arely used in
Europe. But it is so commonly used in America as to be regarded as a
distinctive American feature. With pin connexions some weight is saved in
the girders, and erection is a little easier. In early pin bridges
insufficient bearing area was allowed between the pins and parts connected,
and they worked loose. In some cases riveted covers had to be substituted
for the pins. The proportions are now better understood. Nevertheless the
tendency is to use riveted connexions in preference to pins, and in any
case to use pins for tension members only.

On the first English railways cast iron girder bridges for spans of 20 to
66 ft. were used, and in some cases these were trussed with wrought iron.
When in 1845 the plans for carrying the Chester and Holyhead railway over
the Menai Straits were considered, the conditions imposed by the admiralty
in the interests of navigation involved the adoption of a new type of
bridge. There was an idea of using suspension chains combined with a
girder, and in fact the tower piers were built so as to accommodate chains.
But the theory of such a combined structure could not be formulated at that
time, and it was proved, partly by experiment, that a simple tubular girder
of wrought iron was strong enough to carry the railway. The Britannia
bridge (fig. 16) has two spans of 460 and two of 230 ft. at 104 ft. above
high water. It consists of a pair of tubular girders with solid or plate
sides stiffened by angle irons, one line of rails passing through each
tube. Each girder is 1511 ft. long and weighs 4680 tons. In cross section
(fig. 17), it is 15 ft. wide and varies in depth from 23 ft. at the ends to
30 ft. at the centre. Partly to counteract any tendency to buckling under
compression and partly for convenience in assembling a great mass of
plates, the top and bottom were made cellular, the cells being just large
enough to permit passage for painting. The total area of the cellular top
flange of the large-span girders is 648 sq. in., and of the bottom 585 sq.
in. As no scaffolding could be used for the centre spans, the girders were
built on shore, floated out and raised by hydraulic presses. The credit for
the success of the Conway and Britannia bridges must be divided between the
engineers. Robert Stephenson and William Fairbairn, and Eaton Hodgkinson,
who assisted in the experimental tests and in formulating the imperfect
theory then available. The Conway bridge was first completed