Case Study
Seismic analysis of viaduct sub-structures for the Dubai
Metro light rail project

2D and 3D modelling of concrete viaduct structures

Seismic analysis to AASHTO LRFD

Pile and pier design moments obtained from seismic and modified BS 5400 load combinations
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 various
structures on the project including seismic analysis of the viaduct substructures, where maximum bending
moments were derived for use in designing and reinforcing the pier and pile sections.
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Case Study
The Red Line runs from Jebel Ali Port, through the city centre, to Al Rashidiya - a distance of 52km in total. Of this
length, some 42km is of elevated viaduct construction. The Green Line runs from Al Qusais, through the city centre,
to Al Jaddaf - a distance of 24km. Of this length some 16km is of elevated viaduct construction.
Overview
Viaducts on the Red and Green Lines comprise a combination of simply supported single-span post-tensioned
precast decks of U-shaped cross-section supported on elastomeric bearings, and two and three-span continuous
decks on fixed and free guided sliding pot bearings. Most viaduct spans are of a standard 28m, 32m or 36m length
and are typically supported on single, circular, reinforced concrete columns of 1.75m or 2m diameter. Flared
pierheads support the deck. For speed of construction most single piers are supported on 2.2m and 2.4m diameter
bored monopiles. Pierheads for the single spans, twin spans and stations spans are of precast construction
(requiring in-situ concrete fill and prestressing). Pierheads for single track and three-span continuous viaduct
structures are constructed solely in-situ and for these structures large diameter bored piles were used. The majority
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Case Study
of deck sections are assembled by overhead gantries but, for the pier and pile design, the temporary construction
loading from the gantries was generally found to be less onerous than that caused by any potential seismic loading.
Design basis
The construction programme called for the initial design of over 1200 unique foundations in the first nine months of
design to take the viaduct construction off the critical path. Initially, to keep ahead of construction, this was done
using automation and conservative design methods but as time permitted more refined calculations were introduced
to optimise the design of foundations yet to be constructed. Design of the viaducts was based on BS 5400 and
associated standards with the AASHTO LRFD Bridge Design Specification being used for the seismic design. Using
LUSAS Bridge, both 2D and 3D modelling of sections of viaduct was carried out according to span type.
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Case Study
Single-span modelling
Initially, 2D transverse and longitudinal models were created in LUSAS to model the simply-supported single-span
structures. From these, natural frequencies were obtained for the first 3 mode shapes for use with the AASHTO
LRFD design response spectrum in order to generate acceleration values for the design of pile and pier sections. For
the transverse model, beam elements represented the piles, piers, and crossbeams. Pile modelling was done using
the equivalent cantilever method (as opposed to modelling the soil/structure interaction as a beam with elastic
springs) with appropriate static and dynamic values provided by Atkins geotechnical team. Elastomeric bearings
were modelled with joint elements of appropriate stiffness. In accordance with the Design Basis Report the weight of
a train acting on one track only was represented by a joint element having a mass and vertical eccentricity
appropriate to the centroid of the train. Different pier heights ranging from 2m to 20m and pier diameters of 1.75m
and 2m were investigated. Longitudinal models, built using similar modelling techniques to the transverse model,
evaluated a series of 36m long viaduct spans. On these, beam elements also represented the deck members and
used an equivalent mass density to represent the actual mass of the span as well as the additional mass of the
superimposed dead load.
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Case Study
Continuous span modelling
To analyse the dynamic behaviour of numerous two and
three-span continuous viaduct structures 3D models were
developed from the initial 2D transverse model and used
a very similar modelling methodology (see the image
right).
To allow for torsional effects the deck was modelled using
thick shell elements. Whilst the deck profile varies in
thickness along its length, for modelling purposes an
average thickness was calculated for each part of the
deck and mass density variations along the span were
used to ensure that the mass of the deck was correctly
distributed for the seismic assessment.
Three simply-supported structures each side of the two or
three-span continuous structure were also included in
each model to reduce boundary effects, something
additionally verified to be conservative.
3D modelling concept (2D modelling similar)
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Case Study
3D modelling of a multi-span continuous section of viaduct in LUSAS
(with three additional simply supported spans included each side of the continuous structures)
Seismic analysis
For each 3D model an eigenvalue analysis produced the first 30 mode shapes. These were then checked to ensure
the percentage of mass excited in each direction was not less than the 95% required by the AASHTO LRFD code.
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Case Study
By using the LUSAS Interactive Modal Dynamics (IMD) facility, and specifying a CQC combination using the
AASHTO response spectra and relevant damping values, the maximum bending moments at the base of all piers for
the response spectra were obtained. Results from the IMD analysis were ultimately combined with modified BS5400
load combinations to give final design values. From these, pile and pier reinforcement was designed and curtailed to
suit specific force moment envelopes generated for each support.
Typical deformed viaduct plot from a LUSAS Interactive Modal Dynamics analysis.
Validation of results
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Case Study
LUSAS output was validated using spreadsheet calculation via the SRSS method and also by an additional
equivalent static analysis. Manuela Chiarello, an engineer on the project, explains: "For each model we saw how
much each mode was participating to the total response of the structure and ran an SRSS analysis using a
spreadsheet for just the dominant modes." She continues: "After that we used the frequency of the dominant mode
and ran an equivalent static analysis using the corresponding acceleration from the AASHTO response spectrum".
Whilst these methods should give close and conservative results respectively in relation to the IMD results they
served to help confirm results and highlight any modelling anomalies.
David A Smith, Regional Head of Bridge Engineering at Atkins said: "We had to solve several design and
programme challenges on this project to enable the viaduct substructure and decks to be erected to an agreed
construction schedule." He continued: "Ultimately the use of precast concrete elements for the deck and for key
substructure components, and the use of LUSAS software to assist with our design process, all helped to ensure that
the viaduct construction progressed as planned.
Official opening
The Red Line was officially opened on 9 September 2009 by His Highness Shaikh Mohammad Bin Rashid Al
Maktoum, Vice-President and Prime Minister of the UAE and Ruler of Dubai. Ten key stations out of a total of 29
were opened to passengers initially, with the remaining stations being opened in batches over the following few
months. All Red Line stations are set to be operational by February 2010. The Green Line is scheduled to be opened
in June 2010.
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Case Study
"The use of precast concrete elements for the deck and for key substructure components,
and the use of LUSAS software to assist with our design process, all helped to ensure that
the viaduct construction progressed as planned."
David A Smith, Regional Head of Bridge Engineering, Atkins
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