378
DOCK ENGINEERING.
The lightness and slenderness of the floor call for the best materials and
the most careful workmanship. The concrete should be composed of the
best Portland cement, with an aggregate of broken brick or clean gravel and
sand or crushed granite, in the proportion of 1 to 3. The best metal for the
bars is hard steel; a soft iron does not possess a sufficiently high coefficient
of elasticity. “ Expanded metal,” which is a network sheared out of a solid
steel plate, may be used instead of disconnected bars.
Hennebique System.—This system differs from that just described more
in détail than in principle. There is the same network of bars, but the
meshes are larger, the bars thicker, and the parts are generally set obliquely
with reference to the supporting beams. These beams are themselves
constructed on the same system as the flooring.
Figs. 365 and 366 are the plan and section, respectively, of a bay of
Hennebique flooring.* It will be seen that the main beam is composed of
SECTION OF FLOOR AND BEAM
Figs. 365 and 366. —Hennebique Floor.
three vertical rows of bars, each row containing two bars, of which the lower
one is straight and the upper curved. These bars are bedded in concrete of
a rectangular section, adhesion between the parts being assisted by U-shaped
clips of hoop iron, which enclose the bars and extend almost to the upper
surface of the beam. The model is that of a trussed beam. The concrete
takes the compressive duty ; the bars are simply tension rods.
The ends of the bars are either turned up or split to a fish-tail to increase
the hold.
The floor illustrated has its beams 8 feet 4 inches apart, centre to centre.
The latter are 8 inches wide by 14 inches deep. The floor is 5 inches thick,
and was tested to a uniform load of 18f cwts. per square yard.
* Hope on “ Construction in Fortified Concrete,” Min. Proc. L.E.S., vol. xxii.