The design of Padma Multipurpose Bridge - Bangladesh Group of ...

More documents

Recommendations

Info

Figure 3. MIDAS model of an extradosed bridge 3.2 Alternative bridge forms for two-level bridge Alternative concrete deck 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 all cases a two-level structure was adopted as there are significant advantages over the single level structure: - Separate highway and railway envelopes enable improved operation, inspection and emergency evacuation procedures 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 section), the length of the railway approach viaducts can be minimized. - Construction cost – a two-level structure is more efficient, with a much reduced overall width of the structure. Table 1. Comparison of Alternative Bridge Forms Bridge Form Advantages Disadvantages Extradosed Truss Bridge Concrete Truss structure enables significant weight savings over box girder solutions. Use of stay cables increases potential span lengths. Truss connection details would be difficult to construct leading to a longer construction period and additional cost. Heavier than steel deck solutions. Twin Box Girder with the Railway Carried by an In Situ Concrete Slab Spanning between the Boxes Steel Truss Bridge with Composite Concrete Top Slab Straightforward erection method, similar to other major bridges in Bangladesh. Steel truss is the lightest option, leading to a reduced number of piles and lowest overall cost. Truss is rigid and does not deflect excessively under rail loading. Heavy girder leads to increased demand on the foundations and shorter span lengths. Increased cost for foundations and deck due to additional weight Completely enclosed railway is a potential safety hazard. Steelwork will require repainting at regular intervals 161
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 studies were conducted on this option to determine the optimum span length. Overall deck weight and foundation loads were compared for three span lengths: 120m, 150m and 180m. From this data a construction cost was estimated for each span length with the optimum span length found to be 150m. The conclusion of the studies carried out on 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 bridge. 3.3 Foundation form In conjunction with these studies, further analysis has been carried out on the optimum form for the foundations (Sham, Yu & De Silva,2010). Two types of pile were investigated: - Large diameter (3m) raking steel tubular piles and - 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 lateral loads are resisted as axial forces in the steel piles. For the concrete bored piles the lateral loads are resisted by the flexural capacity of the piles. The very large bending moments generated by a seismic event dictated that insufficient flexural capacity could be generated by reinforcement alone, a permanent steel casing would required to enhance the capacity down to 10m below the riverbed level, which for a 100- year scour event would be -61mPWD. It would also be necessary to have more than fifteen 3.0m 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 and hence the preferred solution was recommended to the raking steel tubular piles. 4 METHODOLOGY OF SEISMIC DESIGN 4.1 Analytical method and model A 3-dimensional non-linear time history dynamic analysis has been performed for the Main Bridge to determine the impact on the structure of seismic action. For the plan alignment of the main bridge, the subtended angle is less than 18 degree (the radius is 3000m and one module of the Main Bridge is 900m) and consequently the structure may be modelled as a straight line in plan. The behaviour of the bridge is complex due to its height (120m when the effects of scour are considered) 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, has been adopted for carrying out the design. This model is divided into two parts, the structure and the free field soil. The interactions between the structure and the free field are 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 has been carried out beforehand by program SHAKE. Subsequently, a 3-dimensional dynamic analysis has been carried out using the equivalent shear modulus and effective damping as input data. Figure 4. Modified Penzien Model and Results for Raking Pile Foundations 162
Page 1 and 2: IABSE-JSCE Joint Conference on Adva
Page 3: For such a major river crossing a s
Page 7 and 8: Figure 6. Typical cross section of
Page 9 and 10: ers being modelled discretely. The

The design of Padma Multipurpose Bridge - Bangladesh Group of ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?