A Revolutionary Design
When the government announced that the Forth Road Bridge – a similar proposal for a tolled suspension bridge – would take priority over the Severn, the design team was able to take advantage of the opportunity that arose out of a situation that had initially seemed to be a minor disaster, to develop a revolutionary design.
As the spans of suspension bridges grew longer, the stiffening girders located under the bridge deck, were being designed as deeper open trusses. In fact, the Severn Bridge was initially intended to have such a truss, similar to the one used on the Forth Bridge. But disaster struck when a model of this truss was smashed to pieces in a wind tunnel, when It broke free while undergoing tests.
What to do? The engineers were able to use the time that had been booked in the wind tunnel before the accident, to explore a radical new idea. Instead of allowing the wind to blow through the bridge, why not streamline the deck using aeronautical technology to produce a design similar to that of the wing of an aircraft. A shallow box section could be made both aerodynamic and strong in bending and torsion. This would enable very large savings to be made in the weight of steel required for the deck, the towers and the cables – as well as making subsequent repainting of the deck much easier. This revolutionary idea, to use an aerodynamic box section for the stiffening girder, was tested in the wind tunnel and it proved to be very effective!
Design of the Severn Bridge
The design for the Severn Bridge that eventually emerged from the developments described above, includes two 400 ft (125 m) high towers, one on either side of the estuary. Each tower consists of a pair of tall hollow boxes, linked at three levels by deep hollow portal beams, all with walls of stiffened steel plates, up to 1 inch (25 mm) thick. The mass concrete foundations for these towers are of crucial importance; on the Aust (east) side, the foundation is located on a rocky outcrop that only emerges at low spring tides. On the Beachley shore to the west, ground conditions were far less favourable and the foundation was laid onto the surface of steeply dipping (nearly vertical) bands of carboniferous rock that was exposed by the excavation of softer material above.
Other major elements of the bridge include the two catenary cables, one on each side of the bridge, to which the hangers, which ultimately support the deck of the bridge, are attached – and the deck itself. The abutments, at the very ends of the bridge, are massive in-situ concrete blocks, to anchor the main cables securely. Each completed cable, approximately 20 inches (50 cm) in diameter, contains 8322 individual strands of 0.196 inches (approx. 5 mm diameter, similar to a typical wooden pencil) galvanised steel wire. These wires were taken across the estuary, two bights (i.e. four wires) at a time, from one abutment to the other, over the tops of both towers, in a process known as ‘spinning the cable’.
In an innovative move, all the hangers used to suspend the deck from the main cable, were fixed in an inclined pattern, rather than hanging vertically. This was to increase the systems ability to prevent oscillations from breaking out on the bridge deck. The choice of a welded box girder for the bridge deck was widely welcomed but there were some concerns about the ability of the structure to dampen any oscillations that might try to gain a foothold.
To check the extent of the problem, a comprehensive study of wind conditions on this part of the estuary was undertaken. The results of the study were then used in further wind tunnel tests on the chosen form of deck. The tests indicated that the box girder design possesses excellent aerodynamic qualities but a slight movement was registered at a wind speed of about 15 miles per hour, at an angle of 7.5 degrees.
The presence of this small amount of movement concentrated attention on the amount of damping available, especially as the wind flow over the welded box girder at 15 miles per hour will be so smooth and undisturbed that there is no chance of the deck providing any damping. So the search was on for a fresh energy-absorbing element – and the possibility of switching to inclined hangers was suggested.
Suspension bridges react to the imposition of an additional heavy load in the same way that a clothes line reacts to a heavy garment being hung off centre. What happens is that the point on the clothes line from which the garment hangs, moves a short distance towards the nearest end of the line. On a suspension bridge, the point on the main cable that takes the brunt of the load, will move marginally closer to the nearest tower. The bridge deck will follow the cable by moving this same small amount because it hangs, freely suspended from the main cables, via the hangers. This enables the deck to move backwards or forwards a small amount, assisted by the sliding joints at each end, albeit restrained laterally at the towers. And any movement of the deck can be shown to cause the transfer of part of the burden of carrying the deck, from one half of the pair of inclined hangers (that share a common upper socket on the main cable) to the other half. This switching of the tension between adjacent hangers provides the necessary additional energy-absorbing element and increases the dampening potential of the bridge.
Calculations show that hysteresis in the inclined hangers will now provide sufficient additional damping to prevent any unstable and runaway oscillation of the bridge deck from emerging. The absence of any such movement for more than fifty years (since the bridge was first opened to traffic), suggests that the designers were amply justified in including this innovation.
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A simple and revolutionary concept was developed to produce the renowned Severn Bridge, with its distinctive and elegant shape. It was the first suspension bridge in the world to use the idea of an aerofoil-shaped box girder deck, a feature that has subsequently been adopted for many other world class bridges, including those on the Humber and the Bosphorus. The significance of this major step forward in bridge design has been recognised by the award of the Grade 1 Listing for the bridge.
Design of the Wye Bridge and Viaducts
The location chosen for the Severn Bridge committed the government to providing a new bridge across the nearby River Wye, together with two adjacent stretches of viaduct. These structures are integral parts of the Severn road crossing and therefore worthy of mention here. The Wye Bridge has a main span of 770 ft (235 metres) and, although it is rather dwarfed by its majestic neighbour with a main span of 3240 ft (988 metres), it was, on completion, the fifth longest span road bridge in Britain.
The particular form of design chosen for the Wye Bridge – a cable stayed bridge – will take centre stage as our story of the bridges across the Severn unfolds. It was, at that time, new to the United Kingdom although another cable stayed bridge was being built at the same time across the River Usk in Newport, only a few miles west. The cables that support the roadway, unlike a suspension bridge, are straight and are fixed to the pylon, or tower, at one end, and the bridge deck at the other. They act rather like guy ropes on a tent or the stays supporting the mast of a sailing boat.
The decking for the stretches of viaduct on either side of the Wye Bridge consists of standardised steel box girder units that were assembled and welded together on site. This deck is supported by steel box trestles at 210 ft centres (64 m centres), with splayed legs that are pivoted at the top and the base. Similar box girder units were used to construct the deck of the Wye Bridge but with an important addition that is described in the additional information that can be accessed by following the link immediately below.
With construction underway in 1961, the Severn Bridge was to become an enduring symbol of the connection between England and Wales, providing stimulus to industrial growth, enhanced prospects for the region, and success beyond anything that could realistically have been envisaged. The revolutionary design, using an aerofoil box girder for the bridge deck, has since been adopted around the world. British engineering led the way!