Strengthening of the M48 Severn Bridge

M48 Wye Bridge in the Foreground
M48 Wye Bridge in the Foreground

Developments in road transport between 1962 and 1977 led to a review of the original bridge design, as there were dramatic changes in freight transport patterns soon after the Severn and Wye Bridges were designed.

  • The amount of goods moved by road measured in tonne per km virtually doubled;
  • The number of goods vehicles with a gross weight of 28 tonnes rose from an insignificant amount to 90,000;
  • those under 11 tonnes gross weight fell substantially in number

Investigation into Bridge Structure in the 1970’s and 80’s

The Department of Transport commissioned independent consultants Flint & Neill to carry out an appraisal of the Bridge structures in the light of changes to traffic loading pattern. There were also changes to bridge design criteria to he addressed as a result of the findings of the Merrison Committee. The final report of the Consultants indicated that in order to maintain the required margin of safety under the worst possible loading conditions various parts of the Bridges would require strengthening or improvement. This involved the following changes to the various structures in the crossing.

(a) Installing a tubular column approximately 120 meters in height at each corner of the four Towers so that some 25% of the loading initially in the tower shell was transferred to the foundation via the tubular columns.

(b) The inclined hangers which transfer the load in the roadway deck boxes to the two main suspension cables were replaced with larger ones with improved designed sockets at either end.

(c) A particular welded joint detail in the roadway deck boxes immediately below the wheel tracks had been found to he susceptible to fatigue damage. These welds were removed and replaced with an improved designed welded joint.

(d) The ends of the roadway deck boxes at the Towers are supported and connected to the Towers via the lower horizontal portal between the Towers The connection is made via a vertical beam, one end that is attached to the end deck box and the other to the portal. The bearings at each end of the beams were replaced with modified bearings to allow for rotation about the longitudinal and transverse axis as a result of the movement of the roadway deck.

(e) The original single cable stay system on the Wye Bridge was replaced with a harp‑ like arrangement, which involved increasing the height of the Towers and the constructing a further four cable anchorage’s to make up a total of eight.

(f) The complete crossing was re-surfaced using a mastic asphalt specification above a waterproof membrane bonded to the steel deck plate. This arrangement is designed to act in composite action with the steel deck plate to distribute vehicle wheel generated stress in the deck plate structure.

Reference 1 Inquiry into the basis of design and method of erection of steel box girder bridges HMSO 1971 Interim Report.