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RE: Concrete Code Interpretation

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Tom, ACI 318 Section 10.5.4 is not limited to "structural slabs on grade"
but rather applies to elevated "structural slabs ... of uniform thickness"
as well. The commentary to ACI 318-89 stated the basis for this as: "The
minimum reinforcement required for slabs is somewhat less than that required
for beams, since an overload would be distributed laterally and a sudden
failure would be less likely." Most building slabs do not see full uniform
live load applied to the surface. 

However, if your slab is likely to be stressed along its length uniformly to
full design load (such as a one-way slab supporting a fluid), I would
recommend ignoring this reduction in minimum reinforcement, since the full
slab cross-section would be loaded similar to a beam. (I wish ACI 318 had
kept this wording in the commentary, as it helps understand the code's

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

Thomas Hunt, S.E. wrote:

 Section 10.5.4 gives a break if you have a slab on grade that does not
transmit vertical loads from other parts of the structure to the soil.  The
feeling being that since the slab is on grade if it fails it is not going
anywhere like the second floor of a parking structure might.  

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