Steel Crane Girder Design — Runway Beams, Loads, and Fatigue

Crane runway girders support overhead travelling cranes and must resist vertical wheel loads, lateral crane thrust, longitudinal traction forces, and fatigue from repeated load cycles. AISC Design Guide 7 (Industrial Buildings) and CMAA 70/74 provide the primary design guidance, with fatigue provisions from AISC 360-22 Appendix 3.

Crane load types

The maximum wheel load is calculated by placing the lifted load (hoist capacity) at the maximum trolley eccentricity and distributing the total bridge reaction to the wheels. For a double-girder bridge crane:

P_w_max = [(Hoist capacity + trolley weight) * (bridge span - trolley position) / bridge span + bridge girder weight / 2] / number of wheels per end truck

Worked example — crane runway beam sizing

Given: 20-ton (40 kip) cab-operated bridge crane. Bridge span = 60 ft. Runway beam span = 30 ft. Bridge weight = 25 kips (distributed to 4 wheels, 2 per end truck). Trolley weight = 5 kips. Wheel spacing along the runway = 12 ft (center-to-center of end truck wheels). CMAA Class C (moderate service).

Step 1 — Maximum wheel load: With trolley at extreme position (assume full eccentricity for worst case): P_w = (40 + 5) * 1.0 + 25/4 = 45 + 6.25 = 51.25 kips per wheel (conservative, assumes all hoist load on one end truck)

More accurately: P_w = (40 + 5) * 0.87 + 6.25 = 39.15 + 6.25 = 45.4 kips (using 87% eccentricity)

Step 2 — Impact factor: Vertical impact = 25% (cab-operated) P_w_design = 45.4 * 1.25 = 56.8 kips per wheel

Step 3 — Maximum moment (two concentrated loads on a simple span): Using the influence line method for two wheels at 12 ft spacing on a 30 ft span, the maximum moment occurs when the midpoint between the loads and the span center are aligned:

Place the heavier wheel at 12 ft from the left support (with the second wheel at 24 ft). The reaction at the left support: R*A = P * (30 - 12)/30 + P _ (30 - 24)/30 = 56.8 _ 18/30 + 56.8 _ 6/30 = 34.1 + 11.4 = 45.4 kips

M*max (at the wheel at 12 ft) = R_A * 12 = 45.4 _ 12 = 545 kip-ft

Alternatively, using the critical position formula: Mmax = P * L/4 _ (1 - s/(2L))^2 for two equal loads: M_max = 56.8 _ 30/4 _ (1 - 12/(230))^2 = 426 * (0.80)^2 = 426 _ 0.64 = 273 kip-ft per wheel Wait — the correct formula gives M*max = 2P * L/4 when s/L is small. Using exact calculation:

Place loads at positions that give max moment. With two 56.8 kip loads at 12 ft spacing, the resultant is 113.6 kips at the center of the two loads. Maximum moment occurs under one of the loads, offset from midspan by s/4 = 3 ft:

R*A = 113.6 * (15 + 3)/30 = 113.6 _ 18/30 = 68.2 kips M_max = 68.2 * 12 = 818 kip-ft (under left wheel)

Check: M*max = 68.2 * 18 - 56.8 _ 12 = 1228 - 682 = 546 kip-ft (under right wheel, nearer support)

Use M_max = 818 kip-ft (governs).

Step 4 — Required section modulus: S*req = M_u / (phi * Fy) = 818 _ 12 / (0.90 * 50) = 218 in.^3

A W24x84 (S_x = 196 in.^3) is slightly undersized. A W24x104 (S_x = 258 in.^3) works with margin for the lateral load check. Alternatively, a W27x94 (S_x = 243 in.^3) provides adequate capacity.

Step 5 — Lateral load on top flange: H_lat = 0.20 * (40 + 5) = 9.0 kips per side (total 18 kips, but applied to one runway) Top flange bending from lateral load is checked using the top flange section modulus about the weak axis.

Fatigue considerations

Crane runway girders are one of the few building-type steel structures that require explicit fatigue checks per AISC 360-22 Appendix 3. The number of load cycles depends on the CMAA service class:

The fatigue stress range is checked at the detail category of the most critical welded connection (typically the stiffener-to-web or flange-to-web weld). For a fillet-welded stiffener to the web (Category C), the allowable stress range at 2,000,000 cycles is approximately 10 ksi.

Code comparison

Key clause references

• AISC Design Guide 7 — Industrial Buildings (comprehensive crane load guidance)
• AISC 360-22 Appendix 3 — Fatigue design provisions
• CMAA 70 — Top Running Bridge and Gantry Type Cranes
• ASCE 7-22 Table 4.7-1 — Crane impact factors
• EN 1991-3 — Actions induced by cranes and machinery
• EN 1993-6 — Crane supporting structures

Topic-specific pitfalls

• Omitting the top flange lateral bending check — lateral crane loads produce weak-axis bending in the top flange (which acts as a beam between stiffeners). This check frequently governs and often requires a channel or plate cap on the top flange.
• Using L/360 deflection limit instead of L/600 — crane runway deflection limits are much tighter than typical building beams. Excessive deflection causes rail misalignment, wheel binding, and accelerated wear on crane components.
• Neglecting fatigue at stiffener welds — stiffeners welded to the tension flange create Category C or C' fatigue details. For Class C and D cranes, the fatigue stress range often governs over static strength.
• Applying impact factor to dead load — the 25% impact factor applies only to the crane live load (wheel loads), not to the self-weight of the runway beam or supported dead loads.

Run this calculation

• Section Properties Database
• Beam Calculator

Related references

• How to Verify Calculations
• steel fabrication tolerances
• steel fatigue design
• stiffener design
• steel beam capacity calculator
• LTB Reference
• Structural Load Path

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.