irrups or loops at frequent intervals. Piles made on
the Williams system have a steel rolled joist of I section buried in the
heart of the pile, and round it a series of steel wire hoops at regular
intervals (fig. 5). Whatever system is used, care must be taken not to
batter the head of the pile to pieces with the heavy ram. To prevent
this an iron "helmet" containing a lining of sawdust is fitted over the
head of the pile. The sawdust adapts itself to the rough shape of the
concrete, and deadens the blow to some extent.

[Illustration: FIG. 12.]

[Illustration: FIG. 13.]

[Illustration: FIG. 14.--Stirrup (Hennebique System).]

[Illustration: FIG. 15.]

But it is in the design of steel concrete _beams_ that the greatest
ingenuity has been shown, and almost every patentee of a "system" has
some new device for arranging the steel reinforcement to the best
advantage. Concrete by itself, though strong in compression, can offer
but little resistance to tensile and shearing stresses, and as these
stresses always occur in beams the problem arises how best to arrange
the steel so as to assist the concrete in bearing them. To meet tensile
stresses the steel is nearly always inserted in the form of bars running
along the beam. Figs. 6 to 9 show how they are arranged for different
loading. In each case the object is to place the bars as nearly as
possible where the tensile stresses occur. In cases where all the
stresses are heavy, that portion of the beam which is under compression
is similarly reinforced, though with smaller bars (figs. 10 and 11). But
as these tension and compression bars are generally placed near the
under and upper surface of the beam they are of little use in helping to
resist the shearing stresses which are greatest at its neutral axis.
(See BRIDGES.) These shearing stresses in a heavily loaded beam would
cause it to split horizontally at or near the centre. To prevent this
many ingenious devices have been introduced. (1) Perhaps one of the most
efficient is a diagonal bracing of steel wire passing to and fro between
the upper and lower bars and firmly secured to each by lapping or
otherwise (fig. 12); this device is used in the Coignet and other French
systems. (2) In the Hennebique system (which has found great favour in
England) vertical bands or "stirrups," as they are generally called, of
hoop steel are used (fig. 13). They are of U shape, and passing round
the tension bars extend to the top of the