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Re: Heavy line loads on thin slabs on grade

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This is exactly what I'm now planning.  It will be an 8" thick slab reinforced with #4 bars at 12" o/c. A uniform load (reaction from soil) can be handled by the slab in bending and shear. I am going to let it "float" in the sawcut opening rather than pin the sides to the adjacent slab as is normally done.  The result may be some minor settlement and a small "lip" under the raised floor, but it will prevent the adjacent slab from taking part of the load.  The allowable subgrade design pressure is 3000psf, and the net force over the new slab area is about 175-180psf, so the settlement should be minimal - the final effect may be a flush condition after accounting for the slab curl ;-)

Sklar, Mitch wrote:


>From one of my textbooks (Reinforced concrete design, Limbrunner & Spiegal) Table 2-1 indicates a 4" slab w/minimal WWR is good for less than 100 psf depending on the fill material and compaction. I have designed a lot of these shelving system and suggest you double check the true loading. The average loading could be approximately 250 psf and therefore the 4" slab w/unknown fill is not going to work.

You could try to distribute the shelving loading to the slab by installing grillage beams on top of the slab but this will push the shelving up 4- 8";
You could try limiting the height of the units to reduce the weight. What about the tracks? Are they surface mounted or recessed? Ultimately, I think the best solution is to saw cut the existing slab, excavate and install nominal 6" slab. If the tracks are recessed than an 8" total slab with blockouts for the tracks would be required.

Mitchell J Sklar, PE, MBA
Structural Engineer

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)]
Sent: Wednesday, August 30, 2006 9:44 AM
To: seaint(--nospam--at)
Subject: Heavy line loads on thin slabs on grade

Has anyone here done an analysis of existing slabs on grade (4" thick, minimal WWR) to support high density filing systems?  I've got a unit with a pair of 750plf loads about 7' o/c and reading ACI 360 has clouded the issue more than helping.  Using the plain concrete formulas for shear in ch22 of ACI 318 and looking at the allowable subgrade bearing all look good (Vu/phi-Vn=0.4, q/q-all=0.35). If this were new construction I'd put a thickened slab in an skip the calcs and hand wringing, but the building is in service and we are trying to minimize the amount of "dirty" work in the area. Suggestions?


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