If no other method of finding the approximate stresses in an arch existed, the elastic theory might be classed as the best available; but this is not the case.  There is a method which is both simple and reliable.  Accuracy is not claimed for it, and hence it is in accord with the more or less uncertain materials dealt with.  Complete safety, however, is assured, for it treats the arch as a series of blocks, and the cementing of these blocks into one mass cannot weaken the arch.  Reinforcement can be proportioned in the same manner as for chimneys, by finding the tension exerted to pull these blocks apart and then providing steel to take that tension.

The fifteenth point concerns steel in compression in reinforced concrete columns or beams.  It is common practice—­and it is recommended in the most pretentious works on the subject—­to include in the strength of a concrete column slender longitudinal rods embedded in the concrete.  To quote from one of these works: 

“The compressive resistance of a hooped member exceeds the sum of the following three elements:  (1) The compressive resistance of the concrete without reinforcement. (2) The compressive resistance of the longitudinal rods stressed to their elastic limit. (3) The compressive resistance which would have been produced by the imaginary longitudinals at the elastic limit of the hooping metal, the volume of the imaginary longitudinals being taken as 2.4 times that of the hooping metal.”

This does not stand the test, either of theory or practice; in fact, it is far from being true.  Its departure from the truth is great enough and of serious enough moment to explain some of the worst accidents in the history of reinforced concrete.

It is a nice theoretical conception that the steel and the concrete act together to take the compression, and that each is accommodating enough to take just as much of the load as will stress it to just the right unit.  Here again, initial stress plays an important part.  The shrinkage of the concrete tends to put the rods in compression, the load adds more compression on the slender rods and they buckle, because of the lack of any adequate stiffening, long before the theorists’ ultimate load is reached.

There is no theoretical or practical consideration which would bring in the strength of the hoops after the strength of the concrete between them has been counted.  All the compression of a column must, of necessity, go through the disk of concrete between the two hoops (and the longitudinal steel).  No additional strength in the hoops can affect the strength of this disk, with a given spacing of the hoops.  It is true that shorter disks will have more strength, but this is a matter of the spacing of the hoops and not of their sectional area, as the above quotation would make it appear.