Main design responsibility for the strengthening of the Avonmouth Bridge's twin steel box girders fell to Hyder Special Structures, whose 'grandparent', Freeman Fox & Partners, carried out the original design in the late 1960s and early 1970s. The 1,400m long crossing is made up of three sections: the two approach viaducts, and the main river span with its two backspans. The challenge was to come up with a strengthening solution which caused minimum disruption to the holiday traffic flowing across the bridge.
On the approach viaducts the box girders are a tight 6m wide by only 3m deep. To relieve stresses in the webs and diaphragms caused by hogging moments over the piers, fans of Macalloy bars run from near the top of the pier diaphragms down to the bottom flange of the box girder. Before these are tensioned, stresses are reduced by propping at third points along the spans.
A more efficient strut and tie design was chosen for the main piers each side of the 174m central span. Here the boxes are haunched to 7.5m deep, and the deck is a steel orthotropic design rather than the concrete on the composite approach spans. Tubular steel struts measuring one third span are installed on top of the bottom flange transverse stiffeners each side of the pier diaphragms. Prestressing cables run from the end of the struts over saddles installed near the top of the diaphragms and are tensioned after the box is jacked at the third span points.
Throughout the box girders around 150,000 individual strengthening plates weighing 3,000t are welded into place. Most have to wait on stress relief from either jacking or tensioning before they can be installed and for any defective welds to be repaired.