The Severn Crossing in England, the United Kingdom

1. Field of application

The Severn Crossing over Severn River in England, the United Kingdom.

2. Circumstances of Repair

Fatigue cracks were found in the Crossing in 1971 at some welded joints after 5 years of service. Then in 1977, fatigue cracks were found in the trough to cross-beam and trough to deck plate joints.

There was also extensive cracking of the road surfacing above the webs of Beachley viaduct and the Wye Bridge boxes, and consequent corrosion pitting of the deck plate. At about the same time, fractures of outer wires in some of the hangers began to occur with increasing frequency in the vicinity of the lower sockets. Because it was required that repair were to be made without lane closures and it was not feasible to replace the deck, methods of repairing and strengthening was developed after numerous fatigue tests, stress measurements, load tests, monitoring and so on.

 

3. Types of structure

The Severn Crossing comprises a dual two-lane motorway carried over Aust viaduct, the Severn Bridge, Beachley viaduct, the Wye Bridge and Wye viaduct (Fig.1).With the exception of Aust viaduct, which is of composite twin steel box construction, the decks of the bridges are streamlined single cell stiffened steel plate boxes (Fig.2).

The Severn Bridge has shallow aerodynamically streamlined deck profile and with inclined hangers to increase structure damping. The cable-stayed Wye Bridge has the stays aligned along the center of the deck and torsional resistance provided solely by the wide deck boxes. In the Severn bridge and the Wye bridge, the steel deck plates were made with closed trapezoidal section stiffeners ( troughs ) (Fig.3:similar deck view.). 

The tower and saddles of the Severn Bridge is shown in figure 4.

 

 
Fig.1 The Severn Bridge.

 Fig.2 Cross section.

 

Fig.3 An orthotropic deck.

Fig.4 The tower and saddles of the Severn Bridge. 

4. Details of loading 

Fluctuating loads due to vehicles live load.

The Severn bridge was in lightweight design before changing in the freight transport pattern. 

5. Description of damage

Fatigue cracks occurred in the weld connections between flotation diaphragms and the underside of trough stiffeners in the deck (Fig.8). This was followed by cracking in the trough to cross-beam and in the trough to deck plate connections.

Various tests indicated that fatigue cracking was a design problem and not one of weld quality and fatigue cracking of the welds in the bridge decks occurred at about one-fifteenth of the design life of the bridge. Cause of fatigue crack in the trough to cross-beam, for example, was longitudinal bending of the trough and significant bending of the cross-beam due to the passages of individual heavy vehicles over the last decades years. 

6. Repair method applied

 

In case of trough to cross-beam.

Repair was performed by increasing the weld size, replacing the fillet by penetration weld, special jigs and drilling equipment. By means of repairing, only a small increase in fatigue strength could be achieved. The repair finally adopted was that of fitting a steel strap around the end of the trough on each side of the cross-beam (Fig.5). The strap was fillet welded to the cross-beam and trough in a controlled sequence. The weld toes were then treated in the most highly stressed locations. Hammer peening was used initially, but, alternative processes have been investigated due to the noise and so on.

  This repair showed the required fatigue strength (class D).

Fig.5 Welded strap repair for trough to cross-beam joint.

 

  In case of trough to deck plate.

  Repair for the trough to deck plate joints was to replacing the 6mm fillet weld with a larger weld on a machined preparation (Fig.6). This repair increased the fatigue strength from BS 5400, class F, to the above class D for stress in the web of the trough at 15mm from the deck plate.

Fig.6 Trough to deck plate weld repair machine.

  In case of web to deck joint strengthening.

  For cracking of the surfacing over the longitudinal web of the box on the Wye Bridge, the welded plate stiffening were carried out (Fig.7). This repair indicated that stress ranges recorded under normal traffic were very low, and estimated fatigue lives were well over 120 years for all welds.

Fig.7 Welded plate strengthening on Wye Bridge web to deck joint.

  In case of flotation diaphragm.

  A temporary diaphragm was fitted to the end of each box section during construction. It was welded to the soffit of the troughs, and not removed on completion.

  Next, the diaphragms were cut away from the troughs, and cracks were repaired by air arc gouging and then by rewelding. Finally, the damaged area of the trough was cut out and the resulting gap bridged with steel plates bolted in position (Fig.8).

   If cracks propagate up the trough web beyond the cut-out section, a complete section of trough will be able to be removed and replaced with a new section butt welded in place.

 

Fig.8 Flotation diaphragm.

  In case of other details.

  For the deck-plate butt welds, a machine, similar to the trough to deck repair machine, was developed to remove the backing bar of the transverse butt weld.

  At the cross-beam to deck joint, strengthening of the joint was performed by over welding and treating the weld toes. On the Wye bridge, vertical stiffeners were added to the cantilever beams (Fig.9).

 

Fig.9 Cantilever web stiffener on Wye Bridge.

Strengthening of the bridges was performed as follows.

	Steel deck boxes throughout the crossing were reinforced.
	The towers of the Severn suspension bridge were relieved of load by jacking in new internal columns.
	All the hangers were replaced.
	The towers of the Wye bridge were extended and its stay system replaced.
	The Aust viaduct and the Wye bridge was provided.
	Refurbishment included provision of new access facilities, improvement of welds in the orthotropic decks, renovation of moving parts, installation of new safety barriers and wind shields, and resurfacing.

 

   References

  Chatterjee S. Strengthening and refurbishment of Severn Crossing. Part 1:introduction. Proc. Inst. Civil. Engrs Structs & Bldgs,1992,94,Feb.,1-5.

  Flint A. R. Strengthening and refurbishment of Severn Crossing. Part 2:design. Proc. Inst. Civil. Engrs Structs & Bldgs,1992,94,Feb.,7-22.

  Cuninghame J. R. and Beales C. Strengthening and refurbishment of Severn Crossing. Part 4:TRRL research on Severn Crossing. Proc. Inst. Civil. Engrs Structs & Bldgs,1992,94,Feb.,37-49.