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.

Crane types and their design implications

Bridge cranes (top-running and underhung)

Top-running bridge cranes are the most common type in industrial buildings. The crane bridge travels on rails mounted on top of the runway beams, which are supported by building columns or separate crane columns. The wheel loads are transmitted directly downward through the rail to the top flange of the runway beam.

Underhung bridge cranes suspend the bridge from the bottom flanges of the runway beams. This arrangement produces downward wheel loads on the bottom flange and introduces additional lateral instability concerns because the beam is loaded on the tension flange (for a simply supported beam).

Monorail cranes

Monorail cranes consist of a single beam from which a hoist and trolley travel. The beam is typically suspended from the building roof framing or supported on brackets from columns. The trolley travels on the bottom flange of the monorail beam.

Design considerations for monorails include the concentrated moving load (hoist + load) at any point along the span, lateral swing of the load producing weak-axis bending, fatigue from repeated trolley passes, and the need for continuous lateral bracing of the top flange (which is typically in compression).

Jib cranes

Jib cranes consist of a horizontal boom (jib) mounted on a vertical mast, which is typically attached to a building column. The boom rotates around the mast, and a hoist travels along the boom. Jib crane design requires checking the supporting column for the combined axial load, bending from the boom reaction, and torsion from the boom swing.

Crane loading — detailed load types and impact factors

Vertical loads

The vertical wheel load is the primary design load for crane runway girders. It includes the hoist capacity (rated load), the trolley weight, the bridge weight distributed to each end truck, and the impact factor.

Maximum wheel load calculation (conservative method):

P_w = [(Rated load + Trolley weight) * eccentricity / bridge span + Bridge weight / 2] / wheels per end truck

The eccentricity is the horizontal distance from the trolley to the nearest runway. For the maximum wheel load, the trolley is placed at its maximum eccentricity (as close to one runway as possible).

Impact factors per ASCE 7-22 Table 4.7-1:

The impact factor applies only to the crane live load (hoist capacity + trolley weight), not to the bridge dead weight or the runway beam self-weight.

Lateral loads

Lateral crane loads are produced by the trolley stopping and starting, the swinging of the lifted load, and the skewing of the bridge on the rails. ASCE 7 specifies the lateral force as 20% of the combined rated load and trolley weight, applied at the top of the rail.

The lateral load produces weak-axis bending in the top flange of the runway beam. The top flange acts as a horizontal beam spanning between vertical stiffeners, with the lateral wheel load applied as a concentrated force. The lateral bending moment at the stiffener locations is:

M_lat = H_lat * a / 4  (for a wheel at midspan between stiffeners)

where a is the stiffener spacing. This moment produces lateral bending stresses that are additive to the vertical bending stresses from the wheel loads.

Longitudinal loads

Longitudinal traction forces act along the runway axis and are transmitted through the driven wheels to the runway beams and the supporting columns. ASCE 7 specifies the longitudinal force as 10% of the total vertical load on the driven wheels. This force is typically resisted by the runway beam-to-column connections and the column base plates.

Combined loading diagram for crane runway beam

Runway beam cross-section:
                    <-- Lateral load H_lat
                    |
    ====================  <-- Top flange (resists vertical + lateral bending)
    |     Rail          |
    |                   |
    |                   |  <-- Web (resists vertical shear)
    |                   |
    ====================  <-- Bottom flange (resists vertical bending only)

    Vertical: P_w (wheel load downward)
    Lateral: H_lat (at rail level)
    Longitudinal: H_long (along beam axis, resisted by connections)

AISC Design Guide 7 — crane runway beam design reference

AISC Design Guide 7 (Industrial Buildings: Roofs to Anchor Rods) is the primary reference for crane runway beam design in the US. The design guide covers:

Key sections of AISC DG7

CMAA crane service classifications

The Crane Manufacturers Association of America (CMAA) classifies cranes by service severity, which directly affects the fatigue design of the runway beam:

Crane girder design procedure

The following step-by-step procedure covers the complete design of a crane runway beam per AISC DG7 and AISC 360:

Step 1: Determine design loads

1. Obtain the maximum wheel loads from the crane manufacturer or calculate them from the rated capacity, trolley weight, and bridge weight.
2. Apply the appropriate impact factor (1.25 for cab-operated, 1.10 for pendant-operated).
3. Calculate the lateral load (20% of rated load + trolley weight, applied at rail level).
4. Calculate the longitudinal traction force (10% of total vertical on driven wheels).
5. Determine the dead load of the runway beam, rail, and any attachments.

Step 2: Determine maximum moments and shears

1. Use influence line analysis for the moving wheel loads to determine the maximum moment and maximum shear for the worst-case wheel position.
2. For two equal wheel loads at spacing s on a simple span L, the maximum moment occurs when the center of the wheel group is offset from midspan by s/4.
3. The maximum shear occurs when the heaviest wheel is at the support.
4. Add the dead load moment and shear.

Step 3: Select trial section

1. Calculate the required section modulus for vertical bending: S_x_req = M_u / (phi * F_y).
2. Add approximately 20-30% to account for lateral bending (top flange) and reserve.
3. Select a W-shape with S_x at or above the required value.
4. Check that the section is compact per AISC Table B4.1b.

Step 4: Check vertical bending capacity

1. Calculate phi _ M_n = phi _ F_y * S_x for the selected section (assuming compact, laterally supported).
2. Verify phi * M_n >= M_u for the factored vertical moment.
3. If the beam is not continuously braced (top flange not laterally supported), use the LTB provisions of AISC F2 with the appropriate effective length.

Step 5: Check lateral-torsional buckling with wheel loads

Crane runway beams are unique because the top flange is loaded by concentrated wheel loads that can induce lateral-torsional buckling (LTB). The top flange is typically braced by the rail and the building framing, but the effective bracing point spacing must be carefully evaluated.

LTB check per AISC F2 with modifications for crane girders:

1. Determine the unbraced length L_b of the top flange. For top-running cranes, the rail provides continuous lateral support if the rail clips are properly designed and the rail-to-flange contact is maintained. For underhung cranes, the bottom flange is loaded and may not have continuous lateral support.
2. Check if Lb <= L_p (fully plastic limit). If yes, phi * Mn = phi * M_p (no LTB reduction).
3. If Lp < L_b <= L_r, use the inelastic LTB equation: phi * Mn = phi * [M_p - (M_p - 0.7 * F_y * S_x) * (L_b - L_p) / (L_r - L_p)].
4. If Lb > L_r, use the elastic LTB equation: phi * Mn = phi * (0.7 _ F_y _ S_x) * (L_r / L_b)^2.
5. Verify phi * M_n >= M_u including the lateral bending contribution.

Step 6: Check top flange lateral bending

The lateral crane load produces weak-axis bending in the top flange. The combined stress check is:

f_b_vertical + f_b_lateral <= phi * F_y

where f_b_vertical = M_vertical / S_x and f_b_lateral = M_lateral / S_y_top_flange. The top flange section modulus about the weak axis is:

S_y_top = b_f^2 * t_f / 6

If the lateral bending stress exceeds 20-30% of the vertical bending stress, a channel cap or plate cap on the top flange may be required to increase the weak-axis capacity.

Step 7: Check shear capacity

1. Calculate the maximum factored shear V_u from the wheel load positions (heaviest wheel near the support).
2. Verify phi * V_n >= V_u per AISC Chapter G.
3. For unstiffened webs: phi _ V_n = 0.90 _ 0.60 _ F_y _ A_w (for compact webs with h/t_w <= 2.24 * sqrt(E/F_y)).

Step 8: Check deflection

1. Calculate the live load deflection under the unfactored wheel loads (without impact factor).
2. Verify delta_vertical <= L/600 (minimum for crane runways; tighter for heavy service).
3. Verify delta_lateral <= L/400 (minimum for lateral deflection).
4. Verify the differential deflection between the two runway beams is within the crane manufacturer's tolerance.

Step 9: Check fatigue

1. Identify the critical fatigue detail categories at the runway beam connections (stiffener welds, flange-to-web weld, bracket connection, splice plate welds).
2. Calculate the stress range at each detail for the unfactored wheel load passage.
3. Determine the number of cycles from the CMAA service class and the design life.
4. Verify the stress range is below the threshold stress range F_TH for the applicable detail category and number of cycles.

Fatigue considerations for crane runways

Fatigue is a critical design consideration for crane runway girders because they experience thousands to millions of load cycles over their service life. AISC 360-22 Appendix 3 governs fatigue design.

Fatigue detail categories for crane runway beams

Fatigue stress range calculation

The fatigue stress range is the difference between the maximum and minimum stress at the detail under consideration for one complete load cycle (crane passage):

Delta_sigma = sigma_max - sigma_min = M_max * y / I - M_min * y / I

For a crane runway beam with a single crane, one load cycle = one crane passage across the beam span. The maximum stress occurs when the wheels are at midspan; the minimum stress occurs when the crane is off the beam entirely.

For multiple cranes on the same runway, the fatigue cycles accumulate from all crane passages. The number of cycles over the design life is:

N = passes_per_day * operating_days_per_year * design_life_years

Fatigue design recommendations

• Avoid welding stiffeners to the tension flange — this creates a Category C or C' detail with a low allowable stress range. Use stop-weld distances (leave the stiffener weld 2-4 in. short of the tension flange) to improve the fatigue category.
• Use bolted splices instead of welded splices where possible — bolted splices have Category B fatigue performance (better than Category D for groove welds).
• Specify smooth weld profiles — the as-welded profile of transverse stiffener welds can be ground to improve the fatigue category from C to B.
• Consider the rail connection detail — rail clips welded to the top flange create Category B' or E details. Bolted rail clips avoid this problem.
• Provide generous radius at gusset and bracket connections — re-entrant corners at bracket connections create severe stress concentrations. A minimum radius of 2 in. is recommended.

Lateral-torsional buckling with wheel loads

Crane runway beams are particularly susceptible to LTB because the top flange is loaded by concentrated forces (wheel loads) that can induce both vertical and lateral bending simultaneously. The interaction between vertical bending, lateral bending, and torsion makes the LTB check more complex than for a typical building beam.

Top flange bracing considerations

Combined bending and torsion from wheel loads

When the wheel load is applied eccentrically to the beam (e.g., the rail is not centered on the top flange), a torsional moment is introduced:

T = P_w * e_rail

where e_rail is the eccentricity of the rail from the beam web centerline. This torsional moment produces additional lateral bending in the top flange that must be added to the lateral bending from the crane lateral load.

The combined stress check becomes:

f_b_vertical / (phi * F_y) + f_b_lateral / (phi * F_y) + f_torsion / (phi * F_y) <= 1.0

For most practical crane runway designs, the torsional stress from rail eccentricity is small (e rail is typically less than 1 in.) and is often neglected. However, for heavy cranes with large eccentricities, this check should be performed.

Stiffener design for crane runway beams

Transverse stiffeners in crane runway beams serve multiple purposes: they prevent web shear buckling, they provide bearing surfaces for the rail clips, and they brace the top flange against lateral bending.

Minimum stiffener requirements for crane runway beams (per AISC DG7):

• Intermediate stiffeners should be provided at a maximum spacing of d (beam depth) to control web shear buckling and brace the top flange.
• Stiffeners should extend to at least 2/3 of the beam depth.
• Stiffener-to-web welds should be continuous (not intermittent) for crane service classes C and above to prevent fatigue cracking at weld terminations.
• Stiffener-to-tension-flange welds should be stopped 2-4 in. short of the tension flange to avoid fatigue Category C details.

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

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