| -------- | ------------------------ | --------------------- | ---------- | | W8x31 | 110 | 14.5 | 12.8 | Deflection | | W10x30 | 170 | 17.0 | 14.8 | Deflection | | W12x26 | 204 | 18.2 | 15.7 | Deflection | | W12x40 | 310 | 22.0 | 18.2 | Deflection | | W14x30 | 291 | 22.5 | 17.8 | Deflection | | W14x48 | 484 | 28.0 | 21.5 | Deflection | | W16x36 | 448 | 27.5 | 20.9 | Deflection | | W18x40 | 612 | 31.0 | 23.5 | Deflection | | W18x55 | 890 | 37.0 | 26.8 | Deflection | | W21x44 | 761 | 35.0 | 25.5 | Deflection | | W21x57 | 1,030 | 39.5 | 28.5 | Deflection | | W24x55 | 1,350 | 43.0 | 31.5 | Deflection | | W24x68 | 1,830 | 49.0 | 35.0 | Deflection | | W27x84 | 2,850 | 56.0 | 40.5 | Deflection | | W30x99 | 3,990 | 63.0 | 45.0 | Deflection |

Approximate values for preliminary sizing. Verify with full calculation.

Roof Purlins — L/240 Deflection Limit, 50 ksi Steel

Snow + dead load = 40 psf, tributary width = 20 ft → w = 0.8 kips/ft

Light Commercial — L/360, w = 1.0 kips/ft

Quick Span Estimation Rules

Rule of Thumb: Depth per Span

For preliminary sizing of simply supported W-shapes under typical floor loads:

Beam depth (in) ≈ Span (ft) / 2 for floor beams (deflection controlled) Beam depth (in) ≈ Span (ft) / 2.5 for roof beams

Examples:

• 20 ft span → W10 (10 in depth)
• 25 ft span → W12 (12 in depth)
• 30 ft span → W14-W16 (14-16 in depth)
• 40 ft span → W18-W21 (18-21 in depth)
• 50 ft span → W24-W27 (24-27 in depth)

Required Moment of Inertia

For L/360 deflection limit under UDL:

I_required = 5wL³ × 360 / (384E) = 5wL³ × 360 / (384 × 29000)

Simplifying with w in kips/ft and L in feet:

I_required (in⁴) ≈ 5 × (w/12) × (L×12)³ / (384 × 29000 × (L×12)/360) ≈ wL³ × 12³ / (384 × 29000 / (5 × 360)) ≈ 0.163 × wL³

where w is in kips/ft and L is in ft.

For L/240: I_required ≈ 0.109 × wL³

Example: Quick Sizing

Span = 30 ft, w = 1.5 kips/ft, L/360:

I_required ≈ 0.163 × 1.5 × 30³ = 0.163 × 1.5 × 27,000 = 6,602 in⁴...

Wait — let me recalculate. The correct formula:

I_required = 5wL⁴ / (384E × Δ_limit)

With Δ_limit = L/360, w in kips/in, L in inches:

I_required = 5 × (1.5/12) × (360)⁴ / (384 × 29000 × (360/360)) = 5 × 0.125 × 16,796,160,000 / 11,136,000 = 10,497,600 / 11,136 = 943 in⁴

A W18x55 (Ix = 890) is close but slightly under. A W21x57 (Ix = 1,030) works.

Span Tables by Application

Residential Floor Beams

Typical: 40 psf live + 15 psf dead, tributary width varies.

Commercial Office Beams

Typical: 50 psf live + 20 psf dead + beam self-weight, tributary width = 30 ft.

Steel Roof Framing

Typical: 30 psf snow + 15 psf dead, tributary width = 25 ft.

Cantilever Span Limits

For cantilevers, the maximum span is typically much shorter than simply supported spans. A rule of thumb:

Cantilever length ≤ 1/4 to 1/3 of the back-span length

The cantilever tip deflection is checked against L/180 or L/240.

Frequently Asked Questions

How far can a W12 beam span? A W12x26 can span about 16 ft (floor, L/360) or 21 ft (roof, L/240) under typical loads. A W12x40 extends this to about 18 ft (floor) or 24 ft (roof).

What is the maximum span for a steel beam? There is no absolute maximum. W-shapes up to W44 are available, and plate girders can span 200+ ft. In practice, typical building beams span 15-60 ft. Longer spans use trusses, plate girders, or composite construction.

Does steel grade affect maximum span? Yes, but only for strength-governed spans. Increasing from 50 ksi to 65 ksi steel increases bending capacity by 30%, which helps if strength governs. For deflection-governed spans (most cases), steel grade does not matter because Ix is a geometric property.

What is the rule of thumb for steel beam depth? For simply supported beams: depth (inches) ≈ span (feet) / 2 for floor beams. This gives L/d ≈ 24, which usually satisfies L/360 deflection limits for typical loading.

How do I select a beam for a specific span?

1. Determine the load (w in kips/ft)
2. Calculate the required Ix for deflection: I_req = 5wL⁴/(384E × Δ_limit)
3. Select a W-shape with Ix ≥ I_req
4. Verify bending strength: φMn ≥ Mu
5. Verify shear: φVn ≥ Vu

Code References

• AISC 360-22 Section B3 -- Member properties and dimensional requirements for flexural members
• AISC 360-22 Chapter F -- Flexural strength provisions including Lb limits for lateral-torsional buckling (Lp, Lr, Cb)
• ASCE 7-22 Table 4.3-1 -- Minimum uniformly distributed live loads by occupancy for beam span selection
• IBC Table 1604.3 -- Deflection limits for structural members supporting floors, roofs, and exterior walls

Try it now: Check your beam span with our free Beam Span calculator →

Related Pages

• Beam Calculator — Full beam analysis with SFD, BMD, deflection
• Beam Capacity Calculator — Strength checks per code
• Steel Beam Sizes — W-shape dimension chart
• Beam Deflection Calculator — L/360, L/240 checks
• Deflection Limits — Code deflection criteria
• I Beam Sizes — Popular I-beam dimensions

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) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.

Design Resources

Calculator tools

• Steel Beam Capacity Calculator
• Beam Deflection Calculator
• Beam Calculator — SFD, BMD & Reactions
• Beam Optimizer — Find Most Efficient Section
• Beam Span Table Tool

Design guides

• Steel Beam Calculator Guide
• Beam Capacity Worked Example
• Beam Deflection Worked Example
• Steel Beam Design Workflow
• Steel Beam Span Reference