What is Crane Girder Design used for in structural engineering?

Crane Girder Design provides values required for steel design calculations including capacity checks, connection design, and detailing. It is referenced in structural design codes and used by practicing engineers during the design of buildings, bridges, and industrial structures.

Can I use these reference values directly in my design?

Yes, provided the values match your governing code edition and project specifications. Always cross-reference against the primary source (code document, manufacturer data, or published standard). The information on this page is for educational use — final verification by a licensed engineer is required.

How often are these reference values updated?

Reference content is maintained to align with current code editions. When a new edition of AISC 360, AS 4100, EN 1993, or CSA S16 is published, values are reviewed and updated. Check the last-modified date at the bottom of the page and verify against the current edition for your jurisdiction.

Crane Girder Design Guide -- AISC DG7 and CMAA 74

Crane runway girders are arguably the most demanding structural steel members in industrial buildings. They must resist vertical wheel loads with impact, lateral surge forces, longitudinal traction, skewing forces from misaligned cranes, and they must do so through millions of load cycles without fatigue cracking -- all while maintaining deflection limits tight enough that the crane operates smoothly. This guide follows AISC Design Guide 7 (Industrial Buildings -- Roofs to Anchor Rods, 3rd Edition) and CMAA 74 (Specification for Top-Running Bridge and Gantry Type Multiple Girder Cranes).

Crane Classification and Duty Cycles

Not all cranes are equal. CMAA 74 defines six crane service classes that determine the fatigue design approach:

Class A (Standby/Infrequent): 0-1 lifts/hour, manual operation, total 20,000-100,000 cycles over life. No fatigue check needed for runway beams.
Class B (Light Service): 2-5 lifts/hour, occasional full capacity. 100,000-500,000 cycles.
Class C (Moderate Service): 5-10 lifts/hour, frequently at or near capacity. Machine shops, fabricating plants. 500,000-2,000,000 cycles. Fatigue check required.
Class D (Heavy Service): 10-20 lifts/hour, continuous operation near capacity. Steel mills, foundries. 2,000,000-5,000,000 cycles. Detailed fatigue assessment required.
Class E (Severe Service): 20+ lifts/hour, capacity lifts frequent. Scrap yards, heavy fabrication. 5,000,000-10,000,000 cycles. Finite-life fatigue design mandatory.
Class F (Continuous Severe): Continuous operation at full capacity. Custom-designed for specific process. Fatigue is the governing design criterion.

The runway beam designer must know the crane class before beginning -- it determines whether strength, deflection, or fatigue will control the beam size.

Wheel Load Calculation

Crane wheel loads are derived from the bridge crane's rated capacity, bridge and trolley weights, and geometry per AISC DG7 Chapter 13. The maximum wheel load per end truck occurs when the trolley is positioned at the minimum hook approach distance from the runway beam:

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

Where: W_bridge = total bridge weight (kips), W_trolley = trolley weight (kips), W_lifted = rated capacity (kips), S = bridge span (ft), a = minimum hook approach (ft).

For a 20-ton capacity bridge crane: capacity = 40 kips, bridge span S = 30 ft, trolley weight = 8 kips, bridge weight = 12 kips, minimum hook approach a = 3 ft. P_max = (12/4) + (8 + 40) x (30 - 3) / (2 x 30) = 3 + 48 x 27 / 60 = 3 + 21.6 = 24.6 kips per wheel. This is the static unfactored wheel load.

Impact factors per AISC DG7 Table 13.2 and ASCE 7 Section 4.10: 25% for cab-operated cranes, 10% for pendant-operated (pendant pushbutton), and 0% for manually operated chain hoists. For monorails: 25%. The design vertical wheel load: P_vertical = P_max x (1 + IF). With 25% impact, P_vertical = 24.6 x 1.25 = 30.75 kips. Note that impact factor applies to the wheel load only, not to the dead load of the runway beam itself.

Lateral surge force per AISC DG7 Section 13.7: Lateral force = 20% of the sum of the lifted load and trolley weight, distributed equally to each runway rail. Surge_per_rail = 0.20 x (W_lifted + W_trolley) / 2 = 0.20 x (40 + 8) / 2 = 4.8 kips. The lateral force is applied at the top of the crane rail (typically 3-5 inches above the runway beam top flange), introducing a small torsional moment in the beam. For design, the beam top flange and the lateral support (cap channel, bracing, or box section) must resist this lateral load.

Longitudinal traction force per AISC DG7: Longitudinal force = 10% of maximum wheel load (with impact) = 0.10 x 30.75 = 3.1 kips per rail. This force is parallel to the runway beam but is typically resisted by the runway beam-to-column connections and the building's longitudinal bracing system, not by the runway beam in bending.

Skewing forces per CMAA 74 Section 3.4.3: When a bridge crane travels along the runway, minor misalignment between the rails causes the bridge to skew (yaw). This introduces horizontal forces at the guide rollers. CMAA 74 provides a simplified approach: the skewing force F_s = 0.05 x (Lifted load + Trolley weight) applied at the top of the rail, acting perpendicular to the runway beam axis. This is additive to the lateral surge force and must be combined in the load cases per AISC DG7 Table 13.3.

Runway Beam Flexural Design -- Biaxial Bending

The runway beam is subjected to simultaneous vertical bending (from wheel loads) and lateral bending (from surge + skewing forces). The AISC 360 interaction equation (Chapter H) governs:

For doubly-symmetric sections: Pr/Pc + Mrx/Mcx + Mry/Mcy <= 1.0. Since there is negligible axial load in the runway beam, this reduces to: M_ux / (phi_b x M_nx) + M_uy / (phi_b x M_ny) <= 1.0.

Strong-axis moment: M_ux is the maximum moment from the wheel loads positioned using influence lines. For a simply supported runway beam with two equal wheel loads from the end truck (spaced at wheelbase B), the maximum moment occurs when one wheel is at a distance B/4 from mid-span. M_max = P x L / 4 x (2 - 4B/L + B^2/L^2) for a beam with two equal moving loads.

Weak-axis moment: M_uy = (Lateral surge + Skewing) x L / 4 (assuming the lateral load is also a point load applied at mid-span from a single wheel axis, which is conservative). For the top flange of a W-shape, the lateral bending capacity is phi_b x Fy x S_y (top flange alone, since lateral bending is resisted primarily by the top flange and cap channel).

Worked example -- 20-ton crane runway beam: Runway span L = 30 ft. Wheelbase of end truck B = 6 ft (two wheels spaced 6 ft apart). Vertical wheel load P_v = 30.75 kips. Maximum strong-axis moment: M_ux = 30.75 x 30 x 12 / 4 x (2 - 4 x 6/30 + 36/900) = 2767.5 x (2 - 0.8 + 0.04) = 2767.5 x 1.24 = 3,431 kip-in = 286 kip-ft. Required Sx >= 286 x 12 / (0.90 x 50) = 3,432 / 45 = 76.3 in^3. Try W24x68: Sx = 154 in^3. OK for strong-axis flexure. Lateral surge = 4.8 kips. Weak-axis moment: M_uy = 4.8 x 30 x 12 / 4 = 432 kip-in. Top flange S_y = b_f x t_f^2 / 6 = 9.0 x 0.585^2 / 6 = 0.51 in^3 (conservative, ignoring cap channel). phi x M_ny = 0.90 x 50 x 0.51 = 23.0 kip-in. Interaction: 3,431/(0.90 x 50 x 154) + 432/23 = 3,431/6,930 + 18.8 = 0.495 + 18.8 = 19.3 >> 1.0! The lateral bending exceeds the top flange capacity without a cap channel. This illustrates why runway beams ALWAYS require a cap channel -- the flange alone cannot carry lateral surge. With a C12x20.7 cap channel providing S_y = 1.90 in^3: phi x M_ny = 0.90 x 50 x 1.90 = 85.5 kip-in. Interaction: 0.495 + 432/85.5 = 0.495 + 5.05 = 5.55 -- still > 1.0. This demonstrates that crane runway beams are lateral-load dominated for moderate-to-heavy cranes and often require a box girder or a specially fabricated section with a robust top flange. For this 20-ton crane, a W24x76 with a C15x33.9 cap channel or a fabricated box girder would be required.

Fatigue Design of Runway Beams

Fatigue in crane runway beams follows AISC 360 Appendix 3, applied per AISC DG7 methodology. The critical insight: a runway beam experiences one stress cycle per crane pass, and for a busy industrial facility with a Class D crane (2.5 million cycles), the fatigue check controls over the strength check for connections.

Stress range calculation: The stress range Delta_f = f_max - f_min, where f_max = stress at mid-span when the crane wheels are positioned for maximum moment, and f_min = minimum stress (typically zero or very small for a simply supported beam when the crane is at the opposite end). The stress range is based on the unfactored wheel load (impact factor DOES apply for fatigue per AISC Appendix 3 because impact cycles with the load). For the W24x68 example with P_v = 30.75 kips producing M = 3,431 kip-in at mid-span: Sx = 154 in^3. f_max = 3,431/154 = 22.3 ksi (tension in bottom flange). Delta_f = 22.3 - 0 = 22.3 ksi.

Fatigue capacity: The runway beam bottom flange in pure bending (plain material, no attachments) is Category A with a constant-amplitude fatigue threshold (CAFT) of 24.0 ksi. Delta_f = 22.3 ksi < 24.0 ksi -- the bottom flange has infinite fatigue life. The governing detail is the stiffener-to-web fillet weld (Category C, CAFT = 10 ksi) and the cap channel-to-beam fillet weld (Category C or D depending on weld quality). The stiffener and channel welds must be checked at their specific locations.

For Class D and heavier cranes (> 2 million cycles), finite-life fatigue applies: The allowable stress range (Delta_F)_n = (C_f / N)^0.333 >= CAFT/2 for finite-life design. C_f for Category C = 44 x 10^8. For N = 2.5 million cycles: (Delta_F)_n = (44 x 10^8 / 2.5 x 10^6)^0.333 = (1,760)^0.333 = 12.1 ksi > 10 ksi (CAFT). The detail is fatigue-acceptable if Delta_f <= 12.1 ksi at the critical location.

Deflection Limits

Crane runway beams must satisfy stringent deflection limits because excessive vertical deflection causes the crane to climb an incline, increasing drive motor loads and wheel flange wear. CMAA 74 and AISC DG7 specify:

Vertical deflection: L/600 under maximum static wheel load (without impact). For automated storage/retrieval cranes with high positioning accuracy: L/800 to L/1,000. For L = 30 ft: L/600 = 0.60 inches. Using the simplified maximum deflection formula for two equal point loads: Delta_max = (PL^3)/(48EI) x (3a/L - 4a^3/L^3) where a = distance from support to nearest load. I_req = (30.75 x 30^3 x 1,728)/(48 x 29,000 x 0.60) x factor = approximately 1,800 in^4, requiring a W24x76 or larger. This demonstrates that deflection often governs runway beam sizing -- the beam must be stiffer than strength alone requires.

Lateral deflection: L/400 under lateral surge. For L = 30 ft: L/400 = 0.90 inches. Lateral deflection is checked using the combined top-flange-plus-cap-channel moment of inertia I_y.

Runway Beam-to-Column Connections

The runway beam connection to the building column transmits vertical reaction, lateral surge, and longitudinal traction. The connection is typically a bolted seated connection (stiffened seat angle) with a top clip angle for lateral stability. For fatigue-sensitive crane classes (D and above), the connection bolts must be pretensioned slip-critical (Class A or B faying surface) per AISC 360 Section J3.8. The connection must also provide adequate stiffness to prevent the beam from rotating about its longitudinal axis under the eccentric wheel load.

Try it now: Calculate beam capacity for crane girders

Related Tools and References

Disclaimer

This page is for educational and reference use only. Crane girder design must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) for the specific crane, facility, and jurisdiction. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.