Bridges, hydraulic design, structural design, communication bridges

1. Communications-Bridges
 Definition:
 Bridge: A Structure having a total length above 6m between the inner faces of the
dirt walls for carrying traffic or other moving loads over a depression, a obstruction
such as a channel, road or a railway.
• Minor Bridges: A bridge having a total length of 60m.
• Major bridges: A bridge having a total length more than 60m.
 Culvert:
A structure having a total length less than 6m between the inner faces of the
dirt walls.
 Foot bridge:
A bridge exclusively used to carry pedestrians, cycles and animals. width
shall not be less than 1500mm.
 High Level Bridge:
A bridge which carries the road way above the HFL of a
channel.
 Submersible bridge:
A submersible bridge or vented causeway is a bridge designed to
be overtopping during floods.

2. Communications-Bridges
 Width of carriage way:
The minimum clear width measured at right angles to the longitudinal centre
line of the bridge between the inside faces of roadway kerbs of wheel guards.
 Width of footway:
The minimum clear width any where within a height of 225mm above
the surface the footway or safety kerb. Normally 1.5 m from outer rounding of kerb to
inner fce of the parapet/railing.
 Safety Kerb:
A road way for usage of pedestrians. High Level Bridge: A bridge which carries
the road way above the HFL of a channel.
 Super elevation:
Transverse inclination given to the cross section of a carriageway on a horizontal
curve in order to reduce the effect of centrifugal force on a moving vehicle.
 Crust level of the bridge:
It shall be the highest of the following:
• Road crust level
• TBL of the canal
• Ground level

3. Communications-Bridges
 Submersible bridges and vented Causeways:
Railing shall be either collapsible or
removable.
 Crash Barriers:
Suitable designed crash barriers provided to safe guard against errant
vehicles. Metal or RCC.
• Multilane bridges and bridges on a urban area
• Flyover and interchanges
• ROBs across railway line
• Open sea, breakwaters, deep valleys
 Types:
• Vehicle cross barriers.
• Combination Railway/Vehicle Pedestrian Crash Barriers
• High Containment Barriers

4. Communications-Bridges
 Basic Data:
• Site plan with contours showing the flow direction of the canal, road way angle
(direction of skew if any), and the approach of the road for 200m on either side.
• Names of the village/town connected on either side.
• Hydraulic particulars of the canal both upstream and downstream.
• LS of the canal and the road for at least 250 m on either side of crossing.
• Cross sections of the canal and the road duly marking, Levels, such as BL, FSL,
TBL, GL, road crust level etc.,
• TPs Particulars, taken up to hard strata or to a minimum depth of 2m below
CBL or ground level which ever deeper with soil classification.
• Bearing capacity of the foundation strata.

5. Communications-Bridges
 Design Criteria:
• Hydrology of the drain or stream.
• Hydraulic design of
i. The stream or drain
ii. The hydraulic deign of the canal
• Structural Design.
i. Super structure
ii. Sub structure
References: IRC: 5-1998, 6-2000, 21-2000,78-2000, 83 (Part-1)-1987,

6. Communications-Bridges
 Design Criteria:
 Hydraulic design of
• Design of vent way
• Bridge crust level
• Afflux by Molesworth’s formula (max.50mm).
• Check for Scour
 Structural Design:
• Super structure
• Substructure.
References: IRC: 5-1998, 6-2000, 21-2000,78-2000, 83 (Part-1)-1987,

7. Bridges-Hydrology
 Hydrology of the stream or drain:
Table-1
Category Canal Discharge Stream Discharge Flood Frequency
in cumecs in cumecs
A 0.0 - 0.5 All discharges 1 in 25 years
B 0.5 – 15 0 – 150 1 in 50 years
Above 150 1 in 100 years
C 15 – 30 0 – 100 1 in 50 years
Above 150 1 in 100 yeas
D Above 30 0 - 150 1 in 100 years
Above 150 Detailed study
• IS: 7784 (part-1)-1973.

8. Communications-Bridges
Hydrology of the Drain/Stream: Detailed study in the case of drain discharge > 150
cumecs and canal discharge > 30 cumecs.
S.No. Type of Canal Catchment Area (CA) in ‘M’ in Sq. Miles
Up land Areas Deltaic Tracts
1. Main Canal Dickens’s formula, Rye's formula
Q = CM 3/4 Q = CM 2/3
C=1400 for CA<1.00 C=1000
C=1200 for CA=1 to 30 Velocity shall not exceed 10 ft/sec
C=1060 for CA=30 to 500
Q=7000 M1/2 for CA>500
Velocity in the barrel up to
12 to13 ft/sec
2. Branch Canal Q=CM 2/3
Q > 500 c/s C=1000 and Velocity<10’/sec same as upland area
3. Distributaries Q = CM 2/3 same as upland
area
Q < 500 c/s C=750 and Velocity< 10”/sec
• Lr. No. CDO/EE-C1/1084/83-3 dated 23.08.1983.

9. Bridges-Hydrology
 Hydrology of the stream or drain:
Table-1
Category Canal Discharge Stream Discharge Flood Frequency
in cumecs in cumecs
A 0.0 - 0.5 All discharges 1 in 25 years
B 0.5 – 15 0 – 150 1 in 50 years
Above 150 1 in 100 years
C 15 – 30 0 – 100 1 in 50 years
Above 150 1 in 100 yeas
D Above 30 0 - 150 1 in 100 years
Above 150 Detailed study
• IS: 7784 (part-1)-1973.

10. Bridges-Hydraulic design
 Linear Waterway:
Width of the water way between the extreme edges of water surface at the
highest flood level measured at right angle to the abutment faces.
Layce’s wetted perimeter (Pw) in meters using the formula
Pw = C(Q)1/2
Where C = a coefficient, a value 4.8 (4.5-6.3)and
Q is the flood discharge in cumecs
 Effective linear waterway:
Total width of the waterway at HFL minus the effective width of the
obstruction.
 Length of the bridge:
Over all length measured along the centre lline of the bridge between inner
faces of dirt walls.

11. Bridges-Hydraulic design
 Vertical Clearance:
• No part of the bearings shall be at a height less than 500mm
• Vertical clearance above the roadway in any traffic lane up to the lowest point 5.5
 Free board:
• It shall not be less than 750mm for approaches to high level bridges.
 Scour Depth:
• Mean scour depth is the depth (dm) below HFL or FSL in m
d = 1.34[q2 /f]1/3
Where, q = Discharge per meter width with or without concentration of flow in
cumecs,
f = Layce’s silt factor expressed as f = 1.76 (d m )1/2
dm = average grain size

12. Bed material Weighted mean diameter Value of silt
of particle in mm-dm factor- f
Coarse silt 0.040 0.350
Fine silt 0.081 0.500
Fine silt 0.120 0.600
Fine silt 0.158 0.700
Medium silt 0.233 0.850
Standard silt 0.323 1.000
Medium sand 0.505 1.250
Coarse sand 0.725 1.500
Fine bajira & sand 0.988 1.750
Heavy sand 1.290 -2.00 2.000 – 2.42
Bridges-Hydraulic design

13. Bridges-Hydraulic design
 Maximum Scour depth or Designed Scour Depth 9dorR) in m:
• Straight reaches for individual foundations without floor protection
In the vicinity of pier 2.00 d
Near abutments 1.27 d approaches retained
2.00 d scour all round
Floods with seismic combinations the values may be reduced by 0.9
For floor protection works, for raft foundations and shallow foundations
In straight reaches 1.27 d
At moderate bends 1.50 d
At sever bends 1.75 d m
At right angle bends 2.00 d
 Depth of Foundation:
• In Soils Up to safe bearing capacity or a minimum of 2.0m below the scour level or
the protected bed level.
• Hard rock with crushing strength 10 MPA: 600mm
• All others : 1500mm

14. Bridges – Structural design
 Loading Classification
• IRC Class AA Loading or Class 70-R Loading
• IRC Class A Loading
• IRC Class B Loading – adopted for temporary structures only
 Loads, Forces and Stresses:
1. Dead Loads 2. Live Loads 3. Snow loads
4. Impact and Dynamic Loads 5. Vehicle collusion load 6. wind load
7.Impact due to floating bodies 8. Water currents 9. Breaking force
10. Centrifugal forces 11. Buoyancy 12. Temperature effects
13. Deformation effects 14. Secondary effects 15.Errection effects
16. Seismic force 17. Wave pressure 19. Grade effects
19. Earth Pressure & LL surcharge

15. Bridges – Structural design
 Loads, Forces and Stresses:
i. Wind Load:
Horizontal force:
• For deck- area as seen in elevation including floor and railing, less area of perforation
in the hand railing
• For through or half trough structures- The area of elevation of the wind ward truss as
specified as above plus half the area of elevation above he deck level of all other
trusses or girders.
The intensity of wind force based on wind pressure and wind velocity.
• It shall be doubled for Guntur, Krihna, Godavri, Visakha, Vijayanagaram and
Srikakulam districts along the coast line

16. Wind Pressure and Wind Velocity
H V P H P V
0 80 40 30 147 141
2 91 52 40 155 157
4 100 63 50 162 171
6 107 73 60 168 183
8 113 82 70 173 193
10 118 91 80 177 202
15 128 107 90 180 210
20 136 119 100 183 217
25 142 130 110 186 224
Where W=Average height in m of the exposed suface above ground, bed level
or water level
V= Horizontal velocity f wind in Km per our at height H
P= Horizontal wing pressure in Kg/Sq.m at height H
(con….)
Bridges – Structural design

17. Bridges – Structural Design
 Wind Load:
• The lateral wind force against any exposed moving live load as acting 1.5m above road way
and shall be assumed to have the following value.
a. Highway bridges , ordinary : 300 Kgs/linear meter
b. Highway bridge carrying tramway: 450 Kgs/linear meter
• The bridge no carrying any live load when the wind velocity at deck level exceeds 130 Kms
per hour.
• The total assumed wind forces as calculated in accordance above cl.1 to 4, shall however ,
not less than 450 Kg per linear meter in plane of the load chord and 225 Kg per liner meter
in the plane of unloaded chord on through or half through truss, lattice or other similar
spans, and not les than 450 Kg per linear meter on deck slab.
• A wind pressure f 240 Kg/Sqm on the unloaded structure, applied as specified in cl1, 2,
shall be used if it produces greater stresses than those produced by the combined wind
forces as peer cl. 1, 2,4 or by the wind force as per cl.5

18. When the current strikes the pier at an angel it resolved in to
two components.
1.Presur parallel to pier- as above
2. Normal to the pier, acting on the area of the side elevation of the pier- as
with K as 1.5, except for circular piers which shall be 0.66.
Possible variation of water current direction inclined at (20±Ə) to length of pier
Bridge having pucca floor static force due to difference in head of 250mm between
the two faces of the pier.
Bridges – Loads, Forces,& Stresses

19. Bridges – Loads, Forces,& Stresses
 Longitudinal Forces:
 Force arising from any one or more of the flowing:
a. Tractate force due to acceleration
b. Breaking effect (invariably greater than tractate force)
c. Frictional resistance offered by the free bearings due to temperature change.
 The Breaking effect:
i. In the case of single lane or two lane bridges:
a. 20% of first train load plus 10% of the succeeding train or part thereof on one lane
only
b. If the entire train is not on the full span, breaking force shall be 20% of the loads
actually on the span,
ii In the case of more than two lanes:
• As in ‘A’ above for the first two lanes plus 5% of the loads on the lane in excess of
two.
• The force due to breaking effect acting at 1.2 m above parallel to road way.

20. Bridges – Loads, Forces,& Stresses
• The change in vertical reaction at the bearings to be accounted for.
 Simply supported spans on unyielding supports:
• For spans of fixed and fee bearing other than Elastomeric bearings, longitudinal forces
Fixed bearing Free bearing
(i). Fh-µ(Rq+Rg) µ(Rq+Rg)
Or (ii). Fh/2 + µ(Rg+Rq) µ(Rg+Rq)
Where Fh= Applied horizontal force
Rg= Reaction due to dead load at free end
Rq= Reaction due to live load at fee end
µ = a coefficient
For steel roller bearings 0.03
concrete roller bearings 0.05
sliding bearings 0.30 to 0.50
Teflon on stainless steel 0.03 to 0.05
 Plate bearings up to 15m span for RCC or Pre stressed super structure. :

21. Bridges – Loads, Forces,& Stresses
 Centrifugal Forces:
 Determined from the following formula:
C = W V2/ 127 R
Where C= Centrifugal force in tonnes
W= live load in tonnes in case of wheel loads and tonnes per linear meter in
case of UDL
V= Designed seed in km per hour
R= Radius of curvature in meters
 Consider to act at a height of 1.2 m above the level of the carriageway :
 No increase for impact effect.

22. Bridges – Loads, Forces,& Stresses
 Buoyancy:
 For full Buoyancy a reduction is made in the gross weight of the member:
• Member displaces water only in shallow foundations, the reduction in weight equal to
the volume of displaced water.
• Member under consideration displaces water and also silt and sand (deep piers and
abutment), the upward pressure causing the reduction in weight shall be
a. Full hydrostatic pressure due to a depth of water equal to the difference in level
between the free surface and the foundation
b. Upward pressure due to the submerged weight of the silt or sand in accordance
with Rankin's theory.
• In design of submerged masonry or concrete , the buoyancy through pore pressure
may be limited to 15% of full buoyancy.
• In case of submerged bridgeless, the full buoyancy of super structure be considered.

23. Bridges – Structural Design
 Earth Pressure:
• In accordance with any rational theory. Coulomb’s theory is accepted.
• All abutments and return walls shall be designed for a live load surcharge equivalent to
1.2m earth fill.
 Approach slab:
• RCC approach slab with 12mm dia. 150mm c/c in each direction both at top and bottom
as reinforcement in concrete grade in M30 for the entire width of road way for a length not
less than 3.5m.
 Temperature:
 Seismic Forces:
• Both the horizontal and vertical forces acting simultaneously.
• Horizontal seismic force:
• Feq = α β λ G
• Where α= Horizontal seismic coefficient.
• β= Coefficient depending on the soil foundation
• λ= coefficient - important bridges… 1.5 and other bridges..1.0
• Horizontal Seismic coefficient α;

24. Zone I II III IV V
α 0.01 0.02 0.04 0.05 0.08
• Seismic forces shall not be considered in the direction of live load but in the direction
perpendicular to the traffic.
Bridges – Structural Design

25. Bridges – Structural Design
 Super structure:
 Design of Deck slab or girder
• As per MOST drawings
• IRC:6-2000, IRC: 21-2000
 Sub structure:
• Piers:
• Minimum thickness 1000mm
• All abutments and return walls shall be designed adopting coulomb’s/Rankin’s theory,
with top width 500mm.
• All abutments and return walls shall be designed for a live load surcharge equivalent to
1.2m earth fill.
 Approach slab:
• RCC approach slab with 12mm dia. 150mm c/c in each direction both at top and
bottom as reinforcement in concrete grade in M30 for the entire width of road way for
a length not less than 3.5m.

27. Bridges – Foundations
 Factor of safety:
 Factor of safety on Soils … 2.5.
 Factor of safety on Rock .… 6 to8
 Allowable Settlement (differential settlement)
• Not exceeding 1 in400 of the distance between two foundations.
 Permissible Tension:
• No tension on soils
• In rock the base area to be reduced to a size where no tension will occur such reduced area not < 67%
 Factor of safety for stability:
• For open foundations:
With out Seismic with Seismic
i. Against overturning 2 1.5
ii. Against sliding 1.5 1.25
iii. Against deep-seated failure 1.25 1.15
 Frictional coefficient Tan Ø, Ø being angle friction:
• Between soil and concrete … 0.5
• Between rock and concrete…0.8 for good rock and 0.7 for fissured rock.

28. Bridges – Foundations
 Well Foundations:
 Cutting edge: In mild steel not < 40 Kg. per cum.
 Well Curb:
• In variably in RCC grade not < M25 with minimum steel 72 Kg. per cum.
• The internal angle 300 to 370
• In case of blasting anticipated steel plate of thickness not < 10mm up to top of well curb.
 Bottom Plug:
• Top shall be 300mm above top of kerb with suitable sump (shear Key) below the level of cutting
edge.
• CC with minimum cement 330 Kg. per cum. Increase cement for Tremie concrete.
 Filling of well:
• Refill with excavated earth or sand
 Plug over fill:
 300mm thick in CC M15.
 Well Cap:
• Bottom of well cap be below LWL
• Reinforcement from steining shall be anchored in well cap
• Design on any acceptable rational method.
 Sinking of well:
• Sinking of well can not be started till the cured for at least 48 hours.

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