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Cantilevered Beams and Trusses- Uses and Advantages

Cantilevered Beams and Trusses

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A cantilever is a rigid structural member such as beam fixed at one end to a commonly vertical support from which it protrudes. It can also be constructed with trusses or slabs or metal girder. When subjected to a structural load, the cantilever carries the load to the support.

Cantilever construction allows overhanging structures without external bracing, in contrast to constructions supported at both ends with loads applied between the supports, such as a simply supported beam.

In residential architecture, cantilever design is often used for creating balconies and other extensions above ground level. The use of cantilevered setups is also often seen in bridges and similar projects.

What is a Cantilever Beam?

Cantilever beams are members that are supported at one end and carrying a load at the other end or distributed along the unsupported portion. In order to ensure the structure is static, the support must be fixed that means it can support forces and moments in all directions.

The upper half of the thickness of cantilever beam is subjected to tensile stress, tending to elongate the fibers, the lower half to compressive stress, tending to crush them. Beams are allowed to be cantilevered beyond their supports by a distance equal to one-fourth the span between supports.

A good example of a cantilever beam is a balcony. A balcony is supported on one end only, the rest of the beam extends over open space; there is nothing supporting it on the other side.

Cantilevers deflect more than most other types of beams because they are only supported from one end. This means there is less support for the load to be transferred to. Cantilever beam deflection can be calculated in a few different ways for instance simplified cantilever beam equations or cantilever beam calculators and software.

Fig. 1: Cantilever Beam

What is Cantilevered Truss?

A cantilevered truss is supported on one side only which is designed to be effectively much heavier than the unsupported one, so even adding weight to the unsupported side does not cause it to sag.

The parts of a truss are chords (top and bottom) and the web, in a pitched truss, there are two top chord pieces. The pieces of the web in a truss are always in opposition to each other, so if one is in tension, its neighbor is in compression. 

The top chord is always in compression and the bottom chord is always in tension.  The triangles of the truss can be in a variety of configurations, each undoubtedly with its own advantages and disadvantages, but all work on the same general principles.

Fig. 2: Cantilever Truss

Cantilevered Roof Truss

A Cantilevered Roof Truss occurs where the main body of the truss projects outside the support. All the standard roof truss profiles can be cantilevered at one or both ends. The structural treatment of the cantilevered roof truss varies with increasing cantilever distance.

The Cantilever Web can be added to strengthen the bottom chord. The maximum cantilever distance permitted is normally limited to the lesser of a quarter of the setting out points span, or the first internal node point. In some instances, a cantilever causes the outer bottom chord bay to be in compression and a lateral brace may be required.

Fig. 3: Cantilevered Roof Truss

Advantages of Cantilevered Beams and Trusses

  1. Cantilever beam is simple in constructions.
  2. It does not require a support on the opposite side.
  3. Cantilevered structure generates a negative bending moment which counteracts positive bending moment of back-spans.
  4. Cantilevered trusses use less material.
  5. It provides greater clear height in the center than can be obtained with any other types of trusses.
  6. Cantilevered truss is light and has graceful appearance
  7. No horizontal thrust and consequently no tie-rods required.
  8. The particular advantage of cantilevered truss for very great spans is that it can be erected without scaffolding under the center, and in bridge work this is considered as its only advantage.
  9. For roofing platforms and grand stands, where an outer support is not desired, it is the only type of truss available.
  10. Building out from each end enables construction to be done with little disruption to navigation below

Disadvantages of Cantilevered Beams and Trusses

  1. It is claimed that the cantilever is not an economical type of truss.
  2. Cantilevered structures deflect largely.
  3. Generally, cantilever structure results in larger moments
  4. Cantilevered structure needs a fixed support, or a back-span and check for uplift of the far support
  5. Cantilever beams keep their shape by the opposition of large tensile and compression forces, as well as shear, and are therefore relatively massive

Application of Cantilevered Beams and Trusses

1. Building Construction

There are various applications of cantilevered beams and trusses in building constructions such as cantilevers carrying a gallery, roof, canopy, runway for an overhead travelling crane, or part of a building above. They are used in various structures such as sun shed, shelves, large halls, armories, and exhibition buildings

Fig. 4: Cantilever Beams in Buildings

2. Bridge Construction

In bridge building, a cantilever construction is employed for large spans in certain sites, especially for heavy loading. For instance, the Forth Bridge, Scotland composed of three cantilevers with two connecting suspended spans.

Fig. 5: Cantilevered Truss Bridge
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