Analysis and design of footbridges
for the Dubai Metro light rail project
3D modelling of
steel truss footbridges
analyses to derive mode shapes and mass participation factors
the design of large number of footbridges in a timely and
The Red and Green Lines of the Dubai Metro light rail scheme are being constructed as a Design-Build contract by a consortium of international contractors.
Atkins, one of the world’s leading engineering and design consultancies, is the lead designer to the major civil contractor of the Dubai Rapid Link (DURL) Consortium and is carrying out the full multi-disciplinary design and project management of the civil works for the project. Atkins used LUSAS Bridge analysis software to assist with its analysis and design of a
large number of steel truss footbridges used to provide pedestrian access to the elevated stations on the project and to permit the crossing of roads along the route.
The 52km long Red and 24km long Green
Lines of the Dubai Metro are accessed via 47 stations, some of which
are at-grade, most are elevated, and ten are underground. A total of
205 steel truss footbridges provide 24-hour, air-conditioned access
for the general public to enter the elevated stations and also permit
the crossing of a variety of roads along the route.
Prior to installation each footbridge
is fully assembled (incorporating glazing, external cladding, internal
fixtures and finishes, and electrical and mechanical items) resulting
in the largest footbridge weighing well in excess of 200 tonnes.
Special self-propelled modular transporters are then used to transport
the footbridges into position from their off-site assembled location
and to subsequently lift them onto piers that have flared pierheads
similar in style to those seen on the main viaduct structures of the
Modelling and analysis
A modular design approach was used to
assist with the structural design of the footbridges. Using LUSAS
Bridge, the longest of each of the main footbridge types were
analysed. These comprised a non-travelator type, a travelator type,
and a low height option. Additionally five ‘special cases’ had
also to be assessed.
non-travelator footbridge cross-sections
For each footbridge, 3D thick beam
elements modelled the primary structural members of the streel truss,
crossheads, and concrete pierhead and pier. Piled supports were
modelled using the equivalent cantilever method. Joint elements of
appropriate stiffness and freedoms represented the articulation of the
bearings. Lumped masses, applied throughout the model using joint
elements, and with an appropriate eccentricity, modelled the mass
distribution of the cladding and finishes to the roof, walls and
3D LUSAS model of a long-span non-travelator footbridge
From a close inspection of the
mass-participation factors, eigenvalues and frequencies obtained from
each LUSAS analysis the primary longitudinal, vertical, lateral and
torsional modes could be identified. Graphs of total mass
participation plotted against frequency showed quite clearly when
particular amounts of structural mass became excited for certain
freqencies. In addition, animations of selected eigenmodes permitted
easy visualisation of the true structural response.
First lateral mode
Design and checking
Steel design was to BS 5400-3:2000 and
BS EN 1993-1-8:2005. Post-LUSAS analysis calculations carried out on
the main footbridge types included vibration serviceability checking
to BD 37/01to ensure that the structure was not overly excited by
pedestrian use, and aerodynamic checking to BD 49/01.
Manuela Chiarello, engineer on the
project said: "When we did the analysis of the longest
non-travelator footbridge type with LUSAS and checked for
susceptibility to aerodynamic excitation as defined by BD 49/01 we
found that, whilst the structure passed the vortex excitation check,
it didn’t comply with the divergent amplitude response check (for
galloping) essentially because it was an enclosed structure that was
just taking too much wind load, so a wind tunnel test was additionally
carried out. This proved the structure wasn’t actually susceptible
David A Smith, Regional Head of Bridge
Engineering at Atkins said: "The use of a modular design approach
in conjunction with detailed LUSAS analyses of the main footbridge
types enabled us to design the large number of footbridges required
for the project in a very timely and efficient manner."
The Red Line was officially opened in September
2009. The Green Line is scheduled to be opened in early 2010.
Aerial view of Dubai Metro Financial Centre station and footbridge
"The use of a modular design
approach in conjunction with detailed LUSAS analyses of the main
footbridge types enabled us to design the large number of footbridges
required for the project in a very timely and efficient manner."
David A Smith, Regional
Head of Bridge Engineering, Atkins
Other analyses undertaken by Atkins on
the Dubai Metro light rail project using LUSAS include: