n. The roadway is 651/2 ft. wide and footways 33 ft. (_Proc. Inst. C.E._
cxxx. p. 335).

[Illustration: FIG. 30.--Viaur Viaduct.]

The largest three-hinged-arch bridge constructed is the Viaur viaduct in
the south of France (fig. 30). The central span is 721 ft. 9 in. and the
height of the rails above the valley 380 ft. It has a very fine appearance,
especially when seen in perspective and not merely in elevation.

[Illustration: FIG. 31.--Douro Viaduct.]

Fig. 31 shows the Douro viaduct of a total length of 1158 ft. carrying a
railway 200 ft. above the water. The span of the central opening is 525 ft.
The principal rib is crescent-shaped 32.8 ft. deep [v.04 p.0543] at the
crown. Rolling load taken at 1.2 ton per ft. Weight of centre span 727
tons. The Luiz I. bridge is another arched bridge over the Douro, also
designed by T. Seyrig. This has a span of 566 ft. There are an upper and
lower roadway, 164 ft. apart vertically. The arch rests on rollers and is
narrowest at the crown. The reason given for this change of form was that
it more conveniently allowed the lower road to pass between the springings
and ensured the transmission of the wind stresses to the abutments without
interrupting the cross-bracing. Wire cables were used in the erection, by
which the members were lifted from barges and assembled, the operations
being conducted from the side piers.

[Illustration: FIG. 32.--Niagara Falls and Clifton Bridge.]

The Niagara Falls and Clifton steel arch (fig. 32) replaces the older
Roebling suspension bridge. The centre span is a two-hinged parabolic
braced rib arch, and there are side spans of 190 and 210 ft. The bridge
carries two electric-car tracks, two roadways and two footways. The main
span weighed 1629 tons, the side spans 154 and 166 tons (Buck, _Proc. Inst.
C.E._ cxliv. p. 70). Prof. Claxton Fidler, speaking of the arrangement
adopted for putting initial stress on the top chord, stated that this
bridge marked the furthest advance yet made in this type of construction.
When such a rib is erected on centering without initial stress, the
subsequent compression of the arch under its weight inflicts a bending
stress and excess of compression in the upper member at the crown. But the
bold expedients adopted by the engineer annulled the bending action.

The Garabit viaduct carries the railway near St Flour, in the Cantal
department, France, at 420 ft. above low water. The deepest part of the
valley is crosse