IS Standards Series · Part 3 of 9

Load Combinations & Partial Safety Factors

Complete reference for ULS and SLS load combinations per IS 456:2000 Table 18, IS 875 Parts 1–3, and IS 1893:2016. Covers gravity-only, wind, and seismic combinations with partial safety factor rationale.

Load Types and Standards
IS 456 ULS Combinations (Table 18)
IS 1893 Seismic Combinations
IS 800 Steel Load Combinations
Serviceability Combinations
Comparison with Other Codes

1. Load Types and Standards

Load Type	Symbol	Reference	Notes
Dead Load	DL (or G)	IS 875 Part 1	Self-weight of structure + permanent fixtures
Imposed (Live) Load	LL (or Q)	IS 875 Part 2	Floor: 1.5–5.0 kN/m²; roof: 0.75–1.5 kN/m²
Wind Load	WL	IS 875 Part 3:2015	V_b from wind map, k₁k₂k₃k₄ factors
Seismic Load	EQ	IS 1893 Part 1:2016	Zone-based pseudo-static base shear
Snow Load	SL	IS 875 Part 4	High-altitude regions only
Temperature Effects	T	IS 875 Part 5	Restrained structures, long spans

Wind vs. seismic: IS 1893 Cl.6.3.1 states that wind and earthquake loads need not be considered simultaneously. Design for whichever produces the more critical demand (typically seismic for zones III–V, wind for coastal/tall structures).

2. IS 456 ULS Combinations — Table 18

IS 456:2000 Table 18 defines the load combinations for Ultimate Limit State (Limit State of Collapse) design of reinforced concrete structures:

#	Combination	DL factor	LL factor	WL / EQ factor
1	DL + LL	1.5	1.5	—
2	DL + LL + WL	1.2	1.2	1.2
3	DL + WL (WL adverse)	1.5	—	1.5
4	DL − WL (DL stabilising)	0.9	—	1.5
5	DL + LL + EQ	1.2	1.2	1.2
6	DL + EQ (EQ adverse)	1.5	—	1.5
7	DL − EQ (DL stabilising)	0.9	—	1.5

Key Observations

Gravity-only (Comb. 1): The most common combination for beam/slab design. Factor 1.5 on both DL and LL.
Three-load (Comb. 2, 5): When wind or seismic is present with gravity, all factors reduce to 1.2 to reflect the lower probability of all maxima occurring simultaneously.
Overturning check (Comb. 4, 7): DL factor reduces to 0.9 (stabilising), WL/EQ factor is 1.5 (destabilising). Critical for retaining walls, foundation uplift, and tall buildings.

Most Common ULS Design Demands

GravityF_d = 1.5·DL + 1.5·LL

Wind/SeismicF_d = 1.2·DL + 1.2·LL + 1.2·WL(EQ) [or] 1.5·DL + 1.5·WL(EQ)

OverturningF_d = 0.9·DL + 1.5·WL(EQ)

3. IS 1893 Seismic Combinations

IS 1893:2016 Cl.6.3 specifies additional combinations where the seismic force EQ is the primary variable load. EQ acts simultaneously in ±X, ±Y directions (horizontal); vertical seismic is 2/3 of horizontal for vertical members.

#	Combination	DL	LL	EQ_X	EQ_Y
1	DL + LL + EQ_X	1.2	1.2	±1.2	0
2	DL + LL + EQ_Y	1.2	1.2	0	±1.2
3	DL + EQ_X	1.5	0	±1.5	0
4	DL + EQ_Y	1.5	0	0	±1.5
5	0.9DL + EQ_X (uplift)	0.9	0	±1.5	0
6	0.9DL + EQ_Y (uplift)	0.9	0	0	±1.5

Live Load Reduction for Seismic

IS 1893 Cl.7.3 permits reduction of imposed loads when computing seismic mass:

Roofs: 0% of LL (not included in seismic mass)
Floors with LL ≤ 3.0 kN/m²: 25% of LL
Floors with LL > 3.0 kN/m²: 50% of LL

4. IS 800 Steel Load Combinations

IS 800:2007 Table 4 lists combinations consistent with IS 456 but expressed for steel frame design. The partial safety factors for loads (γ_f) are the same as IS 456 Table 18.

Limit State	DL	LL	WL/EQ	T
Strength (ULS) — DL + LL	1.5	1.5	—	—
Strength — DL + LL + WL/EQ	1.2	1.2	1.2	—
Strength — DL + WL/EQ only	1.5	—	1.5	—
Strength — DL + LL + T	1.2	1.2	—	1.2
Serviceability — all loads	1.0	1.0	1.0	1.0

5. Serviceability Combinations

For deflection, cracking, and vibration checks (Limit State of Serviceability), IS 456 uses unfactored (characteristic) loads:

SLS Load Combination — IS 456 Cl.35

SLSF_service = 1.0·DL + 1.0·LL

SLS + WindF_service = 1.0·DL + 1.0·LL + 1.0·WL

Deflection limits (IS 456 Cl.23.2):

Span/250 — total deflection (dead + imposed) for beams/slabs
Span/350 or 20 mm — post-construction deflection (to avoid damage to partitions)
Height/500 — horizontal sway at each floor level

6. Comparison with Other Codes

Combination	IS 456 (India)	ACI / ASCE 7 (US)	Eurocode (EU)
Gravity only (DL+LL)	1.5DL + 1.5LL	1.2DL + 1.6LL	1.35DL + 1.5LL
DL + LL + Wind/EQ	1.2(DL+LL+W)	1.2DL + 1.0LL + 1.0W	1.35DL + 1.5LL + 0.9W
DL + Wind/EQ	1.5DL + 1.5W	0.9DL + 1.0W	1.0DL + 1.5W
DL stabilising	0.9DL	0.9DL	1.0DL
Seismic	IS 1893 Cl.6.3	ASCE 7 §12.4	EN 1998-1 §4.3

IS 456 vs ACI 318: The IS 456 gravity combination (1.5DL + 1.5LL) applies the same factor to both DL and LL, which is simpler but less nuanced than ACI's differentiated 1.2/1.6 approach. For DL-dominant structures (heavy RC), IS 456 may be slightly more conservative than ACI; for LL-dominant structures (warehouses), IS 456 may be slightly less conservative.

Load Combinations Guide → Next: RC Design →

Preliminary design only. Always verify load combinations with the current edition of IS 456, IS 875, and IS 1893 and a licensed structural engineer.

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Load Combinations & Partial Safety Factors

Contents

1. Load Types and Standards

2. IS 456 ULS Combinations — Table 18

Key Observations

3. IS 1893 Seismic Combinations

Live Load Reduction for Seismic

4. IS 800 Steel Load Combinations

5. Serviceability Combinations

6. Comparison with Other Codes

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