Inspection and Maintenance of Steel Girders-5

8/12/2019 Inspection and Maintenance of Steel Girders-5

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Bottom flange (tension member) construction is similar to topflange. Bottom flange resists bending tensile stresses and net

area is considered for a cross section. Greater cross section isrequired in the middle portion of the span which is provided withadditional flange plates.Web is made up with 3/8" or 10 mm thick plate for full lengthand depth of beam section. Web plate is stiffened againstbuckling with stiffener angles. Web resists shear stress which ismaximum at the end of beam, hence closer pitches of rivets andcloser interval between two intermediate stiffeners than in middleportion are provided. The load on the entire span is transferred

through bearings to bed block and in turn to substructure. Hencebearings stiffeners are provided with 2 angles back to back oneither side of web plate over the bearing. Bearing stiffeners arestraight and outstanding leg is snug fitting between top andbottom flange. Packing plate is provided between web andstiffener connecting leg equal to flange angle thickness.Intermediate stiffeners are single angle on either side of web andoutstanding leg is snug fitting between top and bottom flanges.Intermediate stiffeners are either straight with packing plate or

joggled without packing plates. Outstanding leg of stiffener actslike a strut between top and bottom flange to resist web bucklingcaused by cross bending of sleeper and top flange.

Secondary Members :Cross frame consists of angle section for horizontal anddiagonal members connected with gussets to stiffener legs forlateral rigidity. Top lateral bracings consist of angles connectingdiagonally (in horizontal plane) to top flanges to reduce theunsupported length of compression flange to resist buckling.Bottom lateral bracings consist of angle connected to bottomflange for spans 24.4 metre and above. All these cross frames,top lateral and bottom lateral bracings are for lateral rigidity andare secondary members.

Centralised articulated (sliding type) bearings are provided withsteel base plates which are anchored to bed blocks with anchor

bolts.Wooden channel sleepers with pad plates are provided on topflanges to support track and anchored with hook bolts. Notchesare required on bottom face of wooden sleeper to accommodate


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rivet heads, hence this poses a problem for frequent painting ofsleeper seats which are more prone to corrosion due to steel in

contact with wood/steel and abrasion of sleepers.

Rivetted deck type plate girders are strong and sturdy, andsimple for fabrication and launching. All standard spans aregenerally fabricated in engineering workshop of each zonalrailways and transported to site for launching. To facilitatetransportation from workshop to site of bridge, spans 9.15 m and12.2 m are fabricated as complete span. 18.3 metres span is

provided with one central joint in I-section, i.e. four numbers ofI-sections are fabricated for one span, 24.4 metres and 30.5metre spans are provided with two joints in I-section, i.e. sixnumbers of I-sections are fabricated for one span. The maximum13.5 metre long component can be loaded in a BFR. Therefore,splice material such as cover flange plates, cover flange angles,cover web plates are required to be assembled with the I-section to form span at site with field rivets for spans 18.3metres and above.

Steel used for existing rivetted plate girders is to conform to IS2062 Grade A 1992 (old IS 226) and rivet steel conforming to IS1148. Fig. 2.3 indicates the details of a rivetted girder.

NOTE : Semi through non-standard plate girders are similar todeck type (mentioned above) except that the sleepers are restingon floor system consisting of stringer and cross girders but no

bracings are provided to top flanges of girder.2.3.1.2 Welded plate girder-deck type :On Indian Railways, majority of steel girders are of plate girdertype. Till 1980, these plate girders were fabricated as rivettedgirders. The rivetted construction tends to make the structureheavy and costly, besides posing maintenance problems due torivet heads on top flange plates which leads to heavy corrosion.During 1980-82, with proven welding technology for fabrication ofdynamic structures and availability of indigenous automaticwelding equipments and consumables for sound welding, thefabrication of welded girders has been taken up for railway trackbridges. RDSO, Lucknow has issued standard welded plate


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girder drawings for spans of 12.2, 18.3 and 24.4 metres for BGand 6.1, 9.2, 12.2, 18.3, 24.4 and 30.5 metres for MG.

Advantage of welded girdera) Welded girder with rivetted intermediate stiffeners result

in saving of steel upto 25%.b) Welded girder eliminates the need of rolled sections of

non-standard sizes and shapes. Also due to non-availability of particular rolled section using highersection leading to uneconomical construction is avoided.

c) Eliminates drilling of holes and hence gross areabecomes net area for tension members (Bottom Flange).d) Welded girder eliminates cumbersome connections and

water pockets.e) Welded girder requires less maintenance due to

elimination of rivet heads on top flange thereby facilitatingthe painting of sleeper seats frequently thus reducingmaintenance and repairs cost.

f) Aesthetic superiority and higher production rates withlesser input are added advantages.

Components of welded girderFollowing are the components (similar to rivetted girder)

a) Welded I-sections (including welded or rivettedintermediate stiffeners)

b) Cross framesc) Top lateral bracingsd) Bottom lateral bracings for spans 24.4 metre and abovee) Bearings

Main components are welded I-sections which are simiilar torivetted plate girders except the following :

i) Web and flange connection is provided with continuousfillet weld (instead of flange angles with rivettedconnections)

ii) Earlier stiffeners were made up of plates instead of anglesections and are welded with fillet weld to the web butnow revised drawings are followed for welded girders withrevitted angle striffeners for intermediate stiffeners.


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1) Top flange plate is made with one flange plate andrequires increase in cross section in middle portion of

span, which is met with by increasing the width of flangeplate to cater for bending (compression) stresses.Sometimes due to non-availability of full length of flangeplates, "Butt" welding is provided with double "V" groove.Full penetration weld on both sides of plate is provided at

junction to increase in width of flange plate.

2) Bottom flange plate is made with one flange plate similar

to top flange but it is a tension member.To avoid stress concentration resulting in crack initiationduring service, at the junction of different widths of topand bottom flange plates, (at butt weld) 1:5 side slopeis provided. Butt weld in flange plates should be avoidedby arranging longer plates for economical fabrication andmaintenance during service life, if possible.

3) Web plate is made with 10 mm thick plate for full lengthand depth of girder. Sometimes due to non-availability offull length of plate, square butt weld on both sides ofweb plate is provided. Web plate is stiffened with platestiffeners. Bearing stiffeners consist of 2 numbers platestiffeners on either side of web over the bearings, and iswelded with continuous fillet welding to web, top andbottom flanges. Intermediate stiffeners are also providedwith plate stiffeners on either side of web plate atstaggered locations to avoid cruciform welding. Weldedbridge code clause No. 5.3.2.1 stipulates that filtet weldsat right angles to the line of principal stress in platesubjected to tension shall be avoided in dynamicallyloaded structures, hence intermediate stiffeners arewelded to web plate and top flange plate with continuousfillet weld and stiffener is terminated 50 mm short ofbottom flange fillet weld (tension flange). This type ofdetailing leads to classification of welded girder in Fclass under fatigue consideration reducing permissiblestress to a very low value resulting in increased crosssection of structure. To achieve economical welded


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girders these are provided with rivetted intermediatestiffener (angles section with rivetted connections) which

classify welded girders in class D under fatigueconsideration with higher permissible stress than ClassF.

RDSO, Lucknow has issued welded girder drawings with(i) Intermediate stiffener welded, and (ii) Intermediate stiffenerrivetted. Presently welded girders are fabricated with intermediaterivetted angle stiffener.

In welded girders, connections made during fabrication inworkshop are only welded. Remaining all connections at site,such as cross frames, top lateral bracings and splice materialfor 18.3 metre span and above are provided with rivettedconnections only. No field welding is permitted on welded girdersof track bridge without prior approval of competent authority evenfor repairs.

Cross frames, top and bottom lateral bracings are similar torivetted girder with rivetted connection for site joints.

Centralised articulated bearings (sliding type) are provided towelded girders similar to rivetted girders.

Welded connections are strong enough to resist the static loadsbut are prone to failure under dynamic loading due to fluctuationof stresses causing crack initiation. Welded plate girder of trackbridge therefore requires careful design and detailing. Fatiguestrength of welded structure depends upon the constructiondetails.

In general, welded girders are provided with following welded joints :

1) Butt Weld :a) double V groove - For top & bottom flangeb) square - For web plates


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2) Fillet Weld :a) T fillet (continuous) - For top & bottom flange

connections to web plateb) Side fillet (continuous)- All stiffeners and bearings.

Fig. 2.4 shows details of welded joints with weld parametersused for welded girder. Butt welds provided in top and bottomflange plates and web plates are to resist the same stress asparent metal and hence these welds are to be testedradiographicaily during fabrication. Welded Bridge Code clause

No. 5.2 & 5.3 stipulates that all butt welds in dynamic structureare examined radiographicaily or by any other equally effectivemethod. Fillet welds connecting top and bottom flanges to web,resist shear stress (resultant of horizontal and vertical shear) andhence full penetration continuous fillet weld is required.

Steel used for welded plate girders for built up I-section withwelded connections should conform to IS 2062 Grade B -1992

fully killed and normalised (old IS 2062) duly tested. Steel usedfor members like intermediate stiffeners cross-frames, lateralbracings, splice material with rivetted connections shouldconform to IS 2062 Grade A -1992 (Old IS 226) and rivet steelto IS 1148.

Fig. 2.5 and 2.6 show the details of welded plate girders (withwelded intermediate stiffeners and rivetted intermediatestiffeners).

2.3.1.3 Composite girder of RCC slab and steel girderFor high speed trains and increased volume of traffic,strong track structure with concrete sleepers is essential foreconomical maintenance. Such track structure on bridges ispossible only with ballasted deck provided on composite girders.

Following are the advantages of composite girders :a) Ballasted deck with PSC sleepers provides similar track

structure on bridge as on the approach which results inbetter maintainability and greater riding comfort.


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b) Superelevation for curved track can be provided withgreater ease, being ballasted deck.

c) Machines can be deployed for maintenance of track orbridges.

d) Use of other type of sleepers, become possible aswooden sleepers is a scarce commodity now on IndianRailways,

e) Concrete is rich in compressive strength and steel isrich in tensile strength. Composite girder consistingof RCC deck with steel beams would be generally

economical for spans ranging from 9.15 to 24.4 metres.However, the actual choice depends on site conditions.f) Less maintenance cost due to slab deck being of RCC.

RDSO, Lucknow has designed and issued compositegirder drawings for use on zonal railways for spans 9.15,12.2, 18.3, 20 and 24.4 metres for BG and 12.2 metresfor MG with welded construction.

Composite girders consist of the following components:A) RCC deck (Cast in situ at the site of bridge) with shearconnectors embedded in the slab.(Presently channel shear connectors are used anddepending on availability of stud welding gun, use of studshear connectors can be adopted)

B) Steel beams with shear connectors welded on top flange(Fabricated in workshop).

C) Cross framesD) Bearings

Details :a) The RCC deck is cast in situ after launching

steel girders on substructure. Shear connectors areprovided on top flange plates of steel beams withchannel sections welded all round with side fillet welds.Shear connectors are required to make cast in situ RCCslab and steel girder act together under load. Drawingsare available for channel shear connectors as well asstud shear connectors.

Documents

Inspection and Maintenance of Steel Girders-5