Have you ever seen a wooden log placed across a small water stream to let you go over the other end? That’s a simple example of a beam bridge. By definition, a beam bridge is a simple and common bridge type consisting of girders or beam of wood or concrete hanging over vertical piers/posts as supports.
A bridge is a structure which is provided across an obstacle such as river, valley etc. in order to safely pass from one end to the other without closing the way beneath like body of water, valley or road. It is provided for easy passage over obstacle.
There are different types of bridges which are constructed depending upon the location, type of traffic, loading conditions. One of the types is Beam Bridge which is more commonly constructed nowadays.
Let us discuss in detail about its function, its types, examples, advantages and disadvantages.
Also Read: Post and Lintel Construction – Advantages & Disadvantages
Beam Bridge Definition
A beam bridge is the simplest structural form of bridge which is supported by piers or abutments at each end. In this bridge there is no moment transfer through supports because beam bridge is simply supported at its ends.
Also Read: Interesting facts about bridge Abutments
Beam Bridge Examples
Here’re some common beam bridge examples from across the world:
- Lake Pontchartrain Causeway – It consists of two parallel bridges across Lake Pontchartrain in Louisiana, United States.
- Manchac Swamp Bridge – It’s a twin concrete trestle beam bridge in the US State of Louisiana.
- Tianjin Grand Bridge, China – It’s a railway viaduct bridge beam in the Beijing-Shanghai area of China.
- King Fahd Causeway – It is a 25 km long series of bridges connecting Saudi Arabia and Bahrain.
Material of Beam Bridge
A beam bridge can be made by different materials depending upon the use of bridge for type of traffic and its location. For light traffic and small spans wooden beam bridges are used because beams made of wood can take light load but does not have high load bearing capacity. For long spans and heavy traffic bridge of concrete and steel are constructed. Bending moments are calculated for beams and slab and then according to the load and moment the size of bridge structural members is selected.
Weight and strength of Bridge Beams
Weight of a bridge depends upon the size of structural members like beams, slabs and type of traffic. While its strength depends upon the size of beams and spacing of piers and Abutments. Larger size member with less spacing of pier can take heavy load with less chances of sagging of beams and their strength increase.
While beam bridges of long span with large spacing have more chances of sagging of beams due to its long span and small size of structural members.
Types of beam Bridge
Beam bridge can be classified into various types depending upon the geometry and cross section of beam.
Type of bridge based on geometry
- Straight beam
- Curved beam
- Tapered beam
Type of bridge based on shape of cross section
- I beam
- T beam
- C section beam
Type based on equilibrium condition
- Statically determinate beam
- Statically indeterminate beam
Type of beam bridge based on geometry
Straight beam
A straight beam is the simplest beam design which is normally used for ordinary conditions where only vertical loads are acting on the bridge with moving loads of traffic. There is a shear and bending moment occur in beam and beams are designed to resist shear and moments.
Curved beam
This type of beam design is chosen in case of traffic loading when there are chances of torsion in beam with shear and bending. In this type of beam is designed curved by taking some assumptions which are
- A plane section before bending remains plane after bending.
- The modulus of elasticity of material of beam is same in tension and compression.
Tapered beam
The design of tapered beam is different as compared to simple beam and their stress analysis is also different. The tapered beam is also subjected to tip bending load due which is analyzed to predict the distribution of stresses.
Type of beam bridge based on shape
I section beam
This beam section is more used as compared to the T beam and C channel section because of its high load carrying capacity and it is easy to fabricate. I section is economical, and it is symmetrical on both the axes and because of its symmetry it is more efficient to resist load. The moment of inertia of I section is also greater, so it resists moment in a better way.
T section
This type of beam section is also constructed nowadays. A T beam is designed monolithically with slab. The top of T beam is a flange of greater width as compared to bottom web. The flange resists compressive stresses and web or rib resists tension stresses.
C channel section
A C channel section beams can also be used in beam bridge. A channel beam is also good in taking load. The moment of inertia of channel beam is less than I section due to its asymmetry due to which it is not preferred for high moment design bridges.
Type of beam bridge based on Equilibrium condition
Statically determinate beam bridge
For this type of equilibrium condition for beams the reaction forces are calculated using equilibrium equation alone. There is no need of other methods to solve for reaction forces while calculating reaction forces of bridge.
Statically indeterminate beam bridge
This type of Beam Bridge consists of a greater number of reaction forces as compared to equilibrium equations. This type of structure is designed by using other methods because number of unknowns are greater than equilibrium equations. The indeterminate structures are good in resisting loads. They ensure greater stiffness and stability than determinate structures.
Advantages of Beam Bridges
- It is beneficial and good for short spans
- The design and construction is simple than other bridge construction.
- Construction is economical for short span while other bridge constructions are expensive.
- The beam bridge is most suitable for providing pathway for railway line.
Disadvantages of Beam Bridges
- It is not preferred for long spans
- A beam bridge faces a significant amount of wear and tear.
- Maintenance cost is more as compared to other bridges.
- The beam bridge construction is expensive for longer span.
