Relocated Building - Slab on Grade becomes Elevated

[Steel2Steel]

We are relocating a building in which a slab on grade is going to become and elevated slab. The building mover is proposing a system of masonry piers 24" SQ spaced at 4'-0" OC each way to sit the new slab on. They will grout beneath the bearings at the piers to ensure direct bearing. They said they have done this numerous of times and have had no problems during or after the move nor did they have any problems with the building officials.

We were asked to provide foundation design for the relocated building. When reading ACI 318, Chapter 22, specifically says that structural plain concrete shall be limited to members that are continuously supported by soil or supported by other structural members capable of providing continuous vertical support. Even though the slab may be reinforced with 6x6 W1.4 W1.4 WWF, it is still considered "plain" concrete per definitions section of ACI. I checked the flexural and shear capacities of the 4" SOG for the 4' clear span and they are all within the limits, however the code says you can't do this.

One way is to fill the space underneath the slab with grout or a flowable fill essentially turning it back into a slab on grade, however, this would be very costly. Maybe you could compact the site beneath the building and pump sand under the building since sand doesn't compact. Any other ideas?

Has anyone come across this? This is a commercial building (Dr. Office) and not a house.

Any tips or advice would be appreciated.

Thanks in advance

 

[kslee1000]

You obviously can't count on the WWF even it was indicated and installed. How about add wood joists, or steel beams, from pedestal to pedestal?

 

[civilperson]

How did you determine the flexural capacity of plain concrete? What tensile strength did you assume was in the concrete. I agree with ACI, can not use plain concrete for elevated slab!

 

[cvg]

why can't you pour a new, elevated slab and set the old one on top?

 

[lkjh345]

I guess I am surprised that it is cheaper to save a 4" SOG (and all the trouble that this would seem to entail) than to just demo the existing slab and pour a new one when the building is in its new location. 4" SOG is pretty cheap.

 

[apsix]

I am surprised that a 4" SOG could even be relocated AND the OP does mention 'new slab'.

If the existing slab is being reused it would be advantageous to place as much fill as possible prior to placing the slab. I doubt that pumped sand will not compact.

 

[hokie66]

Tip: building movers will tell you anything.
Advice: don't get involved in silly schemes.

 

[msquared48]

Unless this is a Matt footing, which at 4" it is not, I don't see the value of saving the slab. What about any stem walls, strip or spread footings? How is the slab to separated from these if present? Sounds real suspicious to me too. Roundfile the idea.

[flush]

Mike McCann
MMC Engineering

 

[apsix]

And it's not even April 1.

 

[CTSeng]

Not that funny,
I've actually seen this more than once in Florida where houses are built on slabs with thickend edges at the perimeter below a masonry wall, difficult to move without slab. They decided to put in a new road or bridge and had a bunch of reasonably decent homes in the way so they moved them. Then gave me a call to look at cracking slabs and masonry walls where the slab was set on masonry piers 4' o.c. Told them to fill the crawl space with flowable concrete fill.

They didn't like that answer and called another guy.

good luck

 

[CTW]

How do they move an entire slab on grade and get it properly supported during the move?

 

[bigmig]

Steel2Steel,

lkjh345 has a good point. Economics. Perhaps a sit down meeting with the moving contractor to have him show you the cost of shoring and supporting the slab during excavation, lifting, moving and setting back down would be a good place to start. And also ask him what he will do if the slab cracks somewhere in the middle of this process. Will he pay you for it? Or just say "act of God, so give me my $"?

Then call up the local concrete guys, who according to my global economic outlook are pretty slow and will cut you a good deal just to stay competitive.

Your answer will be at the end of those two numbers.

The reason the moving contractor wants to move it is because he knows that he will profit $$ big time.

 

[msquared48]

"They decided to put in a new road or bridge and had a bunch of reasonably decent homes in the way so they moved them. Then gave me a call to look at cracking slabs and masonry walls where the slab was set on masonry piers 4' o.c."

Therein lies the innate problem with this idea - the viability of the walls and slab after the move. The slab alone has top be at least 50% of the dead load of the building.

No mention in the original post about CMU walls - I was assuming wood. The CMU would complicate it further, but actually stiffen the edge of the slab, in addition to any thickening of the slab. Any thickened edge of the slab would make it hard to swhore under the slab prior to any lift. I would not want to be the tunnel jockey that had to crawl under the slab next to the shoring/lifting beam to install the shims to the slab...

So...what is the actual existing construction of the foundation here?

Mike McCann
MMC Engineering

 

[msquared48]

Sorry...the word intended was "shore".

Mike McCann
MMC Engineering

 

[kslee1000]

Saw house moving several times before. Some were un-rooted then transported by low-flat bead truck-trailer, usually occurs at night for traffic concerns. One was most impressive - the 3 story masonry building was un-rooted and set on top of round wood logs, then rolled to the new site couple hundred feet away. All were in so called "third world" countries, the owners wanted to preserve the buildings, and move was the most cheap means. It happens in the US as well, just not too often. As pointed out by CTSeng, the SOGs were kept during the whole process to maintain the stability and geometry. I didn't know if they re-use the slab or not.

Extension to my original suggestion to add wood joists on top of the pedestals - how about make it a wood sub-floor to sustain the floor loads including the 4" concrete slab? The house can directly set on the new foundation and sub-floor, the 4" SOG can be left as flooring, or demo later. For 4'x4' spans, wood expert, now is your time to say something.

 

[hokie66]

As CTSeng says, this is not funny. But it would be hilarious if some of these guys weren't serious.