The footnote on page 510 of the December 1971 issue of Concrete Construction concerns a formula stated as:
     As = -----

In the text it is stated that common values for f and S are 1.5 and 30,000 psi, respectively. The f is stated to be the coefficient of friction between the slab and subgrade. Is it possible that this value might be 0.5, i.e., a value less than 1 rather than 1.5?

The coefficient of resistance between the slab and subgrade is not the coefficient of sliding friction in its strictest sense. The coefficient depends upon the temperature drop as well as the length of the slab. The coefficient most widely used for design is 1.5 and on a cement treated subbase may be as high as 50. For slab thicknesses of 6 inches, lengths between 20 and 100 feet, the coefficient is usually between 1.06 and 2.10.