Partial safety factors are multipliers and divisors used in Limit State Design (LSD) to account separately for uncertainty in loads (γf on load) and materials (γm on material strength). The Indian Standards IS 456:2000 Cl. 36.4 and IS 800:2007 Cl. 5.4.1 specify partial safety factors calibrated to give a target structural reliability of approximately 10⁻⁴ failure probability over the structure's service life.
For RCC per IS 456 Cl. 36.4: γf = 1.5 for combined dead + live load (ULS strength); 1.5 for dead + wind or earthquake; 1.2 for combined dead + live + wind/earthquake; 0.9 dead − 1.5 wind/earthquake (the 0.9 factor on dead load when it opposes wind/earthquake — this is the uplift case where dead load works in favour of stability). Material safety factors: γm = 1.5 for concrete (so design strength = 0.45 fck for flexure); γm = 1.15 for steel (so design strength = 0.87 fy). For serviceability: γf = 1.0 (unfactored loads).
For structural steel per IS 800:2007 Cl. 5.4.1: γf = 1.5 (DL + LL); γf = 1.5 for environmental loads alone; γf = 1.2 (DL + LL + EL/WL); γf = 1.5 (DL + EL/WL); γf = 0.9 DL − 1.5 EL/WL (uplift). Material γm = 1.10 (yield strength for tension and bending); γm = 1.25 (compression at extreme fibre, ultimate strength). The product γf × γm gives the total safety margin: for typical RCC with DL + LL: 1.5 × 1.5 / 0.87 (steel design) = 2.59 — meaning the structure has 2.59× the demand at design strength. The system is calibrated to give similar reliability across different load combinations and member types — far more uniform than the older WSM single-factor approach.
Typical values
γf — RCC ULS DL+LL1.5
γf — RCC ULS DL+EL1.5
γf — RCC ULS DL+LL+EL1.2
γf — RCC uplift 0.9 DL0.9 (DL) + 1.5 (EL/WL)
γm — concrete (RCC)1.5
γm — steel (RCC tension/bending)1.15
γm — steel (structural tension/bending)1.10
γm — steel (ultimate)1.25
Where used
All RCC design per IS 456:2000 Section 5
All structural steel design per IS 800:2007 Section 5
Pre-stressed concrete per IS 1343:2012
Bridge design per IRC 112:2020 and IRC 24:2010
Industrial structures and water-retaining structures (modified factors)
Acceptance / threshold
Per IS 456 Cl. 36.4 + IS 800 Cl. 5.4: design check with all applicable combinations; ULS and SLS independently satisfied; total reliability ~10⁻⁴ failure probability.
Site example
Site reality: a Pune commercial project's structural drawings showed 'design moment 145 kNm' on a beam. The site engineer compared against yield strength 500 MPa. WRONG. The 145 kNm was factored (Mu); design must compare against 0.87 × 500 = 435 MPa with all applicable safety factors. Reading the factored moment as service load is a 17% miscalculation that often leads to under-reinforcement. Always read drawings as factored values; never substitute service loads.
Frequently asked
What are partial safety factors in IS 456?
Per IS 456 Cl. 36.4: γf (load factors) on demand side — 1.5 for DL+LL, 1.5 for DL+EL, 1.2 for DL+LL+EL, 0.9 DL + 1.5 EL for uplift. γm (material factors) on capacity side — 1.5 for concrete (so design strength = 0.45 fck), 1.15 for steel (design = 0.87 fy). Both factors applied; product gives total safety margin.
Why are concrete and steel given different safety factors?
γm = 1.5 for concrete reflects higher variability — concrete strength varies with cement, water, aggregate, curing, age, exposure, workmanship. γm = 1.15 for steel reflects lower variability — yield strength of mill-test-certified rebar varies less. The lower steel factor encourages higher design strain capacity and ductile failure modes, while the higher concrete factor protects against the concrete's greater variability.
What is the difference between γf and γm?
γf is on the load (demand) side — multiplies service load to give factored load. γm is on the material (capacity) side — divides material strength to give design strength. The two work together: factored load ≤ design strength (capacity). Modern LSD applies these separately for more rigorous reliability calibration; older WSM applied a single combined factor (allowable stress).