Cable-Stayed  Bridges

22nd July 2001    Back to Home Page    Back to Bridges

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 Arch    Beam    Box Girder    Cantilever    Pre-Stressed    Suspension    Truss

Four anchorages

Both types of bridge have two towers and a suspended deck structure.  Whether the towers are equivalent may become apparent.  There is a difference in the deck structures.  The deck of a suspension bridge merely hangs from the suspenders, and has only to resist bending and torsion caused by live loads and aerodynamic forces.  The cable-stayed deck is in compression, pulled towards the towers, and has to be stiff at all stages of construction and use.

The greater inherent rigidity of the triangulated cable-stayed bridges, compared with the suspension type, makes life easier for their designers and builders.  On the other hand, if a cable-stayed bridge is built by the cantilever method, it is very vulnerable when the structure is very long but has not yet been joined together.

Although the popularity of the cable-stayed bridge is a fairly recent phenomenon, the principle is not new.

The great Brooklyn Bridge combines cable-stays with conventional suspension cables, while other bridges have used stays, even below the deck, to resist aerodynamic forces.  To see a really beautiful picture of Brooklyn bridge by Anney Bonney, click here.  The Albert Bridge, a small suspension bridge across the river Thames in London, also employs some stay-bars as well as a suspension chain.

The diagram below shows graphs of the bending moments along a cantilever caused by point loads at nine different distances from the point of attachment at the left.  The free end of the cantilever is at the right.  The moment at the attachment clearly increases as the load moves out.  This principle is used in the steelyard. 

Instead of building a rigid cantilever we can use a set of cables to support the deck.

In fact, many cable-stayed bridges have other arrangements of the cables.  Some smaller bridges even have only one or two cables per half-span.  Some examples are shown below.

The penalty for the sloping cables is the compression induced in the deck.  This very simple arrangement is, as usual, not the whole story:  Very long cables oscillating in their fundamental mode can store a great deal of energy, so the larger bridges are equipped with light cables that run across the planes of main cables and connect them all together, and eventually to the deck.

Ail  Groesfan  Hafren – Second  Severn  Crossing

The most southerly bridge over the river Severn is the viaduct and cable-stayed bridge which carries the motorway M4 between Wales and England.     It offers an alternative to the earlier suspension bridge, which carries the earlier motorway M4, now called M48.  The designers made use of a large area of hard rocks on the western side of the channel, which are exposed at low tide, to enable a viaduct to be built.  The main channel, called The Shoots, is spanned by the actual cable-stayed bridge.

The bridge is not far from the line of the Severn tunnel, which was a great feat of engineering, built from 1874 to 1886.  Huge pumps were, and are, needed to remove water, and very large fans were installed to provide ventilation.  The construction of the tunnel is described in "Track Topics – A Book of Railway Engineering for Boys of All Ages", by W G Chapman.  This book also provides insights into some famous bridges of the Great Western Railway, and includes a drawing by W Heath Robinson depicting the assembly of Saltash bridge.

The bridge has high baffles on each side to deflect the wind.  This greatly reduces the number of occasions on which any type of vehicle has to be banned from the bridge because of high winds.

Click for big JPEG.

The approach spans are based on post-stressed hollow beams, made from 3.5-metre match-cast sections which were floated out on a barge at high tide.

It is so well integrated into the motorway that it is very easy to reach the cable-stayed section without realising that you have already crossed a long approach viaduct.

The next picture shows a small part of the Severn cable-stayed bridge.  The picture has been tilted and compressed horizontally to show that, although the cables look straight, they sag.  There are few perfectly straight lines in engineering, with the possible exception of verticals.  Every part that is not vertical will sag a little, though of course "rigid" struts will not deflect visibly.  The truth is that there are no rigid bodies.  You can also see that two of the cables (the fifth on each side) are not evenly spaced with the others.

North of this bridge, just upstream of the mouth of the River Wye, a cable-stayed bridge with a main span of 770 feet takes the M48 (ex M4) across that river, at the Welsh end of the Severn suspension bridge.  Here are two pictures.  The second picture has been squashed sideways by a factor of a quarter, to show the undulations in the steel deck.  The high points are at the anchorages of the bundled cables into the deck, and at the towers, and are marked by horizontal black lines.  Each cable contains twenty spiral strands, arranged in a triangular cross section with five layers, with 6, 5, 4, 3 and 2 strands per layer, respectively.  The flat bottom of the section allows the cables to rest on the flat tops of the towers, held by simple clamps, avoiding the need for specially shaped saddles.

The stiffness of the box girder span is used to transmit torsional forces to the abutments, which are the only supports that are not on the centre line.  This technique is used in many modern concrete spans and steel spans, supported either by piers or cables, because the simplicity provides a cost saving that is not overcome by the cost of the torsional stiffness.  For box girders on piers, the potential untidiness of two rows of piers is avoided.

The towers are also of steel box construction.

Note the light traffic on this road, the motorway M4 which runs from London into Wales.  These pictures were taken almost thirty years ago.  As traffic built up, it became clear that a new crossing was needed.  This has been described above.  The road over the earlier crossing was renamed M48, and the M4 now follows the new route.  It is much harder now to take pictures with no vehicles, and to avoid the vibration which persists after vehicles have passed.

Sabrina  Foot-Bridge  at  Worcester

This beautiful and interesting little footbridge is found to the north of the railway bridge in Worcester, joining Le Vésinet Promenade to the west bank of the river.  It is an asymmetrical cable-stayed bridge with one tower at the west end.  An ingenious feature is the use of hinges where the cables join the deck.

This allows the use of rigid trusses without the necessity for extremely precise setting of the cable lengths.  With a through truss, imprecise cable lengths would produce uneven tension in the cables, and unwanted bending stress in the deck.

The next picture shows that the cables are not perfectly straight.  They cannot be straight, because of their weight.  Each one follows a part of a catenary.

If only bridge design were as simple as making a diagram like the ones below, for two tower designs and one tower designs.  Do you think that any of these designs have advantages or disadvantages compared with the others?

If the cables fan from a point, as seen from the side, they must originate from a horizontal line.  However short this is, it will affect the appearance from certain angles, because the cables are not coplanar.  In fact, in most cable-stayed bridges, the multiplicity of sloping cables is liable to lead to a disordered appearance unless care is taken.

To achieve a vertical plane of cables, the second arrangement can be used, but now the tower is not elegant.  Another solution is to abandon the idea of a vertical plane and make an A-frame, as in the right hand pair of diagrams.  An A-frame is very rigid.

A third way is to use only a single plane of cables, relying on the deck to provide stiffness against torsion.

The picture below shows a part of the Sabrina bridge in Worcester.  Although this is an elegant bridge, this view shows the difficulty of maintaining a tidy and ordered appearance from all directions.  The suspension bridge, with its clear distinction between the dominant main cable and the thin hangers, does not suffer from this problem.

In this picture you can see another cable-stayed bridge with one tower – this time a road bridge.

On entering upon a suspension bridge, you will generally be confronted with an orderly view, comprising the graceful sweep of the thick main cables, and the parallel lines of the thin hangers.

Oscillation of Cable-Stayed Bridges

Although the cable-stayed bridge is inherently stiffer than a suspension bridge, the relationship is reversed during construction.  Construction of the deck of a suspension bridge does not begin until the cables are complete, and so all parts of the bridge are connected, however tenuously.  But the cable-stayed span is built out in stages from each tower, and when the span is almost complete, the long cantilevers are at the mercy of the wind.

The diagrammatic plan view below, showing a part of a bridge, suggests what might happen.  The amplitude is exaggerated.  The deck could also oscillate in other modes with higher frequencies.  In principle there could be horizontal oscillations allowed by torsion in the towers, and vertical ones allowed by bending of the towers.

The lower diagram suggests that when the two halves of the span have been joined, the resultant rigidity reduces the amplitude of any oscillations.  It also increases the frequency.  We can see this from the shorter wavelength, about equal to the span.

In principle an active damping system could be created using movable masses near the ends of the cantilevers during construction.  Small signals from sensors on the deck would be amplified and used to control hydraulic or electric motors to move the masses.  The system would require emergency power generators in case of a power supply failure.  Such a system has been used in tall narrow buildings.  Because the moving mass is much smaller than the effective mass of the structure it must move more quickly.

Upside Down Cable-Stayed Bridge

The rotary clothes line is like a bridge which has three self-anchored spans.  If the clothes are hung on the cords, which is the normal usage, the system is like a suspension bridge, but if heavy clothes were hung on the struts, it would be more like a cable-stayed bridge.  The second picture shows numerous newly hatched caterpillars; a moth must have mistaken the line for a plant stem.  Another kind of endless span is a childrens’ roundabout which consists of a polygonal seat suspended from a pivot by a conical array of stays.

Invisible Cable-Stayed Bridges

Before the invention of steel-framed buildings, people sometimes wanted to build a library on an upper floor in a large building.  To take the great weight  of many shelves of books, cables or rods were used.  The shelves were built back to back, at right angles to the walls.

The cables sloped down from the wall to the floor between the shelves, and were invisible.  The inward pull of the cables at the top of the wall was taken by the beams in the ceiling.  Thus the appearance of the room was not spoiled by the engineering.

Visible Cable-Stays

During the 1980s and the 1990s, highly visible cable-stays and tie-bars were very popular with architects.  These two examples are at Gloucester Docks and the Indoor Arena near Birmingham Airport, seen in the middle picture and at the far right of the third picture.

During those decades the classical ideal of hiding certain things was dropped by many designers.  The Pompidou centre is a well-known example.  This type of construction does not always create a restful feeling.

Here is a type of crane which can be seen on almost any large building site.  The truss provides lightness.  The stays allow a longer reach than the truss alone could support.