Scaffold – Maximum Deflection of a Platform

Deflection is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

To assure that scaffold planking remains within its safe load-bearing capacity, it may not be allowed to deflect more than 1/60th of its span between supports.

Span of Plank Between SupportsCalculationMax Deflection
10 Feet120 Inches/60 Inches2 Inches
7 Feet84 Inches/60 Inches1⅜ inches
5 Feet60 inches/60 inches1 inch

Deflection 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

what is deflection - cantilever beam setup

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

what is deflection or displacement simple cantilever beam example

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;

  • 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)

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 formula and equations that can be used to calculate a basic value for deflection in different types of beams.

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).

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 beams are special types of beams that are constrained by only one support. 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.

simple 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