Industrial ground floors and soil bearing capacity

[Vallenato]

Hello!
I have a question about industrial ground floors on grade subjected to concentrated loads. I have found complete information about how to handle moment and punching failure but I cannot find any information about how to determine if the concentrated loads will induce a ground pressure failure, i.e. overpass the soil bearing capacity or induce exaggerated settlements on very bad soil. This information is very well explained for isolated spread footings but I cannot find any easy to follow equations for larger floors such as industrial floors subjected to for example wheel loads. The moments and punching failure differ between internal-, edge- or corner loading so it would be good to have the soil capacity for the same three cases. And normally there is a 150 mm macadam layer directly under the floor so I guess the method I am searching for has to include the positive effect of the load spread through the strong macadam layer onto the underlying soil that might me critical.

I guess there will be some kind of cone pressure that should be compared to the bearing pressure of the soil calculated with the Terzaghi/Meyerhof/Hansen/Vesic bearing capacity equation but which width should be inserted in the formula and compare with which load distribution? And is it possible to make the load go into plastic failure to "cut" the top of the cone pressure I guess will be induced and how is this calculated? Can someone please guide me step-by-step?

 

[MillR]

The Army has a good document for this:

https://www.wbdg.org/ccb/ARMYCOE/COETM/ARCHIVES/tm_5_809_12.pdf

gives how to design a heavy slab for point loads, linear loads, and traffic loads.

 

[SlideRuleEra]

Take a look at how the subgrade for concrete-paved highways are designed. Here is one place to start, a booklet by the American Concrete Pavement Association: Subgrades and Subbases for Concrete Pavement.
See the "References" on page 67. Another source of information may be the U.S. Federal Highway Administration.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

Agree with SRE and MillR....approach as a pavement design. Look at Westergaard and Boussinesq approaches to stresses under the slab. Bearing capacity is rarely an issue for large slabs. The slab itself actually provides confinement for the soil around point loads, thus mitigating the potential for a localized bearing capacity failure of the soil. It is similar to having a footing deeper in the soil....the overburden increases the bearing capacity of the soil with depth.

 

[Vallenato]

The document published by American Concrete Pavement Association from MillR declared that this is rarely an issue just as Ron also responded. Later on I also found this text in the ACI360R "3.5.3 Bearing support—Calculated bearing pressures under loaded slabs-on-ground are typically significantly lower and are not critical to typical designs as compared with the allowable foundation contact pressures for building elements controlled by ACI 318." So now I feel more secure and will continue by neglecting the soil pressure and just consider moment and punching failure for the floor slab. Thank you all for responding.