Art in Iron and Steel - Pushing the Limits: New Adventures in Engineering - page 5

as the newly stretched piece of canvas. And the greatest of bridge engineers, especially, have quite
explicitly written and spoken of the aesthetic criteria and human values that influenced the shapes,
forms, textures, and functions of their structures; the spans themselves stand as tributes to their suc-
cessful applications of their ideals.
Iron Bridge was only the first in a long line of cast-iron, wrought-iron, and steel structures that
have continued to grace the British landscape. Although its form was borrowed from the ancient
lines of stone bridges and the details from the classic lines of timber construction, it did not owe its
structural success to their principles. In particular, the strength of iron enabled subsequent bridges
to be built with much lower profiles, thus marking them as daring and at the same time making them
more user-friendly. Thomas Telford was one of the first masters of the shallow metal arch, with his
1814 Craigellachie Bridge over the River Spey near Elgin, Scotland, being an outstanding example.
The crossed struts between the thin arch and the equally thin deck give the bridge a transparency
and accessibility unknown in stone structures. The crenellated towers of the abutment, Telford's ac-
knowledgment of the setting in which the bridge might otherwise have appeared to be an intruder,
tie it into the culture of its place. Telford used a similar architectural motif on his Conwy Suspension
Bridge, in deference to the Welsh castle to which it leads. Of course, Telford looked well beyond
castles for his inspiration. His Menai Bridge married massive stone towers, which appear to have
evolved naturally from the piers under the approach spans, with wrought-iron chains to produce a
profile of near-perfect proportions that served as an aesthetic model for suspension bridges well in-
to the twentieth century.
The Brooklyn Bridge, completed in 1883, is almost three times as large as the Menai, and during
construction its towers dominated the New York skyline. The technical challenge that John A.
Roebling faced in spanning the East River was to design a structure that would not interfere with
shipping. This demanded not only a high roadway beneath which tall-masted ships could pass but
also a great span to provide a wide, unobstructed channel. Suspension bridges were Roebling's
forte, but the combination of constraints in New York called for one of unprecedented size. Rather
than design a purely utilitarian structure, he produced a masterpiece. The tall stone towers pierced
by the twin Gothic arches through which traffic passes are necessarily massive, but their monument-
al design makes them feel architecturally right. Roebling's patented steel-wire cables hang with a
well-proportioned sag, counterpointed by the taut diagonal stay cables that the engineer included
out of respect for the wind and what it could do to an unstayed bridge deck. The bridge deck was
designed not only for horses and carriages but also for people, and the elevated walkway that puts
the walkers above the road traffic makes the bridge at the same time a brilliant work of engineering,
art, and humanity.