I think) can handle a wall load of
300 lbs/ft, so I'm trying to figure out whether I need to add a footing.Thanks, Wayne
I'm no expert here, but it seems like a case could arise where a single vertical stud would take virtually all the load from a point source (it's slightly taller than it's neighbors for example).
So the better question might be, can the concrete spread the load effectively? In compression, the concrete strength is likely to be 2500-3000 psi. So, the question would be can the concrete effecitvely spread the load from that one vertical stud out to the underlaying soil?
-- Tom
If the top plate was absolutely inflexible (not possible), and all studs were of PERfectly equal height, the weight would distribute evenly. It would have nothing to do with the height of the wall. I am not an engineer, but I can tell you from framing a number of houses that every load must be carried to the ground with sufficient stud width (a stud must be doubled or tripled or quadrupled or quintupled based on the load it carries at that point, with NO reliance on other studs in other parts of the wall. The one point on the floor where the doubled (or tripled, ...) stud touches must be strong enough to hold everything up. The top sill is not to carry weight. The top sill is really something to stabilize the studs and something to nail the sheetrock to. I don't think you should design this project yourself. Now ... put $75 in that little slot on your computer and press Send.
-B
OK, that makes sense. Now, does putting a solid header in the wall spread out a point load, and how much?
The building code says that that a loadbearing wall above a floor diaphragm and perpendicular to the joists can be offset up to one joist depth from a loadbearing wall beneath the floor diaphragm. So that would suggest that the load spreads out at 45 degrees through the solid member.
On the other hand, it seems like the two nearest studs on either side would pick up all the load.
Well, I'm trying to do as much of the design as I can, and then I'll run it by a civil engineer if I end up with anything usual, like a point load. Plus I like to understand how everything works.
Thanks, Wayne
A discussion of the load bearing capabilities of concrete slabs I read seemed to indicate that the critical property is the "flexural strength" which is on the order of 500-700 psi. Anyone know the difference?
Thanks, Wayne
See ASTM C 42 and ASTM C 78
And it it weren't, the weight would distribute fairly evenly.
We can tell.
You tried other techniques, and houses fell down? :-) Shark repellent works well around here. Most houses are way overbuilt. > I have an engineering question: if a point load bears on the top plate
How many pounds of point load? What kind of studs, on what centers?
Maybe less.
300 lb/ft sounds weak. You might check that again.Nick
In the scenario I was considering, a header would pick up two 12' floor joists 16" on center and one end would be supported by the stud wall. As to the specifics of the stud wall, that is flexible. Anyway, it is not a lot of weight, as 12' x 4' = 48 sq ft, but the end of the header only carries 1/4 of that, so 12 sq ft @ 50 lb/sqft = 600 lbs.
Yeah, I picked the smallest number I saw, so I could err on the side of safety, since I didn't know the flexural strength of the concrete or the strength of the subsoil.
Cheers, Wayne
P.S. I'm probably not going to go with this design, but I'm still interested. It's been fun learning how to size beams.
Like this, viewed in a fixed font like Courier?
| 16" | floorfloorfloorfloorfloor j j o o i i s s t t headerheaderheaderheaderheader s s t t u u d d . . . . . .
One end of what? The joists? The header? Is the stud wall parallel to or perpendicular to (as above) the joists?
Where does the 4' come from?
Where do the 1/4 and 12 square feet come from?
Sizing beams is easy... W, L, M, f, S, b, d. Figuring how they share loads can be harder.
Nick
Close, sorry if my descriptions were not clear. The stud wall would be parallel to the joists, so eliminate the lefthand "stud..." in your diagram; the righthand "stud..." is the stud wall viewed on edge. If I understand correctly, that's why the header presents a point load to the stud wall; if the stud wall is perpendicular to the floor joists, they present a distributed load to the stud wall.
One end of the header would be supported by the stud wall.
Sorry, 4' is the header length, since it spans 4 joists (3 * 16"), carrying the middle two. Actually, I guess I'm assuming it carries 3 joists if I use the 4' figure.
The 12' x 4' floor area I'm considering is supported on one short side by a concrete retaining wall, and on the other short side by a header. One end of the header bears on another concrete retaining wall; the other end would bear on a stud wall. So the header carries half the floor area, and the stud wall carries half of the header, hence 1/4.
12 square feet = 1/4 * 4' * 12'.Anyway, this is not my current design, but a reality check on the analysis is welcome. The original question was "does the stud wall spread out the point load?". If the 4" conrete slab can support 300 lbs/foot, and if the point load from the header is 600 ft, then it would be OK as long as the stud wall spread the point load out over at least 2 feet.
Thanks, Wayne
P.S. In my current design, I just have beams big enough to carry everything and bearing on the concrete retaining walls. The header still presents a point load to the beam parallel to the joists, but I've figured out how to do the analysis of a beam with a point load.
This is a warning, friends. You are speculating the hell out of some tried and true building techniques that are covered by very specific building codes. Building codes cover the minimal standards so that you or your children will not get killed when there's snow on the roof, or water, or a worker, or some other exceptional (or normal) condition. There are standards on the depth and width of footings, and you can't build much of anything on a 4'' slab. There are standard on the number of studs and where they must go to hold up such and such a type of ceiling or whatever. I can see the code book from where I sit at this desk, but I'm not going to offer any more specifics because I don't want to wonder whether you get killed, or even if your 4-inch slab cracks and the studs settle and cause your house to crack.
-B
Doom and gloom is not very informative. I believe there is a place for "why can or can't I do this?"
Yours, Wayne
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