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About Robert Cummings, P.E.
Expertise
I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience
30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

You are here: Experts > Education > Votech Education > Construction & Contractors > Concrete loading

Construction & Contractors - Concrete loading

Expert: Robert Cummings, P.E. - 11/2/2009

Question
Even though concrete is given (for example) a 3500 psi load rating, where does the thickness become relevant? I have a small shop with only 5" thick 6 bag concrete. I need to construct a hoist and transfer system that is 25000lbs spread over 10 legs with one foot square floor pads. (2500 lbs per sq foot) The 5" varies, what is minimum thickness I should be looking for?

Answer
The strength of concrete is measured in uniaxial compression on either a 4x8 right cylinder (small aggregate) or, for conventional 3/4 inch aggregate, a 6x12 right cylinder and refers to the strength after a 28-day curing period. (Concrete gains strength as it cures so the 28-day strength, which is 70-100 percent of the long-term strength, is used.) This compressive strength value is an index only. Unreinforced concrete is almost never used structurally in uniaxial (that is, unconfined) compression. The value appears in various civil engineering calculations with discount factors to account for the actual, mechanical loading depending on the application (slab, column, etc.) This means that a slab composed of 3500 psi concrete will not necessarily withstand 3500 psi of applied load.

In some cases concrete is tested in 3-point bending to estimate the tensile or flexural strength, but it is more common to estimate the tensile strength from the compressive strength, again using standard civil engineering relationships.

For a slab-on-grade as is the case in your shop, the thickness becomes relevant in the solution of the differential equations involving the flexure of the slab beam and the resilient modulus of the subgrade. Simplified approximations of these calculations can be found in books and manuals on concrete design. The simplified equations are not difficult and you can probably find them on the Internet or pick up a concrete design book at a used book store.

In your case the issue is simpler because you did not mention any reinforcement in the shop slab.

In your case, assuming all legs are loaded uniformly, the load per leg is 2500 lb and the area is 1 sq ft (144 sq in.) so the average loading density under each pad will be 2500 psf (about 17.5 psi) which is not very high. There are a lot of compacted soils that exhibit allowable bearing capacities of 2500 psf. My guess is that you will find a pretty good safety factor for the slab. Remember, this assumes the load per pad is uniform, which it probably is not. If the frame is flexible, and almost all steel frames are under appreciable load, the load per pad will not be the same. You will want to assume a decent safety factor (3-4 at least) unless you are prepared to analyze the actual load distribution among the pads.

You should consider group loading if the pads are not spread very far apart. Obviously, if you have 10 pads but they are skin to skin, you don't have a 1 sq ft loading area any more, you have a 10 sq ft loading area. The average load is the same but because all 25,000 lb is more concentrated the loading configuration on the slab and subgrade are different. You should consider that the pads will interact (contribute to the loading of each other) if they are closer together than about 3 diameters.

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