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.
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 for the stiffening girder, consisting of a horizontal concrete slab, above a latticed steel-work construction (a truss) that supports it.
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, had drawn attention to the need for wind tunnel testing during the design of suspension bridges. We can now appreciate that the disaster at Tacoma Narrows, and the accidental damage 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 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.
Inclined Hangers. Initial considerations.
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. This led to doubts about the ability of the box girder to dampen any embryonic oscillation that might form. This was at the time when news of the disaster at Tacoma Narrows was still at the forefront of the minds of most civil engineers. More work was needed to confirm the viability of a box girder, with this particular shape.
It was realised that the particular design being considered for the deck of 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 might be needed. The idea then emerged that it might be possible to extract additional damping from the hangers if, instead of being vertical, they were all inclined, as shown on the following diagram. This would be a second major innovation for the Severn Bridge, as all previous suspension bridges had been provided with vertical hangers .
The decision to use Inclined Hangers.
It is a feature of the Severn Bridge and many other suspension bridges that, because the bridge deck is freely suspended from the main cable, it will move backwards and forwards for short distances under the influence of the ever-changing live traffic load. This is because, like a simple clothes line, the bridge will react to any change in the load that it carries. The addition of another garment will not only cause the line to sag a little more at the point at which the new load was added, it will also cause that point to move a short distance towards the nearest point from which the line is suspended which, in the case of the bridge, is likely to be one of the towers! This feature is explained more fully in the additional information which is available through a special link in the main text. If you are interested in this, go up to the top of the present text and hover over “The First Road Bridge” in the main menu. When the list drops down, click on “Design of the Severn Bridge” and the link to supplementary information entitled “For more on how does a suspension bridge work?” appears, as the first item. Click on it.
In the case of a suspension bridge with conventional vertical hangers, the longitudinal movements mentioned in the previous paragraph, will have relatively little effect on the tensions in the bridge hangers because, with vertical hangers, there is no means of transferring to the hangers, the energy produced by the live load of traffic and dissipated in producing the longitudinal movement of the deck. When the hangers are hung in an inclined pattern, the total load on each cable clamp from both hangers will be fairly constant but the manner in which that load is distributed between the two hangers involved can vary enormously, depending upon the distribution of live load along the full length of the bridge deck, at any moment in time. The direction of travel of the vehicles that make up the live load is not, in itself, relevant to the movement of the bridge deck. What matters is the instantaneous pattern of live load on the whole bridge deck and how it changes with time.
Consequence of longitudinal movements of the bridge deck on tensions in the hangers
Consider the situation that is illustrated in the above diagram. It was provided to demonstrate what could be achieved, if the hangers for the Severn Bridge were all inclined. The diagram shows the support for one side of a short section of deck, supported by a pair of hangers, which are both fixed to the base of the cable clamp fitted around the main cable. The lower ends of both hangers are locked into sockets fixed to the deck, directly beneath the main cable. It might be worth looking again at the diagram in the previous section of this chapter, entitled “The Deck Cross Section chosen for the Severn Bridge”, as it shows the positioning of the hangers, the clamps and the sockets from a different angle. The cable clamp is located directly above the mid point between the two adjacent sockets.
There would, in practice, be an identical set-up on the other side of the deck but that can be ignored for the time being. Small longitudinal movements of the deck will take place in response to the changing live loads on the bridge, as previously described. The movement that is illustrated on the diagram, will have been made in the direction of the tower that was nearest to the cable clamp, in this case the one to the left side of the diagram, irrespective of the vehicle’s direction of travel. During these movements, the lower ends of the hangers will remain fixed in their sockets on the bridge deck and because the deck moves to the left, the cable clamp that is holding the tops of both hangers will also move to the left, relative to the bridge deck. This means that the cable clamp will move longitudinally to the left, relative to the bridge deck (and as seen on the diagram) stretching the length of the hanger on the right-hand side of the clamp, thus adding to the tension in that hanger. The total amount of stretch in this right-hand-side hanger is shown coloured blue on the diagram.
At the same time, the longitudinal movement of the cable clamp to the left, described above, will remove some of the tension from the hanger on the left hand side. It will reduce the amount of stretch (usually called the strain) in that hanger by about the same amount as the added extension to the hanger on the right hand side. It also removes some of the load previously carried by the left hand hanger. The reduction in the amount of strain in the left hand hanger is shown coloured red on the diagram. The passing live load must have been exceptionally heavy, because the whole of the strain that had been in the left hand hanger before the heavy live load arrived, had been completely eliminated. In such cases, the hanger would just lay dormant, subject only to its own dead weight, until the heavy vehicle had moved out of range and the previous situation had resumed. This is exactly what must have happened to create the exceptional situation illustrated on the diagram. The fact that the red coloured portion of the left hand hanger, had extended further than its original length, confirmed that all its previous tension had been removed so that, for a short time, the hanger would have gone slack.
The previous two paragraphs illustrate how the idea of suspending the deck of the Severn Bridge using hangers erected in an inclined pattern, had made it possible for large tensile stresses to be redistributed within each pair of hangers that share an upper cable clamp. It was exactly the kind of new energy source that the consultants had been seeking to combat the possibility of an oscillation developing within the completed structure, similar to the one that destroyed the Tacoma Narrows bridge in 1940. It is particularly intriguing that the actual source of that additional energy should be the ‘live load’ that crosses the bridge. However, the consultants concluded that the most sensible approach would be to avoid situations that could lead to a hanger becoming slack, or otherwise overstretched under extreme live loading conditions, and they were supported in that decision by the government Department for Transport.
Manufacture of the hangers. The above demonstration of the ability of the inclined hangers to provide a new source of energy that must be generated from the live load of traffic, raises questions about the construction of individual hangers. The dead load of the bridge deck is extremely uniform throughout its length and so all cable clamps (at 60 ft centres) would carry basically the same share of that weight. At the same time, the average daily live load borne by each cable clamp would be virtually identical. It was also clear, at that stage, that more than 50% of all the hangers would be long enough to be treated in exactly the same manner as if they had been hung vertically. This all suggested that the current practice of the day, based on a multi-strand configuration, would probably be adequate. At the same time, there was strong support for developing a strand that would further strengthen the structure against the possibility of oscillations, by maximising the amount of hysteresis in the hangers (this work was undertaken, but very little has been said about it since). These deliberations resulted in a strand that contained a total of 178 separate wires, made up from the following three sizes, 0.118 ins, 0.133 ins and 0.339 ins with a lay of 7.5 d. The resulting strand would provide a maximum working load of 100 tons and a breaking load of 225 tons. At this stage, the way forward was clear for the suspending of all the longer hangers from cable clamps and making the necessary connections to the sockets that would be fixed to the deck to receive them. But before that work started, the possible need for a modification to the system outlined above, in order to cope properly with all the shorter hangers, was undertaken.
New simple diagram.
The initial system for supporting the bridge deck, that was used to suspend all the longer hangers, is indicated in the above diagram. It represents a section of main cable on one side of the deck, with the two hangers in one pair, coming together directly underneath the main cable to which they are both fixed using a special connection attached to the base of the cable clamp. Lower down, on the top of the deck, a line of sockets had been fixed to the deck, at 60 ft centres, to secure the lower ends of the hangers. Each cable clamp was positioned directly over the mid-point between two sockets. An initial examination confirmed that that the model, in the form shown above, would not be able to cope with the shortest hangers. This meant that a modification was required, together with a decision concerning the point at which the modification would need to be introduced.
The Included angle. It soon became clear that the critical item when dealing with shorter hangers is the angle ‘included’ between the two hangers at the point where they come together under the cable clamp to which they are both fixed.. This angle will be quite small In the case of the longer hangers but consider what would happen if it reached the 60 degrees shown on the diagram above (note, an angle of 60 degrees has been chosen just to simplify the mathematics). The triangle that includes both of the hangers and the line of the deck beneath them, approximates to an isosceles triangle. This means that these hangers are inclined at an angle of 30 degrees to the vertical. Because of that, only 86% of the tensions in the hangers (i.e., the sum of the vertical components of these tensions) would be used to support the weight of the bridge deck, together with the live load of traffic on it. The other 14%, the horizontal components of the tensions in the two hangers, would cancel each other out, being dissipated into the bridge deck. And at the mid point of the span, where the main cable drops lower, approaching the centre of the bridge, the included angle between the two hangers, would need to be larger still, in order to conform with the model.
The chosen modification involved a simple change to the rules that are implied in the model that is described above. Having reached the point chosen to make the change, the consultants extracted the included angle at that point and decreed that it should apply to all subsequent operations that would otherwise have adopted a wider angle between hangers.
The above diagram shows an elevation of a short length of the support mechanism developed for connecting the bridge deck to the main catenary cable for use by for the deck, when viewed from one side. The cable clamps, that transfer the loads from the hangers to the main cable, are positioned along the cable at 60 ft intervals, horizontally, and they each have to secure two hangers, one from either side. The sockets that are used to fix the hangers to the deck are located mid-way, horizontally, between the cable clamps to either side. They are fitted along a line just off the flat area on top of the deck, where an additional section of box girder has been welded on to the main box girder, to provide the short cantilever that carries the footpath and cycle track at the side of the deck. (this can be seen clearly on the drawing of the deck section which is located near the top of the previous section of this document).
Once decisions were taken on the dimensions of the spans and the towers, together with the shape of the main cable and the horizontal space between the cable clamps, there will be no scope for adjusting hanger lengths. They will simply be a matter for calculation
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 could 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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