What dynamic factors apply to crane loads per EN 1991-3?

EN 1991-3 specifies five dynamic factors: phi_1 (vibration, typically 1.10), phi_2 (hoisting dynamics, 1.05-1.30 depending on hoist speed and crane class), phi_3 (sudden payload release, 1.0 for building cranes), phi_4 (travel on rails, typically 1.0), and phi_5 (proof load test, 1.5). For a Class Q general-purpose building crane, phi_1 = 1.10 and phi_2 = 1.08-1.15.

UK Crane Runway Beam Design -- EN 1993-6 Crane-Supporting Structures

Q: Do I always need a channel cap on a UK crane runway beam?

For overhead cranes above 50 kN capacity, a channel cap (surge girder) is standard UK practice. It provides lateral stiffness for the top flange to resist surge loads, a flat surface for crane rail mounting, and intermediate torsional restraint. Below 50 kN with light-duty class, a UB alone with rail clip restraint may suffice, but flange bending under the wheel load must still be checked per EN 1993-6.

Q: What fatigue detail category applies to UK crane runway beams?

The governing detail is typically the crane rail attachment. For a clipped rail with discontinuous fillet weld (most common UK practice), detail category is 160 per EN 1993-1-9 Table 8.10. Continuously welded rail drops to category 71. Bolted rail clip systems (e.g., Gantrail) are typically category 125. The choice of rail fixing significantly impacts fatigue life — category 160 vs 71 provides approximately 2.5:1 endurance ratio.

Q: How do I account for skew forces in UK crane beam design?

Skew forces from misaligned crane travel are applied as longitudinal forces at the rail top. EN 1991-3 gives HS = 0.05 x SUM(Qr_max) per end carriage. This induces weak-axis bending in the runway beam, additional shear in the rail connection, and longitudinal force resisted by building bracing. The UK NA adopts EN 1991-3 skew force provisions without modification for building cranes.

Crane runway beams are subjected to a unique combination of vertical gravity loads, horizontal surge loads from crane acceleration and braking, and skew forces from misaligned crane travel. BS EN 1993-6 "Design of Steel Structures: Crane Supporting Structures" provides specific provisions beyond EN 1993-1-1 for the design of runway beams, including fatigue verification, dynamic load factors, and the interaction of local flange bending with global beam stresses. This reference presents the complete design method for a simply supported crane runway beam supporting an overhead travelling bridge crane in a UK industrial building, with a fully worked example.

Crane Loading

Vertical Loads

The vertical wheel loads from the crane are dynamic, not static. EN 1991-3 "Actions Induced by Cranes and Machinery" specifies dynamic factors applied to the hoist load Qh and the crane self-weight:

phi_1: Vibration factor applied to crane self-weight (typically 1.10 for overhead cranes)
phi_2: Dynamic factor for the hoist load, accounting for inertial effects when lifting
- phi_2 = phi_2,min + beta_2 x vh
- Where vh is the hoisting speed in m/s and phi_2,min = 1.05 for HC1/HC2 (most building cranes)

For a Class Q (formerly Class 3) general-purpose bridge crane:

phi_1 = 1.10
phi_2 = 1.15 (hoist speed up to 0.25 m/s, HC2)
phi_3 = 1.0 (sudden release of payload -- only relevant for grab/magnet cranes)
phi_4 = 1.0 (travel on rails -- already included in horizontal loads)
phi_5 = 1.5 (dynamic test load factor for proof load testing)

Horizontal Loads

Surge load (transverse): HL = 0.10 x (Qh + Gcrab) driven by drive wheel slip

For a 100 kN crane capacity with 20 kN crab weight: HL = 0.10 x (100 + 20) = 12 kN per side

Skew load (longitudinal): HS = 0.05 x SUM(Qr_max) where Qr_max is the maximum wheel load per end carriage

For a typical 100 kN crane with 75 kN maximum wheel load per end carriage (two wheels): HS = 0.05 x (2 x 75) = 7.5 kN

Load Combinations for Runway Beam Design

Per EN 1991-3 and the UK NA, the following combinations must be considered at ULS:

ULS 1: 1.35 x self-weight + 1.35 x phi_1 x crane self-weight + 1.35 x phi_2 x hoist load (vertical governing)
ULS 2: 1.35 x self-weight + 1.35 x phi_1 x crane self-weight + 1.35 x phi_2 x hoist load + 1.50 x HL (transverse bending)
ULS 3: 1.00 x self-weight + phi_5 x test load (proof load test without horizontal forces)
FAT: Frequent load combination for fatigue: 1.00 x self-weight + phi_1,fat x crane self-weight + phi_2,fat x hoist load

Design of the Runway Beam

Section Selection

UK practice favours a UB section with a channel cap (surge girder) for runway beams up to approximately 15 m span. Beyond this span, a plate girder or purpose-designed crane beam (with a wider top flange to resist lateral loads) becomes economical. The channel provides lateral stiffness for the top flange and a flat surface for the crane rail mounting.

Typical section for a 100 kN crane, 7.5 m span: 457 x 191 x 82 UB with 229 x 89 PFC cap in S355J2.

Bending Resistance

The runway beam is subject to biaxial bending: major-axis bending from vertical loads and minor-axis bending from horizontal surge loads. The interaction check per EN 1993-6 Clause 5.3:

(My,Ed / My,Rd)^alpha + (Mz,Ed / Mz,Rd)^beta <= 1.0

Where alpha = 2.0 and beta = 1.0 for I-sections (conservative simplification) or determined from Clause 6.2.9 of EN 1993-1-1.

Additionally, the local bending of the top flange under the wheel load must be verified. The concentrated wheel load induces transverse bending in the flange acting as a plate. This is typically checked by limiting the ratio of flange outstand to thickness and verifying the flange stress per EN 1993-6 Annex A.

Lateral-Torsional Buckling

The crane runway beam is unrestrained against LTB on the top flange between crane positions. The channel cap provides restraint only at discrete rail clip locations (typically at 600-750 mm centres). The buckling resistance is:

Mb,Rd = chi_LT x Wy x fy / gamma_M1

The non-dimensional slenderness lambda_LT depends on the elastic critical moment Mcr calculated for the section between restraints, considering the destabilising effect of the load applied above the shear centre (top flange loading). The SCI guide to EN 1993-6 recommends using a conservative Mcr for the unrestrained length between rail clip fixings.

Web Bearing and Buckling Under Wheel Loads

The concentrated wheel load induces local compressive stress in the web. EN 1993-1-5 Clause 6 covers resistance to transverse forces:

FRd = fyw x Leff x tw / gamma_M1

Where Leff is the effective loaded length calculated from the dispersion of the rail load through the flange thickness at a 1:1 slope from the rail foot.

For a typical rail (50 kg/m) on a 457 x 191 UB:

Rail foot width: 132 mm
Load dispersion through flange (tf = 16.0 mm): +2 x 16 = 32 mm
Effective bearing length: 132 + 32 = 164 mm
Leff for web buckling: depends on section geometry (substantially less than 164 mm for slender webs)

Fatigue Assessment per EN 1993-1-9

Crane runway beams are fatigue-critical because each crane passage constitutes a stress cycle. The number of stress cycles over the design life determines the fatigue assessment method:

Safe-life method: Design for a finite number of cycles at known loading -- typical for building cranes where the crane log provides reliable usage data
Damage-tolerant method: Periodic inspection detects cracks before failure -- typical for heavy industrial cranes

For a UK industrial building with a light-to-medium-duty crane:

Class S2 (approximately 100,000 load cycles over 50 years)
Equivalent constant amplitude stress range at 2 x 10^6 cycles: Delta_sigma_E,2
Detail category: 160 for the crane rail attachment to the top flange (clipped rail with discontinuous fillet weld) per EN 1993-1-9 Table 8.10
Damage equivalent factor lambda from EN 1993-6 Annex A

Worked Example -- 100 kN Overhead Crane Runway Beam

A UK industrial building has an overhead bridge crane with 100 kN SWL, Class Q (general-purpose), 18 m crane span, 7.5 m runway beam span at 7.5 m column centres.

Crane Data:

Crane capacity: 100 kN (10.2 tonnes)
Crane span: 18 m
Crab weight: 18 kN
Crane self-weight (bridge + end carriages): 85 kN
Wheel configuration: 2 wheels per end carriage at 3.6 m centres
Maximum static wheel load: 58 kN (from crane manufacturer's data)
Hoist speed vh = 0.15 m/s (HC1)
Crane classification: Class S3 (approximately 250,000 cycles per 50 years)

Step 1 -- Dynamic wheel loads: phi_1 = 1.10, phi_2 = 1.05 + 0.17 x 0.15 = 1.08 (HC1) Qr,d = phi_1 x Qr,self-weight + phi_2 x Qr,hoist

Self-weight portion: 1.10 x 85/4 = 23.4 kN per wheel Hoist load portion: 1.08 x (100 + 18)/4 = 31.9 kN per wheel Total dynamic wheel load Qr,d = 55.3 kN (close to manufacturer's 58 kN maximum)

Step 2 -- Maximum bending moment: Two moving loads at 3.6 m centres on a 7.5 m simply supported span. Maximum moment occurs when the loads are positioned symmetrically about mid-span. Mmax = 2 x 55.3 x (7.5/2 - 3.6/4) = 2 x 55.3 x (3.75 - 0.90) = 2 x 55.3 x 2.85 = 315 kN.m

Step 3 -- Surge moment (transverse): HL = 0.10 x (100 + 18) = 11.8 kN total per side Mhoriz = 2 x 11.8/2 x 2.85 = 33.6 kN.m

Step 4 -- Section selection: Try 457 x 191 x 82 UB + 229 x 89 PFC cap in S355J2. My,Rd = 355 x 1830 x 10^3 / 1.00 = 650 kN.m Utilisation (vertical) = 315 / 650 = 0.48

Mz,Rd for channel + top flange assembly (PFC provides approximately 250 x 10^3 mm^3 minor axis modulus): Mz,Rd = 355 x 250 x 10^3 / 1.00 = 88.8 kN.m Utilisation (horizontal) = 33.6 / 88.8 = 0.38

Interaction: 0.48^2 + 0.38 = 0.23 + 0.38 = 0.61 -- OK.

Step 5 -- Fatigue check: Equivalent constant amplitude stress range at 2 x 10^6 cycles: Delta_sigma_E = phi_fat x sigma_max = 1.0 x 315 x 10^6 / (1830 x 10^3) = 172 MPa Detail category 160. Damage equivalent factor lambda = 0.85 for S3 class. Delta_sigma_E,2 = 0.85 x 172 = 146 MPa < 160 MPa -- OK.

Design Resources

UK Steel Fatigue Assessment — EN 1993-1-9 — Fatigue detail categories and assessment methods
UK Lateral-Torsional Buckling Guide — LTB resistance per EN 1993-1-1
UK Beam Design Guide — Beam design to EN 1993-1-1
UK Portal Frame Design — Industrial building portal frames
UK Steel Design EN 1993 Guide — Complete Eurocode 3 overview

Frequently Asked Questions

Do I always need a channel cap on a UK crane runway beam?

For overhead cranes with capacity above 50 kN, a channel cap (surge girder) is standard UK practice. It serves three functions: providing lateral stiffness for the top flange to resist surge loads, creating a flat surface for crane rail mounting, and providing intermediate torsional restraint. Below 50 kN capacity with a light-duty crane class, a UB section alone with lateral restraint from the rail clips may suffice, but the flange bending under the concentrated wheel load must still be checked per EN 1993-6.

What fatigue detail category applies to UK crane runway beams?

The governing fatigue detail is typically the crane rail attachment to the top flange. For a clipped rail with discontinuous fillet weld (most common UK industrial practice), the detail category is 160 per EN 1993-1-9 Table 8.10. For a continuously welded rail, the category drops to 71 for the fillet-welded attachment. For a bolted rail clip system (e.g., Gantrail type), the detail category is typically 125 for the bolt hole in the flange. The choice of rail fixing has a significant impact on fatigue life -- a detail category 160 vs 71 provides an endurance ratio of approximately 2.5:1 at the same stress range.

How do I account for skew forces in UK crane beam design?

Skew forces arise from misaligned crane travel and are applied as longitudinal forces at the top of the rail. EN 1991-3 gives HS = 0.05 x SUM(Qr_max) for each end carriage. The skew force induces weak-axis bending in the runway beam, additional shear in the rail-to-beam connection, and a longitudinal force that must be resisted by the building bracing (typically via a horizontal truss at crane beam level). The UK NA adopts the EN 1991-3 skew force provisions without modification for building cranes.

What is the difference between a Class Q crane and the older Class 3 designation?

Class Q (BS EN 13001 / EN 1991-3) replaces the older Class 3 (BS 2573). Class Q is the general-purpose building crane class, corresponding to approximately 250,000 load cycles over 50 years, intermittent operation, and loads up to full capacity in a minority of lifts (10-30%). The change from the numerical to letter classification reflects the adoption of EN 13001 as the crane design code. For UK designers accustomed to BS 2573, Class 1 = HC1, Class 2 = HC2, Class 3 = HC3, Class 4 = HC4 in the new system, though the mapping is approximate and the manufacturer's crane classification plate should always be checked.

Educational reference only. All design values are per BS EN 1993-6:2007 + UK National Annex, BS EN 1991-3:2006, and BS EN 1993-1-9:2005. Crane wheel loads must be obtained from the crane manufacturer's data. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent professional verification.