which had bearing walls to curtain off the sides, less
fortunate results were obtained. A temperature drop over night of nearly
50 deg., followed by a spell of alternate freezing and thawing, effected the
ruin of at least the upper 2 in. of a 6-in. slab spanning 12 ft. (which
was reinforced with 1/2-in. round bars, 4 in. on centers), and the
remaining 4 in. was by no means of the best quality. It was thought that
this particular bay would have to be replaced. Before deciding, however,
a test was arranged, supports being provided underneath to prevent
absolute failure. But as the load was piled up, to the extent of nearly
400 lb. per sq. ft., there was no sign of giving (over this span) other
than an insignificant deflection of less than 1/4 in., which disappeared
on removing the load. This slab still performs its share of the duty,
without visible defect, hence it must be safe. The question naturally
arises: if 4 in. of inferior concrete could make this showing, what must
have been the value of the 6 in. of good concrete in the other slabs?
The reinforcing in the slab, it should be stated, was continuous over
several supports, was proportioned for (_w_ _l_)/8 for the clear span
(about 11 ft.), and three-fourths of it was raised over the supports.
This shows the value of the continuous method of reinforcing, and the
enormous excess of strength in concrete structures, as proportioned by
existing methods, when the reverse stresses are provided for fully and
properly, though building codes may make no concession therefor.

Another point may be raised, although the author has not mentioned it,
namely, the absurdity of the stresses commonly considered as occurring
in tensile steel, 16,000 lb. per sq. in. for medium steel being used
almost everywhere, while some zealots, using steel with a high elastic
limit, are advocating stresses up to 22,000 lb. and more; even the
National Association of Cement Users has adopted a report of the
Committee on Reinforced Concrete, which includes a clause recommending
the use of 20,000 lb. on high steel. As theory indicates, and as F.E.
Turneaure, Assoc. M. Am. Soc. C. E., of the University of Wisconsin, has
proven by experiment, failure of the concrete encircling the steel under
tension occurs when the stress in the steel is about 5,000 lb. per sq.
in. It is evident, therefore, that if a stress of even 16,000 lb. were
actually developed, not to speak of 20,000 lb. or more, the concrete
would