What is AISC DG7 and when is it used for crane runway beams?

AISC Design Guide 7 (Industrial Buildings: Roofs to Anchor Rods) provides comprehensive guidance for crane-supporting steel structures. It covers load determination per CMAA 70, fatigue design per AISC 360 Appendix 3, lateral and longitudinal load distribution, serviceability limits (L/888 vertical, L/400 horizontal deflection), and detailing requirements for runway beams supporting overhead bridge cranes.

How is crane runway beam fatigue assessed per AISC 360?

Fatigue assessment uses the stress range approach per AISC 360 Appendix 3. For a Category C detail (continuous fillet weld), the allowable stress range is (Cf/N)^0.333 where Cf = 44x10^8 and N is the number of loading cycles. For Class C crane service (500,000 cycles), the allowable range is 20.65 ksi, and the actual stress range of 8.13 ksi is below the 10 ksi constant amplitude fatigue threshold, confirming infinite fatigue life.

What serviceability limits apply to crane runway beams?

AISC DG7 recommends vertical deflection limits of L/600 to L/1000 for typical overhead cranes, with L/888 commonly used per CMAA 70. Horizontal deflection is limited to L/400. In the worked example, vertical deflection at 97.6 percent utilisation and horizontal deflection at 92.4 percent utilisation both controlled the design, leading to selection of W21x68 over the initially proposed W21x62.

Crane Runway Beam Design Worked Example — AISC DG7 10-Ton Bridge Crane Fatigue

AISC DG7 / AISC 360-22 Crane Runway Beam Design — 10-Ton Bridge Crane Worked Example

Complete step-by-step design of a crane runway beam supporting a 10-ton (20-kip capacity) overhead bridge crane. Covers vertical and lateral load determination per CMAA 70, AISC DG7, and ASCE 7-22, combined stress checks, fatigue assessment per AISC 360 Appendix 3, and serviceability limits per AISC DG7.

Related pages: AISC Fatigue Guide | Combined Loading Guide | Beam Capacity Calculator | Deflection Limits

Problem Statement

Design a simply supported crane runway beam in a steel-framed industrial building for a 10-ton Class C (moderate service) overhead bridge crane. The building has two 50 ft bays with runway beams at 50 ft centres.

Design data:

Crane capacity: 10 tons = 20 kips (hoist + lifted load)
Bridge span: 60 ft (distance between runway beams)
Runway beam span: 25 ft (column spacing)
Crane bridge weight: 15 kips (including trolley and hoist)
Trolley weight: 3 kips
Maximum wheel load (per end truck, 2 wheels): Pmax = 12.5 kips/wheel (loaded)
Minimum wheel load: Pmin = 4.5 kips/wheel (bridge only)
Wheel spacing (along runway): a = 8 ft
Crane classification: CMAA Class C, AISC fatigue stress category C
Lateral load: 20% of lifted load + trolley weight (per AISC DG7)
Impact factor: 25% for cab-operated crane (ASCE 7-22 Table 4.9-1)
Deflection limit: L/888 vertical, L/400 horizontal (AISC DG7)
Fatigue cycles: N = 500,000 over 25-year design life

Step 1 — Vertical Load Determination

Maximum wheel load with impact: Pv_max = 12.5 x (1 + 0.25) = 15.625 kips per wheel

Maximum moment (two equal loads at a = 8 ft on L = 25 ft):

Mu_vert = P x L/4 + P x (L/2 - a) = 15.625 x 6.25 + 15.625 x 4.5 = 97.66 + 70.31 = 168.0 kip-ft

Maximum shear: Vu_vert = P + P x (L - a)/L = 15.625 + 15.625 x 0.68 = 26.25 kips

Step 2 — Lateral Load Determination

Lateral force = 20% of (lifted load + trolley weight) = 0.20 x (20 + 3) = 4.6 kips total. Per runway beam: H_lat = 2.3 kips (per end truck), per wheel: H_wheel = 1.15 kips.

Lateral moment: Mu_lat = 1.15 x 6.25 + 1.15 x 4.5 = 12.37 kip-ft

Step 3 — Combined Stress Check (AISC 360 Section H1)

Beam: W21x62, A992. Section properties: Sx = 127 in^3, Zx = 144 in^3, Sy = 13.9 in^3, Zy = 21.3 in^3.

Vertical moment capacity (W21x62, Lb = 8 ft, Cb = 1.0): phi_b x Mnx = 475 kip-ft (Table 3-10) Lateral moment capacity: phi_b x Mny = 0.90 x (50 x 21.3)/12 = 79.9 kip-ft

Combined interaction (AISC Eq. H1-1a): 168.0/475 + 12.37/79.9 = 0.354 + 0.155 = 0.509 < 1.0 — OK, 50.9%

Step 4 — Fatigue Assessment (AISC 360 Appendix 3)

Service stress range: Sr = (Ms_max - Ms_min) / Sx = (134.38 - 48.38) x 12 / 127 = 8.13 ksi

Fatigue detail category C: Cf = 44x10^8, FTH = 10 ksi. N = 500,000.

Fsr = (4.4x10^9 / 500,000)^0.333 = 20.65 ksi >= FTH = 10 ksi

Sr = 8.13 ksi < FTH = 10 ksi — infinity fatigue life, utilisation = 39.4%

Step 5 — Serviceability Checks

Vertical deflection (L/888 limit): Initial W21x62: delta_v = 0.365 in. > 0.338 in. — FAIL

Upgrade to W21x68 (Ix = 1,470 in^4): delta_v = 0.330 in. < 0.338 in. — OK, 97.6%

Horizontal deflection (L/400 limit): delta_h = 0.693 in. < 0.750 in. — OK, 92.4%

Summary of Checks

Limit State	Capacity	Demand	Utilisation
Combined bending (H1-1a)	—	—	50.9%
Vertical moment (LTB)	475 kip-ft	168 kip-ft	35.4%
Lateral moment	79.9 kip-ft	12.4 kip-ft	15.5%
Fatigue stress range	20.65 ksi (FTH=10)	8.13 ksi	39.4%
Vertical deflection	0.338 in.	0.330 in.	97.6%
Horizontal deflection	0.750 in.	0.693 in.	92.4%

Conclusion: The W21x68 crane runway beam satisfies all AISC 360-22 and DG7 requirements. The controlling limit state is vertical deflection (97.6%), followed by horizontal deflection (92.4%). The fatigue stress range of 8.13 ksi confirms infinite fatigue life. Intermediate diaphragms at 8 ft spacing are recommended.