ertical ones, and thus form a network of steel
which is buried in the concrete. These rods assist in taking the weight,
and the whole network binds the concrete together and prevents it from
cracking under a heavy load. The vertical rods should not be quite in
the middle of the wall but near the inner and outer faces alternately.
Care must be taken, however, that all the rods are covered by at least
an inch of concrete to preserve them from damage by rust or fire. In
the Cottancin system the concrete is replaced by bricks pierced with
holes through which the vertical rods are threaded; the horizontal
tie-rods are also used, but these do not merely cross the vertical ones,
but are woven in and out of them.

[Illustration: FIG. 5.--Steel and Concrete Pile (Williams System).]

[Illustration: FIG. 6.]

[Illustration: FIG. 7.]

_Columns_ have generally to bear a heavier weight than walls, and have
to be correspondingly stronger. They have usually been made square with
a vertical steel rod at each corner. To prevent these rods from
spreading apart they must be tied together at frequent intervals. In
some systems this is done by loops of stout wire connecting each rod to
its neighbour, and placed one above the other about every 10 in. up the
column (figs. 3 and 4). In other systems a stout wire is wound
continuously in a spiral form round the four rods. Modern investigation
goes to prove that the latter is theoretically the more economical way
of using the steel, as the spiral binding wire acts like the binding of
a wire gun, and prevents the concrete which it encloses from bursting
even under very great loads.

[Illustration: FIG. 8.]

[Illustration: FIG. 9.]

[Illustration: FIG. 10.]

[Illustration: FIG. 11.]

That steel concrete can be used for _piles_ is perhaps the most
astonishing feature in this invention. The fact that a comparatively
brittle material like concrete can be subjected not only to heavy loads
but also to the jar and vibration from the blows of a heavy pile ram
makes it appear as if its nature and properties had been changed by the
steel reinforcement. In a sense this is undoubtedly the case. A. G.
Considere's experiments have shown that concrete when reinforced is
capable of being stretched, without fracture, about twenty times as much
as plain concrete. Most of the piles driven in Great Britain have been
made on the Hennebique system with four or six longitudinal steel rods
tied together by st