Drawing from its experience of building a section of the Bogibeel rail-cum-road bridge in Assam, HCC shares the innovative methodology adopted to manage the movement of trusses.
Forty-two pillars with 39 spans of 125 m and two spans of 32.75 m... construction is on full swing on the Bogibeel Bridge, a combined road and rail bridge in the Dibrugarh district of Assam that is connected to NH-37 on the south bank and NH-52 on the north. The rail approach on the south bank is connected to Chowalkhowa railway station at a distance of 5.83 km from Dibrugarh and between Sisi Borgaon and Sirpani railway station on the north bank at 382.005 km of the existing MG railway line. When completed, the 4.94-km bridge will be among the longest bridges of its kind in India and the largest across Brahmaputra River.
Strategically located to enhance India's ability to transport troops and supplies to its border in Arunachal Pradesh, the double-decked bridge has two railway tracks on the lower deck and a three-lane road on the upper deck flanked by footpaths on each side. Highway alignment will meet the top level of the road deck of the bridge suitably beyond the abutments while the rail alignment will continue in the same line and level.
Being built by HCC, the bridge stretches across the 'moving ocean', as Brahmaputra River is often called, and is the first fully welded steel bridge of the Indian Railways. Constructing the two-tier bridge was certainly not easy. The company was faced with the unique challenge of how to place the 125-m, fully welded steel spans, each weighing 1,700 tonne, on the pillars or columns embedded in the riverbed. On-site engineers had two choices: To lift the spans with floating cranes or erect them with a launching truss. After weighing the pros and cons, the HCC team came up with the solution of pulling the steel trusses with two 400-tonne hydraulic jacks installed at Pier 1, 11, 21 and 31 with the help of strand jacks and winches on the pillars. The trusses slide over the launching bearing with the help of sliding plates moving them towards the desired position. To limit the required launching forces, the superstructure is pulled in four launching segments of 10 spans each. Thus, the superstructure is pulled over the pier cap just like a train of 10 spans totalling around 1,250 m in length and weighing 17,000 MT.
This eliminated the need to enter the river, which was often turbulent during monsoons. Besides, it also ensured safer working conditions, precluded the mobilisation of a giant setup on either side of the river with a width of 4.8 km, and accelerated the pace of the project.
While sliding this train of 10 spans, the project team encountered a new challenge. The differential heating owing to the daytime sun was causing the truss train to bend or show horizontal lateral movement. Although the bending was to the tune of 1-5 mm per span, the resultant shifting of the entire train of span was around 50 mm. With this, the first span (attached with a nose) could have missed the desired position on the sliding jack and there was also the threat of a few spans moving out of the sliding plates. To overcome this challenge, the HCC team installed horizontal movement arresters at each sliding jack. Second, to minimise the temperature effect, the last part of the movement of truss trains over the sliding bearing was carried out at night.
Horizontal movement arresters
After the truss train is fully launched into the final position, the process of span lowering is started. At this position, all 10 spans are fully welded together. Before lowering, the span is fully supported on temporary supports on the pier cap. When the span is in the final position, the top and bottom chords at the joint are cut diagonally so that it can be lowered without interfering with the rest of the truss train. At each end, the span is supported on two temporary supports and two jacks. The jacks are fully stroked before initial placement and then lowered till the temporary supports are engaged. Then, the jacks are freed and one layer of the packing below the jack is removed. The jacks are again placed with a little stroke, they engage the span and one layer of packing is removed from the temporary supports. This process is repeated to lower the span on permanent bearings.
Location in Seismic Zone V
In India, Zone V has historically proven to be the most vulnerable to earthquakes with magnitudes in excess of 7.0. To offer good stability to the heavy spans (1,700 MT), they are provided with seismic restrainers. This arrangement works like a male and female connection with the female pockets embedded onto piers and the male parts arranged on the span. For precise lowering of the span so that the male joint fits into the female connections before final lowering of span on the permanent bearings, the HCC team devised a unique circular slider frame made of stainless steel with jack arrangement. With this, the exact movement to fit the seismic restrainers and final lowering of the heavy spans on permanent bearings were achieved at Bogibeel rail-cum-road bridge.
As mentioned above, this is the first bridge of the Indian Railways to be totally steel welded - a total of 70,000 MT of steel (equivalent to about 10 Eiffel Towers) was used to complete the superstructure of the project and the total length of welding carried out was about 12,800 km, equivalent to circling one-and-a-half times around the moon!
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