huangyhg

relocated building - slab on grade becomes elevated
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

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?
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!
why can't you pour a new, elevated slab and set the old one on top?
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.
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.
tip: building movers will tell you anything.
advice: don't get involved in silly schemes.
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.

mike mccann
mmc engineering
and it's not even april 1.
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
how do they move an entire slab on grade and get it properly supported during the move?

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