Robert Allen, Ivo Petrov, Alek Smocynski, Harrison Wallen

Structural Design
The design of a footbridge crossing the mainline railway line in Winchester
Alastair McDonald, Howard Clarke, Professor David Richards

A footbridge designed to span across the mainline railway line, adjacent to the existing Andover Road (B3420) Bridge, towards the northern end of Winchester. Planned residential redevelopment to the north-west of the town is anticipated to increase pedestrian footfall across the existing combined vehicle and pedestrian bridge and this proposal seeks to realise a safer and more enjoyable user experience for pedestrians.

The bridge deck spans between points chosen to minimise the sloping gradient of the deck, suit an efficient construction process, comply with legislative regulations, and to tie into the existing urban context. The combination of existing pavement levels and the National Rail requirements for minimum clearances above the kinematic envelope of the rolling stock resulted in the development of a thin deck section and the decision to locate the majority of the primary structure above finished deck level.

The bridge’s width and height are set by required footpath clearances and the geometry of the form. The resultant triangular cross section of the truss produces an inherently stiff and efficient primary structure that effectively resists any combination of gravity, lateral or torsion forces whilst also limiting its susceptibility to vibration either from the traffic over the bridge or the dynamic effects of the trains passing underneath. Further efficiencies were gained by the introduction of a mid-span column located to prevent any impact on the operation of the line in either the temporary or permanent cases. These key decisions minimise the overall size of each structural element with the chords a minimal 119mm diameter.

The chords are offset in plan to animate the structure, improving the user experience, and to support the folded geometry of the balustrade panels – designed to reduce the potentially imposing impact of National Rail requirements for 1.7m high, near solid, enclosures above railway lines.

The design has been developed to minimise disruption to rail operations. On-site ground works are located at a distance away from the side of the track clearances to allow work to continue whilst in full service and prefabricated elements are designed to be installed during night-time engineering possession hours. Sheet piling is installed to retain the pavement edge; pad foundations are formed, and prefabricated concrete abutments are installed and back filled to avoid potentially contaminated soil being transported off site, while reducing cost. The primary structure can be transported to site in two sections, spliced together adjacent to the site and then lifted into place within the loading and reach parameters of readily available craneage specifications. The prefabricated deck planks and balustrade panels ensure a high quality