design-of-open-web-girder-bridge.pdf

open web girder bridge design

1. DESIGN OF OPEN WEB
GIRDER BRIDGE
ATUL KUMAR VERMA
XEN/SB-I/RDSO

2. INTRODUCTION
Truss bridge:
Usedforspansgreaterthanwhatcanbespanned
economicallybyaplategirderbridge.
Ingeneraltrussbridgesareusedforspans
greaterthan30m.

3. INTRODUCTION (Cont...)
FormsOfOpenWebGirderBridges
ThroughType
DeckType(Underslung)
SemiThroughType

6. COMPONENTS OF THROUGH TYPE
BRIDGE
FloorSystem:
Crossgirder(Bending)
RailbearersorStringers(Bending)
PrimaryMembers:
Bottomchordmembers(Tensionmembers)
Topchordmembers(Compressionmembers)
Endrackers(Compression&Bendingmembers)
Diagonals(Reversiblestressmembers)
Verticals(Tensionmembers&redundantmembers)

7. COMPONENTS (Cont...)
Secondarymembers:
Bottomlateralbracings(Axialforce)
Toplateralbracings(Axialforce)
Swaybracings&kneesway(Axialforce&Bending)
Portalbracings&kneeportal(Axialforce&Bending)
Maingussets
Bearings

8. GENERAL CONFIGRUATION
Typeoftruss
Warrentrusswithverticalsforstandardrailwayspans
Otherformsmaybeadoptedasperdifferentconditions
Numberofpanels
WeightoftrussVsFloorsystem
Optimumnumberis6to10
Lengthofpanel
WeightoftrussVsFloorsystem
Optimumlengthis6m.to9m.
Inclinationofdiagonals
Between45°and60°withthehorizontal

9. GENERAL CONFIGRUATION(Cont…)
Heightoftruss
ThroughtypeVsDecktype
Between1/8and1/5ofspanlength
Spacingoftrusses
Sufficienttopreventoverturningduetolateralloads
>1/3ofheightoftruss&>1/20ofspan

10. ESTIMATION OF LOADS
Dead load
Live load
Dynamic effects
Longitudinal force
Racking force
Wind pressure effect
Forces and effect due to earthquake

11. DEAD LOAD
Deadloadoftrussisassumedbeforedesignonthe
basisofexperience&earlierdesigns
Afterdesignoftrusstheactualdeadloadoftrussis
comparewithassumeddeadload
Ifthereisdifferencebetweentwo,thenassumed
deadloadisrevisedandstructureisdesignedwith
reviseddeadload

12. LIVE LOAD
Clause2.3ofbridgerule.
Estimatedonthebasisofloadingstandard.
EUDL(equivalentuniformlydistributedloads)are
giveninappendixofbridgerulesfordifferentloading
standards.
EUDLisgivenforbendingmomentandshearforce
EUDLforBendingMoment:
ForMaximumforcesinelementsresistingbending
(Bottomchords&topchords)
EUDLforShearForce:
ForMaximumforcesinelementsresistingshearatsection
(endracker,diagonals,verticals)

13. DYNAMIC EFFECTS
Clause 2.4 of bridge rule
Augmentationinloadduetodynamiceffectsis
consideredbyaddingaloadequivalenttoa
coefficientofdynamicaugment(CDA)multipliedby
theliveloadgivingthemaximumstressinmember
underconsideration.
Forsingletrackspans:
CDA=0.15+(8/(6+L))subjecttoamaximumof1.0
WhereL=loadedlengthofspaninmetersfortheposition
ofthetraingivingthemaximumstressinthemember
underconsideration

14. LONGITUDINAL FORCES
Clause2.8ofbridgerule
Valueoflongitudinalforceduetoeithertractive
effortorbrakingforceshallbeobtainedfrom
appendices.
Valuesdependonloadedlengthandstandardof
loading.
Maximumoftractiveeffortorbrakingforceistaken
aslongitudinalforce.

15. RACKING FORCE
Clause2.9ofbridgerule.
Lateralbracingsofloadeddeckofspanstobe
designedforalateralloadduetorackingforceof600
kg/m.treatedasmovingload.
Rackingforcenottobeconsideredforcalculating
stressesinchordsorflangesofmaingirders.

16. WIND PRESSURE EFFECT
Clause2.11ofbridgerules.
Windpressureexpressedasaequivalentstatic
pressureinwindwarddirection
Windpressureshallapplytoallloadedorunloaded
bridges.
Butbridgeshallnotconsideredtocarryanyliveload
whenwindpressureatdecklevelexceeds150kg/m2
forB.G.
Windforcecalculatedforloadedspanswithwind
pressure150kg/m2.

17. WIND PRESSURE EFFECT (Cont…)
WindForce=windpressure*exposedarea
Exposedarea=areaofmovingload+exposedarea
oftrussmembers.
Fullareaoftrussmembersonwindwardside+50%
areaoftrussmembersonLeewardside.

18. SEISMIC FORCE
Clause2.12ofbridgerule
Seismicforces:
Horizontalseismicforce
Verticalseismicforce
Seismicforcescalculatedtakingintoconsideration
seismiczone,importanceofstructureanditssoil
foundationsystem.
Designseismiccoefficients:
αh=βIα0
αv=αh/2

19. SEISMIC FORCE (Cont…)
F=Wm*αh(orαv)
F=Seismicforce
Wm=Weight of mass under consideration
ignoringreductionduetobuoyancy
Horizontalseismicforceduetoliveloadonthe
bridgeshallbeignoredwhenactinginthedirection
oftraffic
Whenactinginthedirectionperpendiculartotraffic,
thisistobeconsideredfor50%ofdesignliveload
withoutimpact.

20. ANALYSIS OF FORCES
Tofindouttheforcesinmembersoftrussdueto
variousloads.
Forcescanbefoundouteitherbysuitablecomputer
programorbyhandcalculation.
Handcalculationisdonebyusinginfluenceline
diagrams(ILD)forvariousmembersoftruss.
ILDarepreparedforamemberoftrussbycalculating
forceinmemberasaunitloadmovesacrossthedeck
ofthetruss.
AreaofILDcalculatedandmultipliedbytheforce
intensitytogetforceinaparticularmember.

21. DEAD LOAD ANALYSIS
Deadloadintensityissameforallthemembersof
truss.
Deadloadintensity(pertrussperunitlength)
=totalassumeddeadload/(2*spanlength)
Forceduetodeadloadineachmemberoftrussare
calculatedbymultiplyingdeadloadintensitywith
areaofILD.

22. LIVE LOAD ANALYSIS
BottomchordmembershavetensioninILD.Loaded
lengthislengthofspan.
TopchordmembershavecompressioninILD.
Loadedlengthislengthofspan.
Liveloadintensityforchordmembers
=EUDLbending/(2*loadedlength)
CDAforchordmembersiscalculatedtakingLasspan
length.
EndrackerhavecompressioninILD.Loadedlengthis
lengthofspan.
Liveloadintensityforendracker
=EUDLshear/(2*loadedlength)

23. LIVE LOAD ANALYSIS (Cont…)
CDAforendrackeriscalculatedtakingLasspan
length.
Diagonalmembershavebothtension&compression
inILD.loadedlengthfortension&compressionis
foundfromILD.
Liveloadintensity&CDAfordiagonalsare
calculatedfortension&compressionbothbasedon
theirrespectiveloadedlengths.
Forceduetoliveload=ILDarea*liveloadintensity
Forceduetodynamiceffect=CDA*forceduetolive
load

24. LONGITUDINAL FORCE
Longitudinalforcetakenforonlybottomchord
members.
Thisdependsonpositionofdifferentbottomchord
members.
Forbottomchordmemberinendpanelloaded
lengthforlongitudinalforceisfullspan.
Loadedlengthreducesbyonepanellengthaswe
takebottomchordsofotherpanelsstartingfromend
tocentre.
Basedonloadedlengthlongitudinalforceisfoundin
bottomchordmembers.

25. Generalconceptofloadtransferandhowthewind
forcesaredistributedamongthemembers
WindLoad=WindpressureXexposedarea
ExposedArea=Areaofmovingload+exposedarea
oftrussmember
WIND LOAD ANALYSIS

26. INCLINATION FACTOR = 13128 /10500 = 1.25
DEPTH OF BC = 620 mm
DEPTH OF TC = 620 + 16 = 636 mm
WIDTH OF ER = 630 + 20 = 640 mm
WIDTH OF VERT. = 280
WIDTH OF DIAGONAL = 400 mm
LA = = 85.3 Cm
LA = 407.8 Cm
LA = 1081 Cm
1
2
3
1440+265
2
610 610
5500
610 610
620
636
10500
1676
4670
3505
1440
265
620
940
100
75

27. WIND LOAD ANALYSIS (Cont…)
EXPOSED AREA TC BC
1.Between RL and bottom of
B.C.
B1
2.Between Moving load and RL
of stringer
ER, Vertical ,Diagonal
(l x b x No.)
B2
3.Moving load B3
4.TC and top of moving load T1
5.Top Chord T2
6.Gusset Top T2
TotalAT=T1+T2+T3AB=B1+B2+B3
Through Type Truss

28. Windforceontopchord=WindpressurexATX1.5=WT
Windforceonbottomchord=WP[1.5(AB-
B3)+B3]=WB
Nodalforceattopchord:
Atintermediatenodes=WT/No.oftoppanel=Tint.
Atendnodes=Tint/2
Nodal force at bottom chord:
At intermediate nodes = WB/No. of bottom panel=Bint.
At end nodes = Bint/2
WIND LOAD ANALYSIS (Cont…)

29. WIND LOAD ANALYSIS (Cont…)
Windloadanalysisisdoneforfollowingsituations:
Horizontalbendingofbottomchordduetowindforceon
bottomchord&movingload
Verticalbendingofspanduetowindforceonbottom
chord&movingload
Horizontalbendingofbottomchordduetowindforceon
topchordtransmittedthroughswaybracings
Verticalbendingofspanduetowindforceontopchord
transmittedthroughswaybracings
Horizontalbendingoftopchordduetowindloadontop
chord
Overturningeffectofportal

30. SEISMIC FORCE ANALYSIS
Seismicforcecalculatedinhorizontal&vertical
direction
Inhorizontaldirectionseismicforcecalculatedfor
bottomchord&topchord
Onbottomchordseismicforceisduetodeadloadas
wellasliveload&ontopchordseismicforceisdue
todeadloadonly
Inverticaldirectionseismicforceisduetodeadload
aswellasliveload
Analysisofseismicforceforforcesinmembersis
sameasthatofwindforce

31. FORCE IN TRUSS MEMBERS
Forceintrussmembersfoundbyaddingforcesdue
todeadload,liveloadwithdynamiceffect,
longitudinalloads,windloadorseismicloads

32. DESIGN OF STRINGER
Loadedlengthforstringer=lengthofonepanel
Bendingmoment&shearforcecalculatedbygetting
EUDLbendingorEUDLshearaspercase
Deadloadofstringer&trackalsoconsidered
Sectionassumedforstringer
Actualstressescalculatedforbendingmoment&
shearforce
Permissiblestressesforbendingisminimumof:
Basicpermissiblestress(clause3.7ofSBC)
Permissiblestressinfatigue(clause3.6ofSBC)
Permissiblestressinbendingcompression(clause3.9of
SBC)

33. DESIGN OF STRINGER (Cont…)
Permissibleshearstress(TableIIofSBC)
Actualstress<permissiblestressthenassumed
sectionissafeotherwiserevisethesection
Designofconnectionbetweenweb&flangeof
stringer:
Calculationofhorizontalshearatthelevelofweld
Permissiblestressinweld(Appendix-GofSBC&clause
13.4ofweldedbridgecode)
Sizeofweldcalculated(Subjecttoclause6.2ofwelded
bridgecode)

34. DESIGN OF STRINGER (Cont…)
Provisionofstiffeners(Clause5.10ofSBC)
Designofstringerbracings:
Calculationoflateralload(Clause2.9.2ofbridgerule)
Analysisforforceinstringerbracings.
Designofstringerbracings(Clause6.2.3&3.8ofSBC)

35. DESIGN OF CROSS GIRDER
LoadedlengthforcrossgirderforEUDL
=2*centretocentredistanceofcrossgirder
LforCDA=2.5*crossgirderspacing
Bendingmoment&shearforcecalculatedbygetting
EUDL
Deadloadofstringer,track&crossgirderalso
considered
Sectionassumedforcrossgirder
Designprocessforcrossgirderissameasstringer

36. DESIGN OF CROSS GIRDER(Cont…)
Connectionofcrossgirderwithstringer
Calculatenumberofrivetsfor:
-onespanloaded
-bothspanloaded
Connectionofcrossgirderwithvertical&Bottom
chord
Findrivetvalue&calculatenumberofrivetsrequiredfor
connection

37. DESIGN OF BOTTOM CHORD
Bottomchordmembersaretensionmember
SectionassumedforBottomchordmembers(Taking
intoconsiderationclause4.5&clause6.7ofSBC)
Effectiveareaofthesectioncalculated(clause4.3.2
ofSBC)
Actualstressescalculatedforaxialtensionfor:
Withoutlongitudinal&seismicorwindforces
Withlongitudinal&seismicorwindforces
Permissiblestressforaxialtensionisminimumof:
Basicpermissiblestress(clause3.7ofSBC)
Permissiblestressinfatigue(clause3.6ofSBC)

38. DESIGN OF BOTTOM CHORD(Cont…)
Permissiblestressforwindorseismiccaseis
increasedby16.667%(Table1ofSBC)
Actualstress<permissiblestressforbothcasesthen
assumedsectionissafeotherwiserevisethesection
Designofstitchingweld:
Calculationofforceatthelevelofweld
Permissiblestressinweld(appendix-GofSBC&clause
13.4ofweldedbridgecode)
Sizeofweldcalculated(subjecttoclause6.2ofwelded
bridgecode)
Designoflacing&batteningoftensionmembers
(Clause6.9&6.10ofSBC)
Designofdiaphragms(Clause6.16ofSBC)

39. DESIGN OF TOP CHORD
Topchordmembersarecompressionmember
Sectionassumedfortopchordmembers(takinginto
considerationclause4.5&clause6.2ofSBC)
Effectiveareaofthesection(clause6.2.2ofSBC)
Actualstressescalculatedforaxialcompressionfor:
Withoutseismicorwindforces
Withseismicorwindforces
Permissiblestressinaxialcompressionisminimum
of:
Basicpermissiblestress(clause3.7ofSBC)
Stressinaxialcompression(clause3.7ofSBC)
Permissiblestressinfatigue(clause3.6ofSBC)

40. DESIGN OF TOP CHORD (Cont…)
Permissiblestressforwindorseismiccaseis
increasedby16.667%(Table1ofSBC)
Actualstress<permissiblestressforbothcasesthen
assumedsectionissafeotherwiserevisethesection
Designofstitchingweld:
Calculationofforceatthelevelofweld
Permissiblestressinweld(Appendix-GofSBC&clause
13.4ofweldedbridgecode)
Sizeofweldcalculated(Subjecttoclause6.2ofwelded
bridgecode)
Designoflacing&batteningofcompression
members(Clause6.5&6.6ofSBC)
Designofdiaphragms(Clause6.16ofSBC)

41. DESIGN OF END RACKER
Endrackersubjectedtoaxialcompression&bending
(Clause6.19ofSBC)
Sectionassumedforendracker(takinginto
considerationclause4.5&clause6.2ofSBC)
Effectiveareaofthesection(Clause6.2.2ofSBC)
Actualstressescalculatedforaxialcompression&
bendingfor:
Withoutseismicorwindforces
Withseismicorwindforces
Permissible stress in compression is minimum of:
Basic permissible stress (Clause 3.7of SBC)
stress in axial compression (Clause 3.7of SBC)
permissible stress in fatigue (Clause 3.6 of SBC)

42. DESIGN OF END RACKER (Cont…)
Permissiblestressinbending(Table2ofSBC)
Permissiblestressforwindorseismiccaseis
increasedby16.667%foraxialcompression&
bendingboth(Table1ofSBC)
Adequacyofsectionischeckedforcombined
stressesforbothcases(Clause3.11.1ofSBC)
Design of stitching weld, design of lacing & battening
and design of diaphragms same as compression
member

43. DESIGN OF DIAGONALS & VERTICALS
Diagonalsarereversiblestressmembers
Sectionofdiagonalshavetobecheckedforboth
tension&compression
Verticalsaretensionmembers
Designdonesimilartobottomchord

44. DESIGN OF PORTAL BRACINGS SYSTEM
Forceanalysisinmembersofportalsystemdonefor
forces:(Clause6.19ofSBC)
50%oflateralforcesontopchord
Lateralshearequalto1.25%oftotalforceintwoend
rackerorintwotopchordsinendpanelwhicheveris
greater
Topmemberofportalsubjectedtoaxialcompression
&bendingmomentboth
Designoftopmemberissimilartothatofendracker
Kneeportalistensionorcompressionmemberasper
thedirectionofapplicationofnodalforce
Kneeportalisdesignedforbothaxialtension&
compression

45. DESIGN OF TOP LATERAL BRACINGS
Forceanalysisintoplateralbracingsystemdonefor
forces:(Clause6.17ofSBC)
Lateralforceontopchord
2.5%offorceintopchordmembers
Bracingmembersaretensionorcompression
memberdependinguponthedirectionofapplication
ofnodalforce
Bracingmembersaredesignedforbothaxialtension
&compression

46. DESIGN OF BOTTOM LATERAL BRACINGS
Forceanalysisinbottomlateralbracingsystemdone
forforces:(Clause6.17ofSBC)
Lateralforceonbottomchord&movingload
50%oflateralforceintopchordtransmittedthroughsway
bracings
Rackingforce
Longitudinalforce
Bracingmembersaretensionorcompression
memberdependinguponthedirectionofapplication
ofnodalforce
Bracingmembersaredesignedforbothaxialtension
&compression

47. DESIGN OF JOINTS
Connectionatintersectionisdoneasperclause6.12
ofSBC
Rivetvalueiscalculatedforrivetstobeused
Numberofrivets=forceinmember/rivetvalue
Arrangementofrivetsatajointisdoneasperclause
7.1to7.9ofSBC
Splicingofmembersisdoneasperclause6.11of
SBC

48. CAMBER
Camberdiagramispreparedasperclause4.16&
appendix-AofSBC
Cambercalculatedfordeadload&fullliveload
includingimpact
Forcesinmembersarecalculatedfortheseloads
Changeinlengthofmembersduetoforcesin
members=FL/AE
Intensionmembersincreaseinlength&for
compressionmembersdecreaseinlength
Straincorrectionisappliedinnominallengthequal
tochangeinlengthofmembers
Fortensionmembersitisnegative&forcompression
membersitispositive

49. CAMBER (Cont…)
Toavoidchangesinthelengthoffloorsystemfurther
changeinlengthdoneinlengthofallmembers
Thischangeequalto((loadedchordextensionor
contraction/loadedchordlength)*lengthofmember)
Forthroughspansthischangeisincreaseinlengthof
members&fordecktypeitwillbedecrease
Nominallengthsalteredasabovegiveagirder
correctlystressedcamber
Nominallengthsandcamberedlengthroundedoffto
nearest0.5mm.

50. DEFLECTION
Deflection<lengthofgirder/600(Clause4.17ofSBC)
Verticaldeflectionatthecentreofspaniscalculated
byapplyingunitloadatthecentreoftruss
Deflectionatcentre=∑((FL/AE)*U)

51. THANK YOU