Training Programme On Introduction To Bridge Engineering: General Principles
Training Programme On Introduction To Bridge Engineering: General Principles
Training Programme On Introduction To Bridge Engineering: General Principles
GENERAL PRINCIPLES
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, water body, road or a railway line.
• Minor Bridges: A bridge having a total length up to 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.
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/stream
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.
Crash barriers can be of Metal or RCC.
Types:
• Vehicle cross barriers.
• Combination Railway/Vehicle Pedestrian Crash Barriers
• High Containment Barriers
Communications-Bridges
Approaches to bridge
Minimum straight length of 15m on either side and width equal to the carriage width.
Bearings:
Expansion Joints
Expansion joints are provided at the end of deck and cater for movement of deck
Foundations:
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.
• LS of the canal/stream 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 top level etc.,
• Hydraulic design of
• Structural Design.
i. Super structure
iii. Foundation
Structural Design:
• Super structure
• Substructure.
Table-1
Category Canal Discharge Stream Discharge Flood Frequency
in cumecs in cumecs
Table-1
• It is the vertical distance measured from HFL or FSL including the afflux o the
underside of deck of the structure..
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
cumec,
f = Lacey's silt factor expressed as f = 1.76 (d m )0.5
dm = average grain size
Bridges-Hydraulic design
Well foundation:
• Sump (Shear Key) diameter 1.5m to 2.0m less than inner hole, anchored 1.5m below
with six dowel bars of diameter 25mm places in 65mm grout hole and projected 1.5m
above
Bridges – Structural design
Loading Classification
• Steel bridges:
• Tracked vehicles: 10% for all sans
• Wheeled vehicles: 25% for spans up to 23m and in accordance with curves for
span > 23m
Bridges – Structural design
Loads, Forces and Stresses:
Impact:
If the earth filling is > 600mm including the road crust the impact shall be reduced to 50%.
Pressure on the top 3m of the structure below the bed block – 50% decrease to Zero at
bottom
Pressure on the portion of the structure > 3m below bed block - Zero
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.
Bridges – Structural design
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
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
Bridges – Loads, Forces,& Stresses
Horizontal Forces Due to Water Currents:
• On piers parallel to the direction of the water current, the intensity of pressure shall be as
follows:
P = 52 KV2
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.
Bridge having pucca floor static force due to difference in head of 250mm between
the two faces of the pier.
Bridges – Loads, Forces,& Stresses
Longitudinal Forces:
• The force due to breaking effect acting at 1.2 m above parallel to road way.
Bridges – Loads, Forces,& Stresses
• The change in vertical reaction at the bearings to be accounted for.
Elastomeric bearings:
• The sub structure and foundation shall also be designed for 10% variation in
movement of the span on either side.
Bridges – Loads, Forces,& Stresses
Centrifugal Forces:
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.
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;
Bridges – Structural Design
Super structure:
Sub structure:
• Piers:
• Minimum thickness 1000mm
• All abutments and return walls shall be designed adopting coulomb’s/Rankin’s theory
• 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.
Bridges – Structural Design
Miscellaneous Items:
• RCC Kerbs
• Railing:
• Expansion, contraction, construction Joints
• Drainage spouts
• Wearing coat
• Pedestals & Drainage arrangements
• Bearings
• Dirt Walls
• Guide posts
• Weep holes
Minimum Concrete grade:
• RCC for Substructure and Foundation: M25
• RCC for Deck slab and Girders: M30
• CC: M15
• Leveling course: M15