CAST-IN-SITU SPECIAL SPAN OF 56.3 M WITH PRE STRESSED CONCRETE IN BANGALORE METRO • NORTH – SOUTH AND EAST – WEST CORRIDORS. • UNDERGROUND IN CORE AREAS AND ABOVE GROUND AS VIADUCT IN PERIPHERAL AREAS. • SOUTH CORRIDOR PASSES THROUGH SOUTH END CIRCLE FROM WHICH SIX ROADS RADIATE. A MAJOR TRAFFIC JUNCTION. • VIADUCT SPANS ARE MOSTLY 28 M THOUGH FEW ARE 22m , 25m & 31m. SEGMENTAL CONSTRUCTION IS POSSIBLE FOR SPANS UP TO 31m USING EXISTING LAUNCHING GIRDERS. • CONSIDERING HEAVY TRAFFIC, THE CIRCLE IS FREE OF PIERS BY DESIGNING A SPECIAL (LONG) SPAN OF 56.3m. • THEREFORE THE 56.3m VIA DUCT HAS TO BE A CAST IN SITU SPAN & POST TENSIONED. SIMPLY SUPPORTED ON P64 & P65 1
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CAST-IN-SITU SPECIAL SPAN OF 56.3 M WITH PRE STRESSED CONCRETE IN BANGALORE METRO
• NORTH – SOUTH AND EAST – WEST CORRIDORS. • UNDERGROUND IN CORE AREAS AND ABOVE GROUND AS
VIADUCT IN PERIPHERAL AREAS. • SOUTH CORRIDOR PASSES THROUGH SOUTH END CIRCLE
FROM WHICH SIX ROADS RADIATE. A MAJOR TRAFFIC JUNCTION.
• VIADUCT SPANS ARE MOSTLY 28 M THOUGH FEW ARE 22m , 25m & 31m. SEGMENTAL CONSTRUCTION IS POSSIBLE FOR SPANS UP TO 31m USING EXISTING LAUNCHING GIRDERS.
• CONSIDERING HEAVY TRAFFIC, THE CIRCLE IS FREE OF PIERS BY DESIGNING A SPECIAL (LONG) SPAN OF 56.3m.
• THEREFORE THE 56.3m VIA DUCT HAS TO BE A CAST IN SITU SPAN & POST TENSIONED. SIMPLY SUPPORTED ON P64 & P65
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FOUNDATIONS & PIERS
• PILE FOUNDATIONS ARE ADOPTED FOR BOTH PIERS.
• END BEARING PILES SOCKETED IN GRANITE OF UNIAXIAL COMPR STRENGTH OF7380 T/SQ M.
• AT P64, DUE TO 1.8 M DIA WATER LINE AT 1.7 M DEPTH, PILE CAP IS DESIGNED WITH 800MM ECCENTRICITY TO PIER.
• PILES M35 WITH SLAG CEMENT. PIERS M45 WITH OPC
• MODE OF TRACTION IS 750 V DC. ONE REBAR IS ‘STRAY CURRENT BAR’
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SINGLE CELL BOX GIRDER AS SUPER STRUCTURE
• TOP SLAB IS 8.8 M TO ACCOMMODATE TWO METRO TRAINS. BOTTOM SLAB IS 3.6 M
• HEIGHT OF GIRDER IS 3.5M. FOR OTHER SPANS IT IS 2 M. • M60 CONCRETE ( OPC, FLY ASH & SILICA FUMES) AND FE 500
ARE USED.
• PRE STRESS CABLES ARE PLACED IN BOTH WEBS AND BOTTOM SLAB.
• CURVED CABLES WILL TAKE PART OF SHEAR FORCES.
• CABLES ARE STRESSED FROM BOTH ENDS.
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WEB SHUTTERING UNDER PROGRESS
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BOX GIRDER
• FIVE DIAPHRAGMS ARE PROVIDED IN THE BOX AT 0, L/4, L/2, 3L/4 AND L TO ACT AS STIFFENERS. DIAPHRAGMS HAVE OPENINGS FOR MAINTENANCE AND RAIN WATER DISPOSAL.
• A SET OF TWO ANCHOR BLOCKS ARE PROVIDED AT L/3 AND 2L/3 FOR FUTURE POST TENSIONING, IF REQUIRED. TWO CABLES/RODS CAN BE STRESSED TO MAKE UP FOR LOSS OF PRE STRESS OVER THE PERIOD.
• WEBS AND BOTTOM SLABS ARE DESIGNED 500MM TO ENABLE
TENSIONING AND TO SUSTAIN HIGH INITIAL PRE STRESS.
• HOWEVER THE SECTION IS TAPERED TO 300MM IN THE CENTER 2/3RD WHERE ONLY SHEATHING PIPES MATTER.
• ONLY NEEDLE VIBRATORS ARE USED FOR COMPACTION AS SHUTTERS FOR WEBS AND BOTTOM OF TOP FLANGE ARE MADE WITH PLYWOOD BECAUSE OF CURVED SHAPE.
• CONCRETING IN STAGES.1ST STAGE. BOTTOM SLAB, QTY 80 CUM.
• 2ND STAGE. BOTH WEBS JUST BELOW TOP SLAB. QTY 97 CUM.
• 3RD STAGE. TOP FLANGE . QTY 200 CUM.
• 4TH STAGE. COVERING OF PRE STRESS ANCHOR BLOCKS BY CONCRETE. JOINING WITH ADJACENT SPANS WITH EXPANSION JOINTS. QTY 30 CUM.
• TOTAL 407 CUM OF CONCRETE.
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DECK SLAB CONCRETING UNDER PROGRESS
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DECK SLAB CONCRETING UNDER PROGRESS
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PRE STRESSING
• PRE STRESSING BY BBR(I) SYSTEM. IN THE BOTTOM SLAB, SIX CABLES AND A DUMMY PIPE IN THE CENTER.
• SEVEN CABLES IN EACH WEB• 15.6MM dia STRANDS.• W1 TO W6 IN EACH WEB - 19 STRANDS• W7 IN EACH WEB - 20 STRANDS• S1 TO S3 IN BOTTOM SLAB - 22 STRANDS• D (DUMMY) IN CENTER OF BOTTOM SLAB - 115MM DIA
SHEATHING PIPE• DUMMY IS NECESSARY IN IMPORTANT STRUCTURES FOR
EMERGENCY REQUIREMENT OF ADDITIONAL STRANDS. • QTY OF PT STEEL - 26MT. i.e 64 KG PER CUM OF CONCRETE.• CABLES ARE STRESSED SYMMETRICALLY.
• PRE STRESSING OF W1 TO W7 CABLES OF WEBS - 19 MAR 2011, saturday
evening
• ON SATURDAY NIGHT, JACKS DEVELOPED PROBLEMS WHEN 50% STRESSING WAS DONE FOR S1 AND STRESSING STOPPED.
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PRE STRESSING COMPLETED
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PRE STRESSING
• 20 MAR 2011 SUNDAY. ONLY CURING WAS GOING ON BY UNSKILLED.
• ON 21 MAR, AT 0900 HRS CRACKS WERE OBSERVED BY ENGINEERS AROUND ANCHOR BLOCKS i.e. W1 TO W7 ON ALL FOUR ENDS.
• W7 HAD BIGGER CRACKS APPROX 3MM WHERE AS IN OTHERS IT WAS 0.1MM TO 2MM.
• CRACKS APPEARED AS SPLITTING CRACKS ORIGINATING FROM ANCHOR PLATES AND MOSTLY IN THE LINE OF PLUMB I.E VERTICAL.
• S1 DID NOT HAVE ANY CRACKS (S1 WAS STRESSED ONLY 50%.)
• PASTED GLASS STRIPS ACROSS THE CRACKS TO KNOW WHETHER CRACKS ARE PROGRESSING.
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IDENTIFICATION OF CRACKS
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PRE STRESSING
• STRESSING WAS STOPPED. OBSERVED THAT CRACKS DID NOT DEVELOP FURTHER. GLASS STRIPS WERE INTACT.
• 28 DAYS CUBE STRENGTH IS AVERAGE 69 MPA AND NONE LESS THAN 65MPA.SD IS 1.1. 56 DAYS STRENGTH IS 70.69MPA.
• PERMEABILITY TEST RESULT IS AVERAGE 11.98MM WHILE PERMISSIBLE IS 25MM AND SD IS 0.9
• AS CONCRETE QUALITY WAS NEVER A DOUBT, THE CAUSES OF CRACKS WERE HARD TO FATHOM.
• ND TEST USING UPV WAS CONDUCTED AROUND THE ANCHORS TO ASSESS CONCRETE QUALITY BUT RESULTS WERE INCONSISTENT DUE TO CRACKS AND REBAR CONGESTION. THE READINGS WERE ERRATIC & NOT DEPICTING TRUE QUALITY OF CONCRETE.
• DETAILED DESIGN CONSULTANTS (DDC)VISITED AND STUDIED THE SITUATION.
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CONDUCTING NDT TEST
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ANALYSIS OF CRACKS
• SPAN HAD 22MM HOG AND CLEAR OF MIDDLE SUPPORT. SELF LOAD WAS TAKEN BY PRE STRESSED WEB CABLES.
• DESIGN WAS FOUND CONFORMING TO CODAL PROVISIONS.
• WORKMANSHIP AND MATERIALS WERE VERY GOOD MORE SO IN 56.3M SPAN. STILL TWO PROBABLE CAUSES OF CRACKS WERE IDENTIFIED .
• REBARS AROUND ANCHOR BLOCKS COULD BE LESS TO RESIST HIGH INITIAL PRE STRESS THOUGH WHAT IS PROVIDED IS AS PER CODES. MAY BE NECESSARY TO PROVIDE MORE REINFORCEMENT “AS GOOD PRACTICE”.
• CONCRETE AROUND THE ANCHOR BLOCKS MAY HAVE DEVELOPED VOIDS DUE TO HIGH STEEL.
• ONE HAND WE DESIRE MORE STEEL ON THE OTHER HAND WE FEAR PROBABLE HONEY COMBING DUE TO HIGH STEEL. BOTH ABOVE REASONS APPEAR PARADOXICAL BUT COULD BE TRUE.
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ANALYSIS OF CRACKS
• DDC OPINED THERE IS NO MAJOR FAULT AND PRE STRESSING SHOULD BE CONTINUED. AS A CAUTION, PROPOSED THAT STEEL U CLAMPS OF 100 MM WIDTH SHOULD BE NAILED TO THE WEBS TO PREVENT FURTHER DEVELOPMENT OF CRACKS DURING BALANCE STRESSING.
• THE PROPOSAL APPEARED CORRECT. HOWEVER IT WAS REFERRED TO THEIR INTERNATIONAL EXPERT FOR CONFIRMATION.
• THE EXPERT VISITED THE SITE, STUDIED AND AGREED TO THE CONCEPT BUT NOT WITH THE METHOD PROPOSED.
• HE ADVISED SANDWICH PLATES I.E. TWO PLATES OF 100 X 150 ON BOTH SIDES OF WEB HELD TIGHTLY BY A MAC ALLOY BAR BY STRESSING TO RESTRAIN FURTHER DEVELOPMENT OF CRACKS.
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PROPOSAL : PLATES & MAC ALLOY BAR BETWEEN ANCHORS. BARS ARE STRESSED, PLATES PREVENT
PROPAGATION OF CRACKS
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REMEDIAL ACTION FOR CRACKS
• W1 TO W7 WERE ALREADY STRESSED, ONLY W1 TO W6 WERE GROUTED. W7 WAS FULLY DE STRESSED AS WIDER CRACKS WERE NEAR W7.
• A HOLE OF 30MM DIA WAS DRILLED ACROSS 500MM THICK WEB BETWEEN TWO ANCHORS USING HILTI EQUIPMENT. SOME REBARS HAD TO BE CUT IF CAME IN LINE OF DRILLING.
• PLATES WERE KEPT AT EACH FACE OF WEB AND A 26.5MM MAC ALLOY BAR WITH HEAD ON ONE SIDE AND THREADS WITH NUT ON OTHER SIDE WAS PLACED.
• THE MAC ALLOY BAR WAS STRESSED USING TEMP STRESSING EQUIPMENTS. AFTER STRESSING, THE NUT WAS TIGHTENED ON THREADS. PLATES EXERTED PRESSURE ON CRACKS AREA.
• AS 26.5MM ROD IS PLACED IN 30MM HOLE, THE REMAINING PORTION OF THE HOLE WAS GROUTED WITH GP2 MATERIAL THROUGH INLET AND OUTLET HOLES PROVIDED IN PLATES.
• THIS WAS DONE BETWEEN THE ANCHORS ON ALL FOUR ENDS.
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DRILLING FOR MAC ALLOY BAR
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STRESSING OF MAC ALLOY BAR
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REMEDIAL ACTION
• W1 TO W6 ARE GROUTED. AFTER PLATES AND MAC ALLOY BARS FIXING, S1 TO S3 WERE THREADED, STRESSED AND GROUTED.
• IN EACH W7 WHICH WAS DE STRESSED, ONLY 50% OF STRANDS ARE PLACED CONSIDERING EXISTING CRACKS.
• BALANCE 50% STRANDS OF EACH W7 ARE THREADED, STRESSED AND GROUTED IN THE DUMMY PIPE.
• AFTER GROUTING OF ALL CABLES i.e.W1 TO W7, S1 TO S3 AND D, THE CRACKS WERE GROUTED WITH EP 10 OF FOSRCOC WHICH IS OF LOW VISCOSITY AND HIGH EARLY STRENGTH.
• THE SPAN DEVELOPED 40MM HOG WHEN FULLY STRESSED.
• GROUTING OF CRACKS CONSUMED ONLY 1.6 KG OF EP 10 FOR APPROX 7.2 SQ M i.e. 0.22 KG/SQ M. THIS PROVES THAT CRACKS WERE NOT LONG AND WIDE.
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PLACING MAC ALLOY BAR WITH PLATE
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STRESSING OF DUMMY CABLE
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S1,S2,S3 AND D GROUTING COMPLETED
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LESSONS LEARNT
• SAFETY AND HOUSEKEEPING. ALL WORKERS AND ENGINEERS USED PPE.
• HARNESS BELTS WERE SECURED WHILE WORKING AT HEIGHT. FLOOD LIGHTS TO FULLY ILLUMINATE.
• TRAFFIC MARSHALS IN ADDITION TO TRAFFIC POLICE WERE PUT TO ASSIST TRAFFIC AND PEDESTRIANS.
• EDUCATION IS NOT ONLY WHEN FAILURES OCCUR AND SOLUTIONS ARE FOUND. SUCCESSFUL COMPLETION OF A WORK IS A PROVEN EDUCATION FOR ADOPTION FROM WHERE FURTHER INNOVATIONS SHOULD TAKE PLACE.
• DESIGN, EXECUTION, SAFETY AND HOUSEKEEPING OF THIS WORK IS WORTHY OF ADOPTION IN OTHER PROJECTS.
• ZERO ACCIDENT WORK
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LESSONS LEARNT
• THE DEVELOPMENT OF CRACKS MADE US TO INTROSPECT DESIGN OF ANCHORS, REINFORCEMENTS AROUND ANCHORS, USING PLATE VIBRATORS, ETC.
• THE EXPERT’S ADVICE WAS EFFECTIVE AND PRACTICAL.
• I STILL GET JITTERY WHEN I IMAGINE ON THE FATEFUL SATURDAY 19TH MAR, HAD THE JACK NOT FAILED AND HAD WE CONTINUED STRESSING OF S1 WHAT KIND OF CRACKS COULD BE SEEN ON MONDAY MORNING.
• HAD WE NOT APPROACHED THE INTERNATIONAL EXPERT AND HAD WE PINNED THE U CLAMPS WITHOUT STRESSING AS PER FIRST PROPOSAL, WOULD THE CRACKS BE ARRESTED OR FURTHER INCREASED, I HAVE NO IDEA. LATER CRACKS WERE FOUND TO BE OF MINOR DEPTH AND LENGTH.
• DEPLOYING BEST DESIGNERS & ENGINEERS TO EXECUTE SUCH COMPLEX WORKS IS A MUST.LET OUR DESIGNERS & PRACTITIONERS LEARN FROM THIS STUDY
• (I THANK SH N SIVASAILAM, MD BMRCL AND SH B S SUDHIRCHANDRA, DPP. DDC ARE MOTT MAC DONALD. ENGINEERS ARE A CONSORTIUM LED BY RITES. CONTRACTORS ARE M/S NCC LTD)
