ISO 20332:2016 Limit State Method for Steel Structure Verification
ISO 20332:2016 "Cranes — Limit State Method — Verification of Steel Structure Strength" is the verification standard for crane steel structures designed using the limit state method. It specifies load combinations, partial safety factors, and strength verification procedures for steel structures under both the ultimate limit state (ULS) and the serviceability limit state (SLS).
Limit State Classification
ISO 20332:2016 classifies the limit states of crane steel structures into three categories: the ultimate limit state (ULS) — the condition at which the structure reaches its maximum load-bearing capacity, covering overall instability, local buckling, material yielding, and fatigue fracture; the serviceability limit state (SLS) — the condition where structural deformation or vibration exceeds permissible service values, including excessive elastic deformation, vibration, and noise; and the fatigue limit state (FLS) — the condition where the structure fails under alternating loads. This article covers ULS and SLS verification methods; fatigue verification is performed in accordance with ISO 24035.
Load combinations for ULS verification — loads to be considered include permanent loads G (self-weight of the structure, fixed equipment weight), variable loads Q (suspended load, wind load, snow load, temperature load), and accidental loads A (earthquake, collision, impact). Each load is multiplied by its corresponding partial safety factor before combination: the fundamental combination is γG × G + γQ × Q, and the accidental combination is G + A + ψ × Q. The partial safety factor γG (for permanent loads) is taken as 1.2 (unfavorable) or 1.0 (favorable), and γQ (for variable loads) is taken as 1.5.
Strength Verification Procedure
Basic strength verification formula — under the design load combination, the stress σd at any point in the structure must satisfy σd ≤ f_d/γM, where f_d is the material design strength (based on the standard yield strength divided by the material partial safety factor γM), and γM is the material partial safety factor (1.0–1.1 for steel, 1.15–1.25 for weld seams). Compression members must also be checked for stability — both overall stability (Euler buckling) and local stability (plate width-to-thickness ratio limits).
Stability verification — the stability coefficient φ for compression members is determined from the slenderness ratio λ and the cross-section classification (Classes a, b, and c), using the formula N/(φ × A) ≤ f_d/γM. For bending members, overall stability may be omitted if the lateral bracing spacing is adequate; otherwise, verification is performed using φb × W × f_d/γM. For local stability of plate elements, the flange width-to-thickness ratio b/t must not exceed 15 (for Q235 steel), and the web plate height-to-thickness ratio h0/tw must not exceed 80 (or 180 when transverse stiffeners are provided). Kelude Heavy Industry applies the limit state method comprehensively in its steel structure strength design.
| Limit state | Load combination | acceptance criteria | partial safety factor |
|---|---|---|---|
| ULSLoad Bearing | γ G×G+γ Q×Q | σd≤fd/γ M | γ G=1.2,γ Q=1.5 |
| SLSNormal | G+Q+ψ×Q | Deformation≤L/400 | ψ=0.7~1.0 |
| FLSFatigue | Stress Spectrum | D≤1.0 | γ F=1.25 |
| Accidental | G+A+ψ×Q | σd≤1.5fd | No Additional Partial Factors |
Serviceability Limit State (SLS) Requirements
SLS verification covers deformation limits: mid-span deflection of the main girder under rated load must not exceed L/400 to L/500 (with the stricter value applied for heavy-duty classifications), while cantilever end deflection is limited to L/300. Relative deflection at the crane bridge and trolley rail contact points is capped at 5 mm. Maximum horizontal displacement of the structure is limited to H/800 in the bridge travel direction and H/500 in the trolley travel direction, where H is the height above the rail. Deformation checks use standard (unfactored) load combinations — G + Q, with the suspended load taken at its rated capacity and wind load based on working-condition wind pressure.
Dynamic stiffness requirements mandate that the primary vertical vibration frequency of the structure during starting and braking under rated load remain at or above 2 Hz to prevent operator discomfort and resonance with the suspended load. After sudden load release from full-load hoisting, structural vibration attenuation must occur within 5 seconds. For cranes requiring precise positioning — such as ladle cranes — stricter vertical stiffness criteria apply, with deflection limits tightened to L/800 to L/1000. Kelude Heavy Industry's crane steel structure designs fully comply with SLS deformation and dynamic stiffness requirements.
Verification Report
The steel structure strength verification report must include the following: the calculation basis (applicable standards and their versions), crane technical parameters (lifting capacity, span, duty classification, etc.), load values and load combinations (load value determination and combination factors for each operating condition), finite element analysis or analytical calculation results (stress contours, deformation diagrams, and stability assessments), and verification conclusions for each limit state — confirming whether ULS and SLS requirements are satisfied — along with recommendations. The report is signed by a certified structural engineer.
| Verification Items | requirements | Calculation Method | Report Content |
|---|---|---|---|
| ULSStrength | σd≤fd/γ M | Finite Element/Analytical | Stress Contour Plot+Conclusion |
| ULSStability | N/(φ A)≤fd/γ M | Euler | Stability Verification Report |
| SLSDeformation | δmax≤L/400 | Deflection Calculation | Deformation Contour Plot |
| SLSvibration | Fundamental Frequency≥2Hz | modal analysis | Frequency Value |
FAQ
Q: What limit states are defined in ISO 20332:2016?
A: The ultimate limit state (ULS) covering strength and stability, the serviceability limit state (SLS) covering deformation and vibration, and the fatigue limit state (FLS) covering fatigue life. ULS and SLS each use different load combinations and partial safety factors.
Q: How are partial safety factors determined for loads?
A: For permanent loads, γG = 1.2 when the effect is unfavorable and γG = 1.0 when favorable. For variable loads, γQ = 1.5. Material partial safety factors are γM = 1.0–1.1 for steel and γM = 1.15–1.25 for weld seams. Accidental load combinations use G + A + ψ × Q.
Q: What are the deformation limits under the serviceability limit state?
A: Main girder deflection under rated load is limited to L/400 for moderate duty and L/500 for heavy duty; cantilever end deflection is limited to L/300. Relative deflection at the crane rail is limited to 5 mm. The fundamental frequency of the structure must be ≥ 2 Hz. Tighter deflection limits apply to cranes used for precision positioning.
Q: What is the strength verification formula for ULS?
A: σd ≤ fd / γM — where σd is the design stress from the load combination, fd is the design strength of the material (based on standard yield strength), and γM is the material partial safety factor. Members in compression are also checked for overall stability using N/(φA) ≤ fd / γM. Kelude Heavy Industry applies the limit state method throughout its steel structure design.
