Deck Erection And Pre-Stressing Of Extra – Dosed Balanced Cantilever Span

A Case Study of High Level Ganga Bridge Connecting N.H.-19 and N.H.-30, Bihar, INDIA
Anand Shah, Faculty of Technology, CEPT University, Ahmedabad

General Project Information
1. Project – High level bridge across Ganga River connecting N.H.-30 to N.H.-19
2. Contractor – S.P.Singla Constructions Pvt. Ltd.
3. Project Commencement Date – July 2010
4. Expected Project Completion Date – December 2016

The Extra dosed balance cantilever span is 1920 metres long and has a straight alignment without any longitudinal gradient and vertical curve. It is 20.50 metres wide, is a dual carriageway which can accommodate four lanes of traffic densities. The 20.50 metres wide bridge consists of 7.5 metres of carriageway and 1.5 metre of footpath having a transverse slope of 2.5 % and 1% on both the sides respectively.

Deck erection work is comprised of five major activities:

Pre- Cast segment construction; segment transport; cantilever construction equipment handling; deck geometry control and deck pre-stressing. These are explained in detail below.

Precast Segment Construction
Precast segment construction is carried out by match cast technique, in which new segment is cast along the side of preceding segment; this arrangement of casting segment allows proper geometry control during segment erection and deck construction. The segment casting process is carried out by long line bed method where the bulkhead is movable and multiple formwork sets are required.

The construction of pre-cast segment starts with preparation of the reinforcement cage. Reinforcement cage is prepared with the help of a jig, which is fabricated as per segment dimensions. The reinforcement cage is sand blasted in case excessive rusting is observed.

Before placing the outer shutter, wax based compound is applied on the match cast face of the adjacent segment to allow proper de-bounding. The total length of each casting bed is 60 metres and the casting yard has an area of 7512 m2. Outer shutters move along the casting bed with the help of a trolley mounted on a rail. Bulkheads and inner shutters have been fabricated as per the post tension duct profile and segment dimensions respectively. Level check and alignment of bulkhead is critical activityas the duct locater on bulkhead is to be placed with respect to the drawing dimensions. These movable bulkheads are placed using a 20-ton capacity goliath crane gantry. Lastly before concreting, guide pipe which acts as a connector for stay cable, connecting it to the anchorage end and post tension ducts which act as guides for high strength post tensioning tendons are installed.

All the pre-cast segments are of M-60 grade concrete having a volume of ranging between 35m3– 40 m3. Concreting is done via concrete boom placer using two transit mixers of 6 m3 capacity at a time. The entire process consumes time of around 8 hours. De-bounding is done with the assistance of120-ton gantry where the segment is transferred from casting yard to stacking yard. Certain precautions are supposed to be taken, which includes checking finish of shear keys and level of deck bed with respect to casting bed level.

Segment Transport and Management

In bridge construction works, execution work gets complicated because of water works and in such cases proper site management and handling is extremely important. The segment is brought to the load out jetty from the casting yard with the assistance of a 200-metric ton trailer, which is designed as per segment dimensions and placed there. The heaviest segment has a weight of 105 ton. The lifting goliath crane gantry of 125-ton capacity lifts the segment and transfers it to the float barge. The segment is positioned and anchored with precision on the barge so that no eccentric loading occurs and possibility of segment and barge overturning can be avoided. A frame is made on the barge on which the segment is positioned, which keeps the segment intact and sturdy.

The river being shallow at certain locations, tug boat which is generally used for pulling the barge gets repeatedly stuck in sand. To avoid such possibilities, the responsible agency used excavator to drive the float barge in river. Excavator is placed on the barge and is driven by grabbing the sand on the way and thus acting as a steering.

Cantilevel Construction Equipment Handling
While the segment placement and shifting works are going on, the C.C.E. (Cantilever Construction Equipment) is auto launched.Auto launching follows the following steps:
1. Freeing of rail beam (rail anchoring beam is distressed under a pressure of 50 kg/cm2 and removed)
2. Shifting of the rail beam ahead by double acting hydraulic jack
3. Anchoring the rail beam to new position (a stress of 50kg/cm2 is applied to stress bar used for anchoring).
4. Freeing the gantry girder
5. Launching girder to required position using double acting hydraulic jack.
6. Anchoring the girder to new positon

Thelifting frame of the C.C.E. is lowered down and is fixed with the segment using locking pins. After completing the preliminary checks which includes checking alignment of segment and stress pressure of lifting bolts, the segment is lifted with equal lift on both the ends so that the segment remains completely levelled in horizontal plane. The C.C.E. is mechanically operated and lifting is monitored with the help of a crab hoist. C.C.E. has a load taking capacity of 300 ton and can do maximum construction up to a length of 8 metres.

Deck Geometry Control
After lifting the segment, it is aligned and matched with the previous segment as per the match casted shear keys. The process is called dry matching.Thixotropic epoxy glue in the ratio of 1:2 (base: hardener) is used in thickness of 1.5mm to glue the two segments. The setting time of glue is around two hours.

Deck Pre-stressing
The segments are held together in absence of post tensioning by temporary stressing. It done using 36 mm diameter stressing bars anchored with help of anchor beams. Temporary stressing is carried out for both deck slab as well as soffit slab with an average pressure of 400kg/cm2. Horizontal stressing keeps the segments together while vertical stressing prevents the anchor beam from toppling due to horizontal stress.

The post-tensioning process takes place once the threading of high tensile strands is done. The Post-tensioning is double end double stroke stressing type. During post tensioning a bearing collar plate is attached to the anchor head and live wedges are used to prevent slippage of cables. Collar tightening bolt is used to tighten the bearing collar plate and lock off plate is used to lock the entire assembly. Jack is inserted and fixed which is followed by fixing of pulling plate and at last master wedges are inserted.The entire process can be briefed into majorly three steps, which are: stressing operation, lock off operation and jack retraction.

Pre-stressing of deck using stay cable is done via ‘strand by strand’ installation method. Strand by Strand installation method is executed based on the following principle:

1. The installation of 1st strand per the predefined load induces a new state of equilibrium in the newly connected members of the pylon and deck. As reaction to the applied force, deck and pylon deflects accordingly. At the same time, the strand gets a defined sag and elongation at the corresponding force.

2. After installation and stressing of the second strand, mainly two things happen:

a. The structure deflects further due to additional load on the second strand.
b. The stiffness of the cables increases, and as consequence, the force level in the first strand decreases.
The entire stay system has a total of three components: Anchorage end assembly, saddle end assembly and stay pipe assembly. During stressing, the anchorage end which is situated below the deck slab is considered as a live end, i.e. from this end, stressing takes place and pylon saddle end is a dead end. Stressing is done simultaneously from both the anchorage ends, so that no eccentric forces are generated in the bridge deck and it is compressed equally.