Cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. A balcony protruding from a building would be an example of a cantilever. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.
The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 460m (1,500 ft), and can be more easily constructed at difficult crossings by virtue of using little or no falsework (temporary supports). The Hassfurt Bridge over the Main river in Germany with a central span of 38m (124 ft) was completed in 1867 and is recognized as the first modern cantilever bridge.
A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be crossed, meeting at the centre. In a common variant, the suspended span, the cantilever arms do not meet in the centre; instead, they support a central truss bridge which rests on the ends of the cantilever arms. The suspended span may be built off-site and lifted into place, or constructed in place using special travelling supports.
A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. The parallel bridges which form the West-Link in West Dublin were built in this manner.
In a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever’s supports, the bridge superstructure often takes the form of towers above the foundation piers. The Commodore Barry Bridge is an example of this type of cantilever bridge.
Structural Behaviour of Cantilever Beam
A cantilever beam bends downwards when it is subjected to vertical loads, as shown in Figure-2. A cantilever beam can be subjected to point load, uniform load, or varying load.
Irrespective of the type of load, it bends downwards by creating a convexity upwards. This bending creates tension in the upper fiber and compression in the lower fibers. Hence main reinforcement is provided to the upper fiber of the concrete beam, as there is high tensile stress as shown in Figure-4.
Shear Force (SF) and Bending Moment (BM) Diagram of Cantilever Beam
The shear force at any section of a cantilever beam is the sum of loads between the section and the free end. The bending moment at a given section of a cantilever beam is the sum of moments about the section of all the loads acting between the section and the free end.
Consider a cantilever beam AB of length ‘l’ subjected to a point load ‘W’ at the end B. A section X-X at a distance ‘x’ from the free end B is placed. Then the shear force at section X-X is Rx, which is equal to W and the bending moment about the section X-X is Mx, which is equal to W.x.
The shear force at the fixed support A is determined by keeping the section at A, which gives the shear force Ra=W; and moment Ma = W.l. based on which the shear force and bending moment diagram are developed.
The bending moment of a cantilever beam is maximum at the fixed end and decreases to zero at the free end. The bending and shear force diagram is determined for all possible load combinations to design a cantilever beam for a structure. The load applied on the beam is a combination of dead load and live loads as per the design standards.
Design of Cantilever Beam
A cantilever beam under the action of the structural load is subjected to moment and shear stresses. The objective of any design process is to transfer these stresses safely to the support.
The bending moment of a cantilever beam varies from zero at the free end to a maximum value at the fixed end support (Figure-3). Hence during the design of cantilever beams, the main reinforcement is provided to the upper fiber of the concrete beam to withstand the tensile stress safely.
The maximum span of a cantilever beam is generally dependent on the following factors:
- The depth of the cantilever
- The magnitude, type, and location of the load
- The quality and type of material used
Usually, for small cantilever beams, the span is restricted to 2 to 3 m. But the span can be increased either by increasing the depth or using a steel or pre-stressed structural unit. The span can be constructed long, given that the structure can counteract the moments generated by the cantilever and safely transfer it to the ground. A detailed analysis and design of the structure can help study the possibility of long spanned cantilever beams.
The cantilever beam must be properly fixed to the wall or support to reduce the effect of overturning.
Applications of Cantilever Beam in Construction
Cantilever beam structures are used in the following applications:
- Construction of cantilever beams and balconies
- Temporary cantilever support structures
- Freestanding radio towers without guy-wires
- Construction of cantilever beam for pergolas
- Lintel construction in buildings
Advantages and Disadvantages of Cantilever Beams
The important advantages of cantilever beams are:
- Cantilever beams do not require support on the opposite side.
- The negative bending moment created in cantilever beams helps to counteract the positive bending moments created.
- Cantilever beams can be easily constructed.
The disadvantages of cantilever beams are:
- Cantilever beams are subjected to large deflections.
- Cantilever beams are subjected to larger moments.
- A strong fixed support or a backspan is necessary to keep the structure stable.
What is a cantilever beam?
What is the purpose of a cantilever?
What is the maximum span of cantilever beams?
What is an example of a cantilever?
How does a cantilever beam behave under loads?
Irrespective of the type of load, it bends downwards by creating a convexity upwards.