Dawlish Station Footbridge
Tony Gee and subconsultant Optima
Projects, with a third-party check by Parsons Brinckerhoff, Dawlish
Station footbridge is an almost identical lightweight Fibre Reinforced Polymer
(FRP) replacement structure for a severely corroded steel predecessor.
It is the first FRP composite
bridge installed at a mainline station in the UK, It is also, notably, the first
Grade II listed FRP bridge. The structure aesthetically replicates the
character of the original steel structure, but provides the client,
Network Rail, with a much lighter
and more durable solution that is expected to result in considerable
through-life cost savings. Tony Gee used LUSAS Bridge analysis
software to assist with its design.
was originally designed by Isambard Kingdom Brunel in 1830 and
is grade II listed. It is situated in an extremely hostile
seaside environment that caused its 17.5 metre (57'-4")
long covered steel footbridge, that was reconstructed in 1937,
to deteriorate progessively over the years. A detailed
inspection in 2004 identified many areas of corrosion. A
follow-up inspection in 2010 found the structure had
deteriorated even further, and a final study in 2011 showed
severe corrosion to the girder / cross girder connections.
Detailed analysis found that not only could the structure not
carry the specified imposed load due to corrosion of the
members, but even in an ‘as new’ condition the bridge had
been under-strength due to a lack of strength and stiffness in
the U-frames providing lateral stability to the top flange of
the plate girders
expensive like-for-like repairs
were considered, as was a totally new steel replacement, but
ultimately due to the extreme coastal environment, Network
Rail decided to install a wholly fibre-reinforced polymer (FRP)
corroded steel footbridge
(Image: Network Rail)
Station location showing
previous steel footbridge and its proximity to the sea
The new bridge structure
is manufactured entirely from FRP materials and mimics its predecessor
in all respects. It spans the same distance, carries a 1.8 metres wide
walkway, and is covered by
a roof of the same style with wide overhanging eaves. The girders of
the old steel bridge comprised riveted built-up sections with webs that had a clearly visible X-brace detail, which was
exaggerated by corrosion patterning. This detail, together with the
riveted construction, was identified as being a key part of the ‘character’
of the structure and discussions with the planning and listed building
authorities identified that these features would need to be carried
forward into any replacement structure. As a direct result, imitation
rivet heads were bonded to the structure, and in some locations
structural bolts, fitted with domed heads, provide a backup to the
The majority of parts are
pultruded with glass fibre reinforcements and fire retardant polyester
resins to achieve the required structural properties. Both the primary structure and the parapet
are made up from 1.66 metre deep side girders, each formed from foam cored shear webs, moulded by film infusion using fire retardant epoxy resin and biaxial glass fibre reinforcement, capped top and bottom with pultruded angles and plates to form the girder flanges.
Pultruded parts, 17.5m long, were
used to fabricate these flanges and avoided the need for joints.
Web stiffeners made from pultruded plate provide additional lateral support to the girders, connected to transverse angles below the deck.
The deck is formed from
lightweight ‘Composolite’ pultruded panels, that span transversely between the
girders. The deck is bonded to the girders and also forms a shear panel to resist horizontal wind loading, removing the need for diagonal bracing below the deck.
The deck terminates 2.7 metres from the ends of the bridge to leave room for the
stairs, resulting in a long length of girder acting as a cantilever. To
enable these cantilevered areas to resist large wind loading from the
side, additional lateral support plates (shown green on the image
below) were fitted to the flanges external to the girder.
The roof transverse frames were fabricated from back-to-back pultruded angles to form T-sections with bonded and bolted joints. The roof frames support longitudinal purlins made from pultruded box section,
which support standard corrugated fibre-cement panels and also provide lateral restraint to the top of the girder.
At each end of the bridge a much stiffer transverse frame is included
to increase the lateral and torsional stiffness. Stair units at either end of the bridge are made from a single FRP moulding, including the stair treads, risers and side panels, hanging from the bottom flange of the bridge girder.
LUSAS half-model showing
illustratative geometric properties (from a model during
Inset shows a separate staircase model used for strength / frequency
Modelling with LUSAS
of LUSAS models were created to assist with the design of the
structure, using appropriate isotropic and orthotropic materials for
the various component parts. Ian Smith, Director
at Tony Gee & Partners, summarises what was done: "We used a
half-structure model using symmetry conditions at mid-span to carry
out a strength / deflection assessment of the deck, and employed slightly simplified
models for eigenvalue natural frequency and buckling analyses. For the
staircases, a strength/deflection analysis of the staircase moulding
(using a half-model, with symmetry along the staircase centerline),
and a dynamic fundamental frequency model of the whole staircase
moulding were required."
The bridge was required to withstand ‘normal’ Eurocode footbridge loading and criteria were agreed between the designers and Network Rail.
Because large numbers of passengers can exit a train and then use the bridge at the same time the full ‘Load Model 4’ loading of 5
kN/m2 distributed live load was applied.
Parapet loading in Eurocodes was not well resolved at the time of the design, so this was taken from older standards such as Highways Agency document TD19/06 ‘Requirements for Road Restraint Systems’.
was conceptually a simple code-compliant situation, although the location is exposed and the wind loads are accordingly relatively high.
Simple aerodynamic stability checks indicated that the critical wind speeds for vertical or torsional vortex shedding induced vibration were above 1.25 x design mean wind speed and therefore did not require more detailed investigation.
However, lightweight bridges of this type are potentially prone to dynamic response from the aerodynamic loads from passing
trains, so a train buffeting assessment was performed using a number
of time domain analyses. These modelled the
effects of the train pressure loading advancing across the width of
the deck as the train passed, where both the location and pattern of the
loading varies with time.
half-model showing beam elements (fleshed in magenta), shell
elements (shaded green) and symmetry supports.
footbridge after completion
checking and approval.
Since the materials used
in the bridge are still considered ‘novel’ by Network Rail, a rigorous design and checking process was implemented. Tony Gee and Partners was appointed to prepare the
Approval in Principle document, complete the design and the Design/Checking
certificate. Design work undertaken by subconsultant Optima Projects was validated by Tony
Gee, and a full ‘Category 3’ independent check was carried out by Parsons Brinckerhoff. Network Rail’s own planning and listed building specialists managed the process of obtaining listed building
consent, justifying a footbridge replacement rather than repair.
The resulting footbridge authentically replicates the
character of the original steel structure, whilst providing a much lighter
and more durable solution than its predecessor. It is expected to result in considerable
through-life cost savings for Network Rail due to reduced maintenance
Interior of footbridge
Image: First Great Western
is my first-choice tool for any of the structural analysis we do.
For the non-standard work we undertake we need a general-purpose
finite element analysis package that is not narrowly tailored to a
particular market segment. LUSAS fulfils that role, letting us
analyse and design with materials or for applications not yet well
defined in civil engineering codes."
Ian Smith, Director, Tony Gee and Partners
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