Sea Link’s Iconic Pylon
Basil Manoj, Senior Program Expert, Consulting Engineers Group
Bandra Worli Sea Link is an amalgam of natural beauty and man-made marvel constructed over open sea spanning across Mumbai’s coastline, which was initiated by Maharashtra Government and implemented by Maharashtra State Road Development Corporation. Designed by the renowned Bridge Engineer Mr. Seshadri Srinivasan of Dar Al-Handasah Consultants, the beauty and elegance of the iconic structure speaks volumes of the efforts taken in achieving its shape, built by joint effort of engineers from various countries like India, Egypt, China, Canada, Switzerland, Britain, Hong Kong, Thailand, Singapore, Philippines, Indonesia and Serbia. One of the main structures discussed in this article is the Pylon, which is also called as Tower that forms an integral part of cable stayed bridge.
Pylons are the most important components of the cable stayed bridge, which holds the entire main span with stay cables, which are socketed to the Pylon head and transfers the load through its foundation. Sea link has two Pylons in the main span, 128m high above pile cap, supporting the superstructure by means of four planes of stay cables in a semi-harp arrangement. Above the Pile cap, each Pylon consists of 2 legs that branches into 4 legs and continues up to the superstructure. Above the superstructure, the 4 legs are inclined towards each other, and finally merge at 70 m height from deck level and continue 30 m height in vertical direction. The unified Pylon head at the top is embedded with anchorages for holding the stay cables. This shape of Pylon complements torsional stiffness as well as gives aesthetic appearance to the cable stayed bridge. Moreover, the section of the Pylon decreases gradually with height. To enhance the aesthetics of the structure, grooves are provided in vertical and horizontal directions.
Pylon is designed as a stiff member subjected to high longitudinal moments. Designers considered various factors like wind speed, tidal effect, seismic effect, etc., while designing the structure that should withstand for 120 years. Consideration was also done for ease in construction with minimum risk using appropriate type of formwork with advanced technology to achieve the dimensions within tolerance.
Main components of Pylon are grouped as follows:
2. Lower legs from pile cap up to the bottom of deck
3. Diaphragms, adjacent segments and legs up to top of deck
4. Pier Table segments between & outside the legs
5. Upper legs above deck up to anchorage zone
6. Pylon head in anchorage zone
Consists of 52 bored cast in-situ piles of 2m diameter socketed into hard rock and encased in permanent steel liner of 16 mm thick. Piles were designed for load carrying capacity of 2000 MT. All piles were drilled from the jack-up barge using RCD rigs, and the pile length was about 25 m. A combined ‘H’ shape pilecap measuring 57m x 30m x 6m for both Pylons was cast over the piles by constructing cofferdam and tremie seal. Post-tensioning was done after the construction of Pile cap.
Pylon configuration is of inverted “Y” shape with the inclined legs oriented along the axis of the bridge constructed with cast-in-situ reinforced concrete using climbing form method. Fusion bonded epoxy coated reinforcement steel of various diameters was used for the Pylon construction. Sacrificial rebar template was used during the Pylon leg construction to guide the reinforcement cage from sagging, and to ensure that there would be no clashes with embedment.
Basil ManojDOKA Formwork for Pylon Construction
Senior Program Expert
Consulting Engineers Group, Mumbai
Automatic Doka climbing formwork was used for Pylon construction which provided efficient solution for the complex crosssections of Pylon. Also, the all-round formwork enclosure made a safe and weather-shielded working at any height. The hydraulic equipment fitted in the formwork facilitated the climbing and repositioning of large number of units. The operations of this formwork were fast so that the cycle time achieved was 3-5 days for each lift. There were 43 lifts for the entire pylon with construction joints at every 3m interval. This formwork was operated for a maximum wind speed up to 70 kmph, and when it exceeded this limit the working and climbing operations were stopped. Sometimes during monsoon, when wind speed crossed 164 kmph, the formwork system was tied to the reinforcement for safety reason. To enhance the aesthetics of the structure, imported form liners were used in the formwork to get groove finish. Custom made formwork to suit the required geometry, inserts and Post tensioning was used to cast the Diaphragms. The formwork for diaphragm and adjacent segments were supported on the staging. Post-tensioning was done after the construction of diaphragms. Pylon Head was embedded with anchorage box for socketing the stay cables. Anchorage box fabricated with high grade steel plates was placed at location to match the exact coordinate and angle of stay cable for accurately fixing the anchorages. M60 grade design mix concrete was used for Pylon construction. Concrete was produced at Batching Plant located at land and was transported through barge and was placed at location using concrete pump. To keep the alignment of all tower legs in required position during construction, struts and cross bracings were provided at various levels of tower legs as per the required and pre-calculated positions of the Pylon. To resist the cable force in the Pylon head, stressing was done locally using stress bars in both the direction.
Geometry control was achieved by an integrated set of procedures undertaken during the planning and construction stages. To ensure significant and acceptable dimensions, geometry and stress state of the Pylon, the factors like environmental and other actions that may occur during the construction were considered at the planning stage. Accurate stage-by-stage construction analysis was carried out for the pylon construction to establish the optimum construction sequence and the requirements for temporary supports, restraints, and jacking forces. The permanent reference points located on the pile cap was used to define the surveying of vertical, longitudinal and transverse reference axis. These reference point were used to position and rotation of the tower cross section at the various control points along the asconstructed tower axis. Due to complex geometry and inclination of Pylon, the coordinates in each lift were controlled through a combination of total station and prisms mounted on pylon legs. The factors like construction stage analysis and temperature were applied to derive the corrected coordinates.
As the Pylon legs were very slender and susceptible to thermal gradient deflections, care was taken to ensure that the surveys are performed in a thermally neutral state. Surveying was carried out with state-of-the-art, high accuracy, vibration tolerant electronic Total Station, with ATR (automatic target recognition) and on-board software for Free Station and Resection.
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