Inspection and Maintenance of Steel Girders-12

8/12/2019 Inspection and Maintenance of Steel Girders-12

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iv) Rivets connecting bottom lateral bracings to endcross girders as well as top lateral bracings and

portal bracings.

Note: Rivets listed in item (i) and (ii) of open web girdersare subjected to direct shear and sometimes inducetension due to excessive deflection of rail bearers andcross girders. Hence any loose rivets noticed in thesejoints should be replaced to avoid overstressing of otherrivets or shearing of rivet heads.

For loose rivets at field joints careful observation shouldbe made and decision should be taken for replacement.If there are only one or two slightly finger loose rivets atany staggered location or less than 10% in a particularconnection, it should be allowed to remain underobservation with white paint mark for identification. Ifmore rivets are loose at one location, they should bereplaced to avoid overstressing of other rivets. This needscareful understanding and good experience anddiscretion.

Rivets connecting bracings are prone to become looseon account of vibrations and these are secondarymembers which do not affect the strength of plate girderor truss but require replacement when found loose.

Many times rivets replaced on account of corrodedheads are due to bad maintenance and not due to

structural defects. Rivets should be replaced only withpneumatic hand driven rivet hammer which works atcompressed air pressure of 6.5 to 7.6 kg/ cm2. No handrivetting is permitted on main member of steel girder.

Maintaining proper record of loose rivets in group 2 & 3,particularly of open web girder, will help in assessingstructural condition in future inspection. Therefore,

proforma shown in Fig. 4.1 may be used for properrecord. One form will be required for each span. This willbuild the history of girder and help the ADEN forNumerical Rating during his annual inspection.


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4.3.3.1 Testing of loose rivets

Rivets which are driven at site and rivets which aresubjected to heavy vibrations like bracings, foot-path,trolley refuge etc. are prone to get loose. Corrosionaround the rivet will indicate the looseness of rivet. Dueto vibration of loose rivet, paint film gets broken andcorrosion sets in. To test rivets at any critical locationsmentioned above or sample testing, both hands arerequired to be free. Therefore, proper staging is essential.Left hand index finger is placed on one side of rivet head

as shown in Fig.4.2. Then strike the rivet head on theopposite side with testing hammer, which is a lighthammer weighing 110 gms with handle 300 mm long. Ifthe rivet is loose, vibration movement is felt by the lefthand index finger. This requires a little experience. Theloose rivets are marked with white paint and entered inloose rivets proforma. Depending upon number of looserivets and their location it will be programmed for

replacement or kept under observation.

4.3.4 Corrosion

Corrosion of steel members is one of the major factorscausing considerable damage to the steel work. Steelstructures are sensitive to the atmospheric conditions.Corrosion eats up the steel section and reduces its

structural capacity. If not attended to in time, it mayresult in -a) need for heavy repairs in the form of strengthening;b) need for replacement of the structure.

Hence it is an important item of inspection, to keepmaintenance expenditure minimum. A paint film isprovided over steel work as an inhibitor of corrosion. Withthe passage of time, due to extreme heat and cold or

abrasions by sleepers or due to stagnation of water, thisprotective paint film starts scaling off and graduallyexposes the original steel surface to the atmosphere.


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Fig.4.1RECOR

DINGO

F

LOOSE

RIVETS


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Fig.4.2TESTING

RIVET

FOR

LOOSENESS


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There are certain locations in girders, which are moreprone to corrosion than the other areas. These are as

follows:

1. Where steel comes in contact with wood.

2. Formation of water pockets on account ofconstruction features.

3. Places where dust can accumulate.

4. The presence of moisture near drainage system,

particularly with concrete deck composite girders,road over bridge girder, etc.

5. Locations where there is a direct impact of waterrepeatedly.

Therefore, special attention should be given duringinspection to the portions of steel work where corrosionis likely to be more prevalent. Such locations are:

1. Sleeper seats - Top flanges of plate girders andunderslung girders or top flanges of rail bearers(stringers) in through spans.

2. Steel affected by fumes of diesel loco like webmembers of truss of through girder.

3. Water pockets due to construction features - openweb span bottom chord panel points form pockets for

stagnation of rainwater, and in plate girders behindbent stiffeners.

4. Steel work in the vicinity of bearings.

5. Troughing of ballasted deck.

6. Top lateral bracings and portal bracings of throughspans.

7. Parts of the bridge steel work exposed to sea breezeand salt water spray.

8. Interface between steel and concrete in compositegirder and ROB girder.


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9. Underside of road over bridges and web plates nearroad deck level of ROB semi through girder.

10.Base of columns, seating of wooden floor in FOBs aswell as staircase steps.

Railway bridge girder painting consists of minimum (a)primer coat (b) cover coat. Therfore, first paint film toscale is cover coat. Hence only cover coat is repaintedwith patch painting in certain locations which will proveto be economical as surface preparation for only covercoat painting is easy rather than unnecessarily scalingdown the good and compact base coat which willincrease the cost of painting as well as wastage of paint.Therefore, the inspecting officials should clearly indicatethe following during inspection:

i) Major portion of steel work requires only cover coatpainting and at some locations patch painting.

ii) Steel girder requires complete painting.

iii) Only patch painting.

iv) Paint in good condition, hence no painting isrequired.

This will help in planning for painting, preparation oftender schedule and providing funds for maintenance

painting.It is needless to say that bridge steel girderseconomical service life is mainly dictated by properinspection and maintenance painting.

4.3.5 Fatigue Cracks

Fatigue is the tendency of metal to fail at lower stress

level when subjected to cyclic loading than whensubjected to static loading. Fatigue failures are morecommon where stress concentration due to sharpcorners and abrupt change of cross section exist.


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Fatigue is becoming important because of growingvolume of traffic at greater speeds and higher axle loads.

Fatigue failure in the form of cracking of steel section isa major cause in Railway track bridges on account ofrepeated stresses. Following are critical locations ofcracking;

i) Ends of diagonal members near middle of the spandue to reversal of stresses.

ii) Sharp edges at cut notch in stringer flanges atconnections with cross girders.

iii) Top flange plates or flange angles of plate girders orrail bearers especially below the sleepers.

iv) Roots of channel and angles on account of rolling

defects.

v) Corrosion pits at any location in tension member.Joints where heavy shear is transmitted.

To arrest the crack propagation, a hole should be drilledat the end of the crack in plate and cover material isprovided with rivets. The usual inspection for thedetection of fatigue cracking are done with magnifyingglass or tapping rivet testing hammer to detect bymetallic sound. It is very easy to detect cracks duringscraping for full painting of girder.

4.4 INSPECTION OF WELDED GIRDERS

4.4.1 History of welded girders

On Indian Railways welded track bridge girders aremanufactured since 1980. Following are the details ofdrawings and spans used as track bridges:


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S. Span RDSO RemarksNo. Drawing No.

(a) Plate Girders:1 12.2 m MBG - 1987 RDSO-B-1528 Drawing with

weldedintermediatestiffener &

2 18.3m MBG-1987 RDSO-B~1529 with rivettedintermediatestiffener

3 24.4m MBG-1987 RDSO-B-1555

Rivettedbracings and

4. 24.4m H.M. Loading RDSO-BA-16001 intermediatestiffeners

(b) Composite Girders:

1 12.2m MBG-1987 RDSO-B-1569/R

2 18.3 1534/R With channel3 20.0 1581/R shear

4 9.15 1623 connectors.

5 24.4 1730

6 9.15 1701 With stud

7 12.2 1731 shear connector

8 18.3 17389 24..4 1730

Basically above welded girders are having following welded joints:

1) Butt Welds - Flange plate joints and web platejoints (if required)

2) Fillet Welds - Web to flange connections and allstiffener connections to web platein I-section of girders, bearings.Composite girder top flange plateand shear connectors (in additionto above)


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Since all welded plate girder spans are less than 35m,no camber is provided in welded plate girders. Footover

bridge welded truss is fabricated as cambered girder(unprestressed)

All welds carried out in workshop are by submerged arcwelding (SAW) which is a fully automatic process andproduces sound weld. Only at some locations whereSAW is not feasible welding is done by manual metalarc welding (MMAW) or CO

2 welding. All railway

workshops have trained staff and all fabricated girdersare inspected by RDSO before despatch from workshopto ensure quality control and economical service life.

Plate girders are provided with centralised articulatedbearings and composite girders are provided with Bronzebearings.

4.4.2 Frequency of inspection

All welded plate girders and composite girders areinspected as per provision in Chaper XI of IRBM. Hencelevel of inspection, items of inspection are the same asthose of rivetted girder. Following are the additionalmanual provisions regarding welded girder inspection.

(1) SE/JE Bridges shall inspect all welded girder bridgesirrespective of length of span once in 3 years.

(2) A detailed inspection of every welded girder bridgeshould be done by SE/JE Bridges after one year ofits laying in the track and permitting traffic on thebridge.

(3) In case of important or special type of welded girderbridges, suitable schedule of inspection should belaid down by CBEs of Zonal Railways.

4.4.3 Equipment for InspectionWelded joints are strong enough like parent metal instatic structure like FOB girder, but in dynamic structurewelded joint is prone to fatigue-failure due to repeated

Inspection and Maintenance of Steel Girders-12

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