Design Considerations

By the time that design work started in earnest in the 1950s, the form of the bridge that would cross the estuary was not an issue.  It had to be a suspension bridge because that was the only form of bridge capable of spanning the navigation channel of the River Severn at that time.  And in the 1940s, the Government Transport Department had instructed the Consultants to develop a crossing on the Aust–Beachley route.

For more on how does a suspension bridge work,  How do Suspension Bridges work,click here

The Aust-Beachley route would require a new bridge to be built across the River Wye, together with some lengths of viaduct across the Beachley peninsular and at the Welsh end. However the alternative route across the English stones would have required two kilometres of viaduct to be built across the English Stones, over an area that is covered by the river for most of each day. And if the route across the English Stones had been selected, it would have been necessary to construct two massive concrete abutments in the river, one each side of the navigation channel, to anchor the main catenary cables of a suspension bridge.

The river crossing between Aust and Beachley is located on one of the narrowest stretches of the estuary below Gloucester, with a deep navigation channel at all states of the tide. Another advantage is the nature of the geological features encountered along that route which are well suited to the construction of a suspension bridge. The eastern approach off the Aust cliff, provided a desirable high-level approach. The exposed limestone platform of the off-shore limestone rock was ideal for the eastern main anchorage and, although it was some 1000 feet (300 m) further off-shore, and over a tidal channel, the Great Ulverstone Rock was ideally placed to become the location of the east pier and tower of the proposed bridge. The west pier could be located at the low spring tide line off the Beachley peninsular. And finally, the higher land of the peninsular, a further 1000 feet (300 m) inland (to allow for the side span), was ideal for the west main anchorage. This anchorage would be taken down, through the overlaying sedimentary sands and gravels of the peninsular, to be founded on the underlying limestone.

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

The Main Elements of the Bridge

The main span that was being considered for the design of the Severn Bridge in the 1930s, and again in the 1950s, would have been the 7th longest in the world, and so would eminently be technically feasible.

The piers for the towers and the cable anchorages were the first parts of the bridge to be constructed. They are the only parts that are founded on, or in, the earth and so everything else is dependent upon them.  The design that eventually emerged in the 1960s included a 400 ft (125 m) high tower on either side of the main navigation channel.  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 plate, up to 1 inch (25 mm) thick.

The two anchorages are located at opposite ends of the bridge. They are massive blocks of in-situ concrete that are needed to resist the tensions in the main cables  These cables are approximately 20 inches (50 cm), in diameter and they each contain 8322 individual strands of 0.196 inch galvanised steel wire (approximately 5 mm in diameter, similar to a typical wooden pencil). The 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’.

Each hanger was supplied to site, pre-constructed and with a cast steel termination (socket) each end, to a precisely determined length.  This enabled each hanger to be connected to an eye under a cable clamp at the top and to an appropriate deck eye at the bottom.

It should be noted that the hangers provide the only vertical support for the weight of bridge deck, together with all the traffic that it carries. A diagram  later in this section shows how the hangers and other items have been arranged, on top of the deck.

The main Cables.  Another major element, comprised the two main catenary cables which are securely fixed to the anchorages at both ends.

The Bridge Deck. This was suspended freely from the two main cables, using 340 separate hangers. Each one was fixed to a cable, using special cable clamps which were positioned to line up with it’s eye at the lowest point on the circumference of the cable. This allowed each hanger to appear as though it came vertically down, out of the very centre of the cable. Other steel eyes were provided at the corners of the deck, ready to secure the bottoms of the hangers to the deck. The lower eyes are arranged in pairs in the corners on opposite sides of the deck.

The Deck Cross Section.

It had previously been assumed that the deck section would follow the standard practice of the period, with a combination of a deep, lattice-work of steel beams, below a stiff platform that carries the running surface. However, a new and exciting innovation emerged, in the form of a simple enclosed steel box girder with the running surface laid directly on top of its hollow aerofoil shape. This design of the deck had several advantages over previous practice, including the promise of major savings in cost. None of the half dozen or so conventional solutions that had previously been favoured, were able to compete.

The final shape adopted for the Severn deck is shown on the diagram above. Unique for the time, it is a completely enclosed box-girder, only 10 ft deep, and the top of the box also acts as the roadway. The importance of this innovation lay in the fact that the box girder design was very significantly lighter than any previous suspension bridge deck. It was also very strong and stiff in both bending and torsion. The corners of the box are 75’ apart and there are side-tracks on each side cantilevered from the apex of the pointed box shape. During wind tunnel trials a total of seven different edge profiles for the box girder were tested. The model with the point approximately one-third of the overall box depth, below the top of the box, was almost completely stable in the wind tunnel and its shape was the one adopted.

The introduction of Inclined Hangers

The choice of the welded box girder for the bridge deck, was widely welcomed, although it would not have been accepted, if there had been any real doubt about finding a reliable source of energy which could be transferred into the deck to ensure that there would be sufficient dampening of all the oscillations that might seek to establish a foothold.  In a second major innovative move, the Consultants decided that all the hangers used to suspend the deck from the main cables, would be hung in an inclined pattern, as shown on the following diagram. The purpose of this move was to increase the structure’s ability to prevent the deck from oscillating.

 

The simple employment of inclined hangers, together with subsequent longitudinal movements of the bridge deck, due to the weight and the distribution of traffic, has improved the dampening potential of the bridge structure adequately  And it is very convenient that, in the short term at least, the structure operates in a completely self-sufficient manner, the energy required being provided autonomously by the traffic that is travelling over the bridge.

Conclusion. The absence of any such uncontrolled movement for more than sixty 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 a more detailed look at these 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 provides many advantages and which has subsequently been adopted for many other world class bridges, including those on the Humber and the first and second Bosphorus bridges. 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 to the 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 to 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

• Building the Severn Bridge