Steel Beam Span Reference Table

Maximum span-to-depth ratios for common steel sections under uniformly distributed loading (UDL). These are L/360 deflection-limited spans — suitable for floor applications.

Quick Span Estimates

For preliminary sizing, use these span-to-depth ratios (steel beams, simple span, L/360):

• Floor beams: L/20 (service load governs)
• Roof beams: L/25 (live load governs)
• Crane girders: L/15 (fatigue + deflection)

For a given section depth d (mm), approximate maximum span = d × ratio / 25.4 (to get span in feet).

W-Shape Maximum Spans (AISC, L/360, 50 ksi)

HSS and Tube Section Spans

Hollow Structural Sections (HSS) are increasingly used as beams where exposed steel is an architectural feature. Their torsional stiffness also makes them suitable for laterally unbraced applications:

Square HSS beams (L/360, Fy = 46 ksi, 50 psf, 8 ft spacing):

Rectangular HSS beams (L/360, Fy = 46 ksi, strong-axis bending):

Note: HSS sections are not typically lateral-braced along their span, so the weak-axis flexural capacity and torsional stiffness must be considered. The higher torsional constant J of HSS (typically 10-50 times that of an equivalent W-shape) provides superior performance for unbraced applications.

Beam Deflection Check

Always verify deflection with the Beam Deflection Calculator using your actual loads and span.

Cantilever Beam Spans

Cantilevered steel beams are governed by stricter deflection limits and higher moment demands at the support:

• Maximum cantilever length: Typically L_cantilever = 0.3 to 0.4 x adjacent backspan length for uniform loading
• Deflection limit: L/180 for roof cantilevers, L/240 for floor cantilevers (tip deflection under live load)
• Moment at support: w x L²/2 (four times the simple span midspan moment for the same span length)
• Lateral bracing: The top flange (negative moment region) requires bracing at the support and at the point of contraflexure

For a W12x26 cantilever extending 6 ft (serving as a balcony or canopy support), the tip deflection under a 50 psf load at 8 ft spacing is approximately 3/8 inch — acceptable for L/180 = 0.4 inch limit.

Continuous Beam Span Advantage

Continuous beams over multiple supports are significantly more efficient than simple spans:

• Positive moment reduction: Continuous interior spans have peak positive moments approximately 50-60% of a simple span with the same total load
• Span increase: For the same section, continuous beams can span approximately 15-25% longer than simple spans
• Deflection reduction: Peak deflection in a continuous beam is approximately 40-50% of the equivalent simple span deflection
• Practical limitations: Continuous construction requires moment connections at supports and is more complex to erect. Thermal effects and differential settlement also need consideration

For example, a W18x35 that spans 18 ft as a simple span can carry the same load over a 22 ft interior span in a continuous configuration — a 22% increase in span length at no additional material cost.

Composite Beam Spans

Connecting the steel beam to the concrete slab through shear studs creates composite action that significantly increases span capacity:

• Stiffness increase: Composite Ix is typically 1.5 to 2.5 times the steel-only Ix depending on the effective slab width (b_eff = min(L/4, center-to-center spacing, 16 × slab thickness + bf))
• Span increase: For the same section and loading, composite beams span 15-25% longer than non-composite
• Deflection reduction: Composite action reduces live-load deflection by 40-60% compared to non-composite
• Shear connection: Full composite action requires sufficient shear studs between the point of maximum moment and each support. Studs are typically 3/4 inch or 7/8 inch diameter, 3-5 inches tall

Per AISC 360 I3, the number of studs required for full composite action is N = V_h / Q_n, where V_h is the horizontal shear force at the steel-concrete interface and Q_n is the nominal strength of one stud.

Crane Girder Spans

Crane runway girders have distinct span and deflection requirements that differ from floor and roof beams:

• Vertical deflection limits: Per CMAA Specification 74, for Class A and B cranes (light service): L/600; Class C (moderate): L/800; Class D (heavy): L/1000; Class E (severe): L/1200
• Lateral deflection limits: L/400 for all crane classes, measured at the top flange
• Fatigue: Crane girders are subject to fatigue loading. The number of stress cycles over the design life determines the allowable stress range per AISC 360 Appendix 3. For a crane with 500,000+ cycles, the fatigue limit state often governs over static strength
• Impact factors: Vertical wheel loads are increased by 25% for electric overhead cranes and 10% for hand-operated cranes per ASCE 7-22
• Torsional effects: Eccentric crane wheel loads induce torsion in the girder. Closed sections (box girders) are preferred for spans over 60 ft (18 m) due to their superior torsional resistance

For a 40 ft crane girder supporting a 10-ton bridge crane with Class C duty, a W18x50 with a channel cap at the top flange provides approximately L/800 deflection control under the crane wheel loads. The lateral-torsional buckling check at the unfactored crane loads is often the limiting strength criterion.

Span Optimization Tips

To maximize beam span without increasing beam weight:

1. Add intermediate lateral bracing: Reducing Lb from the full span to 6-8 ft intervals can increase flexural capacity by 50-100% for moderately deep sections governed by inelastic LTB
2. Use moment gradient (Cb): For uniformly loaded simple spans, Cb = 1.14 provides 14% more capacity than Cb = 1.0. For beams with transverse loading between brace points, always use the actual Cb
3. Select deeper, lighter sections: A W24x55 typically has 2.5 times the stiffness of a W18x35 at only 1.6 times the weight per foot, making deeper sections more efficient for deflection-controlled spans
4. Consider continuous spans: Continuous beams over two equal spans have 40% less positive moment than simple spans of the same length. For a 30 ft bay, using a continuous beam over two 30 ft spans can carry approximately the same load as a simple span beam at 25 ft
5. Design for partial composite action: Even 25% composite connection provides approximately 40-50% of the stiffness benefit of full composite action, which may be sufficient to meet deflection limits at lower cost

Frequently Asked Questions

What is the maximum span for a W12x26 steel beam? At L/360 deflection limit with a 50 psf uniform load (typical office floor), a W12x26 spans approximately 18 feet. At 80 psf load, reduce to 16 feet. Always check bending and shear capacity at the governing load combination.

Can I span longer with deeper beams? Yes. Doubling beam depth roughly quadruples stiffness (I ∝ d³ for rectangular sections, ∝ d² to d³ for rolled shapes). A W24x55 spans approximately 33 feet at 50 psf — nearly double the W12x26 span. Use the Beam Capacity Calculator with your specific section and loading.

What's the difference between span for bending vs. deflection? Deflection usually governs for longer spans (L/360 or L/240). Bending governs for shorter spans with heavy loads. Always check both: bending (moment capacity check per AISC F2) and deflection (serviceability limit). The governing limit is whichever produces the shorter span.

What UB (universal beam) sections give the longest span for a given depth? Among metric UB sections, the deeper profiles such as UB 533x210x82 carry the highest loads at longer spans due to their larger moment of inertia. For a 10 m span with typical office loading (3 kN/m² LL, 1.5 kN/m² DL at 3 m spacing), a UB 457x191x67 is the minimum recommended section. For spans exceeding 12 m, UB 533x210x82 or UB 610x229x101 should be used, and deflection should be verified with the beam capacity calculator.

See Also

• Beam Capacity Calculator
• Beam Deflection Calculator
• Steel Beam Sizes Reference
• Beam Design Guide
• Beam Span Reference