Pont de Normandie - Pushing the Limits: New Adventures in Engineering - page 36

by the British firm of Freeman Fox & Partners. The satisfactory behavior of those bridges, along
with the cost savings associated with the lighter box-girder structure, led to the deck design of the
Pont de Normandie, with its record main span of 856 meters—or 2,808 feet, almost exactly that of
the ill-fated Tacoma Narrows Bridge.
The new bridge's box-girder deck is further innovative in that it is of a hybrid construction. The
approach spans and the 116 meters of the central span cantilevered out from each pylon are made
of concrete, whereas the remaining 624 meters of the central span are of steel. This design was de-
veloped for various reasons, including economics; different construction demands of different parts
of the bridge; the need to balance construction and permanent loads on the structure; and the need
to take into account the stiffness of the structure in the wind, especially during the latter stages of
construction when the free ends of the main span would be most susceptible to movement. Such un-
usual conditions led to some disagreements over who among the various contractors was respons-
ible for guaranteeing the overall safety of the structure and also led, subsequently, to concerns for
the soundness of the project. These concerns were, of course, largely dismissed with the successful
completion of the bridge.
Construction sequence for the Pont de Normandie
Accurately addressing questions relating to the aerodynamic stability of the bridge, both during
construction and after completion, was central to the success of the project. During construction, it
was necessary to steady the incomplete bridge not only to keep it from being overstressed but also
to enable lifting and welding operations to proceed under controlled conditions. The use of steady-
ing cables anchored in the water would have interfered with shipping, so a tuned mass damper sys-
tem, which checked unwanted motions, was employed in the free ends of the bridge deck to steady
it until the final section of the deck could be inserted, an event that took place in mid-1994.