Civil Engineering March 2021 | Vol 29 No 2

Civil Engineering March 2021 13 most notably the large skew angle, high debris load, and adjacent properties in the urban setting. Although several alternative design so- lutions were evaluated, the new tied-arch bridge was finally selected as the most suitable structural configuration for the replacement structure (Ronné et al., 2018). DESIGN CONSIDERATIONS General details The New Ashton Arch Bridge has a single tied-arch structural configuration, with a span of 110 m between support bear- ings. Highly durable, high performance, 50 MPa concrete was specified for the arch rib and tie-beam members, while the remainder of the bridge structural com- ponents utilise durable 40 MPa concrete. The typical cross-section of the arch bridge deck provides for four 3.4 m traffic lanes and two 2.4 m sidewalks with the following key features (Figure 3): Q Q The overall height of the bridge is approximately 23 m from deck soffit to the top of the arch. Q Q The twin parallel arch ribs are con- nected via five 15.5 m wishbone beams that provide lateral stability to the arch ribs, while post-tensioned tie-beams complete the arch structural form. Q Q 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. Q Q Each arch has 24 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. Q Q The anchor plates are, in turn, con- nected to the concrete structure via high strength threaded post-tensioned, stress bars. Structural modelling for design and construction The arch bridge was originally modelled by AECOM’s South African bridge team using Bentley’s RM Bridge software. An internal design review was conducted by AECOM’s UK long-span complex bridge specialist team using SOFiSTiK. These state-of-the-art bridge analysis software packages were also used for the construction stage analysis and in-service analysis. Both software models were up- dated 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 stay cable 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 service life. The updated forward step analysis models were calibrated with as-built infor- mation and was also used for the following assessments: Q Q Stay cable length determination, which requires lengthy procurement lead times applicable to international suppliers. The stay cable length verifi- cation had to consider both intentional and un-intentional variations, relative to the adjustment capacity available in the fork socket assembly. Q Q Hanger force distribution optimisation. Q Q Pre-camber requirements and influence of temporary works displacements. Q Q Movement capacities of temporary and permanent bearings, and expansion joints. Figure 2 Original bridge over the Cogmanskloof River in Ashton Original bridge width Bridge widened in 1950 18 550 22 125 Tie beam Hangers Arch ribs Wishbone beams 3 400 Lane 3 400 Lane 3 400 Lane 3 400 Lane 2 400 2 100 14 950 2 400 2 100 2 400 Sidewalk Figure 3 Typical new bridge cross-section