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
I'm no expert here, but it seems like a case could arise where a single
stud would take virtually all the load from a point source (it's slightly
than it's neighbors for example).
So the better question might be, can the concrete spread the load
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?
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
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.
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
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.
On 2004-11-04, firstname.lastname@example.org
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.
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.
On 2004-11-04, email@example.com
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.
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.
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