Category: Structure & materials
Prepared by Eng. Ahmed Fathy, Projects Manager ·
Definition
Deflection is the vertical movement of a structural member under load. Codes limit it as a fraction of the span, such as span over 240 or 250 depending on the code and member, and it grows over time through creep and shrinkage.
Why it matters on a project
Excess deflection does not always mean failure, but it cracks walls and tiles, jams doors and ponds water on roofs.
Worked example
A 6 m beam with a limit of span over 250: limit = 6000 ÷ 250 = 24 mm.
Common mistake
Striking formwork early before the concrete gains strength, increasing the slab's permanent deflection.
Frequently asked questions
Is visible deflection dangerous?
It needs a structural engineer's assessment, especially if it grows or comes with cracks.
How do I reduce deflection?
Adequate depth, not striking formwork early, and good curing.
Related terms
- BeamA horizontal element that carries the slab and transfers its loads to the columns.
- CantileverA structural element fixed at one end and free at the other, such as balconies and canopies.
- FormworkTemporary moulds that hold concrete until it hardens.
- Dead and live loadsThe permanent and variable weights a building is designed for.
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