The simple design of the covered acoustic barrier belies the complexity of the major highway interchange; the project involved retrofitting a 770m long 8-lane viaduct already under construction in conjunction with 10 interconnecting flyovers across the junction and incorporating 12 entry and exit ramps.

To determine the optimum solution for the project SGA undertook five different design options, all of which were carried out to detailed model stage for the client to assess. The concept of ‘Qianhai Gateway’ was adopted, using a single arch form wrapping each direction of traffic.

The rotation of the primary structural arch frames at an angle to the carriageways was found to offer unique technical and visual benefits. The diagonal orientation provides strong lateral bracing in comparison to a perpendicular moment frame, reducing the need for additional cross bracing. At the same time the ‘twisting’ spatial effect creates a stimulating experience driving to and from Qianhai.

The frames are supported on a concrete impact barrier at low level, above which a 2m high band of perforated acoustic absorption metal panels is positioned where it is most effective in dampening road level noise. Above the vertical panels the arch frames elegantly curve over the roadway, which is fully enclosed using 3m-wide parallelogram-shaped noise control clear acrylic panels.

Working together with LERA Consulting Structural Engineers, various studies were made to determine the optimum shape and material of the main structural frames. Aerospace aluminium was initially evaluated, however painted steel emerged as the most cost-effective direction. Detailed comparison studies were undertaken to determine the best profile of the angled arch members. In comparison to I-frames and box sections, which need to twist to properly follow the angled arch members, circular sections were found to be the ideal technical solution, as the frames could be single-directional curves while allowing connections at any angle. Furthermore circular members were found to be nearly half the cost of steel box frames, and provided a more pleasing architectural shape in keeping with the overall form of the acoustic barrier.

For the main 23m long diagonal spans, 350mm diameter steel sections are used at 3m centres; larger 450mm diameter sections are used at exit and entry lanes where the span increases up to 36m. Continuous steel flanges are added along the circular hollow sections to accommodate integrated linear lighting strips, and also provide additional structural capacity.

The result is an elegant structure that is refined in detail and achieves the initial aim of a simple overall form. The project is to be completed in mid-2020 and will transform the journey by road between the forward-looking Qianhai New District and the neighbouring parts of modern Shenzhen.