CABLE STAY BRIDGE CONSTRUCTION

By-BALKRISHAN KUSHWAH

CABLE-STAYED BRIDGE construction

It is a bridge in which the weight of the deck is supported by a number of cables running directly to one or more towers. These bridges are very economical for long spans (over 100 meters), especially when access to the construction site is restricted.

Water work and coffer dam:-Construction in water generally takes place in between 1st July to 31st October because during the period fewer organisms are present around the river. Water work includes construction of foundation that is constructing piles and pile cap at the position where pylon of tower is to be constructed. For this, focus is made on installation of piles for temporary work bridge and also on getting sheet piles for forming coffer dams and scour protection around them. Sheet piles are installed with the help of pile driving hammers and coffer dam is formed, that is an island in the river with no fishes and is isolated from render of the flowing water and so that work can be done in any season.

Shows cofferdam

Foundation construction: - After the construction of coffer dam, drilling is done to 150` to 170` and temporary casing or drilled shafts of 10` diameter are installed with help of oscillator placed over temporary platform. Now, in these casing cage made of rebar (about 60 tones of steel) is inserted with the help of two big cranes. Rebar stands for reinforcing bars of steel which acts as back bone of concrete and makes structure sound. Finally concrete is poured into the casing with the help of pipe and pump trucks to have piles. Over the piles so constructed pile cap is formed above which tower/pylon can be constructed.

Shows cage of rebar with couplers at ends (about 14 pounds per foot)

Six piles/drilled shafts and pile cap construction

Pile cap and pylon/tower construction:-After the foundation is constructed, the piles are connected by pile cap in which large amount of concrete is required. Above this pile cap, pylons or towers are constructed with the help of prefabricated steel bars as shown below. These are green because of the oxy coating over them which protect them from corrosion. These towers are constructed to about 80ft and over these pylons deck is than constructed. The total height of pylons here shown is 200ft.

Prefabricated bars for pylon construction

The green cages are placed and concrete is poured with proper scaffoldings/ form work. For the construction of pylon above deck tower crane is required. In the construction of pylons above deck, there are precast units called saddles, are inserted in the cage which will act as a way for cable to pass through pylon as shown.

Precast units inserted in pylon cage

Pier table and Deck construction:-After the construction of pylon up to deck level, pier table is constructed. A pier table is the first portion of deck, through which cast units are placed and span is constructed. It is a launching zone for the rest of bridge. Here pier table is of about 65` and requires about 1000 cubic yard of concrete. After the construction of pier table, remaining portion of the deck is constructed with the help of segments either precast or cast in place. For placing segments temporary cables are required which are distressed after stressing permanent cables. Stressing or tensioning is done with hydraulic jacks and cables are anchored by wedges. With the help of temporary cables, hanged by hydraulic jack from temporary precast platform over the tower, the precast segments were held in level of pier table. The segments are connected to the pier table with the help of projecting bars & pipes as shown.

Shows Construction of pier table

Fig shows projecting bars and pipes and precast segment. The segments used in construction of bridge shown above are 75 feet wide and 16 feet long and total 78 segments are used. In the construction of deck form work (anchor block) is provided to pass cable below the deck for stressing.

Fig shows hydraulics for stressing of temporary cables and insertion of wedges.

Installation of permanent cable and its components:-A cable consists of six strands twisted together with resin in between them and coated with black plastic. A group of these cables holds the deck. This group of cable is covered by HDME pipe.

Now installation process is carried out, in which HDME pipes are held in air and with the help of machine cable is forced through HDME pipe to reach saddle of pylon. There are 30 to 110 strands in each saddle from one to other side of tower. After the cables reached the other side they individually are stressed with the help of hydraulic jack and computers on both sides of tower.

Above fig shows stressing while bottom one shows hydraulic jack.

After stressing the bridge is surveyed for stability. If it is not so stable than more stressing is done. If bridge settles after some years the cables can again be stressed. These cables below the bridge are protected by galvanized cap and whole anchor is injected with wax. Finally construction completes and temporary cables are removed.