Background on choice of design for First Road Crossing

As early as 1943, Gloucestershire County Council started lobbying for an early start to the Severn Bridge. They now favoured the long span, high level option on the Aust-Beachley line.  In 1945, the Minister of Transport accepted responsibility for the crossing and, in 1946, published a National Plan showing the crossing on the Aust-Beachley-Newhouse line, with a high-speed road link from the crossing to the A48 at Tredegar Park, west of Newport.

The design of the Crossing was under way by the late 1950s. It was a time unlike any decade since. The war was over. There was hope and aspiration that a better society would arise from the carnage of the war. It was a time of enthusiasm. The proposal to build a motorway network caught the imagination and gained considerable public acceptance. It was to be pursued by the Ministry of Transport as a public programme and a team effort. Some of the personalities involved, engineers and civil servants, attracted public attention not unlike the engineers of Victorian times.

The design of a road suspension bridge needs to incorporate a stiffening girder on which the road deck is carried. This stiff girder maintains the shape of the cable as live traffic loads cross the bridge. Without the stiffening girder distributing the live loads through the hangers to the cable the behaviour would be like the rope bridges of the South American Indians, in which the rope changes shape and the walkway distorts as people cross it.

There had been a collapse of the Tacoma Narrows Bridge in America in November 1940, only some four months after opening. The spectacular failure of that bridge has been shown many times and can be seen below.

The stiffening girder for the Tacoma Narrows Bridge was novel for the time and was constructed from two deep I-sectioned girders, one beneath each of the main cables, which were joined by cross-girders that carried the road deck.

It so happened that the natural frequency of wind-induced, oscillations (of half-span wave-length along the bridge and a half-span twist transversely) were in phase with the frequency of the vortices shed from the deck by the wind blowing up the Tacoma Narrows Channel. In a prolonged period of relatively low but steady wind, this led to increasing deformations longitudinally and transversely. These galloping deflections ultimately led to massive failure of the main girders and more or less total collapse of the deck, although the towers and main cables survived. At the point of failure, the torsional and bending deflections were enormous. Torsionally, the deck was rotating by about +/- 45º and the vertical deflections were several times greater than the depth of the girders.

The designers had overlooked the effect of strong and steady crosswinds. Even Telford, who aimed at lightness in his structures, underestimated this effect in his pioneer bridge across the Menai Straights, opened in 1824. At the end of January 1826, the Menai Suspension Bridge was swept by major gales inducing extreme torsional oscillations in the deck and twenty-six suspension rods were broken. Telford’s modifications included stiffening the deck and strengthening the suspension rods.

The failure of the Tacoma Narrows bridge made designers all over the world mindful of wind-induced effects. Research showed that plate girders are much more vulnerable to wind-induced deformations than the traditional deep truss-girders of earlier American and European suspension bridge designs. If the cross-wind vortex-shedding frequency is in phase with the natural frequency of the stiffening girder, deformations will build-up. However, if they could be designed to be out of phase, potentially dangerous deformations would be prevented.

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