Structural Analyser › ASCE 7-16 › Chapter 2 — Load Combinations

Chapter 2 — Combinations of Loads

LRFD Strength Design §2.3 · ASD Allowable Stress §2.4 · Seismic §2.3.6 / §2.4.5 · Extraordinary §2.5

Overview

ASCE 7-16 Chapter 2 defines load combinations for both strength design (LRFD, §2.3) and allowable stress design (ASD, §2.4). Where seismic loads govern, additional combinations from §2.3.6 (LRFD) and §2.4.5 (ASD) apply. Extraordinary combinations (§2.5) address progressive collapse and abnormal load events. Use the tabs to view each set. All load symbols follow §1.2.2.

Key Load Factors & Variables §2.3–2.4
f₁
= 1.0 for floors in places of public assembly, live loads > 4.79 kN/m²; else 0.5
f₂
= 0.7 for storage, parking; 0.2 for other live loads in roof
ρ
Redundancy factor (§12.3.4): 1.0 or 1.3 for SDC D, E, F
Ω₀
Overstrength factor (Table 12.2-1); used in Eₘ = Ω₀Eₕ ± Eᵥ
SDS
Design spectral acceleration at short periods; Eᵥ = 0.2S_DS·D (§12.4.2.2)
0.9D
Minimum dead load factor used when dead load resists overturning / uplift
§2.3.1 — Basic LRFD Strength Design Combinations §2.3.1
Combo Expression Ref Governs
LRFD-1
1.4D
 Eq. 2.3-1 Dead load
LRFD-2a
1.2D + 1.6L + 0.5(Lᵣ or S or R)
 Eq. 2.3-2 Gravity max
LRFD-2b
1.2D + 1.6L + 0.5W
 Eq. 2.3-2 alt Gravity + wind
LRFD-3a
1.2D + 1.6(Lᵣ or S or R) + (1.0L or 0.5W)
 Eq. 2.3-3 Roof live/snow
LRFD-3b
1.2D + 1.6S + 0.5W
 Eq. 2.3-3 alt Snow + wind
LRFD-4a
1.2D + 1.0W + 1.0L + 0.5(Lᵣ or S or R)
 Eq. 2.3-4 Wind governs
LRFD-4b
0.9D + 1.0W
 Eq. 2.3-4 uplift Uplift / OT
LRFD-5a
1.2D + 1.0E + 1.0L + 0.2S
 Eq. 2.3-5 Seismic
LRFD-5b
0.9D + 1.0E
 Eq. 2.3-5 uplift Seismic uplift
Note on E: In seismic combinations, E = Eₕ + Eᵥ where Eₕ = ρQE (horizontal effect, §12.4.2.1) and Eᵥ = 0.2S_DS·D (vertical effect, §12.4.2.2). For LRFD-5a: 1.2D + 1.0E + ... expands to (1.2 + 0.2S_DS)D + ρQE + L + 0.2S. For LRFD-5b: (0.9 − 0.2S_DS)D + ρQE.

When to Use Each Combination

  • LRFD-1: Rarely governs — applies only where dead load alone is critical (concrete self-weight check).
  • LRFD-2a: Typically governs floor beams with high occupancy live loads.
  • LRFD-3a/3b: Governs roof members with significant snow accumulation.
  • LRFD-4a: Governs lateral-load-resisting members (columns, shear walls) under wind.
  • LRFD-4b: Critical for foundation uplift and overturning under wind — uses 0.9D to minimize stabilizing dead load.
  • LRFD-5a/5b: Apply wherever seismic loads are present; SDC B and above.
§2.4.1 — Basic ASD Allowable Stress Combinations §2.4.1
Combo Expression (unfactored loads) Ref Governs
ASD-1
D
 §2.4.1 (1) Dead only
ASD-2
D + L
 §2.4.1 (2) Gravity
ASD-3
D + Lᵣ (or S or R)
 §2.4.1 (3) Roof live/snow
ASD-4
D + 0.75L + 0.75Lᵣ (or 0.75S or 0.75R)
 §2.4.1 (4) Gravity combo
ASD-5a
D + 0.6W
 §2.4.1 (5a) Wind
ASD-5b
D + 0.7E
 §2.4.1 (5b) Seismic
ASD-6a
D + 0.75(0.6W) + 0.75L + 0.75(Lᵣ or S or R)
 §2.4.1 (6a) Wind + gravity
ASD-6b
D + 0.75(0.7E) + 0.75L + 0.75S
 §2.4.1 (6b) Seismic + grav.
ASD-7a
0.6D + 0.6W
 §2.4.1 (7a) Wind uplift
ASD-7b
0.6D + 0.7E
 §2.4.1 (7b) Seismic uplift

ASD Design Notes

  • Unfactored loads: ASD uses service-level loads directly — no load amplification factors ≥ 1.0 on gravity.
  • 0.6W factor: Wind loads in ASD are reduced to 60% because the basic wind speed in ASCE 7-10/16 was calibrated for LRFD (strength-level). The 0.6 factor converts to service level.
  • 0.7E factor: Seismic loads are reduced to 70% of design earthquake for ASD service-level check (§2.4.1).
  • Combinations 6a/6b: Use 75% of wind or seismic combined with 75% of floor live and roof live/snow, accounting for improbability of all extreme events occurring simultaneously.
  • Combinations 7a/7b: Overturning and uplift checks — use minimum dead load (0.6D) to maximize net uplift effect.
§2.3.6 — LRFD Seismic Load Combinations with Overstrength §2.3.6
Combo Expression Ref Governs
LRFD-6a
(1.2 + 0.2SDS)D + Ω₀Qₑ + L + 0.2S
 §2.3.6 (1) Overstrength
LRFD-7a
(0.9 − 0.2SDS)D + Ω₀Qₑ
 §2.3.6 (2) OT uplift
§2.4.5 — ASD Seismic Load Combinations with Overstrength §2.4.5
Combo Expression Ref Governs
ASD-8
(1 + 0.14SDS)D + H + F + 0.7Ω₀Qₑ
 §2.4.5 (1) Overstrength
ASD-9
(1 + 0.105SDS)D + H + F + 0.525Ω₀Qₑ + 0.75L + 0.75S
 §2.4.5 (2) Combined grav.
ASD-10
(0.6 − 0.14SDS)D + 0.7Ω₀Qₑ + H
 §2.4.5 (3) OT uplift

When Overstrength Combinations Apply

Overstrength combinations (using Ω₀ in place of ρ) are required by ASCE 7-16 for specific components that must remain elastic to protect adjacent yielding elements:

  • › Collectors (drag struts) connecting to lateral-force-resisting elements — §12.10.2.1
  • › Columns supporting discontinuous walls or frames — §12.3.3.3
  • › Diaphragm connections to the LFRS — §12.10
  • › Foundations supporting LFRS elements — §12.13.5
  • › Cantilevered column systems — §12.2.5.2
  • › Batter piles — §12.13.8.3

Expanded Seismic Load E = Eₕ + Eᵥ

E = Eₕ + Eᵥ = ρQE ± 0.2S_DS·D
Eₕ = ρQE : horizontal seismic effect × redundancy factor (§12.4.2.1)
Eᵥ = 0.2S_DS·D : vertical seismic effect (§12.4.2.2) — additive in LRFD-5a, subtractive in LRFD-5b

Eₘ (with overstrength) = Ω₀Qₑ ± 0.2S_DS·D — substituted for ρQE in overstrength combinations.

§2.5 — Extraordinary Load Combinations (Progressive Collapse) §2.5
Combo Expression Ref Purpose
EXT-1
1.2D + Ak + 1.0L + 0.2S
 §2.5.2 Eq. 2.5-1 Extraordinary
EXT-2
0.9D + Ak
 §2.5.2 Eq. 2.5-2 Uplift check

Ak — Extraordinary (Abnormal) Load

Ak is the load effect resulting from an extraordinary event such as: an explosion, vehicular impact, equipment failure, or other event not normally considered in design but required for structural integrity per the contract documents. Ak is not combined with seismic or wind loads in the same combination.

When Extraordinary Combinations Are Required

  • › Risk Category III/IV buildings where progressive collapse analysis is required
  • › Buildings with parking at ground floor (vehicle impact)
  • › Structures near explosive or high-hazard industrial processes
  • › When specified by the authority having jurisdiction (AHJ)
  • › Buildings using alternate load path method (UFC 4-023-03 for federal)

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