The purpose of the new A8 bridge is to provide pedestrian and vehicle access to The Royal of Scotland Group HQ at Gogarburn and act as a visual statement and gateway to Edinburgh. The location of the bridge was decided by the limited available land on the north side of the A8, while a number of constraints impacted on the design of the bridge and contributed in determining its form. Maximum permissible gradient to gain access to the bridge dictates the deck level, radar signature from the nearby airport determines a height limit to the bridge, whilst the proximity to historical protected trees and land not in RBS ownership determined where the bridge could be founded.
With the constraints placed on the levels of the top of the road surface and underside of the deck to attain the necessary clear headroom on the A8 (5.66m) there is insufficient depth within the deck to achieve a simple clear span across the road. To avoid the introduction of a pier in the central reservation, an arch solution with a suspended deck was adopted.
The structure consists of a two-pin parabolic arch, spanning clear across the A8 while supporting a steel and concrete composite deck. The arch has a clear span of approximately 60m, rises to a height of approximately 20.55m and spans diagonally on plan across the line of the bridge. The arch is formed using fabricated circular hollow section of 1300m diameter and 40mm thick.
The arch is supported on stainless steel bearings and restrained at the deck by virtue of a sliding connection consisting of three ‘fingers’ projecting from the arch interconnecting with two deck fingers, which are, in turn, restrained by a square steel pin, thus longitudinal and vertical deck movement is allowed. Two large lighting optics have been incorporated within the arch at third points to discreetly illuminate the deck. The pedestrian and traffic containment barriers have been developed to reflect the overall aesthetics of the bridge and have lighting fully integrated within the uprights to illuminate the pedestrian footpath.
The deck consists of a grillage formed from two box spine girders (800dp x 1500 wide) with plate girder ‘rib beams’ at 3.75m centres connected to the hangers. The deck is formed from Omnia-precast formwork with in-situ topping.