Australian Deflection Limits — Serviceability Criteria
Serviceability deflection limits ensure structural performance under normal use without excessive deformation that damages finishes, causes misalignment of building services, or creates occupant discomfort.
Deflection limits are expressed as a fraction of the span length (L/###) or building height (H/###). The values below represent standard recommendations from the governing design standard.
Code Reference: AS 4100:2020 Clause 16.4
Recommended Deflection Limits
| Application | Live Load Limit | Total Load Limit |
|---|---|---|
| Beams — live load | L/250 | L/125 |
| Beams — total load | L/125 | L/60 |
| Cantilevers — live load | L/125 | — |
| Cantilevers — total load | L/60 | — |
| Trusses — live load | L/300 | L/150 |
| Crane girders — vertical | L/600 | L/300 |
| Crane girders — lateral | L/400 | — |
Worked Example: Deflection Check
Problem: Verify that an Australian 410UB59.7 beam spanning 8.0 m is adequate for deflection. Maximum allowable live load deflection = L/250. Calculated live load deflection = 32.0 mm.
Solution:
For an Australian beam spanning 8.0 m under live load, the deflection limit per L/250 = 32.0 mm.
Actual deflection (32.0 mm) ≤ allowable (32.0 mm) — the deflection criterion is satisfied.
Note: Deflection checks are serviceability limit states. Factored loads for serviceability typically use unfactored or partially factored loads (e.g., ψ₂ψ₀ combinations per AS 1170.0).
Design Resources
Frequently Asked Questions
What are the deflection limits per AS 4100? AS 4100:2020 Clause 16.4 recommends L/250 for live load deflection and L/125 for total load deflection in beams. Cantilevers: L/125 for live load, L/60 for total load. These are recommendations — the National Building Code of Australia or project specification may specify more stringent limits.
Does AS 4100 address dynamic performance? AS 4100 Commentary provides guidance on floor vibration and dynamic response. For floors susceptible to walking excitation, a fundamental frequency > 4 Hz is typically recommended. The commentary references AS 1170.4 for dynamic load factors.
What is the camber recommendation per AS 4100? AS 4100 Clause 16.4 recommends camber for beams where the calculated dead load deflection exceeds L/300. The recommended camber typically equals the dead load deflection plus half the live load deflection, but should not exceed L/200.
How are deflection limits applied to cantilevers? For cantilevers, AS 4100 recommends the deflection at the free end be limited to L/125 for live load and L/60 for total load (where L is the cantilever length). These are more generous than simply supported spans because cantilevers are typically stiffer relative to their span.
Educational reference only. Verify all values against the current edition of AS 4100:2020 Clause 16.4. This information does not constitute professional engineering advice. Always consult a qualified structural engineer.