Padma Multipurpose Bridge Project

site for the construction of the proposed bridge. For Mawa-Janjira, the traffic forecast was much highercompared Paturia-Goalundo. Similarly, the rate of return was also in favor of Mawa site. Despitepotentially larger social impacts in terms of population displacement at the Mawa-Janjira site, the sitewas favored due to higher bankline stability, based on a temporal study of riverine system and banklinechanges in the floodplain. It was considered that the impact could be minimized through adoptingoptions such as adjustment of technical design and consultation with the affected communities. TheGovernment of Bangladesh (GOB) approved the recommendation of the Study Team. So, Mawa-Janjirawas confirmed as the location for the future bridge construction.6.4.2 Type of Bridge17. The FS considered a range of superstructures with span lengths between 100m and 250m andconcluded that concrete superstructures are generally of lower construction cost. Consequently threesuperstructure forms comprising concrete were further considered:Prestressed concrete box girder;Prestressed concrete extradosed girder;Prestressed concrete cable-stayed girder18. The optimal superstructure and span length in FS were determined on the basis of the highwayloadings – the result being a 180m span concrete extradosed girder. This design was then modified toinclude the railway loading.6.5 Alternative Analysis during Detailed Design6.5.1 Design of Main Bridge19. For a combined road and rail bridge, two options exist for how the road and rail can beaccommodated. The FS proposed to combine both road and rail on the same upper deck level of aconcrete box girder. In the design stage, other possible option, i.e. to accommodate rail and road on twolevels (upper and lower), of a bridge has been studied. Hence, the proposed bridge type in the FS(single level bridge - extradosed concrete girder) was further compared with the following four possibleoptions during initial stages of the detailed design.Option 1: Single level bridge - variable depth concrete twin cell box girder;Option 2: Two level bridge - steel truss acting compositely with concrete upper deck;Option 3: Two level bridge - variable depth concrete twin-box girders built precastsegmental and central railway envelop built in situ, with sheltered utility envelopes;Option 4: two level bridges - constant depth single cell concrete box girder with side strutssupporting thin cantilever wings, all precast segmental, with exposed utility envelopes.20. In the interim scheme design, the Consultant studied all the above options and found that concretebridges are generally more expensive for same lengths of spans and requires more construction time.Hence a two level steel truss bridge is recommended in the interim scheme design.21. In the scheme design, two-level steel truss bridge was considered for further analysis forcomparison with FS option, while offering substantiating information on the basis of the schemeselection. Specifically, it addresses:The extradosed concrete box girder scheme developed in the Feasibility Study – description ofthe design criteria adopted at that time, basis for selection of the preferred concept,consideration of rail and satisfaction of the rail design requirements;Subsequent development of the Design Criteria in the detail design process – particularly theincrease in highway and rail loadings;6-10