Bearing Capacity and types of foundation

Types of foundation

1. Shallow foundation
2. Deep foundation
a. Well foundation b. Pile foundation

Factors affecting the selection of type of foundation

 Importance of the Structure.

 Life of the Structure.
 Loads from superstructure.
 Type of construction materials to be used
 Water table level
 Type of adjoining structure
 Soil condition
 Location of structure

Soil condition of site

Borehole data indicates the presence of hard strata below 15m, so open foundation can be
used as the foundation of the bridge.
Salient Attributes of the Bridge
Reduced level of Benchmark (BM 1) =

High flood level =

Coordinates of bridge axis =

Free board =

Sofit level=

Depth of Girder=

Depth of slab=

Level of road=

Number of lanes =

No of primary beams =

Number of cross beams =

Width of lane =

Width of footpath =

Total width =

Length of bridge =

Number of span =

Span length =

Type of bridge =

Foundation type =

Scour depth =

Maximum afflux =

Railing post height =

Additional = Wearing course on top of RCC deck.

Loads and Loads combination

Types of Loads for Design of Bridge Structures

Various design loads to be considered in the design of bridges are:

1. Dead load
2. Live load
3. Impact load
4. Wind load
5. Longitudinal forces
6. Centrifugal forces
7. Buoyancy effect
8. Effect of water current
9. Thermal effects
10. Deformation and horizontal effects
11. Erection stresses
12. Seismic loads

The dead load is nothing but a self-weight of the bridge elements. The different elements of
bridge are deck slab, wearing coat, railings, parapet, stiffeners and other utilities. It is the first
design load to be calculated in the design of bridge.

2. Live Load Irc 06 clause 204.1

The live load on the bridge, is moving load on the bridge throughout its length. The moving
loads are vehicles, Pedestrians etc. but it is difficult to select one vehicle or a group of vehicles to
design a safe bridge.

So, IRC recommended some imaginary vehicles as live loads which will give safe results against
the any type of vehicle moving on the bridge. The vehicle loadings are categorized in to three
types and they are

 IRC class AA loading

 IRC class A loading
 IRC class B loading
IRC Class AA Loading

This type of loading is considered for the design of new bridge especially heavy loading bridges
like bridges on highways, in cities, industrial areas etc. In class AA loading generally two types
of vehicles considered, and they are

 Tracked type
 Wheeled type
IRC Class A Loading

This type of loading is used in the design of all permanent bridges. It is considered as standard
live load of bridge. When we design a bridge using class AA type loading, then it must be
checked for class A loading also.

IRC Class B Loading

This type of loading is used to design temporary bridges like Timber Bridge etc. It is considered
as light loading. Both IRC class A and Class B are shown in below figure.
3. Impact Loads

The Impact load on bridge is due to sudden loads which are caused when the vehicle is moving
on the bridge. When the wheel is in movement, the live load will change periodically from one
wheel to another which results the impact load on bridge.

To consider impact loads on bridges, an impact factor is used. Impact factor is a multiplying
factor which depends upon many factors such as weight of vehicle, span of bridge, velocity of
vehicle etc. The impact factors for different IRC loadings are given below.

For IRC Class AA Loading and 70R Loading

Span Vehicle type Impact factor
 25% up to 5m and linearly reducing to 10% from
Tracked vehicle
Less than 9 meters 5 m to 9 m.

Wheeled vehicle 25% up to 9 m

Tracked vehicle (RCC

10% up to 40 m
bridge)

Greater than 9 Wheeled vehicle (RCC

25% up to 12m
meters bridge)

Tracked vehicle (steel

10% for all spans
bridge)

Wheeled vehicle (steel

25% up to 23 m
bridge)

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.

For IRC class A and class B loadings

Impact factor If = A/(B+L)

Where L = span in meters

A and B are constants

Bridge A B
type

4.
RCC 6.0
5

9. 13.5
Steel
0 0

Apart from the super structure impact factor is also considered for substructures

 For bed blocks, If = 0.5

 For substructure up to the depth of 3 meters If = 0.5 to 0
 For substructure greater than 3 m depth If = 0

4. Wind Loads

Wind loads 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.

5. 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.
6. 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.

Centrifugal force can be calculated by C (kN/m) = (WV2)/(12.7R)

Where

W = live load (kN)

V = Design speed (kmph)

R = Radius of curve (m)

7. Buoyancy Effect

Buoyancy effect is considered for substructures of large bridges submerged under deep water
bodies. Is the depth of submergence is less it can be negligible.

8. 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)

9. Thermal Stresses

Thermal stresses are caused due to temperature. When the temperature is very high or very low
they induce stresses in the bridge elements especially at bearings and deck joints. These stresses
are tensile in nature so, concrete cannot withstand against this and cracks are formed.

To resist this, additional steel reinforcement perpendicular to main reinforcement should be

provided. Expansion joints are also provided.
10. Seismic Loads

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.

11. Deformation and Horizontal Effects

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
being considered as design loads in bridge design.

12. Erection Stresses

Erection stresses are induced by the construction equipment during the bridge construction.
These can be resisted by providing suitable supports for the members.

Loads IRC 006 202.1

Components of Bridge

1. Sub structures
2. Superstructures

a. Abutments
b. Wing walls
c. Pier

a. Beam/Girder (main/cross)
b. Deck slab
c. Kerb
d. Railing post

Bearings