CABLE STAY BRIDGE CONSTRUCTION
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
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.
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
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.