At the Padma Bridge site in Bangladesh, AECOM used
state-of-the-art technology and innovative disaster prevention and mitigation
solutions to tackle some severe challenges. At 6.15 kilometers (3.8 miles) in
length, the Padma Bridge is a landmark structure and one of the longest river
crossings in the world. The Padma River is the third largest river in the
world, and has the largest volume of sediment transport. During monsoon
seasons, the Padma River becomes fast flowing and is susceptible to deep scour,
requiring deep-pile foundations for bridge stability. The Padma Bridge site is
also in an area of considerable seismic activity, resulting in significant
earthquake forces being exerted on the bridge. This combination, together with
other forces of nature, posed a unique challenge. The multipurpose Padma Bridge
detailed design project has been successfully completed. AECOM developed
alternative concrete deck forms, including an extradosed concrete truss bridge,
a concrete girder bridge and a steel truss bridge. In all cases, a two-level
structure was chosen, having significant advantages over a single level
structure. These included segregated highway and railway envelopes to offer
enhanced safety, improved operation, inspection, maintenance, and emergency evacuation
procedures, as well as efficient provisions for utilities. With the railway in
the lower deck, the structural depth beneath the railway is reduced, allowing
the lengths of the railway approach viaducts for tie-in at the north and south
banks to be minimized. With a two-level structure the construction cost is
reduced, making the structure more efficient. 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
efficient with the lightest deck. Further details of this option were developed
to determine the optimum span length. Total deck weight and foundation loads
were compared for span lengths of 120 meters, 150 meters and 180 meters (394
feet, 492 feet and 591 feet, respectively). From this data, a construction cost
was estimated for each span length with the optimum span being 150 meters. In
conclusion, the most economic and appropriate form for the bridge was found to
be the steel truss bridge with a concrete top slab acting compositely. The
multipurpose bridge also has many utilities built into it, including a gas
pipeline, telecommunications and a high-voltage power transmission line.
Additionally, it has emergency access points in order to facilitate evacuation
of a train on the lower deck.A detailed study of seismic hazard at the site was
performed to determine suitable seismic parameters for use in the design. Two
levels of seismic hazard were adopted: Operating Level Earthquake and
Contingency Level Earthquake. Operating Level Earthquake has a return period of
100 years with a 65 percent probability of being exceeded during that period.
Contingency Level Earthquake has a return period of 475 years with a 20 percent
probability of being exceeded during a 100-year bridge life period. Any damage
sustained from such an earthquake would be easily detectable and capable of
repair without demolition or component replacement. In conjunction with these
investigations, AECOM carried out further analysis to determine the optimum
foundation design, and two pile types were investigated; large diameter (3
meter; 10 feet) raking steel tubular piles and large diameter cast-in-situ
concrete bored piles. Raking piles were more efficient in resisting lateral
loads resulting from earthquake motions. This type of load is resisted as axial
force in the steel piles, while the lateral load is resisted by the flexural
capacity of the piles for the concrete bored piles. The very large bending
moments generated by a seismic event dictated that insufficient flexural
capacity could be created by reinforcement alone, and a permanent steel casing
would be required to enhance the capacity down to 10 meters (33 feet) below the
riverbed level, which for a 100-year scour event would be -61m PWD. It would
also be necessary to have more than fifteen 3-meter (10 feet) diameter vertical
concrete piles, compared to eight raking steel tubular piles. The large number
of piles increased the weight of the pile cap and also the local scour. All of
these factors had an adverse effect on the cost and constructability of the
foundations, therefore the preferred solution was recommended as being the
raking steel tubular piles. The behavior of the bridge is complex due to its
height, which is 120 meters (394 feet) when the effects of scour are taken into
consideration, and the large mass of the superstructure, pile caps and piles. A
three-dimensional non-linear time history dynamic analysis, using a modified Penzien model, was adopted. It was divided
into two parts, the structure and the free field soil. The interactions between
the structure and the free field 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 Shake analysis program. Subsequently, a three-dimensional dynamic
analysis was carried out using the equivalent shear modulus and effective
damping as input data. Ground motions were applied to the model to simulate the
earthquake case, and loads were generated in the piles and substructure
accordingly. Although other load combinations were considered, such as ship
impact and wind, these effects were not found to be critical for the
substructure, and the seismic load combination dictated the design.A further model was developed to investigate the global
behavior of the bridge. The bridge is divided into six span modules, each span
150 meters (492 feet) long, so the global analysis model examined an individual
six span module and applied different levels of scour at each pier. A scour
hole may form around an individual pier, or around two or more piers. The
global model looked at various combinations in order to determine the critical
axial, shear and bending loads on the foundations of any particular pier. Initial
studies of the bridge were based on the deck being supported by traditional
sliding bearings, with the point of fixity being the central pier of the six-span
module. To avoid the fixed pier being heavily loaded during a seismic event by
a longitudinal translation, shock transmission units were proposed at the free
piers to ensure even load distribution between the piers. But under this
system, the loads applied to the piers were still large; therefore, as part of
the value engineering process, AECOM considered alternative forms of
articulation. The original seismic design strategy was to dissipate seismic
energy through plastic hinges at the bottom of the piers. Further design
optimization identified the benefits of seismic isolation, which allows the
structure to behave elastically without damage. The application of seismic
isolation has reduced the number of piles, the size of the pile caps and the size
of the steel superstructure, resulting in a more cost effective design. Seismic
isolation bearings have been used worldwide to mitigate seismic response by
isolating structures from seismic input. They can accommodate thermal movements
with minimum resistance, but will engage under seismic excitations. In this
strategy, all primary structural members remain elastic without any damage or
plastic hinging. Isolation bearings contain three key elements: one to provide
rigidity under service loads and lateral flexibility beyond service loads, one
to provide self-centering capability, and one to provide energy dissipation.
These key elements have to be properly designed and fine-tuned to achieve
optimal seismic behavior.Analyses indicate that seismic forces can be
greatly reduced by replacing conventional pot bearings with isolation bearings.
Friction pendulum bearings utilize the characteristics of a pendulum to
lengthen the natural period of the isolated structure so as to reduce the input
of earthquake forces. 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 bridge piers significantly reduces pile loads. With
the conventional scheme of bearings and shock-transmission units, eight raking
steel piles were required for each pier; with seismic isolation this was reduced
to six, leading to a savings in foundation costs of more than 20 percent. AECOM
then further developed the design with the inclusion of seismic isolation. The
impact of the seismic isolation scheme is not limited to the substructure; the
reduced seismic loading leads to reduction in section sizes for truss members,
with an overall saving in truss steelwork of greater than 6 percent.