Civil Engineering Reference
In-Depth Information
(1940)
The Tacoma Narrows suspension bridge, nicknamed "Galloping Gertie" for its
propensity to oscillate, opened for traffic on July 1, 1940, and was the most flexible
suspension bridge of its time. After being in service for only 129 days, the bridge
failed late in the morning of November 7, 1940. Under a 68 km/h (42 mph) wind, the
bridge, which normally vibrated in a vertical plane, began to oscillate with the
opposite sides out of phase (torsional mode). The oscillation became extremely
violent, until the failure began at mid span, with buckling of the stiffening girders and
lateral bracing. The suspenders snapped, and sections of the floor system fell to the
water below. Almost the entire suspended span between the towers fell into the water.
The side spans, which remained, sagged about 9 m (30 ft) bending back the towers
sharply by the pull of the side span cables. The towers, which were fixed at the base
by steel anchors deeply embedded in concrete piers, survived but were damaged.
The dramatic collapse of this bridge, captured on film by Professor F. B.
Farquharson of the University of Washington, Seattle, who was on the main span just
before the failure taking motion pictures of the violent twisting of the bridge deck,
provides a spectacular example of the forces developed when dynamic resonance
occurs. Because of its excessive motions, the bridge was closed to traffic sometime
before the failure occurred. Hence, there was no loss of human life.
The contract drawings consisted of 39 sheets and were signed by the highway
and bridge engineers of the State of Washington. The drawings carried the signatures
of Moran, Proctor and Freeman, the consultants on substructure and of Leon
Moisseiff as consultant of superstructure. In addition, the engineer retained by the
Reconstruction Finance Corporation to review the plans commented on the flexibility
of the suspended structure, but deferred to the wider experience of Moisseiff. The
design was also reviewed and approved by a board of consultants appointed by the
Washington Toll Bridge Authority.
The total cost amounted to $6,400,000 of which $2,880,000 was a grant from
the Public Works Administration and $3,520,000 was a loan from the Reconstruction
Finance Corporation to be repaid by the tolls.
The suspended span was 853 m (2,800 ft) and the two side spans were 335 m
(1,100 ft) each. The 853 m (2,800 ft) main span made this the third longest
suspension bridge of its time. The roadway was 7.9 m (26 ft) wide and the sidewalks
were 1.5 m (5 ft) wide on each side. The spacing between the cables and stiffening
girders was 11.9 m (39 ft), a 1:12 span ratio. The main piers were 19.7 m (64.5 ft) by
35.8 m (117.5 ft) in plan. The towers were 128 m (420 ft) in height. Each cable
consisted of nineteen strands of 332-No. 6 cold drawn galvanized wires. The diameter
of each cable under wrapping was 435 mm (17 1/8 in.) with a net area of 1228 sq. cm
(190 sq. in.). The original design specified stiffening trusses, but was changed to
stiffening girders 2.4 m (8 ft) deep, a 1:350 span ratio.
Based upon previous experience with suspension bridges with shallow
stiffening girders, the engineers anticipated oscillations. From the time it was
constructed, however, the bridge developed a reputation for disturbing oscillations,
