Lessons · Engineering · How far it moves, and why that governs
Strong enough and still unacceptable
Deflection is how far a member moves under load. A beam can be nowhere near failure and still be unusable: floors bounce, doors jam, plaster cracks, and people believe a building is unsafe because it moves. Deflection is checked separately from strength and frequently governs.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
A long, shallow steel beam over a shop front was comfortably strong and deflected enough that the glazing below it cracked twice. Nothing about the strength calculation was wrong. It was the wrong calculation to have stopped at.
How to think about it
Check strength, then check deflection as a separate question against a limit expressed as a fraction of the span. Deflection depends on the cube or fourth power of span, so lengthening a beam hurts deflection far faster than it hurts strength.
Worked example
Strength asks: will it break?Peak stress against capacity.
Serviceability asks: will it be usable?Deflection against a fraction of span.
Limits are given as span over a number, e.g. span/360.The problem supplies the number.
Deflection grows with span far faster than moment does.Which is why long spans are governed by it.
Your turn
A 7200 mm span has a deflection limit of span/360. Write the limit in millimetres.
7200 / 360 = mm
Solve one, graded on the server
The trap
Stopping at strength. A member that passes strength and fails deflection is a complaint, a crack and sometimes a replacement.