Different Types of Bridges and Criteria of Site Selection
Definition
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle.
Designs of bridges vary depending on
- The function of the bridge,
- The nature of the terrain where the bridge is to be constructed,
- The material used to make it and
- The funds available to build it.
Different Types Of Bridges
- Beam bridges
- Cantilever bridges
- Arch bridges
- Tied arch bridges
- Suspension bridges
- Cable-stayed bridges
- Movable bridges
Beam Bridges
Made up of simply supported horizontal Beams I-beams, trusses, or box girders.
End supports are known as abutments and intermediate supports are known as piers.
Weight on top of the beam pushes straight down on the abutments at either end of the bridge
Types of construction could include having many beams side by side with a deck across the top of them.
Cantilever bridges
horizontal beams supported on only one Support.
simple cantilever
formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center.
balanced cantilever
cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction.
Arch bridges
bridge with abutments at each end shaped as a curved arch.
Arch bridges transfers the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side.
Tied arch bridges
an arch-shaped superstructure,
Instead of transferring the weight of the bridge and traffic loads into thrust forces into the abutments, the ends of the arches are restrained by tension in the bottom chord of the structure. They are also called bowstring arches.
Suspension bridges
Deck, the load-bearing portion, is hung below suspension cables on vertical suspenders.
This type of bridge has cables suspended between towers, and vertical suspender cables.
The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables.
Cable-stayed bridges
consists of one or more columns (referred to as towers or pylons), with cables supporting the bridge deck.
Two types of design–
harp design
The cables are made nearly parallel by attaching them to various points on the tower
fan design
the cables all connect to or pass over the top of the tower
Movable bridges
designed to move out of the way of boats or other kinds of traffic
powered by electric motors, whether operating winches, gearing, or hydraulic pistons
Survey for bridge site selection
- Social feasibility survey
- Level of local participation
- Size of area of influence
- Size of traffic flow
- Socio-economic benefits produced by the proposed bridge
Technical survey
It includes Bridge site selection and Topographic Survey of the selected bridge site
Bridge site selection
The bridge site should fulfill a number of general conditions:
– The bridge site should be selected at or near to the traditional crossing point
– minimum free board
– maximum bridge span
– space for the bridge foundations
River condition
River condition
The selected bridge site must have favorable river conditions. Accordingly, a bridge should be located:
• on a straight reach of the river
• beyond the disturbing influence of larger tributaries
• on well defined banks
Slope and bank condition
If the slope and bank is soil, potential instability features and failure modes are:
• bank erosion
• toppling instability of the bank
• erosion of the slope
• land slide
If the slope and bank is rock, potential instability features and failure modes are:
• plain failures in a rock slide along the slope.
• wedge failure leading to the fall of rock mass.
• toppling leading to the fall of rock blocks.
• rotational slide is similar to the landslide in a soil slope. Such failure is likely when the material of therock is very weak (soft rock) and the rock mass is heavily jointed and broken into small pieces
General data collection
General data is required for needs assessment and construction planning of the proposed bridge.
Collect the following general data & information:
- Location of bridge site
- Transportation distance,
- Nature of crossing and fordability
- Availability of local materials
- Availability of local bridge builders
Local participation
Traffic volume
Width of walkway
Temporary crossing
Evaluation of the bridge site
Good – All or most of the features are favorable and if the surveyor is confident about the
stability of the slopes. Proceed with further survey work.
Bad – Most of the features are unfavorable. Reject site.
Questionable – Most of the features are favorable and some are unfavorable. The site is questionable. In this case, further detailed investigation by an experienced geo-technical engineer is necessary.