Factored load is the design load for limit state ultimate strength check, obtained by multiplying service (working) loads by partial safety factors. Per IS 456:2000 Cl. 36.4 + IS 800:2007 Cl. 5.4: γf (load factor) = 1.5 for combined dead + live load (ULS); 1.5 for dead + wind/earthquake; 1.2 for dead + live + wind/earthquake; 0.9 dead − 1.5 wind/earthquake (uplift). Factored loads are higher than service loads, representing the design strength demand at ultimate limit state.
For a typical residential 5 m simply-supported beam with: DL = 25 kN/m (own weight + finishes), LL = 12 kN/m: service load = 37 kN/m (UDL); factored load = 1.5 × (25 + 12) = 55.5 kN/m. Service moment = 37 × 5²/8 = 116 kNm; factored moment Mu = 55.5 × 5²/8 = 174 kNm. Design uses Mu compared against design strength (0.87 × fy × Ast × leverarm) at the section. Without applying factor, the design would be inadequate at ultimate (collapse) limit state.
Where used
All ULS strength design — flexure, shear, axial, combined
Foundation bearing capacity check (against ultimate)
Stability check — overturning, sliding, uplift
Pre-stressed concrete pre-stress check at ultimate
Steel structure connection design (bolts, welds)
Acceptance / threshold
Per IS 456 Cl. 36.4 + IS 800 Cl. 5.4: appropriate γf applied based on combination; design forces ≤ design strength × (1/γm) at ULS; serviceability checked separately at unfactored loads.
Site example
Site reality: a Pune commercial drawing 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 factored moment as service load is a 17% miscalculation that often leads to under-reinforcement.
Frequently asked
What is factored load?
Factored load is the design load for limit state ultimate strength check, obtained by multiplying service loads by partial safety factors (γf). Per IS 456 Cl. 36.4: γf = 1.5 for DL+LL; 1.5 for DL+EL/WL; 1.2 for combined; 0.9 DL+1.5 EL for uplift. Used for ULS strength design.
What is the difference between service load and factored load?
Service load is the actual expected load (unfactored). Factored load = service × γf, used for ULS strength design. Service load is used for serviceability check (deflection, crack width). Both must be satisfied: ULS at factored load and SLS at service load.
Why are loads multiplied by safety factors?
Loads are multiplied by γf (typically 1.5) to account for uncertainty in load magnitude (variability of dead load, occasional excessive live load, extreme wind/earthquake). Materials are simultaneously divided by γm (1.5 for concrete, 1.15 for steel) to account for material variability. Combined γf × γm gives target reliability ~10⁻⁴ failure probability.