Bridge Engineering
Design of the Padma road and rail bridge,
Bangladesh
Sham
ice | proceedings
Proceedings of the Institution of Civil Engineers
http://dx.doi.org/10.1680/bren.14.00029
Paper 1400029
Received 24/06/2014
Accepted 08/01/2015
Keywords: bridges/seismic engineering/steel structures
ICE Publishing: All rights reserved
Design of the Padma road
and rail bridge, Bangladesh
S. H. Robin Sham BSc, PhD, DIC, CEng, FCGI, FRSA, FICE, FHKIE
Global Long Span and Specialty Bridges Director, AECOM, Shatin, Hong
Kong
The 6?15-km-long Padma road and rail bridge will become a landmark structure in Bangladesh and one of the largest
river crossings in the world. The design encountered significant engineering challenges, particularly from the hostile
site conditions and the merciless forces of nature. During the monsoon season the Padma River becomes fast flowing,
and is susceptible to deep scour, demanding deep, piled foundations. The bridge site is also in an area of considerable
seismic activity, leading to significant seismic loads being exerted on the structure. In the design, extensive
engineering studies were conducted, advanced computational analyses were employed and innovative engineering
solutions were developed to ensure that the bridge will be able to survive the challenges of nature in its long design
working life. The project has accumulated a significant body of knowledge in seismic-resilient and scour-tolerant
design, and it has advanced understanding of bridge behaviour in conditions of severe earthquake and deep riverbed
scour.
1.
Introduction
1.1
The significance of the project
Bangladesh is a nation in a region stricken by poverty, famine,
flooding, earthquakes and natural disasters beyond description. The light would have gone out on that nation had it not
been for the dignity and perseverance of its people. However,
dignity and perseverance have to be supported by vision and
strategic planning. The construction of Padma Multipurpose
Bridge, a top-priority project in Bangladesh, is exemplary of
that vision and strategic planning. The 6?15-km-long river
crossing will replace an existing hazardous ferry link and a
saturated make-shift roadwork. The bridge will stimulate the
economic growth of Bangladesh, increasing gross domestic
product (GDP) by 1?2%. It will also provide a reliable and
relatively fast link to the poverty-stricken south west, increasing its GDP by 2?3%. The Padma Bridge is a two-level,
combined rail–road river crossing, the construction cost of
which was estimated at approximately US $2?97 billion. When
the freight railway is in operation, it will connect to the transAsian railway route, contributing to the multimodal international transport network and enabling cargo movement between
India and the container ports at Chittagong on the south coast
of Bangladesh. The project is a most compelling example of
poverty relief and disaster prevention, as recurring catastrophic
events around the world are a poignant reminder that nature can
be destructive to infrastructure. When completed, the Padma
Bridge will become a landmark in Bangladesh. Never in a single
campaign has the present author been so convinced of the
project’s significance to the people and the nation involved
(Sham, 2013).
Some of the extreme difficulties faced by the design and
construction are deep-rooted in the characteristics of the
Padma River. The Padma River is the third largest river in the
world but it has the most significant sediment transport and
that poses recalcitrant problems in the design and construction
of a bridge crossing. The dramatically changing nature of the
river during the monsoon season accelerates the flow rate
and causes major fluctuations in riverbed level; threatening to
undermine the bridge foundations. At the crossing point of the
bridge the river has an overall width in excess of 6 km. Both for
the protection of the bridge and for the disaster prevention of the
connecting highway network, extensive river training work is
required. The bridge is also located in a region of strong seismic
activity, which when combined with the deep riverbed scour leads
to a very onerous design condition. Effectively an earthquake
could strike a piled foundation at its most vulnerable, when the
piles have lost substantial embedment as a result of deep scour.
(Sham, 2012).
In January 2009, The Bangladesh Bridge Authority appointed
AECOM as Design Consultant for the project. AECOM
carried out a rigorous review of the previous studies conducted
for the project, before investigating in detail a series of different
bridge forms. A number of feasible schemes were developed,
and the design evolved into a two-level steel truss bridge,
acting compositely with a concrete top slab. This scheme was
judged to be the most appropriate form of structure for the
project, with the highway configured in the upper deck and
the railway in the lower deck. This two-level, combined railroad bridge scheme was adopted for the detailed design which
was completed in 2010 (see Figure 1). The construction
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Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
trucks are often heavily loaded, matching the load patterns
predicted within the British standards. The Eurocodes would
have been a possibility because there was a significant body of
knowledge in their application and they had been compared to
give similar results to BS 5400. However, at the detailed design
stage of the Padma Bridge project, some of the principles in the
Eurocodes had not been scrutinised for a project of this scale
and nature outside Europe; also there was no detailed study of
the potential application of the Eurocodes in Bangladesh, let
alone development of its national application document.
Figure 1. Padma Bridge will be a two-level structure with the
highway running on the top concrete slab and the railway running
in the lower deck, between the truss planes
contract of the Padma Bridge was awarded in June 2014,
marking a significant milestone in the history of modern
bridge engineering.
1.2
A multipurpose bridge
The Padma Bridge is a multipurpose structure carrying a
highway, railway and utilities, including a gas pipeline and
telecommunications cables. The two-level bridge enables the
road, railway and utilities to be arranged in a logical manner
with safe segregation of the highway and railway, together with
good access for maintenance and inspection. The bridge is also
provided with emergency access points in order to facilitate
safe and efficient evacuation from a train on the lower deck.
2.
Design criteria
2.1
Selection of suitable design codes
At the outset of the project, an extensive study was conducted to
develop a comprehensive set of project-specific design criteria,
underpinned by international best practices for bridge design.
Detailed investigations were undertaken to establish the most
suitable set of international bridge codes, to be incorporated
into and used in conjunction with the project-specific design
criteria. The three options available were
The freight railway crossing the bridge will connect to the
Indian national railways and hence railway loading was based
on the standards in that system. In particular, the bridge was
designed to be part of a dedicated freight corridor, which
implies an even higher railway loading than usual, with a load
of 32?5 t per axle.
The design therefore was primarily in accordance with the
British standards, while the seismic loading and design were
according to a combination of Japanese and American codes.
2.2
Seismic design criteria
The Padma Bridge is sited in an area of high seismic activity and
consequently earthquakes were a critical consideration in the
design. Bangladesh University of Engineering and Technology
(BUET) carried out a site-specific seismic hazard assessment, for
determination of suitable seismic design parameters. Two levels
of seismic hazard were adopted, as described below.
& Operating level earthquake (OLE) has a return period of
100 years with a 65% probability of being exceeded during
that period. In such an earthquake the bridge will
experience a peak ground acceleration of 0?052g and shall
remain operational for all traffic after such an event.
& Contingency level earthquake (CLE) has a return period of
475 years with a 20% probability of being exceeded during
the design life of the bridge (100 years). The peak ground
acceleration for such an event is 0?144g in the dense sand at
2120 m PWD (metres above public works datum). Any
damage sustained from such an earthquake shall be easily
detectable and repairable without demolition or component
replacement.
& The British standard BS 5400: Steel concrete and composite
bridges
& The American Association of State Highway and
Transportation Officials (Aashto) load and resistance factor
design (LRFD) bridge design specifications
& The Eurocodes.
A step-by-step non-linear time history analysis was undertaken
based on five Aashto spectrum-compatible acceleration–time
histories representing the earthquake loading at the elevation
of 2120 m PWD (refer to Figure 2).
2.3
British standard BS 5400 was selected because it was judged
that its highway loading criteria most closely corresponded to
the practical situation experienced in Bangladesh, in which
2
Foundation scour
Another critical design criterion for the bridge was riverbed
scour. There are large fluctuations in the volume of water
flowing along the Padma River, causing an impact both on the
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
0.50
0.45
Japan code ground motion I
Japan code ground motion II
Japan code ground motion III
BUETI 2009
BUETIII 2009
Aashto
0.40
0.35
CS: g
0.30
0.25
0.50
0.15
0.10
0.05
0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Period: s
Figure 2. Response spectra for CLE were derived from the sitespecific study conducted by BUET in 2009 and also from the
ground motions given in the ‘Highway bridge design specification,
part V: Seismic design’ by the Japan Road Association
course of the river and on the depth of the riverbed. These
phenomena are particularly acute where the presence of bridge
pier foundations causes blockage to the hydraulic regime. As a
result extensive river training works are required around the
bridge (Neill et al., 2010) and the bridge foundations were
designed for potentially severe scour conditions.
Scour can essentially be divided into two parts
& general scour – due to the action of the river and
independent of any structure built in the river
& local scour – due to an obstruction to the flow, such as a
bridge pier and its foundation.
General scour was studied by examining the data from the
river over the past 40 years. River depth measurements were
taken on a regular basis and they provided a good indication of
how the river changes during the monsoon season. For local
scour model tests were carried out by Northwest Hydraulic
Consultants at its laboratories in Canada. Various foundation
schemes were investigated, with the potential scours varying by
over 7 m depending on the piling configuration.
For a 100-year return period, the riverbed level was determined
to be 246?7 m PWD in the regions near the river banks and
235?0 m PWD at the central region of the river. From the
experiments carried out, local scour was estimated to deepen
the scour by a further 15 m for an eight-raking-pile arrangement, and 20 m for 15 vertical piles.
2.4
Ship impact design criteria
A study of current shipping patterns in the river identified a
design vessel of 4000 DWT (deadweight tonnage) for determining the ship collision loads to be applied to the bridge. The 4000
DWT design vessel was larger than all the vessels listed in the
ship register and also the future coal-handling ships which may
transit the river. Based on the provisions in Aashto LRFD
impact forces of 23?3 MN and 11?7 MN were derived, respectively, for head-on impact and sideways impact with the bridge
substructures and foundations. The possibility of ship impact
with the bridge superstructure was studied and provisions were
made to mitigate ship impact with any vulnerable and exposed
essential utilities carried by the bridge. The provision of fenders
will be considered, either on the bridge substructures or possibly
on separate structures to absorb impact energy and reduce peak
load effects, if such options prove economic.
2.5
Combining loads and environmental effects
The load combinations given in BS 5400, Part 2 were generally
followed, but this code does not adequately address the
combination of seismic loading, ship impact and scour of the
foundations. The effect of scour was given special attention in
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Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
the design, because the nature of the Padma River is unique.
Scour can occur over prolonged periods and when infill of
scour holes subsequently occurs, the material that fills the holes
is loose and remains uncompacted for a long period after the
event. The loose material will be susceptible to liquefaction and
therefore cannot be relied upon for foundation resistance in a
seismic event.
stiffened, reducing the benefit of the stay cables and increasing
the weight of the deck. This conflicted with another constraint –
with poor ground conditions and onerous loading combinations, it was imperative to minimise the loads transferred to the
foundations, to curb the cost escalation in foundation construction, which already constituted a high proportion of the overall
cost of the bridge. Because of these severe constraints the
extradosed bridge was found not to be the preferred option.
With liquefaction of the compacted fill material being a serious
concern, scour with a 100-year return period was adopted, to
be combined with earthquake loading. In the case of ship
impact, liquefaction of the infill material is not considered a
problem, and therefore a lesser return period of 10 years for
scour was adopted. Suitable partial safety factors were adopted
to reflect the respective probability of occurrence of the events.
3.
Concept creation
3.1
Severe constraints on the initial scheme
3.2
Alternative bridge deck forms for two-level
bridge
Alternative concrete superstructure forms were investigated.
Three examples are shown in Table 1, an extradosed concrete
truss bridge, a concrete girder bridge and a steel truss bridge.
In each case a two-level structure was proposed because it has
significant advantages over the single-level structure.
& Separate highway and railway envelopes enable improved
In previous studies for the bridge a number of options for the
bridge form had been examined, with a single-level extradosed
bridge with spans of 180 m being proposed. Then, in 2009,
AECOM undertook detailed investigation of this bridge form
by extensive finite-element modelling, an analysis model for
which is illustrated in Figure 3.
The extradosed bridge investigated consisted of a concrete box
girder superstructure, supported by stay cables in order to
reduce the girder depth. The freight railway, however, posed
very tight tolerances on displacement and rotation, and in
attempting to meet these tolerances the girder would need to be
Figure 3. Global analysis model of an extradosed bridge
4
operation, inspection and emergency evacuation for the bridge.
& The maximum permissible gradient on the railway is 0?5%,
requiring long lengths of approach viaduct for the railway
to descend to ground level. By reducing the structural depth
beneath the railway (in a two-level structure the railway
runs inside the structural cross-section), the length of the
railway approach viaducts can be minimised.
& Construction cost – a two-level structure is more efficient,
with a much reduced overall width of the structure.
Analytical models were developed for each of the bridge forms
to determine member sizes and in particular the weight of the
superstructure. The steel truss bridge was found to be the most
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
Advantages
Disadvantages
Extradosed concrete truss
bridge
A truss structure enables
significant weight savings over
concrete box girder schemes
Use of cable supports will
increase potential span lengths
Truss connection nodes would
be difficult to construct,
leading to longer construction
period and additional costs
Heavier than steel deck girder
schemes
Twin-box concrete girder
with the railway carried by
a perforated beam-andslab system spanning
transversely between the
boxes
A heavy girder leads to
A straightforward erection
method, similar to that used in increased demand on the
foundations and limits the span
other major bridges in
lengths. Increased costs for
Bangladesh
foundations and deck girder
owing to additional weight
An enclosed railway is a
potential safety hazard
The steelwork will require
A steel truss is the lightest
repainting at regular intervals
girder option, leading to a
reduced number of piles
and lowest overall cost
The truss is relatively rigid and
does not deflect excessively
under railway loading
Steel truss bridge with
composite concrete top
slab
Table 1. Comparison of alternative bridge forms
efficient with the lightest deck. Further studies were conducted
on this scheme to determine the optimum span length. Overall
deck weight and foundation loads were compared for three
span length modules: 120 m, 150 m and 180 m. From these
data a construction cost was estimated for each span length
module, and the optimum span length was found to be 150 m.
The conclusion of the studies carried out on the bridge deck was
that the steel truss bridge, with a concrete top slab acting
compositely with the truss, would be the most economic and
suitable bridge form for the river crossing. The fundamental
principles are that this structural form has a relatively high
stiffness-to-mass ratio and it therefore has advantages in (a) the
control of deflections and instantaneous longitudinal gradient
under the freight railway loading, and in (b) its seismic
performance, by virtue of the reduced sprung mass to be carried
by the pier-and-piled foundation system in the event of an
earthquake. This two-level, combined rail–road bridge scheme,
comprising a steel truss superstructure acting compositely with a
concrete roadway slab, was adopted for the detailed design.
3.3
Bridge geometry
3.3.1 Carriageway and railway cross-section
A dual two-lane highway is carried on the upper deck, with a
highway design traffic speed of 100 km/h. Each carriageway
comprises two 3?5-m-wide traffic lanes with a 2?5-m shoulder
and 0?5-m shoulder adjacent to the median.
Provision was made for future addition of a single-track broad
gauge railway along the bridge, potentially with electrification. The
design rail speed is 160 km/h for passenger trains and 125 km/h for
freight trains. The rail corridor clearance is based on the fixed
structure gauge diagram of 5500 mm width and 7410 mm height to
top of rail. The clearance diagram will allow for wide rolling stock,
double-stack containers and future electrification.
3.3.2 Vertical alignment and gradients
The minimum level of the approach roadway and railway at
each end of the bridge was set at design high-water level
(DHWL) + 1?50 m 5 +8?85 m PWD.
The maximum gradient for the railway was limited to 0?5% and
this tended to govern the slope of the whole structure, including
the roadway over the entire length of the main bridge.
3.3.3 Navigation clearance
The required horizontal and vertical navigation clearances for
Padma River at the bridge site are as follows
& a minimum horizontal clearance of 76?2 m
& a minimum vertical clearance above standard high-water
5
Bridge Engineering
level (SHWL) of 18?3 m, and this required minimum value
was provided over a length of 4500 m of the main bridge
crossing.
An allowance of 0?3 m was added to the required minimum
vertical navigational clearance. The allowance was to account
for future changes to the present value of the SHWL of 5?9 m
PWD due to climatic effects. The effects of live load deflections
were considered in determining the soffit level of the bridge.
3.4
Foundation types
In parallel with these investigations, further analysis was
carried out in search of the optimum foundation scheme (Sham
et al., 2010). Two types of piled foundations were investigated
& large-diameter (3 m), raking, tubular steel piles
& large-diameter, cast-in-situ, concrete bored piles.
The raking piles were found to be more efficient in resisting
lateral loads resulting from earthquake motions. The fundamental principles are that lateral loads are resisted through axial
loads in the raked tubular steel piles, but in contrast lateral loads
are resisted by flexure in the vertical concrete bored piles. The
very large bending moments in the concrete bored piles, induced
by horizontal earthquake loads, would mean that the required
flexural capacity could not be generated by the provision of steel
reinforcement alone. A permanent steel casing would be
required to increase the bending resistance of the bored pile
down to 10 m below the riverbed level (which for a 100-year
scour event would be at 261 m PWD). It would also be
necessary to have more than 15 in number 3?0-m-diameter
vertical concrete bored piles, compared to eight raking tubular
steel piles. The large number of piles increased the weight of the
pile cap and also the local scour. The underlying phenomenon is
that, intrinsically, a group of raking pile causes less blockage to
the flow than a group of vertical piles does. The larger the
number of vertical bored piles, the higher the blockage to
the flow and hence the deeper the local scour will be. In turn, the
deeper the scour, the greater the loss of pile embedment will be
and, if that is compensated structurally by an increase in pile
size or number, further hydraulic blockage will result, causing
deepening scour. The design would be locked in a vicious circle.
All of these factors had significant effects on the viability, cost
and constructability of the piled foundations and therefore the
raking tubular steel pile scheme was found to be the preferred
solution, and it was carried forward into the detailed design.
4.
Methodology of seismic design
4.1
Analytical methods and models
A three-dimensional non-linear time history dynamic analysis
was performed for the main bridge to determine the impact of
seismic actions on the structure.
6
Design of the Padma road and
rail bridge, Bangladesh
Sham
For the plan alignment of the main bridge, the subtended angle
is less than 18 ˚ (the radius is 3000 m and a typical six-span
module of the main bridge is 900 m). Therefore the structure
was modelled as a straight line in plan.
The behaviour of the bridge is complex due to its height (120 m
when the effects of scour are taken into account) and the
substantial mass of the superstructure, pile caps and piles. The
non-linear time history dynamic analysis was based on a
modified Penzien model (see Figure 4). This model was divided
into two parts, the structure and the free-field soil. The
interactions between the structure and the free-field soil were
simulated by lateral spring links. In order to determine the
equivalent shear modulus and effective damping ratio between
each layer of the soil, free-field analysis was carried out
beforehand using the program Shake. Subsequently, a threedimensional dynamic analysis was performed using the
equivalent shear modulus and effective damping as input data.
The ground motions shown in Figure 2 were applied to the
model to simulate the earthquakes and loads were generated in
the piers and piles accordingly. Other load combinations were
explored, such as ship impact and wind, although generally
these effects were not found to be critical for the substructure
and foundation design. The seismic load combination governed the design.
A further global analysis model was developed to investigate the
global behaviour of the bridge. The bridge was divided into sixspan modules, with each span being 150 m (Figure 5). The global
analysis model examined a six-span module and simulated
different depths of scour at each pier foundation. The physical
scenarios would represent a scour hole forming around an
individual pier foundation, or a scour hole forming around two
or more pier foundations. The global model investigated various
different eventualities of scour at pier foundations, in order to
determine the critical axial load, shear and bending in the
foundation of any particular pier location.
4.2
Seismic isolation
The initial design of the bridge was based on the deck girder
being supported on its piers by traditional sliding bearings,
with the stagnant point at the central pier of a six-span
superstructure module. To avoid overloading the fixed pier due
to longitudinal displacements in an earthquake, shock transmission units were provided at the top of the free piers to
distribute the loads evenly between all piers. In this articulation
system the loads transferred to the piers and foundations were,
however, still high and therefore alternative forms of articulation were sought.
Isolation bearings mitigate seismic response by isolating a
structure from the seismic input. Isolation bearings can
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
SHWL
+5.81 M PD
Top
Midas/Civil
Post-processor
Beam diagram
5000
Moment-y
5.27960×104
4.32002×104
3.36043×104
2.40085×104
1.44127×104
4.81681×103
0.00000
–1.43749×104
–2.39707×104
–3.35665×104
–4.31624×104
–5.27582×104
Free field
THall: J1
Max: 220507
Min: 220512
File: NewDeck2T_~
Unit: kN m
Date: 09/11/2014
View Direction
x: 1.000
y: 0.000
–120 m PD
z: 0.000
Pile model details
Ground motion input
Figure 4. Modified Penzien model and results for raking pile
foundations
accommodate thermal movements with minimum resistance,
but will engage under seismic excitations. In this seismic-resilient
design strategy, all primary structural members will remain
elastic without any damage (or plastic hinging). A seismic
isolation bearing consists of components which provide rigidity
under the service loads, lateral flexibility beyond service loads,
self-centring capability and energy dissipation. These principal
elements have to be properly designed and fine-tuned to achieve
an optimal seismic behaviour.
Analyses showed that seismic forces can be greatly reduced by
replacing the conventional pot bearings with isolation bearings.
Friction pendulum bearings utilise the characteristics of a
pendulum to lengthen the natural period of the isolated structure,
900 000 (Typical module)
Pier
150 000
(Span A)
Pier
150 000
(Span B)
Pier
150 000
(Span C)
Pier
150 000
(Span D)
Pier
150 000
(Span E)
Pier
150 000
(Span F )
M.J.
M.J.
Pier
Figure 5. Typical six-span bridge module
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Bridge Engineering
Design of the Padma road and
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Single pendulum bearing
cross-section
Single pendulum motion
Maximum credible earthquake
Figure 6. Schematic diagrams of a single friction pendulum
bearing
so as to reduce the input of earthquake forces. Figure 6 provides
schematic diagrams of a single friction pendulum bearing. The
damping effect due to the sliding mechanism also helps mitigate
earthquake response. Since earthquake-induced displacements
occur primarily in the bearings, lateral loads and shaking
movements transmitted to the structure are greatly reduced.
The reduced seismic loading generated at the top of the piers
led to significantly reduced pile loads. With the conventional
scheme of pot bearings and shock transmission units, eight
raking steel piles were required for each pier, but with seismic
isolation this number of piles was reduced to six, achieving a
saving in foundation cost in excess of 20% (see Figure 7).
The advantages of the seismic isolation scheme were not,
however, limited to the piers and foundations. The reduced
seismic loading resulted in a reduction in section sizes of the
Figure 7. Piled foundation for the seismic isolation scheme: six
raking steel piles
8
truss members, with an overall saving in the truss steelwork
tonnage of more than 6%.
5.
Structural analysis of superstructure
The work drew on the art and science of computer modelling for
structural design, in that a hypothesis of the structural behaviour
was investigated by a model, or part model, especially developed
with a view to verifying the hypothesis. To this end, different
models or part models were carefully formulated to verify
different phenomena.
5.1
Bridge deck cross-section
Figure 8 depicts the typical bridge deck cross-section. The
highway is configured in the upper deck, on a concrete roadway
slab designed to act compositely with the steel truss for live load
effects. The railway is configured in the lower deck, supported
between the truss planes. A precast concrete railway slab system
is used; composite action being formed between the channelshaped slab and four longitudinal steel stringers. The railway
composite deck will include fixings for connection to an in situ
concrete pour for the track-form. The cross-section also shows
the intended locations of utilities such as a high-pressure gas
pipeline and telecommunications ducts. Walkways will also
be provided to each side of the railway for inspection and
maintenance purposes, as well as emergency evacuation routes.
The top chords, bottom chords and diagonal members of the
main trusses are fabricated in hollow steel box sections. The
plate thicknesses of the boxes vary according to the location of
the member. For thin plate thicknesses, longitudinal stiffeners
are provided to increase the efficiency of the section in resisting
compressive stresses. Box sections are also adopted for other
members in the superstructure, including the lower cross beams
and upper cross beams. The concrete roadway slab is a
prestressed member in the longitudinal direction, and a
reinforced member in the transverse direction. The longitudinal
prestressing will be carried out before the composite (shear)
connection is established, such that no additional stresses will
be induced in the steel truss members. The railway concrete
slab is of reinforced concrete construction with no prestressing.
For all steel members except the steel stringers supporting the
railway slab, the steel grade is S420M for plate thicknesses up
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
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Upper cross beam
(near pier only)
Concrete roadway slab
Main truss
Main truss
Lower cross beam
Railway steel beams
Concrete railway slab
Figure 8. Typical cross-section of the main bridge
to 40 mm and S420ML for plate thicknesses over 40 mm. For
the railway slab support girders, the steel grade is S355M.
to the longitudinal prestress being applied and the steel–concrete
composite action being achieved.
5.2
There were three models, each representing different stages in
the construction of the bridge.
M.J.
Pier
Pier
Truss lifting
frame
Connection
established
Pier
& Model A was an initial model of a simply supported span of
Pier
Lifting
vessel
Spreader
the truss without the concrete top slab, representing the
stage at which a single span of the truss is lifted into place
(see Figure 9).
Pier
Pier
Pier
M.J.
Global model
Global analysis models were developed using the Midas software
for the superstructure design. The envisaged method for superstructure erection is by whole-span lifting and placement.
Placement of the deck slab will be made after completion of
the steel truss for a six-span girder module. The self-weights of
the steel structure and concrete roadway slab are activated prior
Figure 9. Deck erection is anticipated by lifting and placing
individual spans
9
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
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bending moments and shears in the slab under highway
loading. To limit the overall thickness of the slab, transverse
ribs at 2-m centres were provided to enhance the transverse
bending capacity of the deck. Figure 12 illustrates the solid
element model.
& Model B was a second model of a complete continuous
module of the bridge without the concrete top slab,
representing the stage at which the steel trusses are connected
together but prior to the placing of the concrete roadway
slab. Figure 10 depicts progressive placement of the deck
slab after completion of the steel truss for a six-span girder
module. Application of the self-weight of the deck slabs on
the steel truss module was simulated in Model B.
& Model C was a final-stage model of a complete bridge
module including the concrete roadway slab (and composite
action).
The bridge was modelled for the tightest bridge curvature, at a
radius of 3000 m. Although the deck is curved the trusses are
straight over each span, the angular changes for deck curvature
are concentrated at the support locations.
At each pier location in the analytical model, the truss was
supported on elastic springs. Only translation stiffness was
provided in the model and there was no rotational restraint,
because the actual bridge truss is supported on seismic isolation
bearings. The stiffness of the elastic springs represented the
stiffness of the pile group. Figure 11 shows part of the global
model and the location of the support springs. The values of
spring stiffness were derived from a separate pile group analysis
and are given in Table 2.
Two further models were used in the analysis of the superstructure.
5.3
Railway deck
The railway is supported by four steel stringers acting
compositely with a concrete deck slab. A special model was
assembled, consisting of four longitudinal composite sections,
connected by transverse members and a cross beam at each end.
The cross beams are connected between the steel truss lower
chords. The composite sections comprise steel universal beams
(UB) – 914 6 419 6 388 mm. The concrete slab is 1295 mm
wide and 200 mm thick.
Beam-end releases were introduced at the connection points of
the composite railway decks and cross beams, so that no
hogging moment would be induced at these points in the
composite decks, and a shear connection between the decks and
cross beams could be simulated (see Figure 13). Three different
types of railway live loads were applied on the composite railway
deck section to represent different wheel patterns. Since the
bridge is curved on plan, there exists a maximum lateral shift of
500 mm between the railway alignment and the composite deck
centreline where the horizontal curvature is at its tightest.
Therefore, for each load type, two cases were examined: one for
the railway load applied along the deck centreline, and the other
for the same load but accounting for the lateral shift of 500 mm.
& Plate element model of roadway slab: the concrete roadway
slab is prestressed longitudinally to ensure there is no
significant cracking of the slab over the piers when acting
compositely with the steel truss. A finite-element model
consisting of plate elements was developed to investigate the
stresses in the slab, wherein the steelwork and slab members
were modelled discretely. The model was also used to
examine the time-dependent effects in the concrete and the
effect of transverse wind acting on the slab.
& Solid element model of roadway slab: a separate finiteelement model consisting of solid elements was developed to
represent the concrete roadway slab to determine the critical
6.
Superstructure design
In the superstructure, two main truss planes, transversely
spaced at 12 m, form the major structural component. At the
lower deck level, transverse lower cross beams at 18?75 m
spacing connect the two bottom chords and form a platform
for the railway track. At the upper deck level, a concrete slab
approximately 22 m wide is placed on top of the top chords
and carries the highway carriageways.
Longitudinally, the main truss is in the form of a warren truss.
To further increase the member stiffness, the concrete roadway
900 000 (Typical module)
Pier
M.J.
Truss lifting
frame 150 000
(Span A)
Pier
150 000
(Span B)
Pier
150 000
(Span C)
Figure 10. Progressive placement of the deck slab after completion
of the steel truss for a six-span module
10
Pier
150 000
(Span D)
Pier
150 000
(Span E)
Pier
150 000
(Span F)
M.J.
Pier
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
Spring support
Figure 11. Extract of analytical model of superstructure
Figure 12. Finite-element model of deck slab with 45 units of HB
vehicle load applied
lower non-support node
lower end node
lower support node
upper node without cross beam
upper node with cross beam.
slab is connected to the top chord by shear stud connections
so that they can act compositely together. The railway deck
comprises longitudinal steel beams spanning between lower
cross beams and a reinforced concrete railway slab, which is
also compositely connected to the beams.
&
&
&
&
&
6.1
The finite-element models of various truss nodes were assembled
in plate elements. The design forces for the nodes were extracted
from the global analysis model under different load combinations. For each node, five BS 5400 load combinations were
investigated.
Design of principal structural steel members
The design of the main structural steel members forming the
truss was a direct extract of stresses from the structural analysis
models for the bottom chord, diagonal and crossbeam members.
For the upper chord the section steel box acts compositely with
the deck slab. For the composite top chords the design was subdivided into three parts: a longitudinal stress check of the
concrete roadway slab, a longitudinal stress check of the steel
sections and a shear connection design.
The top chord, bottom chord and diagonal members of the main
truss are in the form of a hollow steel box. Plate thicknesses of
the boxes vary depending on the location of the member. For
thin plate thicknesses, longitudinal stiffeners were used to
increase the efficiency of the section in resisting compressive
stresses. Box sections were also adopted for other members,
including the lower cross beams and upper cross beams.
Table 3 presents the member sizes of the different superstructure member types.
6.2
Design of structural nodes
The detailed designs of truss nodes were carried out for five
types of nodes at the intersection points of truss members
Stiffness
Transverse
Longitudinal
Vertical
Spring constant
(scour depth at
220 m PWD): kN/mm
Spring constant
(scour depth at
262 m PWD): kN/mm
52
19
33
12
1700
Table 2. Spring stiffness for analytical model of superstructure
The steel plates of the node were divided according to the
configuration of the diaphragms. Each plate was designed for
adequacy in strength and stability. The maximum stresses of
each element in the plate were chosen for design against
buckling.
6.3
Concrete roadway slab design
The concrete roadway slab is designed as a reinforced concrete
member in the transverse direction, and as a prestressed
concrete member in the longitudinal direction. The longitudinal prestress will be carried out before the composite
connection is established. The railway concrete slab is of
reinforced concrete construction with no prestress. The design
was based on the use of precast concrete slabs, typically of
panel dimensions 2 m (along the bridge longitudinal direction)
by 20 m (transverse to the bridge), made composite with the
steel superstructure through shear connectors. The envisaged
construction sequence assumed that the self-weights of the steel
structure and concrete roadway slab are activated, before the
longitudinal prestress is applied and the steel–concrete
composite action is achieved. Prestressing before composite
action is established will ensure no additional stresses will be
generated in the steel truss members. Also, the design forces
acting on the roadway slabs derived only from superimposed
dead load and live load.
7.
Conclusions
The Padma Multipurpose Bridge will emerge as an inspiring
landmark in Bangladesh. It will provide a vital link in the
11
Bridge Engineering
Design of the Padma road and
rail bridge, Bangladesh
Sham
Composite railway deck
Shear connection at composite railway
deck-cross beam connection points,
no hogging moment
Cross beam
Cross beam
Figure 13. Modelling of railway composite deck and its connection
to cross girders between truss lower chords
Member type
Box width (horizontal dimension): mm
Box depth (vertical dimension): mm
Plate thickness: mm
1600
1600
1550
2800
1200
1500
1600
1100
1600
1475
25–70
30–70
20–70
25–50
25–70
Top chord
Bottom chord
Diagonal
Lower cross beam
Upper cross beam
Table 3. Superstructure member sizes
transport infrastructure in that geographical region, and will also
mark a significant milestone in modern bridge engineering. The
project provides a compelling example of civil engineering
ingenuity addressing recalcitrant problems in a region of extreme
environmental and natural hazards. The risks of deep scour
aggravated by severe seismic activity were tackled by state-ofthe-art bridge technology and fundamental engineering principles. Bridge design was integrated with construction to ensure
the bridge will not only serve the Bangladesh nation of today,
but also its future generations. The design drew on the art and
science of computer modelling of structures, in that a hypothesis
of the structural behaviour was investigated by a model, or partmodel, especially developed with a view to verifying the
hypothesis. The work has accumulated a significant body of
knowledge in seismic-resilient and scour-tolerant design, and it
has advanced understanding of bridge behaviour in conditions
of severe earthquake and deep riverbed scour.
Acknowledgements
The author wishes to thank the Bangladesh Bridge Authority
for permission to publish this paper. The author would also
like to thank staff members of the Aecom Long Span and
Specialty Bridge Group, for all the work that brought the
design of the Padma Bridge into fruition.
REFERENCES
Neill CR, Oberhagemann K, McLean D and Ferdous QM (2010)
River training works for Padma Multipurpose Bridge,
Bangladesh. Proceedings of IABSE–JSCE Conference on
12
Advances in Bridge Engineering (Amin AFMS, Okui Y and
Bhuiyan AR (eds)), Dhaka, Bangladesh, pp. 441–448.
Sham SHR, Yu GX and De Silva S (2010) Foundation design
methodology for Padma main bridge. Proceedings of
IABSE–JSCE Conference on Advances in Bridge
Engineering (Amin AFMS, Okui Y and Bhuiyan AR (eds)),
Dhaka, Bangladesh, pp. 417–426.
Sham SHR (2012) Padma progress. Bridge Design and
Engineering Issue 69(Fourth Quarter): 34–36.
Sham SHR (2013) In pursuit of new frontiers in bridge
engineering. In Reflections … of the Greatest Bridge
Engineers and Architects of the 20th and 21st Centuries
(Leech T (ed.)). Engineers Society of Western
Pennsylvania, Pittsburgh, PA, USA, pp. 69–76.
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