...from Ashton to Montagu
The newly completed Ashton Arch is South Africa's first concrete tied arch bridge constructed using a transverse launching method. It represents a proud monument for the Western Cape Department of Transport and Public Works, the people of the Ashton-Montagu area, and all the contractors, labourers and suppliers who were involved in the project.
The Department upgrades, rehabilitates and maintains provincial roads as well as national roads located in the Western Cape. The Department also provides Expanded Public Works Programme (EPWP) work opportunities, develops emerging contractors, and contributes to black economic empowerment in local communities where it undertakes projects.The New Ashton Arch Bridge is being constructed in the town of Ashton, Western Cape, on Trunk Road 31 Sections 2 (start of “Route 62”, an important tourist destination and national road) crossing the Cogmanskloof River. The bridge replaces an existing multi-arch bridge, built in the 1930's, which did not fulfil its functional requirements, inter alia service life, width and hydraulic capacity, anymore. The new tied-arch bridge comprises a cable-supported concrete deck which spans 110 metres between supports with arching ribs rising 22 metres above the asphalt road surface. The superstructure, which was completed mid 2020 adjacent to the existing road-alignment, will be launched transversely into its final position onto the adjacent recently completed substructure during August 2021, a first of this kind of construction in Africa.
The objective of this project was to reconstruct parts of Trunk Road 31 Section 2 and 3, from Ashton to Montagu (start of tourist Route 62) to a higher standard in order to improve traffic and pedestrian safety as well as improve flood capacity of bridges and resistance to overtopping of the road at several problem locations.
This included the Cogmanskloof River crossing in Ashton which has experienced substantial flood damage on several occasions.
The original earth-filled multiple arch bridge at this crossing was constructed in 1930 for single lane traffic. In 1950 a substantial structural retro fitment that allowed two-way single carriageway vehicular and pedestrian traffic was undertaken. The superstructure was modified to a cast in-situ beam and slab configuration that maintained portions of the old arch superstructure, including arch-profile and related hydraulic opening configuration. The 1950 upgrade was however retained at an elevation which still resulted in overtopping with enhanced flood risk to the road and adjacent residential properties. The high river debris load combined with deep deck and wide solid wall piers, orientated at unfavourable skew angles relative to the flow direction, exacerbated the effects of flooding.
Key considerations for the latest upgrading was to minimize flow restriction and improve the available free board within the restrictions of adjacent properties and road alignment levels. Through an economic analysis, the technical options for the river crossing and construction strategy were evaluated with due consideration of the impact of construction strategy on road user costs.
The design which was finally adopted consisted of a single span (110m) concrete tied arch solution with a deck suspended by stay cables which accommodates four traffic lanes and two walkways. This largely eliminated the possibility of debris build up and provided the shallowest deck depth option (key considerations).
Construction adjacent to existing bridge and transverse launching after completion minimized traffic disruption during construction.
To minimize traffic disruption, the new bridge was constructed adjacent to the existing bridge whilst maintaining traffic over it. After completion the new bridge has been used as temporary bypass/river crossing while the existing bridge was demolished and new abutments built. The new tied arch bridge will be launched transversely into its final position in less than 24 hours.
The New Ashton Arch has a single tied-arch structural configuration, with a span of 110 metres between support bearings. Highly durable, high performance 50MPa concrete was specified for the arch rib and tie-beam members, while the remainder of the bridge structural components utilize durable 40MPa concrete.
The typical cross-section of the arch bridge deck provides for four 3,4m traffic lanes and two 2,4m sidewalks with the following key features (see section below):
- The overall height of the bridge is approximately 23m from deck soffit to top of the arch.
- The twin parallel arch ribs are connected via five 15,5m wishbone beams that provide lateral stability to the arch ribs, post-tensioned tie-beams complete the arch structural form.
- Post-tensioned longitudinal and transverse beams support the integral deck road slab, resulting in a coffered deck arrangement which transfers load to the tie-beam.
- Each arch has twenty-four fully locked coil strand-type hangers that connect the arch rib and tie-beam by cast steel fork sockets, via a pin connection, to locally manufactured, engineering-grade, welded composite metal anchor plates.
- The anchor plates are, in turn, connected to the concrete structure via high strength threaded post-tensioned, stress bars.
Through a specified laboratory testing programme, the desirable concrete mix properties were selected; favouring structural and thermal performance properties, as well durable and sustainability parameters while also providing technical inputs to a construction stage finite element model. Construction methodology of large volume concrete elements was planned with due care and management of thermal performance aspects. The meticulous planning extended to sustainability considerations by recycling the old fabric of the original bridge within the permanent foundation fill of the new bridge.
Local manufacturers with support from overseas specialists were used to manufacture specialized components such as post tensioning systems, arch stay cables, anchor plates and support bearings.
Structural form and bridge aesthetics received meticulous attention during the conceptual design with due consideration of the following:
- The historical significance of existing multiple-span arch bridge inspired the structural form combined with the major benefit of the arch's ability to span over the entire river and consequently improve the hydraulic efficiency, specifically in view of the large skew angle of the crossing.
- A tied-arch bridge is particularly suitable for this topographic location, since it suspends the roadway and does not require propped arch founding conditions.
- V-hanger-configuration was adopted in preference to the vertical hanger orientation, since it provides an aesthetic advantage.
- Aesthetically, the slender tied arch design is appropriate due to the depth of the proposed roadway above the riverbed with a visually appealing light deck.
- The tied-arch form expresses a visualisation of flowing of forces with light hangers and slender members displaying a transparency accentuated by the surrounding mountains.
State-of-the-art bridge analysis software packages were also used for the construction stage analysis, in-service analysis, and design, a joint effort by AECOM's SA and UK Long-span complex bridge teams.
- Software models were updated with concrete material parameters, as determined from laboratory testing of actual material.
- This allowed an accurate simulation of the time-dependent material behaviour with the possibility of force adjustments during construction.
- Such modelling is important for bridges of this nature to ensure effective behaviour and force distribution of all structural elements during its full-service life.
An extensive structural behaviour monitoring plan was prepared for construction and included system identification by full-scale dynamic testing as well as structural behaviour verification by deformation measurements, requiring continuous modelling and monitoring during all construction stages. This will ensure that proper structural behaviour is achieved during the tensioning of the stay cables and structure's service life.
Modern state of the art structural components was designed, manufactured and installed using complex methodology. These included the concrete arch ribs, post tensioned tie-beams, stay anchors and cables as well as the transverse launching equipment which had to be specially imported.
Many design, procurement and construction challenges were addressed over several years.
Using a transverse launching method of a completed concrete tied-arch road bridge is a first in South Africa when more than 8000 tons of concrete and steel is moved over a distance of 24m in less than 24hours after several years of meticulous planning, design and construction.
The dead weight at each abutment of 4000 tons will be moved using the following equipment:
- Four 322 ton launching strand jacks
- Four pulling cables with 31 strands with a breaking force of 27 tons each
- Supplementary pulling capacity of two 120ton jacks with 50mm dia stress bars
- Four temporary launch sliding bearings with 900mm diameter rotational pots
- Four roller side guides to keep the bridge on track
On completion of the project, a total of 18% of the contract value would have been allocated to the creation of economic opportunities and entrepreneurial capacity in the surrounding areas. This includes a local labour utilisation of 65 000 person days and 300 work opportunities which were created for exempted micro enterprises and qualifying small enterprises (EME/QSEs).
The successful completion of the New Ashton Arch resulted in a proud monument for the region and all involved. A first and unique application of this bridge engineering technique for a concrete tied arch bridge in South Africa.