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Structural Rehabilation & Retrofitting
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RESTORATION OF A FAILED BRIDGE


This  case refers to a bridge which has completely failed in service.  The bridge is a single span T beam slab bridge with 3 nos Beams and is located on a very important connecting road between National Highway 7 and National Highway 9, about 75 K M from Hyderabad, capital of State of Andhra Pradesh.


On a fine day the bridge suddenly failed by developing major vertical cracks in all the 3 beams in the centre and the deflection observed to be about 220 mm. The main cause being

  1. Poor quality of concrete especially in the bottom region of the beam
  2. Overloading of the structure.  (Vehicle load more than designed capacity)

It is a clear failure of the structure and in normal cases the best option would be to re-construct the superstructure.  However, the process involved in re-construction would take about 3-4 months starting from budget sanction to floating tenders and final completion of job.  The road cannot be blocked for such long period with very high inconvenience to heavy vehicles which use this road as the main link between the National Highways.


The structure and its failure are shown in following photographs :


Failed Bridge

close view of cracks in beams
(note the quality of concrete)

Failed Bridge

In view of the same, restoration of the structure was awarded to M/s Varshitha to complete the project in less than 1 month.  The restoration process involved  :

  1. Lifting the beams and correcting the deflection.
  2. Removing the poor concrete at the  bottom region of the beams
  3. Replacing the same with Micro Concrete.
  4. Providing PT strands in the new encasement and stressing
  5. Lifting and replacement of neoprene bearings.

THE EXECUTION  :

As a first step, bed concreting was carried out at the locations where lifting supports are proposed to be placed to provide a firm base for the supports.

Bed concrete carried out (300 mm thick) at the support locations
A) LIFTING AND CORRECTING THE DEFLECTION  :

As a first step, it is important to bring the structure to near level, i.e. the present deflection of approximately 220 mm to be brought to near “Zero”.  The following procedure was adopted for carrying out the same.

 

The supports were provided as shown in following sketch :


The photographs below show the supports and lifting operation

Lifting and deflection correction in progress

Crack before and after lifting

Crack before and after lifting

The Bridge after lifting and deflection correction

The Bridge after lifting and deflection correction
 

B) ENCASEMENT OF BEAMS:

Once the structure is brought to level, the supports below the beam are transferred to below the slab (close to beams) as shown in sketch below and the beams are made free for repairs.


CROSS SECTION OF BRIDGE SHOWING SUPPORTS TO SLAB
 
Brief sketch of the encasement to the beams is shown in following sketch:

SECTION BB (not to scale)
 

6 Nos Mono strands encased in 20 mm dia HDPE pipe are provided in each beam encasement.  The strands are anchored to a bearing plate behind the end diaphragm beams.

 

After fabrication of reinforcement as per specification and fabrication of ply shuttering, Micro concrete was placed to encase each beam.


Beams after encasement
 

C) STRESSING OF BEAMS:

Once the Encasement of beams is complete and cured for 7 days, the strands in the beams are stress to 14.70 Tons each, thus inducing a stress of approximately 90 T in each beam. This stress in the beams will not only strengthen the beams, but also cater to the additional loading on the structure due to use by heavier vehicles than the designed load.


Photograph showing the strands (before concrete)

End Blocks
 

D) LIFTING AND REPLACEMENT OF NEPORENE BEARINGS:

 

Once the strengthening of the beams is completed, using 8 nos of simultaneously operated Hydraulic jacks (placed below the end diaphragm), the structure is lifted and the old bearings removed and replaced with new Neoprene Bearings.


LIFTING IN PROGRESS AND NEOPRENE BEARINGS REPLACED

 

 

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2. Restoration of a Failed Bridge
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4. High Reach Demolition 4th Floor P T Slab Demolished
5. Stabilization of Vertical Pit Faces Using P T Technique
6. Removal of Columns by External Post Tensioning Technique to Increase Spans
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