Thelwall's problems probably began when its entire structural philosophy was changed in the 1996 refurbishment.
The original 1.4km structure was split up into 36 spans by expansion joints. A half-joint also split up the 102m long central span, the longest in the structure.
But by 1995 de-icing salts used on the road had corroded the expansion joints and concrete deck. Increasing traffic loads had also taken their toll on the aging bridge bearings. So the Agency decided to replace all the bridge bearings and reduce the number of expansion joints.
Eight roller bearings per pier was cut down to four roller bearings and two pairs of rocker bearings. The number of expansion joints was also cut to four: one at each abutment and two either side of the central span. This split up the deck into just three sections, with each joint handling 173mm movement from the approach spans and 89mm from the central span.