ion must be studied, and material must be added to
resist erecting stresses. In the case of the St Louis bridge, half arches
were built out on either side of each pier, so that the load balanced.
Skeleton towers on the piers supported chains attached to the arched ribs
at suitable points. In spite of careful provision, much difficulty was
experienced in making the connexion at the crown, from the expansion due to
temperature changes. The Douro bridge was similarly erected. The girders of
the side spans were rolled out so as to overhang the great span by 105 ft.,
and formed a platform from which parts of the arch could be suspended.
Dwarf towers, built on the arch ring at the fifth panel from either side,
helped to support the girder above, in erecting the centre part of the arch
(Seyrig, _Proc. Inst. C.E._ lxiii. p. 177). The great cantilever bridges
have been erected in the same way, and they are specially adapted for
erection by building out.

_Straining Actions and Working Stresses._

17. In metal bridges wrought iron has been replaced by mild steel--a
stronger, tougher and better material. Ingot metal or mild steel was
sometimes treacherous when first introduced, and accidents occurred, the
causes of which were obscure. In fact, small differences of composition or
variations in thermal treatment during manufacture involve relatively large
differences of quality. Now it is understood that care must be taken in
specifying the exact quality and in testing the material supplied.
Structural wrought iron has a tenacity of 20 to 221/2 tons per sq. in. in the
direction of rolling, and an ultimate elongation of 8 or 10% in 8 in.
Across the direction of rolling the tenacity is about 18 tons per sq. in.,
and the elongation 3% in 8 in. Steel has only a small difference of quality
in different directions. There is still controversy as to what degree of
hardness, or (which is nearly the same thing) what percentage of carbon,
can be permitted with safety in steel for structures.

The qualities of steel used may be classified as follows:--(a) Soft steel,
having a tenacity of 221/2 to 26 tons per sq. in., and an elongation of 32 to
24% in 8 in. (b) Medium steel, having a tenacity of 26 to 34 tons per sq.
in., and 28 to 25% elongation. (c) Moderately hard steel, having a tenacity
of 34 to 37 tons per sq. in., and 17% elongation, (d) Hard steel, having a
tenacity of 37 to 40 tons per sq. in., and 10% elongation. Soft steel is
used fo