Point Load On Concrete Slab
Point Load On Concrete Slab
(OP)
I have a client who wishes to build a crawl space under his home which is currently on a slab on grade. (The house has been flooded twice in the last 3 years.) The house was built in the late 1940's, is a single story structure with a flat roof. The exterior dimensions of the house are 60' x 30'.
He wishes to raise the house approximately 40" (5 courses of c.m.u.). The 16" x 8" c.m.u. piers would be spaced at 6'-0" along a triple 2 x 8 girder and 7'-6" between girders. This corresponds to a 2,250 lb load (10psf Dead and 40 psf Live) at the interior piers. The client would like to place the piers on the existing slab. I am not so sure this is a good idea. How do I get a handle on the allowable load capacity for the slab if I assume it is 4" thick, non-reinforced, and adequately supported by the soils? My initial thought is to cut the slab and pour a footing at each pier, but I would like to hear the input of others for possible solutions/alternatives. Thank you.
He wishes to raise the house approximately 40" (5 courses of c.m.u.). The 16" x 8" c.m.u. piers would be spaced at 6'-0" along a triple 2 x 8 girder and 7'-6" between girders. This corresponds to a 2,250 lb load (10psf Dead and 40 psf Live) at the interior piers. The client would like to place the piers on the existing slab. I am not so sure this is a good idea. How do I get a handle on the allowable load capacity for the slab if I assume it is 4" thick, non-reinforced, and adequately supported by the soils? My initial thought is to cut the slab and pour a footing at each pier, but I would like to hear the input of others for possible solutions/alternatives. Thank you.






RE: Point Load On Concrete Slab
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Point Load On Concrete Slab
I'd film this one. This could be on one of the house moving shows.
Might be cheaper to build a new house, with a pitched roof.
RE: Point Load On Concrete Slab
Stillerz: I spoke with the client today and he told me steel beams would run along the 30' length spaced about 10' or so. These beams would be longer than the 30' length of the house to allow lifting equipment on either end. I believe he did mention some other support members to run perpendicular from the steel beams. I would think these could be used to carry the interior/exterior walls.
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
Forgot to mention, in area without frost action, wayniac3 should follow up on your suggestion, which could save the owner a bundle.
RE: Point Load On Concrete Slab
jenDL: When you say increase the square area for the interior piers what do you mean - construct a pier with a larger foot print (16" x 16") or use a steel bearing plate under the 8" x 16" or something else?
RE: Point Load On Concrete Slab
The effective area of soil bearing is approximately 24" x 12", assuming a 45 degree spread through the slab, or an area of 2 square feet. The bearing pressure is approximately 2250/2 = 1125 psf.
I don't think you have to do anything, but, for peace of mind, you might want to start with a 16" x 16" block with voids filled with concrete for the first course, then continue with 16" x 8" above.
I agree with Mike that cutting the slab and installing a footing is necessary for heavier loads, but with your loads, it is not warranted.
BA
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
I am not providing the expertise on the lifting of the existing structure. I was asked to provide assistance with the crawlspace design. My earlier entry talking about the steel beams came from the client who told me the contractor doing that part of the job was going to pass several steel beams through the window openings along the 60 foot length.
RE: Point Load On Concrete Slab
http://www.ideals.illinois.edu/handle/2142/4518
http://www.ideals.illinois.edu/handle/2142/14021
There are some others at this library for flat-slab but not mainly point load.
RE: Point Load On Concrete Slab
RE: Point Load On Concrete Slab
Assuming the roof spans the 30' direction and has no interior bearing walls... assuming the roof DL = 10 psf and LL = 20 psf (minimum) it is possible that each exterior pier would see closer to 3,825 lbs.
(3.25'x6'x50psf)+(15'x6'x30psf)
This is only slightly more significant because you do not have a continuous slab in which to distribute your load since you will be at the edge. 3,825lbs / 2s.f. = 1,913 psf.
Also to consider, may want to dowel the new block piers to the existing slab to reduce potential for movement during flood and high wind exposure. "Coastal" anything sends alarms to my mind. Make sure to use a rot resistant wood for your triple girt unless you intend a barrier between them and the piers.
My first comments may not be valid if a continuous cmu wall was planned for supporting the exterior wall of the home. This seems could be a good option to consider as well.
Even though this is a 1940's home don't forget to think about the load path for lateral... it could be significant to the details of your design.