The Design of Modern Steel Bridges - TEDI

and supported on 8 ft (2.4 m) deep plate girders rather than a lattice structure.
From the opening, very substantial horizontal and vertical movements of the
deck in wave forms were noticeable even in moderate wind and light traffic,
and earned for the bridge a nickname ‘Galloping Gertie’. Before its construction,
tests in a wind tunnel had shown it to be capable of resisting gale forces of
up to 120 mile/h (193 km/h). On 7 November 1940, a storm that raged for several
hours and reached a speed of 42 mile/h (68 km/h) drove the bridge into an
uncontrollable torsional oscillation, culminating in its collapse into the water.
After the great success of long-span bridges in the previous 60 years, this
disaster shook the very foundations of bridge building. The following official
enquiry by three great engineering experts, von Karman, Ammann and Glen
Woodruff, blamed no individuals and pointed out no mistakes; it attributed the
failure to a lack of proper understanding and knowledge of the whole profession.
The deck was too narrow for the span and thus its torsional rigidity was
inadequate, and the plate girders not only provided insufficient flexural rigidity,
but their bluff elevation caused wind vortices above and underneath the
deck even in moderate and steady wind speeds.
Figure 1.17 Tamar Suspension Bridge, England (Brunel’s bridge can also be seen).
Figure 1.18 Mackinac Bridge, Michigan (1957).
Types and History of Steel Bridges 19