beam (figs. 14 and 3). They
are exceedingly thin, but being buried in concrete no danger of their
perishing from rust is to be feared. (3) In the Boussiron system a
similar stirrup is used, but instead of being vertical the two parts are
spread so that each is slightly inclined. (4) In the Coularon system,
the stirrups are inclined as in fig. 15, and consist of rods, the ends
of which are hooked over the tension and compression bars. (5) In the
Kahn system the stirrups are similarly arranged, but instead of being
merely secured to the tension bar, they form an integral part of it like
branches on a stem, the bar being rolled to a special section to admit
of this. (6) In many systems such as the "expanded metal" system, the
tension and compression rods together with the stirrups are all
abandoned in favour of a single rolled steel joist of I section, buried
in concrete (see fig. 16). Probably the weight of steel used in this way
is excessive, but the joists are cheap, readily procurable and easy to
handle.

Floor _slabs_ may be regarded as wide and shallow beams, and the remarks
made about the stresses in the one apply to the other also; accordingly,
the various devices which are used for strengthening beams recur in the
slabs. But in a thin slab, with its comparatively small span and light
load, the concrete is generally strong enough to bear the shearing
stresses unaided, and the reinforcement is devoted to assisting it where
the tensile stresses occur. For this purpose many designers simply use
the modification of the Monier system, consisting of a horizontal
network of crossed steel rods buried in the concrete. "Expanded metal"
too is admirably adapted for the purpose (fig. 1). In the Matrai system
thin wires are used instead of rods, and are securely fastened to rolled
steel joists, which form the beams on which the slabs rest; moreover,
the wires instead of being stretched tight from side to side of the slab
are allowed to sag as much as the thickness of the concrete will allow.
In the Williams system small flat bars are used, which are not quite
horizontal, but pass alternately over and under the rolled joists which
support the slabs.

[Illustration: FIG. 16.]

A concrete _arch_ is reinforced in much the same way as a wall, the
stresses being somewhat similar. The reinforcing rods are generally laid
both longitudinally and circumferentially. In the case of a culvert the
circumferential rods are sometimes laid