platform at this part is
carried by a timber and iron truss. When the headway is great or the river
deep, timber-braced piers or clusters of piles at distances of 50 ft. to
100 ft. may be used. These carry temporary trusses of timber or steel. The
Kuilenburg bridge in Holland, which has a span of 492 ft., was erected on a
timber staging of this kind, containing 81,000 cub. ft. of timber and 5
tons of bolts. The bridge superstructure weighed 2150 tons, so that 38 cub.
ft. of timber were used per ton of superstructure.

(2) The Britannia and Conway bridges were built on staging on shore, lifted
by pontoons, floated out to their position between the piers, and lastly
lifted into place by hydraulic presses. The Moerdyk bridge in Holland, with
14 spans of 328 ft., was erected in a similar way. The convenience of
erecting girders on shore is very great, but there is some risk in the
floating operations and a good deal of hauling plant is required.

(3) If a bridge consists of girders continuous over two or more spans, it
may be put together on the embankment at one end and rolled over the piers.
In some cases hauling tackle is used, in others power is applied by levers
and ratchets to the rollers on which the girders travel. In such rolling
operations the girder is subjected to straining actions different from
those which it is intended to resist, and parts intended for tension may be
in compression; hence it may need to be stiffened by timber during rolling.
The bending action on the bottom boom in passing over the rollers is also
severe. Modifications of the system have been adopted for bridges with
discontinuous spans. In narrow ravines a bridge of one span may be rolled
out, if the projecting end is supported on a temporary suspension cable
anchored on each side. The free end is slung to a block running on the
cable. If the bridge is erected when the river is nearly dry a travelling
stage may be constructed to carry the projecting end of the girder while it
is hauled across, the other end resting on one abutment. Sometimes a girder
is rolled out about one-third of its length, and then supported on a
floating pontoon.

(4) Some types of bridge can be built out from the abutments, the completed
part forming an erecting stage on which lifting appliances are fixed.
Generally, in addition, wire cables are stretched across the span, from
which lifting tackle is suspended. In bridges so erected the straining
action during erect