Deflection limits
Why it is a separate question
Section titled “Why it is a separate question”The strength check answers whether the structure carries the load. The serviceability check answers whether its behaviour is acceptable: that it does not deflect too far, vibrate annoyingly or crack the finish.
The two are separate, and in a thin-walled structure the second is often the stricter.
What deflection limits
Section titled “What deflection limits”The integrity of the finishes
Section titled “The integrity of the finishes”A rigid finish — ceramic tiling, thin render, rigid boards — tolerates little movement. Where such a finish goes on the structure, the limit has to be stricter.
The operation of windows and doors
Section titled “The operation of windows and doors”Too flexible a lintel presses on the window or door frame and makes it stick.
The feel of the building
Section titled “The feel of the building”A visibly sagging floor or a wall that “lives” in the wind is unpleasant even when it is structurally sound.
Drainage
Section titled “Drainage”On a flat roof, deflection can cause ponding, which comes back on the roof covering.
Where it counts especially in a thin-walled structure
Section titled “Where it counts especially in a thin-walled structure”Walls under wind load
Section titled “Walls under wind load”An external wall deflects under wind pressure. If the finish is rigid, the permitted movement is small — often more onerous than the strength check.
Floors
Section titled “Floors”Here vibration enters alongside deflection. Floor vibration.
Lintels
Section titled “Lintels”Strict limits even over short spans, because of the window or door.
How design handles it
Section titled “How design handles it”- The limit is chosen to suit the role of the structure, not applied uniformly.
- Long-term and short-term effects are treated separately.
- The sensitivity of the finish is settled up front: if the finish will be rigid, the structural engineer has to know.
That last point is a common source of misunderstanding: the build-up decision feeds back into the structure, so it cannot be made after the structure is closed.
The direction of the solution
Section titled “The direction of the solution”The most effective way to reduce deflection is to increase stiffness: a deeper section, closer spacing, a shorter span. Increasing the material thickness is less effective — that improves the capacity instead.
The point
Section titled “The point”A structure is good when it not only carries the load but moves in a way nobody notices. The serviceability limits are set by the sensitivity of the finish — which is why it has to be settled early.