Deflection

[edit] Introduction

Deflection – in engineering terms – is the degree to which an element of structure changes shape when a load is applied. The change may be a distance or an angle and can be either visible or invisible, depending on the load intensity, the shape of the component and the material from which it is made.

Deflection is a crucial consideration in the design of a structure and failure to apply due attention to it can be catastrophic.

Different types of load can cause deflections. These include point loads, uniformly distributed loads, wind loads, shear loads as well as ground pressure and earthquakes, to name but a few. When a load produces a deflection that is too great, the component may fail.

Components and structures that suffer deflection include, beams, columns, floors, walls, bridge decks, tunnel walls, dams and so on. San Francisco’s Golden Gate Bridge can sway by as much as 4m laterally under strong winds.

Non-structural components can also deflect, for example cladding panels on a building may deflect inwards when subject to intense wind loading.

Given the possibility of structural failure, building codes usually determine what the maximum allowable deflection should be to ensure the safety of a building’s users and overall structural integrity. For a beam, this is usually expressed as a fraction of the span, eg the beam’s deflection should not be greater than 1/360th of the span; so, if the span is 5m, the deflection should not be greater than 13.9mm. This will usually be measured at the mid-point of the beam.

A structural element will deflect less under load if its stiffness or rigidity is increased. This can usually be achieved by strengthening its section or increasing its size; the latter may also increase its cost.

The material itself must also be considered. For example, because aluminium is around three times more flexible than steel, it is often designed for deflection rather than strength. In contrast, glass is relatively inflexible: even slight deflections in a steel frame could cause the glass to fracture.

RAAC playbook, published by the Manufacturing Technology Centre and the High Value Manufacturing Catapult in 2024 defines deflection as: ‘the degree to which a part of a longitudinal structural element is deformed laterally (in the direction transverse to its longitudinal axis) under a load.’

[edit] Related articles on Designing Buildings

• Approved Document A.
• Concept structural design of buildings.
• Creep.
• Elements of structure in buildings
• Institution of Structural Engineers IStructE.
• Span.
• Structural principles.
• Structures at the end of their design life.
• Substructure.
• Superstructure.
• The development of structural membranes.
• Tower.
• Types of structure.