Slab on Grade allowable distributed load

[penpe]

I have an old 1961 CRSI design handbook which has a chart of slab on grade thicknesses and their typical applications, with recommended reinforcement, and a range of allowable distributed loadings: 4" slab good for light commercial 100 psf; 5" commercial 100 to 200 psf; 6" industrial 400 to 500 psf; 7" industrial plants, gas stations 600 to 800 psf; etc.

I think these are good guidelines, but would like a good way to determine more precisely a 6" slab on grade's distributed loading capacity. I expect that the soil characteristic would play a significant role in that determination.

The case I'm investigating is on clay soil with about 2000 psf compressive strength. Do you think a 6" 3000 psi concrete slab with WWF reinforcing should be able to handle 500 psf? Maybe more?

 

[JLNJ]

Slab capacity depends on the soil stiffness more than the soil allowable bearing capacity. Usually we work with point loads either from racking or forktruck wheels when calculating slab capacities. It's the point loads which cause flexure in the slab. Flexure controls slab thickness, not simple compression from a uniform load.

 

[jayrod12]

Agree with JLNJ, for truly distributed loads, the slab itself is essentially irrelevant, it's just a load transfer medium to the subgrade below.

For wheel and racking loading, or line loads, the strength of the slab itself does come into play when it comes to spreading the load appropriately to the sub-grade. But more important than the slab design, is the sub-grade strength and stiffness.

If you look for a book called Concrete Floors on Ground, it in my opinion is one of the must-have books. It stresses how important the sub-grade prep can be. You could put a 10" slab on quicksand and it wouldn't stand up nearly as well as a 4" slab on a properly prepared base material.

 

[hokie66]

Agree with JLNJ and Jayrod12, but precision is not a word to use with slab on grade design.

 

[phamENG]

Where it can become important is at joints. If you have construction joints or even crack control joints where you assume the section will crack through, the stiffness of the slab begins to have a greater impact on the slab-soil interaction and how much the load spreads away from the joint. This is because, rather than a circular area, your load is now semi-circular or a quarter circle if at a corner. Similarly, slab thickness will impact shear transfer of dowels at these joints. I realize this is more of concentrated load thing, but good slab designs should consider point loads. Uniform loads don't actually exist unless you're looking at the floor of a tank filled with still water.

 

[OldDawgNewTricks]

When considering uniformly distributed loads on slabs-on-grade, you must account for aisles between loaded areas. This pattern loading causes tension at the top of the slab in the aisle areas. The WRI and PCA slab design methods have charts and tables to account for this.

 

[dik]

A lot of slabs on grade are used for racking... and you have to consider the racking 'leg' loads and the pattern of leg loads. I used to use the PCA 'Airport' pavement design program for designing rack loads...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[penpe]

Thanks for the responses. This case is a temporary "staging" location of a 6.86 foot diameter, 42 foot tall vessel weighing about 46 kips. It will be on a 6" thick slab. My old CRSI handbook (1961) says 6" slab is good for up to 500 psf, but the vessel bottom plate area would impose over 1200 psf. (That's dead load only, and there are wind loads too). I proposed supporting the vessel on a big 15' square plate 1" thick, which would keep the worst case distributed load below 500 psf, even on the windiest day. The client asked me if we could get away with something smaller. Smaller plate means bigger loads. I need to determine what maximum distributed load is acceptable. It will be in this staging location for about two months, then will be moved to it's permanent location on the existing foundation.

When the wind blows, the stiff plate will transfer the lateral load to one edge theoretically. I need to make sure the concrete won't fail as a result.

 

[WesternJeb]

If you are billing time and material, I would tell them that they're paying you more to cut the cost on the plate than if they were to just order the 1" plate. My clients slip into this pitfall a lot.

 

[phamENG]

Really? A 1" plate is stiff enough to lower your bearing pressure? That seems...suspicious.

 

[penpe]

phamENG, here's my design approach:
Arrived at 1" thick plate by using AISC Design Guide DG1 "Base Plate and anchor Rod Design" 2nd Edition. Small moment Base Plate design. Treated the round vessel like a steel column, used an equivalent square area to determine variables "d" and "bf". Base plate thickness required is 0.82" (Gravity load 46k, moment 96 ft-k).

WesternJeb,
I agree. I think part of the motivation to reduce the size is in order to take up less space, make it easier to transport and install.

 

[phamENG]

At 6.86ft in diameter, you're well outside of the applicable range of DG1. DG1 requires that the base plate behavior is rigid, it does not ensure that it is rigid. Not sure if you've ever used Hilti Profis software, but they have have a component based finite element model that they run to verify stiffness of plates. I've checked several baseplates that look great based on DG1, but a quick CBFEM check shows the predicted tension or bearing stress diverges from the rigid assumption by 30% or more.

It's one thing to assume an equivalent square area when you're looking at a pipe that's 10" across...it's very different when it's 82.32" across.