Design Issues

Choice of Deck Cross Section.

The previous item of ‘more detailed information’, entitled “Background on choice of design for First Road Crossing”, describes the circumstances which led to the catastrophic collapse of the Tacoma Narrows Bridge in the USA in 1940. Initial work on the Severn Bridge started shortly after that and there can be no doubt that its designers were well aware of the potential consequences of neglecting the lessons learned.

Severn Railway Bridge used to measure wind

In the late 1950s, using an anemometer mounted on a mast 110 feet (33 m) high (the height of the proposed Severn Bridge deck) and an array of instruments mounted on the then existing Severn Railway Bridge, some 3 miles (5 km) upstream, a spectrum of data on local wind speed, direction and inclination, was established over a width of about 300 feet (90 m). This information would be put to good use when the 1/100-scale model options for the Severn Bridge deck were tested in a wind tunnel at Bedford. At that stage, it was assumed that the deck design would follow American practice with a stiffening girder, about 33 feet (10 m) deep and of open-lattice truss construction, as had been proposed in the 1930s.

However, in the late 1950s, the Government stipulated that the new Forth Road Bridge would take priority over the Severn. The Forth Bridge would, not surprisingly, be designed on the same principles as those being developed for the Severn Bridge, although the main span across the Forth would be slightly longer, at 3300 feet (1008 m). Mott Hay and Anderson were appointed in association with Freeman Fox and Partners to design the Forth crossing using a conventional design with a stiffening girder of latticed (truss) construction.

There was a new development on the Forth Bridge in that the road deck was made of stiffened steel panels resting on cross girders at the top chords of the trusses. These panels were light in themselves and the overall dead weight was also kept to a minimum by specifying a thin layer (c 1 ½ inch or 40 mm) of mastic asphalt as the running surface. Nevertheless, the total dead weight of the Forth deck was some 18,000 tons which required the towers, main cables and anchorages to be designed to cope with this load plus the weight of the traffic on the deck. Construction on the Forth proceeded about two years ahead of the Severn.

In the meantime, scale models of several options for the deck of the Severn Bridge were to be tested in the wind tunnel. One of these, a shallower truss configuration, became loose during the initial testing and was destroyed in rather spectacular fashion. It was during the delay caused by the need to rebuild the model and re-schedule the wind tunnel tests, that Freeman Fox found time to develop and test a completely novel design for the deck.

The disastrous fate that befell the bridge over the Tacoma Narrows, drew attention to the need for wind tunnel testing during the design of suspension bridges. Later, the master model of the Severn Bridge was destroyed during wind tunnel testing. We can now appreciate that the disasters at Tacoma Narrows, and in the wind tunnel Laboratory at Bedford, were essential prerequisites for the development of the design of the deck for the Severn Bridge. It was a simple and revolutionary concept that showed British ingenuity in a most favourable light, internationally. The Severn Bridge was a great boost to British engineering.

The Deck Cross Section chosen for the Severn Bridge

The final shape adopted for the Severn deck is shown on the diagram opposite. Unique for the time, it is a completely enclosed box-girder, only 10’ 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.

Detailed wind tunnel tests had indicated that the box girder deck would be aerodynamically stable for all wind speeds and angles that were likely to be encountered, except for a slight movement in a narrow range of steady wind speeds, around 15 miles/hour and at an angle of 7.5 degrees. This was heartening news, so far as the effect of the lateral wind pressure on the whole structure was concerned, but it did not rule out the possibility that oscillations might develop on the bridge deck. Doubts were expressed about the ability of the box girder to dampen any embryonic oscillation that might form. This was at a time when news of the disaster at Tacoma Narrows was still alive in the minds of many in the civil engineering profession. More work was needed to confirm the viability of a box girder with this particular shape, for the Severn Bridge.

Other studies had found that, for the mode of oscillation which has proved most damaging to suspension bridge structures, 7% of the energy of oscillation will be stored in the hangers (at the instant of maximum displacement) and at least 75% of that energy will be dissipated in each half cycle.

At the same time, reviews of experience elsewhere indicated that oscillations would be liable to occur in the deck of a suspension bridge if its structural damping coefficient were much less than 0.05. The particular deck design being considered for the new bridge, would be so exceptionally smooth and aerodynamic, that any air flowing over it at 15 miles/hour, would be virtually undisturbed. This meant that no damping could be expected from the deck itself. So, a fresh energy-absorbing element was needed. The idea then emerged that it might be possible to extract more damping from the hangers if, instead of being vertical, they were all inclined, as shown on the following diagram.

The pattern for inclined hangers

This would be a second major innovation, as vertical hangers had been used on all previous suspension bridges.

The decision to use Inclined Hangers.

It is a feature of the Severn Bridge and many other suspension bridges that the main cable and the bridge deck (which is freely suspended from the cable) will move backwards and forwards for short distances under the influence of changing traffic loads. This feature is described and explained in the main text on the 5th sub-section of the page entitled, “the First Road Bridge – How would a suspension bridge work?”. Such lateral movements, per se, have little effect on the loads carried by vertical hangers. In such cases, hanger loads are almost entirely dependent on the live and dead loading from the bridge deck. However, when hangers are hung in an inclined pattern, although the total load on each cable clamp may be fairly constant, its distribution between the two hangers that connect from the bridge deck to the cable clamp, will vary widely. The following diagram might throw some light on the situation.

Changes in hanger loads due to bridge movements

Consider the situation, illustrated in this diagram, where a section of deck is supported by a pair of converging hangers that carry the load from the bridge deck to a single cable clamp on the main cable. The small longitudinal movements of the deck, mentioned in the previous paragraph, will cause the main cable, and the deck, to move a short distance towards the tower nearest to the location of the new load. During this movement, the cable clamp that holds the top ends of both hangers on the main cable, will move to the left relative to the bridge deck, while the lower ends of the hangers will remain fixed to their sockets on the bridge deck. This means that the cable clamp on the main cable will move relative to the bridge deck and this will stretch the length of the hanger to the right of the clamp, inevitably increasing the tensile load carried by that hanger. The extra extension to this right-hand-side hanger, caused by the lateral movement of the bridge deck, is shown coloured blue on the diagram.

The hanger on the left side will behave quite differently. The constraint imposed by the upper cable clamp reduces the length of hanger required on this side (by about the same amount as the added extension to the hanger on the right hand side). This removes some of the load previously carried by the left hand hanger and allows it to recover from some of the stretch that was caused by the previous heavier load. The reduction in the amount of stretch in the left hand hanger is shown coloured red on the diagram.

Enlarged detail of hanger loads

Loads in the near-vertical cables in the vicinity of the two bridge towers, will be less affected by the longitudinal movements of the deck because the hangers in those areas are much longer than elsewhere. They behave much the same as vertical hangers and, because they are so long, changes in length cause much less change in tension. On the other hand, towards the centre of the bridge, where hangers are quite short, it is possible that certain right hand hangers, as indicated in the above diagram, could be carrying the full burden of the vertical tensile load on the cable clamp, with the left hand hanger ‘hanging’ loose and bearing no weight. Incidentally, this means that some of the hangers, near the centre of the bridge, will occasionally be required to bear at least twice as much load as equivalent hangers that have been hung vertically.

The double cable clamps that transfer the loads from the hangers to the main cable, are positioned along that cable at intervals of 60 ft, horizontally. At the other end, and for all reasonably long hangers, the special clamps fixed to the bridge deck to hold the lower ends, are all located at the mid points, horizontally, between each pair of upper hangers. This arrangement enables the hangers to create the classic pattern but a modification is required when the hanger lengths involved reach a critical short length. If the above arrangement were to continue beyond that point, the ‘dead load’ inclination of the hangers would be such that any movement of the deck would add a significant extra component of load to hangers that would already be stretched. One hanger of each pair could then be required to carry more than the total load passing through the upper cable clamp. To avoid this happening, the double clamps fixed to the deck were rejected in favour of individual clamps, allowing each one to be moved progressively closer to its companion as hangers become shorter, thus avoiding excessive hanger inclinations.

The purpose behind the introduction of inclined hangers is to produce a situation in which the energy stored in the hangers will increase and decrease much more rapidly and by larger amounts than would be the case with vertical hangers. This should provide the structure with much greater potential for dampening embryonic oscillations of the type that destroyed the bridge at Tacoma Narrows. The Severn Bridge has already given good service for more than 50 years and, with adequate maintenance, should continue to do so for the full 120 years of its design life, and beyond.

Unfortunately, there is a downside to the introduction of inclined hangers. It is extremely important that components of the structure that are active in suspending the deck, should remain in service for long periods. And when necessary, they must be capable of being removed and replaced, without requiring traffic restrictions. The rapid fluctuations in traffic loading associated with inclined hangers, and the additional loads that are imposed, make these hangers especially vulnerable to fatigue failure. This is particularly true of the shorter hangers located in the central area of the bridge. The situation is manageable but it requires significantly more maintenance than was originally planned.

It has been shown that the level of stress for hangers in an inclined pattern can rise to at least twice the maximum level experienced in a pattern of vertical hangers, especially where the hangers are short and near the centre of the main span. And it has been established that the relative fatigue life of an individual hanger on that bridge can be expressed, mathematically, as being inversely proportional to the fourth power of the range of stress involved. When this formula is used to explore the effect of doubling the range of stress in a hanger, as in the present circumstances on Severn Bridge, it confirms that we should expect the fatigue life of some of these hangers to be reduced to one sixteenth of the life that would apply if the hangers had been in a vertical pattern (the fourth power of 2 being 2 x 2 x 2 x 2, which equals 16).

It should have been no surprise, therefore, to find fatigue failures of wires within hangers on the Severn Bridge after only six to ten years service. After a period during which individual hangers that had failed, were replaced ad hoc, a wholesale strengthening programme for the bridge put in hand. All hangers were replaced using stronger and less fatigue-prone strand construction, and new designs of socket were installed.

Mott Hay and Anderson continued in their role to design and supervise the construction of the crossing, but now jointly with Freeman Fox and Partners. Motts led overall and designed all the foundations. Freeman Fox were responsible for the steel superstructures. The superstructure of the main suspension bridge was a major technical advance on all previous long-span road bridge designs.

A total of three suspension bridges were built with inclined hangers, between 1965 and 1980, all with Freeman Fox and Partners as main consultants. It is interesting to note that the inclined hangers on the Humber Bridge have proved less prone to such problems, possible because of a less severe traffic loading pattern. On the other hand, those of the first Bosphorus Bridge have been so fatigue prone, probably because of a traffic loading pattern even more extreme than on Severn, that they were all recently replaced by new vertical ones. Suffice to say that, to date, no other major suspension bridge has been fitted with a system of inclined hangers.

At the time, the Severn Bridge was hailed not only for its majestic appearance but also for its technical excellence. The all-up weight of steel in the Severn Bridge, 3,240 feet (988 m) span, was 19,000 tons for the deck, cable suspension system and towers compared with 39,000 tons for the earlier Forth Bridge which had a main span only 60 feet (20 m) longer.

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