Runway beam design for an overhead gantry crane starts with one governing input, the maximum static wheel load Pmax, derived from the bridge self-weight Wbr divided across end-truck wheels and a worst-case hook position that minimises the distance Smin from the trolley to the nearest rail [S1].
The standard closed form is Pmax = (Wrc + Wth) x (Sr - Smin) / Sr + Pbr, where Wrc is the rated crane capacity plus trolley and hoist weight, Wth is the lifted load, Sr is the rail-to-rail span, and Pbr = Wbr / (number of wheels x 2 sides); Pmax-v from the vendor and Pmax-c from calculation are compared and the larger is used [S1]. A typical manufacturer-stated maximum wheel load of 45,000 lb combined with the 25 percent AISC impact factor yields a design wheel load of 56,250 lb per wheel, a figure that consistently governs runway beam sizing for heavy top-running bridge cranes [S2].
Vertical load groups, impact factors, and load combinations
Per CMAA 70 and AISC Steel Construction Manual practice, the vertical wheel load is the sum of the bridge dead load, the trolley and hoist weight, and the lifted load, then multiplied by an impact factor alpha: 25 percent for cab-operated and pendant-operated cranes, 10 percent for floor-operated cranes [S2][S4].
For LRFD runway beam checks the vertical wheel load is separated into a 1.2 dead component (bridge self-weight Pbr) and a 1.6 live component (lift plus trolley, Pmax minus Pbr) multiplied by alpha, so the factored vertical load per wheel becomes Pv-f = 1.2 x Pbr + 1.6 x (Pmax - Pbr) x alpha [S1]. EN 1993-6 takes a parallel but distinct approach, grouping crane actions into sets of characteristic loads with their own dynamic factors phi and combining them per the Eurocode partial-factor framework rather than the AISC single impact multiplier [S3][S6].
Lateral, longitudinal, and tractive load components on the runway
Lateral side-thrust loads on the runway beam are taken as percentages of the lifted load: 20 percent of (lifted load + trolley weight) for cab-operated and pendant-operated cranes, 10 percent of (lifted load + trolley weight) for floor-operated units, applied at the top of rail and resisted by the top flange, cap channel, and column connection [S2]. The runway-beam load template in [S1] lists four working side-thrust cases, H_s1 = 0.4 x Lifted Load, H_s2 = 0.2 x (Lifted Load + Trolley/hoist weight), H_s3 = 0.1 x (Lifted Load + entire crane weight), and H_s4 = 1.0 x Lifted Load, with the most adverse governing the design.
Longitudinal tractive force on the runway is taken as H_tr = 0.2 x Pmax, applied per wheel at the rail head, and combined with vertical and lateral cases through the runway-beam load combinations [S1]. Out-of-plane braking and skew are absorbed into the same 20 percent lateral share, and for outdoor gantries wind is added as a separate case on the lifted load and crane structure per EN 1993-6 fatigue load sets [S4][S6].
Beam selection: rolled W-shapes vs built-up plate girders
Runway beams are usually wide-flange W-shapes for light and moderate cranes, shifting to built-up plate girders (box girders or I-girders with cover plates) once the design wheel load exceeds roughly 50 kip per wheel or the span pushes past 12 m [S4]. Composite steel-concrete sections are used where headroom is tight and the column grid is regular, since the concrete deck shares fatigue-sensitive top-flange stress cycles that otherwise accumulate in bare steel [S4].
Selection is driven by three criteria, with the relative weight shown in the table: stiffness (deflection L/600 to L/1000 vertical, L/400 to L/600 lateral, set by CMAA and FEM duty class), fatigue category (EN 1993-6 detail categories 71 to 112 m-N/mm^2 or AISC fatigue design per CMAA 70), and local flange bending under the rail, which is checked only for class S8/S9 Heavy Duty runways in AS 1418.18 [S3][S5]. The wheel loader reference page covers a different load path, but the same principle of distributing a concentrated point load across a stiffened running surface applies when comparing crane rails versus heavy-equipment bogies.
Fatigue assessment, crane duty class, and standard references
Fatigue is the long-cycle failure mode that drives runway-beam plate thickness, and the governing standard is EN 1993-6 with detail categories in MPa taken against the stress-range spectrum from the crane classification S0 to S9 [S3][S5][S6]. AS 1418.18 sets fatigue structure classifications S1 to S9, utilisation classes U0 to U9 (number of cycles), and state-of-loading categories Q1 to Q4 (load spectrum), with the words "light" and "heavy" in this context referring to the load spectrum and not the SWL [S5].
For a 100-ton gantry crane with a 10 m leg span, trolley position is the main variable in wheel-load distribution, and placing the trolley at the closest hook position Smin produces a peak wheel load roughly 1.4 to 1.8 times the average per-wheel value, depending on bridge self-weight and end-truck geometry [S7]. CMAA classifies cranes by duty cycle A (standby) through F (continuous severe), with higher classes demanding stiffer beams, stronger connections, and tighter deflection limits, while FEM 1.001 and ISO 8686 provide the European equivalent load-spectrum definitions and ISO 12488-1 sets the runway-alignment tolerances that feed back into the lateral load assumption [S2][S4].
Runway geometry, load positioning, and gantry-specific variables
For a gantry crane, the runway beam is supported on the gantry legs rather than a building column, so the wheel loads P1, P2 on the side close to the hook and the corresponding values on the far side must be carried down through the leg to the foundation pad, with leg spacing and tie-back bracing controlling longitudinal stability [S1][S5]. Maximum member forces in the runway beam, Mx and My bending moments and Vy shear, are produced by the moving load positions, and the template in [S1] runs both vertical-span and horizontal-span influence lines to capture the worst-case moment and shear from a moving wheel group.
Wheel base dimensions d1, d2, d3 between adjacent wheels and the end-truck wheelbase L1, L2 along the runway govern the spacing of stiffeners and local flange bending checks, with d10 and d11 typically set by the crane vendor and used directly as the load-pattern inputs [S1]. Verification steps for an engineer working this problem are: (1) compute Pbr from the bridge dead weight and number of wheels, (2) compute Pmax from the worst-case Smin and compare to vendor Pmax-v, (3) apply the 25 percent AISC impact factor to vertical load, (4) add 20 percent lateral of (lift + trolley) at top of rail, and (5) add 0.2 x Pmax longitudinal tractive force, then combine through the AISC LRFD or EN 1993-6 load sets before checking the section for moment, shear, deflection, and fatigue detail class.
Common pitfalls, deflection limits, and rail-beam interaction
The most common error in runway beam design is using the average per-wheel load rather than the maximum from the worst-case hook position, which underestimates Pmax by 20 to 40 percent on typical bridge cranes and leads to undersized top flanges and excessive local bending under the rail [S1][S7]. A second pitfall is forgetting that vertical impact alpha applies only to the live component, not the bridge dead weight, so LRFD combinations should read Pv-f = 1.2 x Pbr + 1.6 x (Pmax - Pbr) x alpha rather than a flat 1.6 alpha on the full wheel load [S1].
EN 1993-6 clause 5.6 explicitly allows credit for the stabilising effect of the rail when the wheel load is applied through a rail without an elastomeric bearing pad, but only where the rail is continuously welded and positively fastened, so designers carrying the rail-stiffness benefit must document the fastening detail [S6]. Deflection limits for runway beams typically run L/600 vertical under static wheel load and L/400 lateral under the 20 percent lateral case for standard duty cranes, tightening to L/1000 and L/600 for CMAA Class D, E, F heavy and severe-duty units [S2][S4].
Trackable signals to watch: AISC is updating its crane runway beam design guide to align with the next CMAA 70 revision expected in this cycle, and EN 1993-6 fatigue detail-category tables are under periodic maintenance by CEN/TC 250/SC 3, with clarifications on rail-bear-pad interaction most likely in the next amendment [S3][S6]. For a related decision matrix on long-span rolled versus welded sections used in runway girders, the Hot-Rolled vs Welded Built-Up Steel Section for Long Spans: Decision Matrix article walks through the same stiffness-versus-fatigue trade-off at the section level.
The underlying component specifications are covered under electronic load.