Bridges - Wind Loading of Structures - page 290

12
Bridges
12.1 Introduction
As discussed in Chapter 1, bridges have featured in some spectacular failures during wind
storms (Figure 1.10). The history of the dynamically wind-sensitive suspension bridge
from nineteen century onwards, including the periodic failures that have occurred, has
been well documented (e.g. Steinman and Watson, 1957; Billington, 1977; Petroski,
1996).
Most of the early interest was in the drag or along-wind forces, and Baker (1884),
Kernot (1893) and others noted that peak wind forces acting on large areas, such as a
complete bridge girder, were considerably less than those on a small plate or board.
However, the great American builder of suspension bridges, John Roebling, was aware of
the dynamic effects of wind as early as 1855. In commenting on the failure of the
Wheeling Bridge, Ohio, in the previous year, he wrote: That bridge was destroyed by the
momentum acquired by its own dead weight, when swayed up and down by the force of
the wind…. A high wind, acting upon a suspended floor, devoid of inherent stiffness, will
produce a series of undulations, which will be corresponding from the center each way'
(Steinman and Watson, 1957).
However, it took over 80 years for the dramatic failure of the first Tacoma Narrows
suspension bridge in 1940 (Section 1.4) to direct serious attention to the dynamic actions
of the wind and other wind actions on bridge decks: vertical cross-wind forces and
torsional moments.
The cable-stayed bridge emerged in the 1950s in Germany, as an efficient method of
spanning intermediate length crossings. Gimsing (1983) and Virlogeux (1999) have
reviewed recent developments in the design of bridges of this type.
As the twentieth century ended, the spans of the long-span suspension and cable-
stayed bridges have been extended to new limits. The longest bridge in the world at the
turn of the century was the suspension bridge across the Akashi-Kaikyo Straits in Japan,
which has an overall length of nearly 4km, with a main span of 1990 m(Figure 12.1). The
design of this bridge was dominated by its aerodynamic characteristics.
The longest cable-stayed bridge was the Tatara Bridge, also in Japan, with an overall
length of 1480 m and a main span of 890 m(Figure 12.2).
As the spans increase, wind actions become more critical in bridge design, and for the
longest suspension or cable-stayed bridges, extensive wind studies are normally
undertaken. The dynamic wind forces will excite resonant response, often in several
modes, and aeroelastic forces, in which the motion of the structure itself generates forces,
are important. Long-span bridges are usually crossings of large expanses of water and
may be exposed to relatively low turbulence flow, at least at low wind speeds. This has
contributed to a number of cases of vibrations of bridge decks induced by