Cast-In-situ Special Span of 56.3 m With Pre Stressed Concrete in Bangalore Metro

7/27/2019 Cast-In-situ Special Span of 56.3 m With Pre Stressed Concrete in Bangalore Metro

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CAST-IN-SITU SPECIAL SPAN OF 56.3 M WITH PRE

STRESSED CONCRETE IN BANGALORE METRO

NORTHSOUTH AND EASTWEST 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 TRAFFICJUNCTION.

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 PIERSBY 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, PILECAP 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 BOTTOMSLAB.

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 HAVEOPENINGS FOR MAINTENANCE AND RAIN WATER DISPOSAL.

A SET OF TWO ANCHOR BLOCKS ARE PROVIDED AT L/3 AND2L/3 FOR FUTURE POST TENSIONING, IF REQUIRED. TWO

CABLES/RODS CAN BE STRESSED TO MAKE UP FOR LOSS OFPRE STRESS OVER THE PERIOD.

WEBS AND BOTTOM SLABS ARE DESIGNED 500MM TO ENABLETENSIONING AND TO SUSTAIN HIGH INITIAL PRE STRESS.

HOWEVER THE SECTION IS TAPERED TO 300MM IN THE CENTER2/3RD WHERE ONLY SHEATHING PIPES MATTER.

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BOX GIRDER

CONCRETE BOOM PLACERS USED. CONCRETE SLUMP - 110-125.

ONLY NEEDLE VIBRATORS ARE USED FOR COMPACTION ASSHUTTERS FOR WEBS AND BOTTOM OF TOP FLANGE ARE MADEWITH PLYWOOD BECAUSE OF CURVED SHAPE.

CONCRETING IN STAGES.1ST STAGE. BOTTOM SLAB, QTY 80CUM.

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 BYCONCRETE. JOINING WITH ADJACENT SPANS WITHEXPANSION JOINTS. QTY 30 CUM.

TOTAL 407 CUM OF CONCRETE.6


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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.

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COMPLETED BOX GIRDER

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CALENDAR OF EVENTS

1ST STAGE CONCRETING - BOTTOM SLAB - 12 JAN 2011

2ND STAGE CONCRETING - WEBS - 04 FEB 2011

3RD STAGE CONCRETING - TOP SLAB - 01MAR 2011

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 ENGINEERSAROUND 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 FROMANCHOR PLATES AND MOSTLY IN THE LINE OF PLUMB I.EVERTICAL.

S1 DID NOT HAVE ANY CRACKS (S1 WAS STRESSED ONLY 50%.)

PASTED GLASS STRIPS ACROSS THE CRACKS TO KNOW WHETHERCRACKS 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 DEVELOPEDVOIDS 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. 17


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ANALYSIS OF CRACKS

DDC OPINED THERE IS NO MAJOR FAULT AND PRE STRESSINGSHOULD BE CONTINUED. AS A CAUTION, PROPOSED THATSTEEL U CLAMPS OF 100 MM WIDTH SHOULD BE NAILED TOTHE WEBS TO PREVENT FURTHER DEVELOPMENT OF CRACKSDURING BALANCE STRESSING.

THE PROPOSAL APPEARED CORRECT. HOWEVER IT WASREFERRED TO THEIR INTERNATIONAL EXPERT FOR

CONFIRMATION.

THE EXPERT VISITED THE SITE, STUDIED AND AGREED TO THECONCEPT BUT NOT WITH THE METHOD PROPOSED.

HE ADVISED SANDWICH PLATES I.E. TWO PLATES OF 100 X 150ON BOTH SIDES OF WEB HELD TIGHTLY BY A MAC ALLOY BARBY STRESSING TO RESTRAIN FURTHER DEVELOPMENT OFCRACKS.

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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.20


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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 ALLOYBARS FIXING, S1 TO S3 WERE THREADED, STRESSED ANDGROUTED.

IN EACH W7 WHICH WAS DE STRESSED, ONLY 50% OF STRANDSARE 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 ANDD, THE CRACKS WERE GROUTED WITH EP 10 OF FOSRCOCWHICH 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 FORAPPROX 7.2 SQ M i.e. 0.22 KG/SQ M. THIS PROVES THAT CRACKS

WERE NOT LONG AND WIDE. 23


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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 ENGINEERSUSED PPE.

HARNESS BELTS WERE SECURED WHILE WORKING AT HEIGHT.FLOOD LIGHTS TO FULLY ILLUMINATE.

TRAFFIC MARSHALS IN ADDITION TO TRAFFIC POLICE WEREPUT TO ASSIST TRAFFIC AND PEDESTRIANS.

EDUCATION IS NOT ONLY WHEN FAILURES OCCUR ANDSOLUTIONS ARE FOUND. SUCCESSFUL COMPLETION OF AWORK IS A PROVEN EDUCATION FOR ADOPTION FROM WHEREFURTHER INNOVATIONS SHOULD TAKE PLACE.

DESIGN, EXECUTION, SAFETY AND HOUSEKEEPING OF THISWORK IS WORTHY OF ADOPTION IN OTHER PROJECTS.

ZERO ACCIDENT WORK

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LESSONS LEARNT

THE DEVELOPMENT OF CRACKS MADE US TO INTROSPECTDESIGN OF ANCHORS, REINFORCEMENTS AROUND ANCHORS,

USING PLATE VIBRATORS, ETC.

THE EXPERTS ADVICE WAS EFFECTIVE AND PRACTICAL.

I STILL GET JITTERY WHEN I IMAGINE ON THE FATEFUL

SATURDAY 19

TH

MAR, HAD THE JACK NOT FAILED AND HAD WECONTINUED STRESSING OF S1 WHAT KIND OF CRACKS COULDBE SEEN ON MONDAY MORNING.

HAD WE NOT APPROACHED THE INTERNATIONAL EXPERT ANDHAD WE PINNED THE U CLAMPS WITHOUT STRESSING AS PERFIRST PROPOSAL, WOULD THE CRACKS BE ARRESTED ORFURTHER INCREASED, I HAVE NO IDEA. LATER CRACKS WEREFOUND TO BE OF MINOR DEPTH AND LENGTH.

DEPLOYING BEST DESIGNERS & ENGINEERS TO EXECUTE SUCHCOMPLEX 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. ENGINEERSARE A CONSORTIUM LED BY RITES. CONTRACTORS ARE M/S NCC LTD) 28

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Cast-In-situ Special Span of 56.3 m With Pre Stressed Concrete in Bangalore Metro