Civil Engineering October 2022 | Vol 30 No 9

Civil Engineering October 2022 9 and developed further by the engineering team. One proposed retaining the central stone pier as a non-structural component in a single span bridge, while the other utilised a two-span structure with a func- tional central pier. AECOM’s analysis confirmed that the bearing capacity of the existing central pier would be significantly exceeded if it were incorporated structurally into the proposed two-span design and would require another row of piles and further structural modifications. By contrast, in a single-span design using prestressed precast concrete beams supported by new abutments, the central pier could be retained in a non-structural capacity, and it was this solution that was finally chosen. The retention of the pier was recommended because, as with the old abutments, it predates the existing upper structure and could therefore serve as a visual link to the old stone bridge. An additional heritage aspect was the re-instatement of four existing handrail bollards, which serve as entry markers at either end of the bridge. Eight metres wide kerb-to-kerb and 22.5 m long (2 m longer than the old bridge), the new bridge will accom- modate two lanes of traffic. In addition, pedestrians and cyclists are being given cantilevered walkways on either side of the vehicle deck. Besides their functional purpose, the walkways enhance the vis- ibility of the central pier and reflect the character of the old bridge. CHOOSING PRECAST The new bridge was designed by AECOM bridge engineer Heinrich van Wijk, who proposed a precast beam design primarily because it saved time, reduced the risk of flood damage to formwork staging, and limited construction debris falling into the riverbed. “In a cast-in-situ bridge option, there is an element of flood risk when staging formwork above a riverbed. Instead, we chose to specify a precast construction technique, which limits disturbance in the riverbed and enhanced our application to the Department of Water and Sanitation for a water-use licence,” says van Wijk. Van Wijk’s design adheres to the South African bridge design code TMH7 for NA and NB loads. One of the design con- straints was the need to keep the bridge open during construction which is why it is being built in two phases. Apart from 1.5 m wide edging which was demolished, the old bridge was left intact during the construction of Phase 1 and was only fully demolished once Phase 1 was opened to traffic. “The abutments and deck are being built in an integral monolithic design, unlike conventional bridges which are mounted on bearings with expansion joints at either end. The integral design approach allows the free articulation of the deck without imposing significant stresses on the abutments. Thermal expansion and contraction is the primary source of movement (elongation and shortening) and the magnitude of this movement is directly related to the length of the deck,” explains Van Wijk. “For shorter decks, such as the Distillery Road Bridge, conventional expansion joints are not needed. Instead, thermal expansion and contraction can be accommodated by buried joints located at either end of the bridge, which allows for the small movement of the flexible abutments. The soffit sides of the Phase 1 beams and the old central pier The retention of the pier was recommended because, as with the old abutments, it predates the existing upper structure and could therefore serve as a visual link to the old stone bridge.