How are crane wheel loads calculated for runway beam design?

The maximum wheel load Pmax for an overhead travelling crane is calculated as: Pmax = (Wbridge / nwheels) + (Ptrolley + Pload) × (Lcrane − a) / Lcrane × (1/nwheels_per_side), where Wbridge is the bridge self-weight, nwheels is the total number of wheels, Ptrolley is the trolley weight, Pload is the rated lifting capacity, Lcrane is the crane span, and a is the distance from the trolley to the nearest wheel (minimum hook approach). The trolley is assumed at its extreme position producing the maximum reaction. Vertical impact factors amplify static wheel loads: 1.25 for cab-operated cranes, 1.10 for pendant-operated cranes per AISC Design Guide 7. Lateral forces are taken as 20% of the lifted load plus trolley weight for pendant-operated cranes (per AISC DG7) or 10% for cab-operated cranes (per AS 1418). Longitudinal braking force is taken as 10% of the maximum wheel load acting along the runway. For multiple cranes, ASCE 7 load combination rules determine which cranes are considered simultaneously.

Why is fatigue critical for crane runway beams?

Crane runway beams are among the most fatigue-critical structural members because they experience hundreds of thousands to millions of load cycles over their service life, with each crane pass producing a full stress reversal. Fatigue design life depends on crane usage classification: light-duty maintenance cranes accumulate under 20,000 cycles per year (design for 100,000 cycles), moderate production cranes see 20,000-100,000 cycles per year (500,000 cycles design life), and heavy steel mill duty cranes exceed 100,000 cycles per year (2,000,000+ cycles design life). Fatigue must be checked at four critical locations: the web-to-flange fillet weld (longitudinal shear stress range), bracket and connection details (geometric stress concentration), the rail contact surface (Hertz contact stresses producing surface cracking), and cope/re-entrant corners (high local stress concentrations). AISC 360 Appendix 3 provides the fatigue design procedure using stress range categories (A through F), with Category A (plain base metal) having the highest fatigue threshold at 24 ksi stress range and Category E (fillet welds) having the lowest at 4.5 ksi. Most runway beam details fall into Categories C or D.

How do I select a trial section for a crane runway beam?

Trial section selection starts with the crane capacity and span. For US W-shapes: 5-ton cranes spanning 20-30 ft typically use W12x26 to W14x22; 10-ton at 30-40 ft use W16x36 to W18x40; 15-ton at 40-50 ft use W21x44 to W24x55; 20-ton at 50-60 ft use W24x68 to W27x84; 30+ tons at 60+ ft require W30x99 or heavier, or built-up plate girders. For Australian sections: 5t at 6-9m span use 310UB32; 10t at 9-12m use 360UB44.7; 15t at 12-18m use 410UB53.6; 20t at 18-24m use 460UB67.1; 30t at 24-30m use 530UB82. The key design check is combined biaxial bending — the beam bends about its strong axis from vertical wheel loads and about its weak axis from lateral surge loads. The interaction equation is (Mux/φMnx) + (Muy/φMny) ≤ 1.0 where Mux is the major-axis moment from vertical loads and Muy is the minor-axis moment from lateral loads. For sections with a cap channel (common for heavier cranes), the channel increases the weak-axis section modulus significantly. Crane runway beams also need lateral bracing at regular intervals — typically the rail and tie-back system provides some restraint.

What special design checks apply to crane runway beams beyond typical beam design?

Crane runway beams require six design checks beyond standard beam flexure and shear. Web local crippling under the concentrated wheel load is checked per AISC J10.2: φRn = 0.75 × 0.80 × tw² × [1 + 3(N/d)(tw/tf)^1.5] × √(E × Fyf × tf/tw), where N is the bearing length of the rail. Web sidesway buckling per J10.4 checks instability of the compression flange when the tension flange is laterally restrained — this often controls for deep, slender runway girders. Lateral-torsional buckling must consider the actual unbraced length segments between intermediate lateral restraints; the top flange is in compression under vertical load but the bottom flange may be critical near supports. Deflection limits are stricter than floor beams: vertical deflection limit is typically L/600 to L/1000 (versus L/360 for floors) to prevent crane rail misalignment and wheel binding. Horizontal deflection limit is L/400 for the lateral load case to maintain crane geometry. Fatigue-sensitive details must use the appropriate stress category per AISC Appendix 3, with special attention to cope geometry, weld terminations, and rail attachment details.

How is lateral-torsional buckling addressed in crane runway beams?

Lateral-torsional buckling (LTB) is a critical limit state for crane runway beams because they are typically long, slender, and loaded on the top flange. The unbraced length Lb is the distance between lateral supports, which depends on the restraint system: tie-backs to the building column at regular intervals (typically 10-20 ft), the crane rail itself (provides some restraint but should not be fully relied upon), and the cap channel on the top flange (increases lateral stiffness). The elastic LTB moment capacity is Mcr = (Cb × π² × E × Iy / Lb²) × √(G × J / Iy + (π × E × Cw)² / (E × Iy × Lb)). AISC 360 Section F2 provides the three-zone LTB curve: plastic moment Mp for Lb ≤ Lp (full plastic capacity), linear transition from Mp to 0.7FySx for Lp Lr. The Cb factor accounts for the moment gradient between brace points — crane runway beams with multiple wheel loads often have Cb values between 1.0 and 1.3. The top flange loading condition (load applied above the shear center) is destabilizing and increases the effective unbraced length. For built-up plate girders, the AISC provisions for slender webs require tension field action or transverse stiffeners to develop post-buckling shear strength.

Crane Runway Beam Design — AISC & AS 4100 Guide

Crane runway beams (also called crane girders) support the rails on which overhead travelling cranes operate. They are among the most heavily loaded and fatigue-critical structural members in industrial buildings.

This page covers the complete design procedure including wheel load analysis, lateral loads, fatigue, lateral-torsional buckling, web crippling, and deflection limits.

Crane Types and Loads

Overhead Travelling Cranes (EOT)

The most common industrial crane. Runs on rails supported by runway beams.

Load characteristics:

Vertical wheel loads: from crane self-weight + lifted load, distributed to wheels
Lateral loads: from crane acceleration, deceleration, and load swing (transverse to the runway)
Longitudinal loads: from crane braking along the runway
Impact factor: dynamic amplification of vertical loads

Load Determination

Crane self-weight (dead load): Obtain from crane manufacturer data. Typically includes:

Bridge weight
Trolley weight
Hoist weight

Lifted load (live load): Maximum rated capacity of the crane.

Maximum wheel load: The reaction at the most heavily loaded wheel when the trolley is at the extreme position carrying the rated load.

Wheel load per AISC: P_max = (W_bridge / n_wheels) + (P_trolley + P_load) × (L_crane - a) / L_crane × (1/n_wheels_per_side)

where a is the distance from the trolley to the nearest wheel.

Load Factors and Impact

Load Type	Factor	Reference
Vertical impact (cab operated)	1.25	AISC DG7
Vertical impact (pendant operated)	1.10	AISC DG7
Lateral force (monorail)	20% of load	AISC DG7
Lateral force (cab operated)	10% of load	AS 1418
Longitudinal braking	10% of max wheel load	AISC DG7

Fatigue Considerations

Crane runway beams are fatigue-critical. The number of loading cycles determines the fatigue category:

Crane Usage	Cycles per Year	Design Life Cycles
Light (maintenance)	< 20,000	100,000
Moderate (production)	20,000 - 100,000	500,000
Heavy (steel mill)	> 100,000	2,000,000+

Fatigue must be checked at:

Web-to-flange welds
Bracket and connection details
Rail contact surface
Cope and re-entrant corners

Design Procedure

Step 1: Determine Factored Loads

Apply load combinations per ASCE 7 / AS/NZS 1170:

LRFD (AISC): 1.2D + 1.6(L_crane + impact)

AS 4100: 1.2G + 1.5Q_crane × impact_factor

Step 2: Select Trial Section

Typical runway beam sections:

Crane Capacity	Span (m)	Typical Section	Weight (kg/m)
5 t	6-9	310UB32	32
10 t	9-12	360UB44.7	44.7
15 t	12-18	410UB53.6	53.6
20 t	18-24	460UB67.1	67.1
30 t	24-30	530UB82	82
50 t	30+	Built-up plate girder	120+

Alternatively, US W-shapes:

Crane Capacity	Span (ft)	Typical W-Shape
5 tons	20-30	W12x26 - W14x22
10 tons	30-40	W16x36 - W18x40
15 tons	40-50	W21x44 - W24x55
20 tons	50-60	W24x68 - W27x84
30+ tons	60+	W30x99+ or plate girder

Step 3: Bending Capacity Check

Strong axis (vertical):

φMnx = φ × Fy × Sx (elastic, for unbraced length > Lr) φMnx = φ × Fy × Zx (plastic, for compact sections with Lb ≤ Lp)

Weak axis (lateral):

The lateral load from the crane creates weak-axis bending. For a W-shape runway beam:

φMny = φ × Fy × Zy ≤ 1.6 × Fy × Sy

Combined check:

(Mux / φMnx) + (Muy / φMny) ≤ 1.0

Step 4: Shear Check

Check web shear capacity at supports and near wheel load points:

φVn = φ × 0.6 × Fy × Aw (for stocky webs)

where Aw = d × tw.

For slender webs, use the web shear buckling capacity per AISC Chapter G or AS 4100 Clause 5.11.

Step 5: Lateral-Torsional Buckling

Crane runway beams are vulnerable to LTB because the top flange is loaded laterally by the crane and may not be continuously braced.

Critical parameters:

Lb: unbraced length of compression flange (between lateral braces)
Lp: limiting laterally unbraced length for full plastic moment
Lr: limiting laterally unbraced length for inelastic LTB

LTB capacity per AISC Chapter F:

If Lb ≤ Lp: φMn = φFyZx
If Lp < Lb ≤ Lr: φMn = Cb[1 - (Lb-Lp)/(Lr-Lp) × (1 - 0.7FySx/φMn)] × φFySx (simplified)
If Lb > Lr: elastic LTB governs

The Cb factor (moment gradient modifier) helps for non-uniform moment diagrams. For crane loads, Cb is typically 1.0 to 1.3.

Step 6: Web Crippling and Yielding

Concentrated wheel loads can cripple the web. Check per AISC Chapter J10:

Web local yielding: Rn = Fyw × tw × (5k + N)

Web local crippling: Rn = 0.8 × t²w × √(E × Fyw / tw) × [1 + 3(N/d) × (tw/tf)^1.5] (for interior loads)

where:

k = distance from outer face of flange to web toe of fillet
N = bearing length (crane rail width)
tf = flange thickness

If web crippling capacity is insufficient, add transverse stiffeners at wheel load locations.

Step 7: Deflection Check

Crane runway beams have strict deflection limits:

Check	Limit	Typical Code
Vertical deflection (live load)	L/800	AISC DG7
Vertical deflection (total)	L/600	AISC DG7
Lateral deflection	L/400	AISC DG7
Differential settlement	L/1000	Project spec

Vertical deflection for simply supported beam under wheel loads:

Use influence line analysis or superposition for moving point loads.

For a single wheel load P at midspan of span L:

Δ = PL³ / (48EI)

For two equal wheel loads at spacing s:

Δ = P × a² × (3L - 4a) / (6EI)

where a = (L - s) / 2 is the distance from support to the nearer wheel.

Step 8: Fatigue Check

Check fatigue at all stress concentrations using AISC Appendix 3 or AS 4100 Section 11.

Fatigue life per AISC:

The nominal stress range must not exceed the threshold stress range for the applicable fatigue category:

Detail Category	Constant C	Threshold (ksi)	Example
A	250 × 10⁸	24	Base metal, rolled surfaces
B	120 × 10⁸	16	Base metal at welds, full-penetration groove welds
B'	61 × 10⁸	12	Longitudinal welds at plate girders
C	44 × 10⁸	10	Transverse groove welds, attachments
D	22 × 10⁸	7	Groove welded attachments, 2-4 in. long
E	11 × 10⁸	4.5	Fillet welded connections
E'	3.9 × 10⁸	2.6	Base metal at short attachments

Design stress range: Δσ = σ_max - σ_min at the detail

Required: Δσ ≤ Δσ_threshold (for infinite life at N > 2×10⁶ cycles)

Rail-to-Beam Connection

The crane rail is attached to the top flange of the runway beam. Common methods:

Direct welding: Rail welded to flange. High fatigue category (E or E'). Not recommended for heavy cranes.
Bolted clips: Rail held by bolted clamp plates. Allows thermal movement. Most common for medium cranes.
Hook bolts: J-bolts hook over the rail flange. Simple but limited lateral capacity.
Punched rail: Rail has pre-drilled holes for direct bolting. Best for heavy cranes.

Rail pad (resilient layer between rail and beam) reduces impact and noise.

Frequently Asked Questions

What is the deflection limit for crane runway beams? AISC Design Guide 7 recommends L/800 for vertical deflection under live load and L/600 for total deflection. These are stricter than typical floor beams (L/360) because excessive deflection causes crane misalignment and rail wear.

How do I calculate crane wheel loads? The maximum wheel load occurs when the trolley is at the extreme position carrying the rated load. Obtain the wheel load from the crane manufacturer's data sheet, which provides the maximum wheel load for each crane configuration.

Do crane runway beams need fatigue checks? Yes. Crane runway beams are fatigue-critical members. The number of cycles over the design life (typically 25-50 years) determines the fatigue category and the allowable stress range at each detail.

What is the lateral force from an overhead crane? Per AISC Design Guide 7, the lateral force is 20% of the crane rated capacity for monorails and 10% for bridge cranes. This force acts at the rail level and creates weak-axis bending in the runway beam.

Can I use a standard W-shape for a crane runway beam? For light to moderate cranes (up to 20 tons), standard W-shapes are common. For heavier cranes or longer spans, built-up plate girders with heavier flanges and thicker webs are used.

What is web crippling in crane beams? Web crippling is a local buckling failure of the web under concentrated wheel loads. It occurs when the web is too slender to transfer the wheel load from the rail to the web without local buckling. Stiffeners can be added to increase capacity.

Crane Runway Beam Calculator — Automated crane beam design
Beam Capacity Calculator — Flexure, shear, LTB checks
Beam Deflection Calculator — L/360, L/800 checks
Steel Beam Sizes — W-shape, UB dimensions and properties
Lateral-Torsional Buckling — LTB theory and checks
Fatigue Design — Fatigue categories and stress ranges
Beam Design Example — Worked beam design example

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE), Chartered Professional Engineer (CPEng), or Structural Engineer before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.