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What is Deflection?

What is Deflection (Deflection Definition)

Deflection, in structural engineering terms, refers to the movement of a beam or node from its original position due to the forces and loads being applied to the member. Deflection, also known as displacement, can occur from external applied loads or from the weight of the structure itself, and the force of gravity in which this applies. It can occur in beams, trusses, frames and basically any other structure. To define deflection, let’s take a simple cantilevered beam deflection that has a person with weight (W) standing at the end:

The force of this person standing at the end will cause the beam to bend and deflect from its natural position. In the below diagram the blue beam is the original position, and the dotted line simulates the cantilever beam deflection:

As you can see, the beam has bent or moved away from the original position. This distance at each point along the member is the meaning or definition of deflection. There are generally 4 main variables that determine how much beam deflections. These include:

  • How much loading is on the structure
  • The length of the unsupported member
  • The material, specifically the Young’s Modulus
  • The Cross Section Size, specifically the Moment of Inertia (I)



Beam Deflection Equations

Deflection of a beam (beam deflection) is calculated based on a variety of factors, including materials, the moment of inertia of a section, the force applied and the distance from support. There is a range of beam deflection equations that can be used to calculate a basic value for deflection in different types of beams.

Generally, deflection can be calculated by taking the double integral of the Bending Moment Equation, M(x) divided by EI (Young’s Modulus x Moment of Inertia).

What is the unit of deflection?

The unit of deflection, or displacement, is a length unit and normally taken as mm (for metric) and in (for imperial). This number defines the distance in which the beam has deflected from the original position.

Cantilever Beam Deflection

Cantilever beams are special types of beams that are constrained by only one support, as seen in the above example. These members would naturally deflect more as they are only supported at one end. To calculate the deflection of cantilever beam you can use the below equation, where W is the force at the end point, L is the length of the cantilever beam, E = Young’s Modulus and I = Moment of Inertia.

cantilever beam deflection equation point load

Simply Supported Beam Deflection

Another example of deflection is the deflection of a simply supported beam. These beams are supported at both ends, so deflection of a beam is generally left and follows a much different shape to that of the cantilever. Under a uniform distributed load (for instance the self-weight), the beam will deflect smoothly and toward the midpoint:

simple supported beam deflection


We hope you have found this short article to define beam deflection in structural engineering. Please feel free to leave a comment below, or try using our beam span calculator to try for yourself and bring this theoretical concept to practice using structural analysis software.

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