Engineers completing the final design for the Severn Bridge, however, realized that serious eddy sheddings might veil be avoided on bridge decks by adopting the principles that had been used for years to avoid it in aircraf structures. The deck of the bridge was designed as a box girder, shaped like an aerofoil, and enclosed by stiffened plates at the top and hottom and inclined plates at the sides, so as to make it in a large measure streamlined. The upper surface constitutes the deck, on which the roadway is constructed. projecting platforms being provided at the sides to carry cycle tracks and footways. Extending throughout the .spun, the deck is only 10 feet (3-metres) deep and is suspended from the cables by inclined hangers at 60-feet (18-metre) intervals. These triangulated hangers damp out the small degree of oscillation that might otherwise occur with winds blowing slightly upwards from the horizontal.
Considerable economy is achieved not only by the saving in weight in the deck itself but also by the fact that its shallow airfoil profile reduces the wind loads on the bridge and, thus, leads to further economies in the weight of cables and towers. Moreover，the tower legs are of single-box section, so that their weight is reduced to a minimum. These innovations have undoubtedly set high standards of economy and sophistication for the future of suspension-bridge design.
Aerodynamic action may also be a source of trouble during the erection of a suspension bridge. When the north tower of the Forth Road Bridge had reached a height of over 400 feet, it began to sway alarmingly under quite light crosswinds of only 20-25 miles per hour, although in a heavy gale it moved only a few inches. The sway developed an amplitude of over 7 feet (2. 1 meters) at the tower top. with a period of 4. 5 seconds, and built up and died down every few minutes. This novel phenomenon was quickly checked by mean:: of a damping device consisting of a 16-ton counterweight that was connected to the top of the tower by long steel cables and arranged so that it could slide up and down a ramp inclined at 45° to the horizontal. Any movement of the tower top of more than a few inches began to pull the counterweight up the ramp; this. interrupted the rhythmic buildup of the oscillations and brought the top of the tower to rest. The steelwork of the suspended deck of this bridge was erected in two passes by lour 15-ton derricks. working outward from each of the towers to the centre and then returning again. Wind-tunnel tests made on models of the deck during erection demonstrated that it was necessary to leave a longitudinal strip 20 feet wide open along the centre line of the bridge, during the first pass of the derricks , in order to maintain the aerodynamic s*ability of the deck, until the first pass had been completed, connecting the stiffening trusses of the deck at midspan.
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