es. Its value is generally
taken at 1,500,000 to 3,000,000 lb per sq. in. For steel E = 28,000,000 to
30,000,000, or on the average about twelve times its value for concrete.
The maximum compressive working stress on the concrete may be 500 lb per
sq. in., the tensile working stress 50 lb per sq. in., and the working
shearing stress 75 lb per sq. in. The tensile stress on the steel may be
16,000 lb per sq. in. The amount of steel in the structure may vary from
0.75 to 1.5%. The concrete not only affords much of the strength to resist
compression, but effectively protects the steel from corrosion.

8. (c) _Suspension Bridges._--A suspension bridge consists of two or more
chains, constructed of links connected by pins, or of twisted wire strands,
or of wires laid parallel. The chains pass over lofty piers on which they
usually rest on saddles carried by rollers, and are led down on either side
to anchorages in rock chambers. A level platform is hung from the chains by
suspension rods. In the suspension bridge iron or steel can be used in its
strongest form, namely hard-drawn wire. Iron suspension bridges began to be
used at the end of the 18th century for road bridges with spans
unattainable at that time in any other system. In 1819 T. Telford began the
construction of the Menai bridge (fig. 10), the span being 570 ft. and the
dip 43 ft. This bridge suffered some injury in a storm, but it is still in
good condition and one of the most graceful of bridges. Other bridges built
soon after were the Fribourg bridge of 870 ft. span, the Hammersmith bridge
of 422 ft. span, and the Pest bridge of 666 ft. span. The merit of the
simple suspension bridge is its cheapness, and its defect is its
flexibility. This last becomes less [v.04 p.0537] serious as the dead
weight of the structure becomes large in proportion to the live or
temporary load. It is, therefore, a type specially suited for great spans.
Some suspension bridges have broken down in consequence of the oscillations
produced by bodies of men marching in step. In 1850 a suspension bridge at
Angers gave way when 487 soldiers were marching over it, and 226 were
killed.

[Illustration: FIG. 10.--Menai Suspension Bridge.]

To obtain greater stiffness various plans have been adopted. In the Ordish
system a certain number of intermediate points in the span are supported by
oblique chains, on which girders rest. The Ordish bridge built at Prague in
1868 had oblique chains support