Point Load On Concrete Slab 2

[wayniac3]

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.

 

[msquared48]

Your instincts are correct here. Cut the slab.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Stillerz]

How does he intend to jack up this house with no floor? Going to need lots of shoring.
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.

 

[wayniac3]

Thanks msquared48. I thought cutting the slab sould be the better way to do this. Could you direct me to a reference manual to use when designing slabs on grade?

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.

 

[jenDL]

Think about using a larger square area for the interior piers, but leave the slab in tact. Basically assume the slab is only as strong as the soil (1000psf is conservative depening on the area, not sure about there with the flooding). This would reduce labor and may be better to keep the slab in tact in an area prone to flooding.

 

[cntw1953]

Good idea, jenDl, however won't frost heave become a problem? The slab could have been designed for heated space, now the space is somewhat open to outdoor environment.

 

[jenDL]

thanks cntw1953, good point...I live in a frost free zone and sometimes forget that this forum reaches other parts of the world...Someone else is going to have to comment on the frozen ground issue or wayniac3 is going to have to say whether it is a factor for this project.

 

[cntw1953]

I am from a cold region ... I think we can use more info.

 

[cntw1953]

jenDL:

Forgot to mention, in area without frost action, wayniac3 should follow up on your suggestion, which could save the owner a bundle.

 

[wayniac3]

Thanks for all the input. The project is located in a coastal area of Virginia where frost action is not an issue. I have since discovered the original blueprints for the house. They call out a 4" thick slab with 6" wire mesh. The code officials in the area enforce a 1500 psf allowable soil bearing when no analysis has been performed.

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?

 

[BAretired]

The load is 2,250#. The piers are 16" x 8". The existing slab is 4" thick. The reinforcement is negligible and should not be considered in assessing strength.

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

 

[jenDL]

a bigger footprint for the piers should do it. BAretired explained it well...

 

[wayniac3]

Thanks to everyone for the valuable information!

 

[SWComposites]

I'm really, really curious - how are you going to get the steel beams under the walls? and how are you going to keep everything level and avoid massive cracking since there are no floor joists??

 

[wayniac3]

SWComposites,
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.

 

[ishvaaag]

http://www.ideals.illinois.edu/bitstream/handle/2142/13811/SRS-398.pdf?sequence=2

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.

 

[bylar]

in Florida, house mover routinely excavate under SOG and move the house and then support it on piers. I DON'T KNOW THE CALCS. but it works

 

[bdSE]

From your description of the design it sounds like there will be three girder lines running the length of the building. One at each exterior wall and one centered under the newly constructed floor. It sounds as though piers will be used at both the interior and exterior girt lines. If this is the case, please note that your exterior piers could experience more load than you are allowing for your interior piers.

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.