2. • The various forces, loads
and stresses on bridge
– Buoyancy pressure
– Centrifugal forces
– Dead load
– Deformation stresses
– Earth pressure
– Impact load
– Live load
– Longitudinal forces
– Seismic Loads
– Water Pressure
– Wind Load
– Erection stresses
– Secondary Stresses
– Temperature
Variation forces
3. Buoyancy Pressure
• When the water table is above the foundation or any part of
the structure, there will be buoyancy force on the structure.
• It shall be considered during the desing as it could affect
significantly the desing of the structure.
• Facts to be remembered while considering Buoyancy
– While designing abutments – especially submerged part –
filling behind the abutment should not be washed away.
– For submerged masonry or concrete structure – the pores
is to be limited to 15% of full buoyancy effect.
– For Design of submerged bridges, the full buoyancy effect
on superstructure, piers and abutments is to be
considered.
4. Centrifugal Forces
• If bridge is to be built on horizontal curves, then the movement of vehicle
along curves will cause centrifugal force on to the super structure. Hence,
in this case design should be done for centrifugal forces also.
5.
6. Dead Load
• This DL depends upon various factors such as live load to be
carried out, length of the span, working stresses developed in
the design, etc.
• Initially this DL is assumed.
– Following are the rules for assumption:
– The DL of the structure is assumed by reference to suitable
empirical formulas
– After the design is finalized, the actual weight of the
structure is worked out. If there is appreciable difference
between the actual and assumed dead loads, the design is
revised accordingly.
10. Live Load
• The standard IRC loads specified in IRC:6-2000 are grouped under
four categories:
– IRC Class AA Loading
– IRC Class 70R Loading
– IRC Class A Loading
– IRC Class B Loading
Highway bridge decks have to be designed to withstand the live loads
spcified above. The different categories of loadings were first
formulated in 1958 and they have not changed in the subsequent
revisions of 1964,1966 and 2000.
11.
12.
13.
14.
15.
16.
17.
18. Impact Load
• The stresses developed due to fast moving of heavy
vehicles over uneven surfaces in case of road bridges or
due to trains moving over uneven rails in case of railway
bridges.
• Following points has to be considered for Impact Load
– Depth of floor – if the depth of the solid floor is more, the effect
is less.
– Filling of arch – in case of spandrel-filled arch bridges, the effect
of impact is reduced due to the absorbing power of filling.
– Footways – the foot ways are need not to be designed for
impact load.
19. – Hammer Blow Action – This action will be less in case of road
bridges and more in case of railway bridges
– Span – the impact load is inversely proportional to the span. It is
therfore, smaller on large span than on the shorter span.
– Speed of the vehicle – Impact load increases with the increase
in the speed of the vehicle.
This impact load will be added to live load in the form of
coefficient of impact or impact factor. There are various
empirical formulas used to calculate the impact factor.
20. • When the span exceeds 45m, the impact factor for RCC bridges and steel
bridges are taken as 0.088 and 0.154 respectively.
21. Span Vehicle type Impact factor
Less than 9 meters
Tracked vehicle
25% up to 5m and linearly
reducing to 10% from 5 m to 9
m.
Wheeled vehicle 25% up to 9 m
Greater than 9 meters
Tracked vehicle (RCC bridge) 10% up to 40 m
Wheeled vehicle (RCC bridge) 25% up to 12m
Tracked vehicle (steel bridge) 10% for all spans
Wheeled vehicle (steel bridge) 25% up to 23 m
For IRC Class AA Loading and 70R Loading
22. • If the length exceeds in any of the above limits, the impact
factor should be considered from the graph given by IRC
which is shown below.
23. Deformation Stresses
• Deformation stresses are occurred due to change is material properties
either internally or externally. The change may be creep, shrinkage of
concrete etc.
• Similarly horizontal forces will develop due to temperature changes,
braking of vehicles, earthquakes etc. Hence, these are also be considered
as design loads in bridge design.
• Deformation stresses are to be considered for steel bridges only.
• It should be designed in such a way that the deformation stresses should
be brought down to minimum.
• For the purpose of assumption only, the deformation stresses may be
taken as not less than 16% of the live load & dead load stresses.
• The deformation stresses should be ignored in case of prestressed girders.
24. Earth Pressure
• Bridge which are required to retain earth has to be designed for
earth pressure.
• The position of LL on earth cause pressure which is called live load
surcharge.
• IRC recommends the theory of Coulomb with slight modification
– The height of the centre of pressure above bottom is to be taken as
0.42 of the height of wall above the base instead of 0.33 of the height
as per coulomb’s theory.
– The design of abutments of railway bridges is based on Rankine’s
principles.
25. Longitudinal Forces
• The longitudinal forces are caused by braking or accelerating
of vehicle on the bridge. When the vehicle stops suddenly or
accelerates suddenly it induces longitudinal forces on the
bridge structure especially on the substructure.
• So, IRC recommends 20% of live load should be considered as
longitudinal force on the bridges.
26. Forces by Water Current
• When the bridge is to be constructed across a river, some part
of the substructure is under submergence of water.
• The water current induces horizontal forces on submerged
portion. The forces caused by water currents are maximum at
the top of water level and zero at the bottom water level or at
the bed level.
• The pressure by water current is P = KW [V2/2g]
– Where P = pressure (kN/m2)
– K = constant (value depending upon shape of pier)
– W = unit weight of water
– V = water current velocity (m/s)
– G = acceleration due to gravity (m/s2)
27. Seismic Load
• When the bridge is to be built in seismic zone or earthquake
zone, earthquake loads must be considered.
• They induce both vertical and horizontal forces during
earthquake.
• The amount of forces exerted is mainly depends on the self-
weight of the structure.
• If weight of structure is more, larger forces will be exerted.
28. Wind Loads
• Wind load also an important factor in the bridge design.
• For short span bridges, wind load can be negligible.
• But for medium span bridges, wind load should be considered
for substructure design.
• For long span bridges, wind load is considered in the design
of super structure.
• Failure of
– Tacoma’s Bridge
