The rule
L/200 — the only number that matters
Every steel beam under load deflects. AS 4084:2023 sets the maximum elastic deflection at L/200 — the beam's clear length divided by 200. The beam may deflect up to that much under its rated working load and still pass. Beyond it, the system is either overloaded, mis-specified or fatigued.
| Beam length | L/200 limit | Plain check |
|---|---|---|
| 1,800 mm (Australian standard short) | 9.0 mm | Pinky-finger gap or less under load |
| 2,400 mm | 12.0 mm | About the thickness of an iPhone |
| 2,700 mm (most common AU) | 13.5 mm | A bit over a centimetre |
| 3,300 mm (heavy duty wide) | 16.5 mm | Just under two centimetres |
How to measure
The string-line field test
L/200 is easy to test on site. You need a string line, a steel rule, and one fully loaded bay at rated capacity.
- Confirm the bay is loaded to (not above) the rated working load shown on the load notice.
- Stretch a string line tight horn-to-horn along the underside of the beam.
- Measure the gap between the string and the underside of the beam at mid-span.
- Record the measurement and photograph it with the rule visible.
- Unload the bay completely.
- Repeat the measurement empty — record any permanent set.
Spring-back is the test
Decision tree
What each measurement means
| Under load | After unloading | Rating | Action |
|---|---|---|---|
| ≤ L/200 | Springs back to zero | Green | Log; recheck at next inspection |
| Slightly over L/200 | Springs back to zero | Amber | Confirm load matches notice; engineering review |
| Within L/200 | Any permanent set | Red | Off-load, isolate, replace beam |
| Over L/200 | Any permanent set | Red | Stop work, off-load entire bay, replace beam |
Common causes
Why beams fail the deflection test
Beam over-deflection rarely happens because the steel itself was bad. The patterns we see at audit across QLD and NSW warehouses come from a handful of root causes — most of them operational, not structural.
- Pallet weights heavier than the load notice allows (mixed CHEP and euro pallets in one bay).
- Beams from a different manufacturer mixed into a single bay or run.
- Beam end-connectors with corrosion or missing safety clips.
- Cold-flow creep in cold storage applications under sustained load.
- Repeated minor forklift impacts to the beam underside.
- Loads concentrated at mid-span instead of distributed over the beam length.
What not to do
The compliance traps
We see three things in the field that look like fixes but make compliance worse: flipping a deflected beam upside-down, adding "support" timber underneath the load, and welding stiffeners onto an existing beam. None of these restore the steel's elastic properties. Beams that have yielded must be replaced with a manufacturer-original, identically-rated part.