CMAA Standards for Deflection in Girders

Deflection criteria for girders under CMAA (Crane Manufacturers Association of America) standards depend on the specific crane classification and the intended application. The CMAA defines deflection limits to ensure safe and reliable operation of overhead cranes and their supporting structures.

CMAA Standards for Deflection in Girders

The two key standards under CMAA for girder deflection are:

1. CMAA Specification No. 70: Specifications for Top-Running Bridge and Gantry Type Multiple Girder Cranes.
2. CMAA Specification No. 74: Specifications for Top-Running and Under-Running Single Girder Electric Traveling Cranes.

General Deflection Limits

1. Vertical Deflection
   The maximum allowable vertical deflection for a crane girder is typically limited to:​Where:
   • $L$: Span of the girder (distance between supports or end trucks).
   • $R$: Deflection ratio, which depends on the crane’s classification and application. Common values:
     • CMAA Class A & B (Light Duty): $R=600$
     • CMAA Class C (Moderate Duty): $R=600$
     • CMAA Class D (Heavy Duty): $R=800$
     • CMAA Class E & F (Severe Duty): $R=1000$

   Example: For a girder with a span of 30 feet ($L=360$ inches), the allowable deflection for Class C cranes is:

   360/600 = 0.6 in

2. Lateral Deflection
   CMAA standards also specify limits for lateral deflection caused by side loading:
   • Typically, lateral deflection should not exceed 1/2 of the vertical deflection limit.

Crane Classification Impact

The classification (A through F) reflects the crane’s duty cycle and operating conditions:

• Class A (Standby Service): Infrequent use, light loads.
• Class B (Light Service): Low operating frequency and light to moderate loads.
• Class C (Moderate Service): Industrial applications with regular use.
• Class D (Heavy Service): High-volume operations, such as manufacturing plants.
• Class E (Severe Service): Continuous service with nearly full-capacity lifts.
• Class F (Continuous Severe Service): Extreme duty, such as steel mills.

The higher the classification, the stricter the deflection requirements.

Finite Element Analysis (FEM) Considerations

When performing FEM for crane girders:

1. Modeling Assumptions:
   • Include both dead load (self-weight) and live load (crane trolley, payload).
   • Apply load combinations per CMAA guidelines.
2. Boundary Conditions:
   • Accurately model the end supports and connections to the crane runway beams.
3. Deflection Measurement:
   • Evaluate mid-span deflection under the maximum load condition.
   • Check both vertical and lateral deflections.

Compliance Check

• Ensure the maximum deflection does not exceed the permissible limits defined by the formula $L/R$.
• Cross-verify results with CMAA design charts or use software capable of structural analysis for cranes (e.g., STAAD.Pro, ANSYS, or SAP2000).