Design of the Severn Bridge

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 near-contemporary Forth Bridge. But disaster struck when a model of this truss was smashed to pieces in a wind tunnel, after breaking 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!

For more on background on choice of design for first road crossing, click here.

The Main Elements of the  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. There are a number of good aerial views of these key foundation elements in the second tier item (ahead), dealing with the building of the Severn Bridge Foundations and Anchorages.

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’.

Design of the Deck Cross Section.

A comprehensive study of wind conditions on the chosen stretch of the estuary was undertaken to provide data for a series of wind tunnel tests on a 1/100 scale model of the bridge, before the main design process could begin. However, two setbacks occurred early in that process. First, the government decided that the Forth Bridge should take priority, and proceed about two years ahead of Severn. And then the programme for essential wind-tunnel tests suffered what appeared to be a crucial delay when the model of the bridge broke loose from its fixings at the beginning of the process and was completely destroyed. As several of the following time slots in the heavy schedule of the Bedford Laboratory, had already been booked, the opportunity was taken to construct, and later test, an additional model for a new and very different design. The consultants, Freeman Fox and Partners, to their great credit, turned the situation around, taking full advantage of the extra time made available by the delay, to produce this new and revolutionary design.

It had been assumed that the Deck Section would follow the standard practice of the period with a stiffening girder based on a truss. However, the new and exciting solution was a simple, enclosed steel box girder that would have several advantages, promising major savings in cost. None of the half dozen or so conventional and previously favoured solutions could compete, even before any wind-tunnel tests. Then the box girder revealed excellent aerodynamic qualities, demonstrating that it should have no difficulty in coping with anticipated lateral wind pressures. However, at a separate stage of the wind tunnel tests, a slight movement was registered at a wind speed of 15 miles per hour, at an angle of 7.5 degrees. This led to doubts about the box girders ability to resist oscillations.

Diagram showing new Deck Cross Section

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 would be so smooth and undisturbed that there would be no chance of the deck providing any contribution to damping. So the search was on for a fresh energy-absorbing element – and the possibility of adopting inclined hangers was explored.

The introduction of Inclined Hangers

The choice of a welded box girder, as favourite for the bridge deck, was widely welcomed but it could only be accepted if there were no real doubts about its ability to dampen any oscillations that might gain a small foothold in a favourable wind. In a further innovative move, it was decided that all the hangers used to suspend the deck from the main cable, should be fixed in an inclined pattern, as shown on the following diagram. This was to increase the structure’s ability to prevent the bridge deck from oscillating.

The pattern for inclined hangers

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 ofany concentrated traffic loading will move marginally closer to the nearest tower. The bridge deck which hangs freely suspended from the main cables (via the hangers), will follow that movement of the cable closely. This means that the deck will be continually moving backwards or forwards by a small amount, assisted by the presence of sliding joints at each end, while being restrained, laterally, at the towers. This applies to all standard suspension bridges

The purpose of employing inclined hangers is to ensure a transfer of part of the tensile load from one hanger to the other, within each pair of hangers that share a single cable clamp fixing with the main cable – every time there is a longitudinal movement of the deck. This switching of the tension between adjacent hangers provides the additional energy-absorbing element and increases the dampening potential of the bridge structure.

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 relying upon the use of inclined hangers to prevent oscillations from occurring.

For more on design issues, click here.

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 Relationship between the Severn Bridge and the Wye Bridge

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.

For more on developing the cable stayed design for wye bridge, click here.

The Result

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!

Next Page