Why did the Bridge Need Strengthening?
The Severn Bridge was designed and built in the early 1960s, to plaudits from professional engineers world-wide, and with anticipation of a design life of 120 years. Yet, within 25 years, a major programme of bridge strengthening works was needed. What had changed?
Traffic Growth had been underestimated.
Consulting engineers Mott, Hay and Anderson were engaged to design the Severn Bridge in 1949 but, for more than ten years, the slow post-war economic recovery prevented the government from releasing funds for its construction. During the whole of that period, the expectation of government, supported by professional opinion, was that road traffic throughout the country would grow at a modest rate every year for the foreseeable future, with heavy lorries making up about 15% of traffic flows on longer distance routes.
We now know that, in the decades that followed the opening of the bridge in 1966, there was an unprecedented growth in road traffic, far outstripping the modest forecasts of the early post-war years. These higher traffic volumes called into question the ability of the existing stock of bridges to cope in the long-term with the weight of traffic they were now being required to carry. The problem was not confined to the Severn Bridge, it affected bridge structures on the whole of the UK major road network.
There were two other factors that exacerbated the problem.
1. Some years after the Severn Bridge was opened, the maximum allowable weight for lorries on UK roads was substantially increased. There had been pressure for this change from the transport and logistics industries in response to the very rapid pace of developments taking place within those industries and it brought the UK into line with most other developed countries.
2. The percentage of lorries in traffic flows on all longer distance routes had increased steadily from the figure of 15% in the fifties and earlier sixties to 30%. This change had been picked up by regular monitoring surveys and data collected in relation to other road improvement schemes.
In the light of the above developments, how would existing bridges cope? They would certainly be subjected to greater forces than had been allowed for in their original designs. The problems would be more critical on longer distance routes because they would have to cope with higher proportions of heavy lorries – and, in general, longer bridges would be more severely affected than shorter ones.
The response from the government’s transport department was to update the traffic forecasting elements of design criteria for bridges and to initiate a programme for close inspections of all bridges on trunk roads and motorways, followed by reassessments of their capabilities in the light of the revised design criteria. In addition, higher design wind speed and an increased temperature range were introduced. Bridges that failed to comply with the new criteria would then be subject to a strengthening programme, to overcome the identified deficiencies. Local highway authorities introduced similar arrangements.
For more information on highway bridge loadings, Click here
The Box Girder Problem.
In the early 1970s, just a few years after the Severn Bridge was opened to traffic but before the problem of lorry weights and numbers came to a head, four bridges across the world that were using box girders in their construction, collapsed with significant loss of life. One of these bridges was in Pembrokeshire. Both the Severn and Wye bridges had steel box girders built into their structures and so, inevitably, were swept into the turmoil that followed.
The government set up a committee of inquiry to look into the failures and to make recommendations on the changes needed to prevent repetition. New design rules were drawn up, the most important being concerned with box girders being built using cantilever construction. There was also a strong call more rigorous independent checking procedures at both the design and construction stages.
Three of the four failures had occurred during the construction phase and involved the building of a cantilever comprised of box girder units. Cantilevered bridges and viaducts are always at their most vulnerable just before completion, when the unsupported front end is ready to be lifted and fixed in position. After completion, the stresses at the root of the cantilevered deck will never be as high again, as at that most vulnerable moment, even when it is fully loaded with traffic.
The government Transport Department ordered scrutinies of all major bridges under their jurisdiction, in which box girders played an integral part. No serious problems were expected by those who were involved with the management and maintenance of the Severn and Wye Bridges and there were good reasons for that.
On the Severn Bridge, box girders, in a distinctive aerofoil shape, were used to create the stiffening girder that acts as the deck of the bridge. However, no cantilever was involved in the construction process. Each unit of the deck was floated out on the river and then lifted into position, where its weight would be transferred to hangers attached to the main cable. Each unit was later welded to its neighbours, in order to provide the required element of deck girder stiffening but this was of a much lower order of magnitude than the degree of stiffening that would have been necessary for the building of a cantilever.
The Wye Viaducts are classical examples of box girder construction and the Wye Bridge itself possesses similar elements. There is a possibility that the deck spans of the viaducts might have been vulnerable when the cantilevered sections were nearing completion but that possibility became irrelevant as soon as the construction process was complete. In the case of the Wye Bridge, two sets of pylon and cables were employed to support the deck before the most vulnerable period. Those additional elements became part of the permanent structure that would carry the weight of traffic. Parts of the Wye structures were found to be at risk under the new box girder design rules and so corrective action was taken. And both the Severn Bridge and the Wye structures were later strengthened to cope with the revised traffic loading requirements.
For more on the steel box girder problem, Click here
Reflections on the strengthening of the Severn and Wye Bridges.
The First Road Crossing needed strengthening. In round terms, it would need to carry twice the weight of traffic for which it had originally been designed. That was a very tall order. To the layman, it might seem to be unachievable. However, the saving grace was that, as in the case of all large bridges, the lion’s share of stresses in the structure is caused by the weight of the bridge itself. At the time of its original design, the weight of the Severn Bridge structure contributed about 85% of the total design load, the other 15% coming from the traffic it was expected to carry. So doubling the weight of traffic would, in round terms, add just 15% to the total load on the bridge. The additional material required for the strengthening process would not add much to the overall weight of the structure.
The other crucial factor that enabled the strengthening project to proceed was the strength of the main catenary cables of the Severn Bridge. The strength of every major element of this particular bridge was carefully reassessed in the light of the revised design criteria and, with the exception of the catenary cables, all were found wanting. The reassessment had indicated that the cables had sufficient strength to cope with the extra load, without the need for any intervention. That was indeed fortunate because the cable was the one element of the structure that would be virtually impossible to augment or replace without extremely high cost, together with major disruption to traffic.
Realisation that the catenary cable of the Severn Bridge did not need strengthening came as a great relief but that should not be allowed to detract from the enormity of the engineering challenges thrown up by the need to strengthen all the remaining elements of the bridge, or from the quality of the professionalism and ingenuity that went into confronting and overcoming them.
The project received the British Construction Industry Supreme Award for 1990 in recognition of the extremely complex nature of the problems that were revealed and the innovative solutions of both an engineering and environmental nature that were employed to resolve them.
Modification and Strengthening of Severn Bridge
The most important elements of the Severn Bridge that required major strengthening were as follows:
1. The main tower legs.
2. The hangers that transfer the weight of the bridge deck (and any traffic on it) up to the main cables.
3. The box girder top flange stiffener welds.
Work was also required in many other areas as shown on the diagram above. Details of all these items are available using the link below.
Modification and Strengthening of Wye-bridge-and-viaducts
The developments and external changes that took place in the first 25 years of the First Crossing’s existence, generated much more traffic than had been expected, requiring the Severn Bridge to be strengthened. The adjacent Wye Bridge and Viaducts had experienced the same increased traffic flows as the Severn Bridge and, not surprisingly, they also needed strengthening. However because their ‘cable stayed’ form was quite different from the ‘suspension’ design of Severn Bridge, their strengthening programme was tackled quite differently.
It is worth bearing in mind that, before the structures were strengthened, the tops of the single towers of Wye Bridge were much lower, with only one pair of inclined cables suspended from each. The position of every significant item of strengthening work is indicated on one of the two diagrams, above, and a short description of each can be found by clicking on the link below.