Civil Engineering March 2022 | Vol 30 No 2

28 March 2022 Civil Engineering be demolished and replaced. With the original deck removed, the abutment seating platforms could be repaired rela- tively easily. The pier columns, pile caps and piles would be retained, since they were still in good condition. FINAL DESIGN SOLUTION Option 3 was selected as the best repair solution and comprised demolishing and rebuilding elements of the bridge (the deck and pier seating beams) and changing the bridge’s structural configu- ration from simply supported to con- tinuous. With this option, the intention was to reduce the number of joints and bearings by making the deck continuous. This was structurally possible because the original simply supported deck was sup- ported by two rows of bearings on each of the piers. This system was replaced by one row of bearings on the centreline of the piers for a continuous deck. The original bridge had joints at each of the nine piers and the two abutments (11 joints in total), with a total of 120 bearings (see Figure 9). For the new deck configuration, joints were used only at the ends (abutments A1 and A2) and the centre of the bridge (Pier P5), resulting in three joints in total. In addition, the number of bearings was reduced to 96 (see Figure 10). By reducing the number of joints, the durability of the structure was substantially increased. For the original bridge, the loads from the deck were applied eccentrically to the piers since there were two rows of bearings offset from the centrelines of the piers (see Figure 11). However, for the new continuous deck, the total loads on the piers are similar to the simply supported design since the bending moment on the pier is reduced by the fact that the load from the deck is applied to the centre of the pier (see Figure 12). The increased movements of the continuous deck (due to temperature and shrinkage) are therefore cancelled by the reduction of the eccentric loads from the deck. The new deck consists of precast reinforced concrete T-beams with an in-situ top slab. The deck is made continuous by a transverse diaphragm beam that connects the beams and the in-situ slab. The precast beams were placed on temporary supports and al- lowed to shrink before the diaphragm could be cast. This method is depicted in Figure 13. This meant that the move- ments from short-term shrinkage did not have to be taken into account by the elastomeric bearings. The following additional measures were taken to enhance the durability of the structure: Q Q An extensive gutter system was provided to drain water away from the concrete elements. Figure 9 Schematic of the structural configuration of the original bridge Figure 10 Schematic of the structural configuration of the new bridge