AISC 360-22 — Specification for Structural Steel Buildings
The primary US specification for structural steel design, covering LRFD and ASD methods for tension, compression, flexure, shear, combined forces, connections, and hollow structural sections across 16 chapters.
Scope
AISC 360-22, the “Specification for Structural Steel Buildings,” is the governing design standard for structural steel buildings in the United States. Published by the American Institute of Steel Construction, it provides requirements for the design of steel members, connections, and structures using hot-rolled structural shapes, plates, bars, and HSS sections. The specification covers 16 chapters: A (General), B (Design Requirements), C (Stability Design), D (Tension Members), E (Compression Members), F (Flexure), G (Shear), H (Combined Forces), I (Composite Members), J (Connections), K (HSS), L (Serviceability), M (Fabrication and Erection), N (Quality), O (Fatigue), and P (Fire Design). The 2022 edition updates include revised provisions for the Direct Analysis Method, updated HSS connection design, and modernised bolt and weld provisions.
Purpose
The specification establishes minimum criteria for the design of steel-framed buildings to ensure safety, serviceability, and constructability. It provides two parallel design methodologies: LRFD (Load and Resistance Factor Design), where φRn ≥ ΣγQ, and ASD (Allowable Strength Design), where Rn/Ω ≥ ΣQ. Both methods are calibrated to provide consistent reliability across different limit states. The specification is referenced by the International Building Code (IBC) and is adopted by all US jurisdictions that have adopted modern building codes. It forms the basis for the design aids in the AISC Steel Construction Manual, 16th Edition.
Engineering Applications
AISC 360 is used in all steel building design in the United States, including office towers, industrial facilities, schools, hospitals, sports arenas, and airport terminals. Its provisions apply to the design of beams and girders (Chapter F), columns (Chapter E), beam-columns (Chapter H), tension members such as bracing and hangers (Chapter D), connections including bolted (Chapter J3) and welded (Chapter J2), composite steel-concrete members (Chapter I), and hollow structural sections (Chapter K). Chapter C provides the Direct Analysis Method for stability design, which accounts for both P-delta and P-Δ effects in a single second-order analysis.
Design Philosophy
AISC 360 is based on limit states design. Each member and connection is checked against all applicable limit states — yielding, rupture, buckling (local, flexural, torsional, and lateral-torsional), and fracture — with design strengths computed as φRn (LRFD) or Rn/Ω (ASD). The nominal strength Rn represents the best estimate of the member or connection capacity, while φ and Ω account for uncertainties in material strength, fabrication, analysis, and loading. The specification employs the Direct Analysis Method (Chapter C) as the primary stability design method, which explicitly accounts for initial imperfections, stiffness reduction due to residual stresses and inelasticity, and second-order effects without requiring separate K-factor calculations. This method supersedes the older Effective Length Method (permitted as an alternative in the Commentary).
Important Requirements
All members must be designed for the applicable load combinations per ASCE 7. Stability must be considered through the Direct Analysis Method, which requires: a second-order analysis capturing both P-Δ and P-δ effects; a reduced stiffness of 0.8τbEI for columns and beam-columns to account for inelasticity and residual stresses; and notional loads of 0.002Yi applied at each story level in all load combinations. Tension members are limited by yielding on gross area (φt = 0.90) and rupture on net effective area (φt = 0.75). Compression members have a maximum slenderness of KL/r ≤ 200 (Chapter E2). Flexural members must satisfy compactness limits (Table B4.1b) to reach full plastic moment, with laterally unbraced lengths limited by Lp and Lr per Chapter F. Connection design must account for bolt shear and tension, weld strength, prying action, block shear, and connection slip-critical requirements where specified.
Key Parameters
The following table summarises the resistance (φ) and safety (Ω) factors for key limit states in AISC 360-22:
| Limit State | Chapter | φ (LRFD) | Ω (ASD) |
|---|---|---|---|
| Tension — yielding (gross section) | D2 | 0.90 | 1.67 |
| Tension — rupture (net section) | D2 | 0.75 | 2.00 |
| Compression (flexural, torsional, etc.) | E3-E7 | 0.90 | 1.67 |
| Flexure — compact sections | F2 | 0.90 | 1.67 |
| Shear — most sections | G2 | 1.00 | 1.50 |
| Tension — bolts (shear, bearing-type) | J3 | 0.75 | 2.00 |
| Weld — filler metal (fillet/groove) | J2 | 0.75 | 2.00 |
| Composite — flexure (positive moment) | I3 | 0.90 | 1.67 |
Warning: The stability design requirements of Chapter C (Direct Analysis Method) are mandatory for all designs. The notional load Ni = 0.002Yi must be applied at every story level in every load combination. The reduced stiffness approach (0.8τbEI) must be used unless the more rigorous stiffness reduction procedure is employed. Designs using the older Effective Length Method require a separate check and additional requirements per Appendix 7.
Practical Engineering Notes
In practice, most steel building designs use the Direct Analysis Method with reduced stiffness, which eliminates the need for K-factors and provides a more consistent treatment of stability effects. The τb factor (stiffness reduction for inelasticity) is 1.0 for αPr/Pns ≤ 0.5 and reduces linearly to 0.5 for αPr/Pns = 1.0. For flexural members, the most economical design typically occurs at 70-90% utilisation with Lb ≤ Lp. For columns, the slenderness ratio KL/r has a dominant effect on capacity — a column with KL/r = 100 has roughly 55% of the capacity of a column with KL/r = 50 for A992 steel. The Cb factor for beams can provide up to 2.27 times the basic flexural strength when the moment gradient is favourable. For bolt design, A325 bolts (now classified as Group A in RCSC terminology) have a shear strength of 330 MPa in threads included condition (N) and 414 MPa in threads excluded (X).
Field Tip: When designing connections with combined shear and tension, use the elliptical interaction formula per AISC 360 J3-3b: ΩVr/Rn + ΩTr/Rn ≤ 1.3. For slotted holes in HSS connections, verify the effective net section and the limits on slot length per Chapter K. The block shear rupture strength per J4.3 often governs for gusset plate connections and beam end connections.
Typical Workflow
A typical AISC 360 design workflow: determine loads and load combinations per ASCE 7. Perform second-order analysis per Chapter C (Direct Analysis Method) with reduced stiffness and notional loads. Design tension members per Chapter D (check yielding and rupture). Design compression members per Chapter E (check flexural buckling, torsional buckling, and local buckling). Design flexural members per Chapter F (check compactness, LTB, and flange/web local buckling). Check shear per Chapter G. Verify combined forces per Chapter H (H1-1a for Pr/Pc ≥ 0.2, H1-1b for Pr/Pc < 0.2). Design connections per Chapter J (bolts, welds, connecting elements, and fillers). Verify serviceability per Chapter L (deflection, drift, vibration).
Common Mistakes
Warning: Common errors include neglecting the Cb factor (defaulting to 1.0 when it should be calculated), using the wrong φ factor for the limit state (e.g., using φ = 0.9 for bolt tension instead of φ = 0.75), not accounting for the stiffness reduction in the Direct Analysis Method, omitting notional loads, forgetting block shear checks in connection design, and ignoring the 0.75φ cap on tension rupture. For composite beams, engineers frequently forget to check the negative moment region at interior supports or to verify the shear stud capacity including deck orientation effects.
Best Practices
Always use three-dimensional modelling with the Direct Analysis Method for multi-story frames. Run second-order analysis with P-Δ and P-δ effects included and with the reduced stiffness provisions. Verify both LRFD and ASD results when transitioning between design methods. For connections, always complete the full set of checks: bolt shear/tension/interaction, weld strength, base metal strength, block shear, prying action (for tee-stub and end-plate connections), and connecting element strength. Keep comprehensive design calculations showing each limit state check with formula references. Use the Steel Beam Section Properties Calculator to rapidly iterate section choices.
Limitations
AISC 360 does not cover all steel structures. Seismic design requires the additional provisions of AISC 341 (Seismic Provisions). Bridge design follows AASHTO LRFD, not AISC 360. The specification does not cover cold-formed steel members (governed by AISI S100), aluminium structures (ADM), or stainless steel (ASCE 8). Fatigue design per Chapter O is limited to provisions for certain connection details and does not cover fracture mechanics-based approaches for crack propagation. Fire design per Chapter P provides simple methods for unprotected steel, but complex or unusual fire scenarios require performance-based analysis per ASCE 29.
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References
- AISC 360-22. Specification for Structural Steel Buildings. American Institute of Steel Construction, 2022.
- AISC. Steel Construction Manual. 16th ed., AISC, 2023.
- AISC 341-22. Seismic Provisions for Structural Steel Buildings. AISC, 2022.
- Salmon, C.G., Johnson, J.E., and Malhas, F.A. Steel Structures: Design and Behavior. 5th ed., Pearson, 2009.
- RCSC. Specification for Structural Joints Using High-Strength Bolts. Research Council on Structural Connections, 2020.