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

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

INTRODUCTION
Truss bridge:
Used for spans greater than what can be spanned
economically by a plate girder bridge.
In general truss bridges are used for spans
greater than 30 m.

INTRODUCTION (Cont...)
 Forms Of Open Web Girder Bridges
 Through Type
 Deck Type (Underslung)
 Semi Through Type

COMPONENTS OF THROUGH TYPE
BRIDGE
 Floor System:
 Cross girder (Bending)
 Rail bearers or Stringers(Bending)
 Primary Members:
 Bottom chord members (Tension members)
 Top chord members (Compression members)
 End rackers (Compression & Bending members)
 Diagonals (Reversible stress members)
 Verticals (Tension members & redundant members)

COMPONENTS (Cont...)
 Secondary members:
 Bottom lateral bracings (Axial force)
 Top lateral bracings (Axial force)
 Sway bracings & knee sway (Axial force & Bending)
 Portal bracings & knee portal (Axial force & Bending)
 Main gussets
 Bearings

GENERAL CONFIGRUATION
 Type of truss
 Warren truss with verticals for standard railway spans
 Other forms may be adopted as per different conditions
 Number of panels
 Weight of truss Vs Floor system
 Optimum number is 6 to 10
 Length of panel
 Weight of truss Vs Floor system
 Optimum length is 6 m. to 9 m.
 Inclination of diagonals
 Between 45° and 60° with the horizontal

GENERAL CONFIGRUATION (Cont…)
 Height of truss
 Through type Vs Deck type
 Between 1/8 and 1/5 of span length
 Spacing of trusses
 Sufficient to prevent overturning due to lateral loads
 > 1/3 of height of truss & > 1/20 of span

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

DEAD LOAD
 Dead load of truss is assumed before design on the
basis of experience & earlier designs
 After design of truss the actual dead load of truss is
compare with assumed dead load
 If there is difference between two, then assumed
dead load is revised and structure is designed with
revised dead load

LIVE LOAD
 Clause 2.3 of bridge rule.
 Estimated on the basis of loading standard.
 EUDL(equivalent uniformly distributed loads ) are
given in appendix of bridge rules for different loading
standards.
 EUDL is given for bending moment and shear force
 EUDL for Bending Moment:
 For Maximum forces in elements resisting bending
(Bottom chords & top chords)
 EUDL for Shear Force:
 For Maximum forces in elements resisting shear at section
(end racker, diagonals, verticals)

DYNAMIC EFFECTS
 Clause 2.4 of bridge rule
 Augmentation in load due to dynamic effects is
considered by adding a load equivalent to a
coefficient of dynamic augment(CDA) multiplied by
the live load giving the maximum stress in member
under consideration.
 For single track spans:
 CDA=0.15+(8/(6+L)) subject to a maximum of 1.0
Where L= loaded length of span in meters for the position
of the train giving the maximum stress in the member
under consideration

LONGITUDINAL FORCES
 Value of longitudinal force due to either tractive
effort or braking force shall be obtained from
appendices.
 Values depend on loaded length and standard of
loading.
 Maximum of tractive effort or braking force is taken
as longitudinal force.

RACKING FORCE
 Clause 2.9 of bridge rule.
 Lateral bracings of loaded deck of spans to be
designed for a lateral load due to racking force of 600
kg/m. treated as moving load.
 Racking force not to be considered for calculating
stresses in chords or flanges of main girders.

WIND PRESSURE EFFECT
 Clause 2.11 of bridge rules.
 Wind pressure expressed as a equivalent static
pressure in windward direction
 Wind pressure shall apply to all loaded or unloaded
bridges.
 But bridge shall not considered to carry any live load
when wind pressure at deck level exceeds 150 kg/m2
for B.G.
 Wind force calculated for loaded spans with wind
pressure 150 kg/m2.

WIND PRESSURE EFFECT (Cont…)
 Wind Force = wind pressure*exposed area
 Exposed area = area of moving load + exposed area
of truss members.
 Full area of truss members on windward side +50%
area of truss members on Leeward side.

SEISMIC FORCE
 Seismic forces:
 Horizontal seismic force
 Vertical seismic force
 Seismic forces calculated taking into consideration
seismic zone, importance of structure and its soil
foundation system.
 Design seismic coefficients:
αh=βIα0
αv= αh/2

SEISMIC FORCE (Cont…)
 F=Wm*αh (or αv)
F = Seismic force
Wm=Weight of mass under consideration
ignoring reduction due to buoyancy
 Horizontal seismic force due to live load on the
bridge shall be ignored when acting in the direction
of traffic
 When acting in the direction perpendicular to traffic,
this is to be considered for 50% of design live load
without impact.

ANALYSIS OF FORCES
 To find out the forces in members of truss due to
various loads.
 Forces can be found out either by suitable computer
program or by hand calculation.
 Hand calculation is done by using influence line
diagrams(ILD) for various members of truss.
 ILD are prepared for a member of truss by calculating
force in member as a unit load moves across the deck
of the truss.
 Area of ILD calculated and multiplied by the force
intensity to get force in a particular member.

DEAD LOAD ANALYSIS
 Dead load intensity is same for all the members of
truss.
 Dead load intensity (per truss per unit length)
= total assumed dead load/(2*span length)
 Force due to dead load in each member of truss are
calculated by multiplying dead load intensity with
area of ILD.

LIVE LOAD ANALYSIS
 Bottom chord members have tension in ILD. Loaded
length is length of span.
 Top chord members have compression in ILD.
Loaded length is length of span.
 Live load intensity for chord members
=EUDL bending/(2*loaded length)
 CDA for chord members is calculated taking L as span
length.
 End racker have compression in ILD. Loaded length is
length of span.
 Live load intensity for end racker
=EUDL shear/(2*loaded length)

LIVE LOAD ANALYSIS (Cont…)
 CDA for end racker is calculated taking L as span
length.
 Diagonal members have both tension & compression
in ILD. loaded length for tension & compression is
found from ILD.
 Live load intensity & CDA for diagonals are
calculated for tension & compression both based on
their respective loaded lengths.
 Force due to live load = ILD area*live load intensity
 Force due to dynamic effect = CDA*force due to live
load

LONGITUDINAL FORCE
 Longitudinal force taken for only bottom chord
members.
 This depends on position of different bottom chord
members.
 For bottom chord member in end panel loaded
length for longitudinal force is full span.
 Loaded length reduces by one panel length as we
take bottom chords of other panels starting from end
to centre.
 Based on loaded length longitudinal force is found in
bottom chord members.

 General concept of load transfer and how the wind
forces are distributed among the members
Wind Load = Wind pressure X exposed area
Exposed Area = Area of moving load + exposed area
of truss member
WIND LOAD ANALYSIS

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

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
Total AT=T1+T2+T3 AB=B1+B2+B3
Through Type Truss

 Wind force on top chord = Wind pressurexATX1.5=WT
 Wind force on bottom chord= W P[1.5(AB-
B3)+B3]=WB
 Nodal force at top chord:
 At intermediate nodes = WT/No. of top panel=Tint.
 At end nodes = Tint/2
 Nodal force at bottom chord:
 At intermediate nodes = WB/No. of bottom panel=Bint.
 At end nodes = Bint/2

 Wind load analysis is done for following situations:
 Horizontal bending of bottom chord due to wind force on
bottom chord & moving load
 Vertical bending of span due to wind force on bottom
chord & moving load
 Horizontal bending of bottom chord due to wind force on
top chord transmitted through sway bracings
 Vertical bending of span due to wind force on top chord
transmitted through sway bracings
 Horizontal bending of top chord due to wind load on top
chord
 Overturning effect of portal

SEISMIC FORCE ANALYSIS
 Seismic force calculated in horizontal & vertical
direction
 In horizontal direction seismic force calculated for
bottom chord & top chord
 On bottom chord seismic force is due to dead load as
well as live load & on top chord seismic force is due
to dead load only
 In vertical direction seismic force is due to dead load
as well as live load
 Analysis of seismic force for forces in members is
same as that of wind force

FORCE IN TRUSS MEMBERS
 Force in truss members found by adding forces due
to dead load, live load with dynamic effect,
longitudinal loads, wind load or seismic loads

DESIGN OF STRINGER
 Loaded length for stringer = length of one panel
 Bending moment & shear force calculated by getting
EUDL bending or EUDL shear as per case
 Dead load of stringer & track also considered
 Section assumed for stringer
 Actual stresses calculated for bending moment &
shear force
 Permissible stresses for bending is minimum of :
 Basic permissible stress (clause 3.7of SBC)
 Permissible stress in fatigue (clause 3.6 of SBC)
 Permissible stress in bending compression (clause 3.9 of
SBC)

DESIGN OF STRINGER (Cont…)
 Permissible shear stress (Table II of SBC)
 Actual stress < permissible stress then assumed
section is safe otherwise revise the section
 Design of connection between web & flange of
stringer:
 Calculation of horizontal shear at the level of weld
 Permissible stress in weld (Appendix-G of SBC & clause
13.4 of welded bridge code)
 Size of weld calculated (Subject to clause 6.2 of welded
bridge code)

DESIGN OF STRINGER (Cont…)
 Provision of stiffeners (Clause 5.10 of SBC)
 Design of stringer bracings:
 Calculation of lateral load (Clause 2.9.2 of bridge rule)
 Analysis for force in stringer bracings.
 Design of stringer bracings (Clause 6.2.3 & 3.8 of SBC)

DESIGN OF CROSS GIRDER
 Loaded length for cross girder for EUDL
= 2*centre to centre distance of cross girder
 L for CDA = 2.5*cross girder spacing
 Bending moment & shear force calculated by getting
EUDL
 Dead load of stringer, track & cross girder also
considered
 Section assumed for cross girder
 Design process for cross girder is same as stringer

DESIGN OF CROSS GIRDER (Cont…)
 Connection of cross girder with stringer
 Calculate number of rivets for:
-one span loaded
-both span loaded
 Connection of cross girder with vertical & Bottom
chord
 Find rivet value & calculate number of rivets required for
connection

DESIGN OF BOTTOM CHORD
 Bottom chord members are tension member
 Section assumed for Bottom chord members (Taking
into consideration clause 4.5 & clause 6.7 of SBC)
 Effective area of the section calculated (clause 4.3.2
of SBC)
 Actual stresses calculated for axial tension for:
 Without longitudinal & seismic or wind forces
 With longitudinal & seismic or wind forces
 Permissible stress for axial tension is minimum of:
 Basic permissible stress (clause 3.7of SBC)
 Permissible stress in fatigue (clause 3.6 of SBC)

DESIGN OF BOTTOM CHORD (Cont…)
 Permissible stress for wind or seismic case is
increased by 16.667% (Table 1 of SBC )
 Actual stress < permissible stress for both cases then
assumed section is safe otherwise revise the section
 Design of stitching weld:
 Calculation of force at the level of weld
 Permissible stress in weld ( appendix-G of SBC & clause
 Size of weld calculated (subject to clause 6.2 of welded
bridge code)
 Design of lacing & battening of tension members
(Clause 6.9 & 6.10 of SBC)
 Design of diaphragms (Clause 6.16 of SBC)

DESIGN OF TOP CHORD
 Top chord members are compression member
 Section assumed for top chord members (taking into
consideration clause 4.5 & clause 6.2 of SBC)
 Effective area of the section (clause 6.2.2 of SBC)
 Actual stresses calculated for axial compression for :
 Without seismic or wind forces
 With seismic or wind forces
 Permissible stress in axial 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)

DESIGN OF TOP CHORD (Cont…)
increased by 16.667% (Table 1 of SBC )
 Actual stress < permissible stress for both cases then
assumed section is safe otherwise revise the section
 Design of stitching weld:
 Calculation of force at the level of weld
 Permissible stress in weld (Appendix-G of SBC & clause
 Size of weld calculated (Subject to clause 6.2 of welded
bridge code)
 Design of lacing & battening of compression
members (Clause 6.5 & 6.6 of SBC)
 Design of diaphragms (Clause 6.16 of SBC)

DESIGN OF END RACKER
 End racker subjected to axial compression & bending
(Clause 6.19 of SBC)
 Section assumed for end racker (taking into
consideration clause 4.5 & clause 6.2 of SBC)
 Effective area of the section (Clause 6.2.2 of SBC)
 Actual stresses calculated for axial compression &
bending for :
 Without seismic or wind forces
 With seismic or wind forces
 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)

DESIGN OF END RACKER (Cont…)
 Permissible stress in bending (Table 2 of SBC)
increased by 16.667% for axial compression &
bending both (Table 1 of SBC )
 Adequacy of section is checked for combined
stresses for both cases (Clause 3.11.1 of SBC )
 Design of stitching weld, design of lacing & battening
and design of diaphragms same as compression
member

DESIGN OF DIAGONALS & VERTICALS
 Diagonals are reversible stress members
 Section of diagonals have to be checked for both
tension & compression
 Verticals are tension members
 Design done similar to bottom chord

DESIGN OF PORTAL BRACINGS SYSTEM
 Force analysis in members of portal system done for
forces: (Clause 6.19 of SBC)
 50% of lateral forces on top chord
 Lateral shear equal to1.25% of total force in two end
racker or in two top chords in end panel whichever is
greater
 Top member of portal subjected to axial compression
& bending moment both
 Design of top member is similar to that of end racker
 Knee portal is tension or compression member as per
the direction of application of nodal force
 Knee portal is designed for both axial tension &
compression

DESIGN OF TOP LATERAL BRACINGS
 Force analysis in top lateral bracing system done for
forces: (Clause 6.17 of SBC)
Lateral force on top chord
2.5% of force in top chord members
 Bracing members are tension or compression
member depending upon the direction of application
of nodal force
 Bracing members are designed for both axial tension
& compression

DESIGN OF BOTTOM LATERAL BRACINGS
 Force analysis in bottom lateral bracing system done
for forces: (Clause 6.17 of SBC)
 Lateral force on bottom chord & moving load
 50% of lateral force in top chord transmitted through sway
bracings
 Racking force
 Longitudinal force
 Bracing members are tension or compression
member depending upon the direction of application
of nodal force
 Bracing members are designed for both axial tension
& compression

DESIGN OF JOINTS
 Connection at intersection is done as per clause 6.12
of SBC
 Rivet value is calculated for rivets to be used
 Number of rivets = force in member/rivet value
 Arrangement of rivets at a joint is done as per clause
7.1 to 7.9 of SBC
 Splicing of members is done as per clause 6.11 of
SBC

CAMBER
 Camber diagram is prepared as per clause 4.16 &
appendix-A of SBC
 Camber calculated for dead load & full live load
including impact
 Forces in members are calculated for these loads
 Change in length of members due to forces in
members = FL/AE
 In tension members increase in length & for
compression members decrease in length
 Strain correction is applied in nominal length equal
to change in length of members
 For tension members it is negative & for compression
members it is positive

CAMBER (Cont…)
 To avoid changes in the length of floor system further
change in length done in length of all members
 This change equal to ((loaded chord extension or
contraction/loaded chord length)*length of member)
 For through spans this change is increase in length of
members & for deck type it will be decrease
 Nominal lengths altered as above give a girder
correctly stressed camber
 Nominal lengths and cambered length rounded off to
nearest 0.5 mm.

DEFLECTION
 Deflection < length of girder/600 (Clause 4.17 of SBC)
 Vertical deflection at the centre of span is calculated
by applying unit load at the centre of truss
 Deflection at centre=∑((FL/AE)*U)

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