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A cantilever beam is a rigid structural element supported at one end and free at the other, as shown in Figure-1. The cantilever beam can be either made of concrete or steel whose one end is cast or anchored to a vertical support. It is a horizontal beam structure whose free end is exposed to vertical loads.
In a building, a cantilever is constructed as an extension of a continuous beam, and in bridges, it is a segment of a cantilever girder. It can be constructed either cast-in-situ or by segmental construction by pre-stressing methods.
Cantilever construction allows overhanging structures without additional supports and bracing. This structural element is widely used in the construction of bridges, towers, and buildings, and can add a unique beauty to the structure.
This article explains some important structural actions and basic concepts of a cantilever beam in construction.
Contents:
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.
FAQs
A cantilever beam is a rigid structural element that is supported at one end and free at the other. The cantilever beam can be made of either concrete or steel whose one end is cast or anchored to a vertical support. It is a horizontal beam structure whose free end is exposed to vertical loads.
Usually, for small cantilever beams, the span is restricted to 2 m 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 to study the possibility of long spanned cantilever beams.
A cantilever beam bends downwards when it is subjected to vertical loads. It 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, 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.
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