CMAA Service Classes (CMAA 70 and 74)

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

CMAA defines six service classes (A through F), each corresponding to a range of load cycles and load spectrum severity. The class is selected by the crane specifier or manufacturer based on the intended use.

CMAA Service Class Definitions

Key principle: A crane's classification is NOT determined solely by its rated capacity. A 5-ton crane in a Class E scrap yard may see more fatigue damage than a 50-ton crane in a Class B maintenance bay. Classification combines load magnitude, load frequency, and total cycles.

AISE Technical Report 6 — Mill Building Runway Design

AISE TR-6 (now maintained by AIST — Association for Iron & Steel Technology) provides specific guidance for runway girder design in steel mill buildings where CMAA classifications C through F typically apply. AISE TR-13 extends this to crane runway structural supports.

CMAA-to-AISE Equivalency

AISE TR-6 includes additional requirements beyond CMAA:

• Horizontal side-thrust from crane acceleration/deceleration: 20 percent of lifted load for top-running, 10 percent for under-running
• Longitudinal tractor force: 10 percent of maximum wheel load
• Impact factors specific to mill duty cycles (see below)
• Deflection limits: vertical L/600 for light duty, L/1000 for heavy mill cranes to prevent rail wear

Impact Factors for Crane Runway Design

The vertical wheel load includes an impact factor to account for dynamic effects (hoist acceleration, snatch loads, rail irregularities). The impact factor is applied ONLY to the lifted load, NOT to the crane dead weight.

CMAA Impact Factors

AISE TR-6 Impact Factors

Crane Wheel Load Calculation

The maximum wheel load P_max controls the design of the runway girder, bracket, and column. The wheel load depends on the crane capacity, bridge weight, trolley position, and number of wheels.

Wheel Load Formula

P_max = (Bridge Weight / Number of Wheels) + (Lifted Load + Trolley Weight) x (Bridge Span - Hook Approach) / (Bridge Span x Wheels per End Truck)

For a typical top-running double-girder crane:

P_max = (W_bridge / 4) + (W_lift + W_trolley) x (S - a) / (2 x S)

Where:

• W_bridge = total crane bridge weight (lb or kip)
• W_lift = rated lifted load (lb or kip)
• W_trolley = trolley weight (lb or kip)
• S = crane bridge span (ft)
• a = minimum hook approach (ft) — distance from rail centerline to hook at closest position
• Assumes 4-wheel, double-girder (2 wheels per end truck)

Worked Example

A Class D crane with rated capacity 30 tons (60,000 lb). Bridge weight 42,000 lb, trolley weight 8,000 lb. Bridge span S = 60 ft, minimum hook approach a = 3 ft. Four total wheels, two per end truck.

P_max = (42,000 / 4) + (60,000 + 8,000) x (60 - 3) / (2 x 60) P_max = 10,500 + 68,000 x 57 / 120 P_max = 10,500 + 68,000 x 0.475 P_max = 10,500 + 32,300 P_max = 42,800 lb (21.4 tons per wheel, unfactored)

Factored Wheel Load for Runway Beam Design

Per AISC 360 and ASCE 7, the factored wheel load for LRFD:

P_u = 1.2 x (P_max from bridge weight) + 1.6 x (P_max from lifted load + impact)

Using the example above with 20 percent impact (Class D):

• P_max bridge weight component = 10,500 lb
• P_max lifted load component = 32,300 lb
• Impact = 0.20 x 32,300 = 6,460 lb
• P_u = 1.2 x 10,500 + 1.6 x (32,300 + 6,460)
• P_u = 12,600 + 1.6 x 38,760
• P_u = 12,600 + 62,016
• P_u = 74,616 lb (37.3 tons per wheel, factored LRFD)

This factored load is used for the runway beam strength design (moment and shear). The ASD approach uses the service-level P_max without factors.

Runway Beam Design Considerations

Girder Type Selection

Lateral Load to Runway Girder

The lateral (side-thrust) load from crane acceleration:

• Top-running crane: 20% of (lifted load + trolley weight)
• Under-running crane: 10% of (lifted load + trolley weight)

This lateral load is applied at the top of the rail and resisted by the runway girder top flange in weak-axis bending. For heavy Class D-F cranes, a separate horizontal truss or surge girder is often added to resist side-thrust.

Deflection Criteria

Stricter deflection limits for heavy cranes prevent rail misalignment and excessive wheel flange wear. A runway girder spanning 30 ft at Class E (L/1000) must limit vertical deflection to 30 x 12 / 1000 = 0.36 inch under service wheel loads.

Fatigue Design for Crane Runways

Crane runway girders are among the most fatigue-sensitive structural elements in building design. Per AISC 360 Appendix 3:

• Fatigue is required when the number of load cycles N exceeds 20,000 (which covers CMAA Class C and above)
• The stress range Sr is computed from the full range of the moving wheel loads (empty to full P_max)
• Fatigue categories for typical details:
  • Stiffener-to-flange fillet welds: Category C (constant-amplitude fatigue threshold = 10 ksi at 2 million cycles)
  • Web-to-flange fillet welds: Category B (16 ksi threshold)
  • Cover plate terminations: Category E (4.5 ksi threshold — AVOID on crane runways)
  • Bolted rail splices: Category B (16 ksi threshold)

For a Class E crane with 3 million cycles, the allowable stress range drops approximately 13 percent below the 2-million-cycle threshold. Continuous fillet welds with smooth transitions are essential. Cover plates and abrupt section changes are strongly discouraged on crane runway girders.

Worked Example — Classifying a Steel Mill Crane

Given: A crane in a steel making facility handles ladles of molten steel. The crane operates 16 cycles per hour (one lift, transfer, pour, return), 24 hours per day, 350 days per year. Design life = 40 years. Each lift is near the crane's rated capacity.

Step 1: Estimate Total Load Cycles

• Cycles per day = 16 x 24 = 384
• Cycles per year = 384 x 350 = 134,400
• Cycles over 40 years = 134,400 x 40 = 5,376,000

Step 2: Determine CMAA Class

Total cycles exceed 5,000,000 with lifts at or near rated capacity. Load spectrum is continuous severe (nearly every lift is full capacity hot metal). Classification: Class F.

Step 3: Determine AISE Duty Group

Class F ladle crane = AISE Duty Group 4 (severe). Impact factor = 50 percent of lifted load.

Step 4: Verify Specific Requirements for Class F / Group 4

• Vertical impact: 50 percent of lifted load
• Vertical deflection limit: L/1200
• Horizontal side-thrust: 20 percent of (lifted load + trolley)
• Fatigue design: Category B details minimum; Category C or below requires detailed fracture mechanics analysis
• Rail attachment: continuous full-penetration weld or bolted clip every 12 inches
• Inspection interval: runway girders every 6 months; crane structure every 3 months

The Class F designation drives the entire runway design: heavier sections for stiffness, stricter fatigue details, and more frequent inspection programs than a Class C crane of identical rated capacity would require.

Frequently Asked Questions

What is the difference between a CMAA Class C and Class D crane if both are rated at 10 tons?

The rated capacity is the same, but Class D anticipates approximately 5-10 times more load cycles and a heavier load spectrum (lifts at or near capacity more often). Physically, the Class D crane has heavier girders, larger wheels, higher-horsepower motors, and more robust end trucks. The runway for a Class D crane must also be designed for higher fatigue stress ranges, potentially requiring a deeper or heavier girder. A Class C crane in a Class D application would experience premature fatigue failure.

What is the minimum hook approach and why does it matter?

The minimum hook approach is the closest distance the hook can approach the runway rail on either side. It is typically 2 to 5 feet, determined by the trolley width and end stop location. A smaller hook approach increases the maximum wheel load (as shown in the worked example) because the lifted load and trolley are closer to one end of the bridge, concentrating the reaction. Specifying a larger minimum hook approach reduces the maximum wheel load and can reduce the runway girder size, but at the cost of reduced usable floor coverage.

Do I need a separate surge girder for lateral crane loads?

For CMAA Classes A-C, the top flange of the runway girder resisting weak-axis bending is usually sufficient, provided the flange width is at least 8-10 inches. For Class D and above, a separate horizontal truss (surge girder) or a channel cap on the top flange becomes economical because the lateral loads are significant relative to weak-axis capacity. AISE TR-6 recommends a surge girder when the factored lateral load exceeds 20 percent of the factored vertical bending capacity of the top flange.

How are multiple cranes on the same runway handled?

When two or more cranes operate on the same runway, the runway girder must be designed for the simultaneous presence of cranes in the worst position. ASCE 7 Section 4.9 provides live load reduction factors for multiple cranes: 100 percent for one crane, 85 percent for two cranes simultaneously in worst position, 70 percent for three cranes. However, AISE TR-6 does NOT permit reduction for ladle cranes in steel mills (Class F): full load from all cranes on the runway must be considered simultaneously due to the catastrophic consequence of failure.

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Related References

• Load Combinations ASCE 7-22 — LRFD and ASD Guide
• Steel Fatigue Design — S-N Curves and Stress Categories
• AISC Steel Construction Tables — Manual Reference
• Steel Fy and Fu Reference — Yield and Tensile Strength by Grade
• Beam Deflection Formulas — Simply Supported, Cantilever, and Fixed
• Bolt Spacing — Minimum Edge and End Distance Guide
• Connection Checklist — Design and Detailing Review
• Guides and checklists
• How to verify calculator results
• Disclaimer (educational use only)

Crane classifications per CMAA 70-2020, CMAA 74-2020, and AIST (formerly AISE) Technical Reports 6 and 13. All crane specifications must be reviewed and stamped by a qualified professional engineer. The classification listed on the crane nameplate controls; do not rely on visual inspection alone.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer or crane engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world crane and runway design depends on project-specific factors (crane manufacturer data, duty cycle analysis, fatigue assessment, rail alignment tolerances, and governing codes and standards). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.

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