Main menu


Cantilever Beams - All you need to know about cantilevers

 Introduction to cantilever beams

A cantilever beam is a rigid structural element that is supported or bolted at one end and free at the other. Cantilever beams can be either concrete or steel/steel cast, bolted, screwed or welded to a support element or header. A cantilever is a horizontal beam structure that is subjected, at its free end, to predominantly vertical loads.

In any building, a cantilever projection is constructed or designed and implemented as an extension of the continuous beam, and in bridges, it is part of a deep projecting beam. It can be carried out either molded on site or by precast construction by prestressing methods.

Cantilevering allows the construction of protruding structures without additional supports and stiffeners. This structural element is widely used in the construction of bridges, towers, and buildings, and it can add unique beauty to a structure.

Structural behavior of cantilever beams

The cantilever beam bends downward when subjected to vertical loads. Cantilever beams may be subjected to point (centered) load, uniform distributed load, or variable load. 

Regardless of the type of load, it curves downward by convexing upwards. This bending creates tensile stresses in the upper layer and compressive stresses in the lower layer. Then the main reinforcement is provided in the upper layer of the concrete beam (ie the main reinforcement is the upper reinforcement), where there is a high tensile stress.

Schematic diagram of the shear strength (SF) and bending moment (BM) of a cantilever beam

The shear strength of any part of the cantilever beam is the sum of the loads between the section and the free end. The bending moment in a given section of the cantilever beam is the sum of the moments around the section of all the loads acting between the section and the free end.

For example: a cantilever beam AB of length “L” is subjected to point load “P” at end B. The X-X segment is placed at a distance “x” from the free end B. Then the shear force of the X-X segment is Rx, which is equal to P and the bending moment Around sector X-X is Mx, which is equal to Px.

The shear force at the fixed end A is determined by keeping the segment at A, which gives the shear force Ra = P; and torque Ma = P.L. Based on the shear strength and bending moment diagram generated by loading.

The maximum bending moment of a cantilever beam is at the fixed end and decreases to zero at the free end. A flexural and shear strength scheme is specified for all possible load combinations for the design of a cantilever beam in the structure. The load applied to the beam is an aggregation/combination of dead loads and live loads as per the design criteria.

Design of cantilever beams

Cantilever beams are subjected to flexural and shear stresses under structural load. The goal of any design process is to safely transfer these stresses to the bolster/bolster. 

The bending moment of the cantilever beam varies from zero at the free end to a maximum value at the fixed end support (Fig. 3). Hence, during the design of the cantilever beams, the main reinforcement is provided in the upper layer of the concrete beam to bear the tensile stress safely.

The maximum seam (maximum length) of cantilevered beams generally depends on the following factors:

     cantilever depth
     The amount, type and location of the load
     Quality and type of materials used in construction

Usually, for small cantilever beams, the seam is limited to 2 to 3 metres. But the span or seams can be increased either by increasing the depth or by using a steel or prestressed structural unit. Long or large spans of cantilevers can be constructed and executed, given that the structure can counteract the moments generated by the cantilever loading and transfer them safely to the ground or soil. Detailed analysis and design of the structure can assist in studying the possibility of extended cantilever beams for large spans or spans.

The cantilever beam must be properly fixed to the wall, support or column to reduce tipping effect.

Applications of cantilever beams in construction

Cantilever beam structures are used in the following applications:

     Building cantilevered beams and balconies.
     Temporary or permanent cantilever support structures.
     Free radio towers without pulling wires.
     Construction of cantilevered beams for pergolas.
     Build thresholds in buildings.
     On bridges and overpasses.

Advantages and disadvantages of cantilever beams

The important advantages of the cantilever beam are:

     Cantilever beams do not require support on the other side.
     The negative bending moment generated in the cantilever beams counteracts the positive bending moment generated in the inner beam.
     Cantilever beams can be easily constructed and implemented.

The disadvantages of cantilever beams are:

     Cantilever beams are subjected to significant deflection.
     Cantilever beams are subject to greater moments.
     A strong fixed support (column) or back span is necessary to keep the structure stable.