Construction World January 2022

ROAD & BRIDGES

The newly completed Ashton Arch in the Western Cape is South Africa’s first concrete tied-arch bridge constructed using the transverse launching method. The new bridge replaced an existing multi-arch bridge, which no longer fulfilled its functional requirements. AECOM were the appointed structural engineers, while the build was contracted to Haw & Inglis Construction. These teams worked closely to develop detailed method statements to ensure compliance and agreement before construction commenced. ASHTON ARCH BRIDGE

T he objective of the project was to improve traffic and pedestrian safety, as well as improve the flood capacity of the bridge and its resistance to overtopping. The original earth-filled multi-arch bridge was constructed in 1 930 for single lane traffic and in 1 950 a substantial structural retro fitment upgrade to facilitate two traffic lanes and pedestrians was carried out. These measures did not fully address the overtopping issue, which still resulted in road closure and flooding of adjacent properties. The tied-arch design has taken these issues into consideration and will also prevent the accumulation of debris during floods. State-of-the-art bridge analysis software packages were used for the design. Key considerations for the upgrade were to minimise river flow restriction and improve the available free board within the restrictions of adjacent properties and road alignment levels. Breaking new ground in South Africa, and after several years of meticulous planning, design and construction, more than 8 000 tons of reinforced concrete and steel was moved over 24 metres in less than 24 hours. To install the sliding and permanent bearings the 8 000 ton deck had to be vertically lifted and lowered by

H&I Construction. This was done in phases by installing the bearings at one abutment before repeating the process at the next. By constructing the new bridge adjacent to the existing one, traffic flow was maintained, and the disruption of its flow was minimised. The bridge comprises a cable-supported concrete deck which spans 110 metres between supports with arching ribs rising 22 metres above the roadway. It has a single tied-arch structural configuration, where the typical cross-section of the arch bridge deck provides for four 3,4 metre traffic lanes and two 2,4 metre sidewalks. The twin parallel arch ribs are connected via five 15,5 metre wishbone beams providing lateral stability to the arch ribs, post-tensioned tie-beams complete the arch structural form. To achieve a higher level of stability and safety on the temporary concrete support columns, the temporary works engineers specified SikaWrap®-300 C for additional structural strengthening. It is a high-quality unidirectional woven carbon fibre wrapping system, that was used to encapsulate the head of the concrete support columns, using Sikadur ®-330, a structural impregnating epoxy resin

26 CONSTRUCTION WORLD JANUARY 2022