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