work, and itself acting only as a stiff floor
and a protecting envelope. Bond, in this case, while, of course, an
adjunct, is by no means vitally important, as is generally the case with
beams unrestrained in any way and in which the reinforcement is not
provided with adequate end anchorage, in which case a continuous bond is
apparently--at any rate, theoretically--indispensable.

An example of the opposite extreme in reinforced concrete design, where
provision for reverse stresses was almost wholly lacking, is shown in
the Bridgeman Brothers' Building, in Philadelphia, which collapsed while
the operation of casting the roof was in progress, in the summer of
1907. The engineering world is fairly familiar with the details of this
disaster, as they were noted both in the lay and technical press. In
this structure, not only were U-bars almost entirely absent, but the
few main bars which were bent up, were stopped short over the support.
The result was that the ties between the rib and the slab, and also
across the support, being lacking, some of the beams, the forms of which
had been removed prematurely, cracked of their own dead weight, and,
later, when the roof collapsed, owing to the deficient bracing of the
centers, it carried with it each of the four floors to the basement, the
beams giving way abruptly over the supports. Had an adequate tie of
steel been provided across the supports, the collapse, undoubtedly,
would have stopped at the fourth floor. So many faults were apparent in
this structure, that, although only half of it had fallen, it was
ordered to be entirely demolished and reconstructed.

The cracks in the beams, due to the action of the dead weight alone,
were most interesting, and illuminative of the action which takes place
in a concrete beam. They were in every case on the diagonal, at an angle
of approximately 45 deg., and extended upward and outward from the edge of
the support to the bottom side of the slab. Never was the necessity for
diagonal steel, crossing this plane of weakness, more emphatically
demonstrated. To the writer--an eye-witness--the following line of
thought was suggested:

Should not the concrete in the region above the supports and for a
distance on either side, as encompassed by the opposed 45 deg. lines (Fig.
14), be regarded as abundantly able, of and by itself, and without
reinforcing, to convey all its load into the column, leaving only the
bending to be considered in the tr