strengthened against lateral yielding by horizontal and diagonal bracing.
In some cases the piers are cast iron cylinders 10 ft. or more in diameter
filled with concrete.

[Illustration: FIG. 35.--Cylinder, Charing Cross Bridge.]

_Cylinder Foundations._--Formerly when bridge piers had to be placed where
a firm bearing stratum could only be reached at a considerable depth, a
timber cofferdam was used in which piles were driven down to the firm
stratum. On the piles the masonry piers were built. Many bridges so
constructed have stood for centuries. A great change of method arose when
iron cylinders and in some cases brick cylinders or wells were adopted for
foundations. These can be sunk to almost any depth or brought up to any
height, and are filled with Portland cement concrete. They are sometimes
excavated by grabs. Sometimes they are closed in and kept free of water by
compressed air so that excavation work can be carried on inside them (fig.
35). Sometimes in silty river beds they are sunk 100 ft. or more, for [v.04
p.0546] security against deep scouring of the river-bed in floods. In the
case of the Empress bridge over the Sutlej each pier consisted of three
brick wells, 19 ft. in diameter, sunk 110 ft. The piers of the Benares
bridge were single iron caissons, 65 ft. by 28 ft., sunk about 100 ft.,
lined with brick and filled with concrete. At the Forth bridge iron
caissons 70 ft. in diameter were sunk about 40 ft. into the bed of the
Forth. In this case the compressed air process was used.

16. _Erection._--Consideration of the local conditions affecting the
erection of bridges is always important, and sometimes becomes a
controlling factor in the determination of the design. The methods of
erection may be classed as--(1) erection on staging or falsework; (2)
floating to the site and raising; (3) rolling out from one abutment; (4)
building out member by member, the completed part forming the stage from
which additions are handled.

(1) In erection on staging, the materials available determine the character
of the staging; stacks of timber, earth banks, or built-up staging of piles
and trestles have all been employed, also iron staging, which can be
rapidly erected and moved from site to site. The most ordinary type of
staging consists of timber piles at nearly equal distances of 20 ft. to 30
ft., carrying a timber platform, on which the bridge is erected. Sometimes
a wide space is left for navigation, and the