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Steel Beam Estimator (Simply Supported, Uniform Load)

Quick beam size estimate for simply-supported uniformly-loaded steel beams. ESTIMATE ONLY.

warning

ESTIMATE ONLY — not a stamped engineering design. Verify with a licensed PE before procurement or construction.

info

Simplified beam estimation only. Final beam sizing requires PE-stamped structural analysis considering deflection limits, lateral bracing, connection design, and load combinations per AISC and IBC.

Pounds per linear foot of beam, including dead + live load (factored or service per your method).

A992 is the standard for W-shapes since 2000. A36 still common for plates, angles, channels.

Default 0.66 per AISC ASD for compact, laterally-braced sections. Reduce if section is non-compact or unbraced.

Default 360 (live-load floor). Common: L/240 total load, L/360 live load, L/180 roof. Per IBC Table 1604.3.

Required Section Properties

Section Modulus S

18.18

in³

Moment of Inertia I

186.21

in⁴

Select a W-shape that meets both values.

Max bending moment M50000 lb·ft
M (in lb·in)600000 lb·in
Yield strength Fy50 ksi
Allowable bending stress σallow33000 psi
Deflection limitL/360 (0.67 in over 20 ft span)

Beam selection

Look up a W-shape in the AISC Steel Construction Manual (Part 1, dimensions and properties of W-shapes) — or a reputable online property table — that satisfies both S ≥ 18.18 in³ and I ≥ 186.21in⁴. Then have a PE verify the selection against shear, lateral-torsional buckling, web crippling, deflection under service loads, and connection design before procurement.

Methodology

Maximum bending momentfor a simply-supported beam with a uniform load: M = w·L²/8, where w is in plf and L in ft, giving M in lb·ft. Convert to lb·in by multiplying by 12.

Required section modulus: Sreq = M (lb·in) ÷ σallow (psi), where σallow= factor × Fy. The default 0.66·Fy follows AISC ASD for compact, laterally-braced sections.

Required moment of inertia for the deflection criterion: starting from δ = 5·w·L⁴ ÷ (384·E·I) for a simply-supported uniform load, and limiting δ ≤ L/N (with L in inches), we get Ireq = 5·win·Lin³·N ÷ (384·E), where win = plf ÷ 12 (lb/in), Lin= ft × 12, and E = 29,000,000 psi for structural steel.

Sources: AISC Steel Construction Manual (15th/16th Ed.) and AISC 360 Specification for Structural Steel Buildings. Deflection limits per IBC Table 1604.3. This tool does not check shear, lateral-torsional buckling, web crippling, connection capacity, vibration, or load combinations — all of which a PE must address.

Frequently Asked Questions

What is section modulus?
Section modulus S is a geometric property of a cross-section, equal to the moment of inertia I divided by the distance from the neutral axis to the extreme fiber (c): S = I/c. It directly relates bending moment to bending stress (σ = M/S), so it's the property you compare against required strength when selecting a beam. Larger S means more bending capacity at a given allowable stress. The AISC Manual lists both Sx (about the strong axis) and Sy (about the weak axis) for every standard shape. For typical beam design with the load applied to the strong axis, use Sx.
What deflection limit should I use?
IBC Table 1604.3 gives standard maximum deflections: L/360 for live load on floors, L/240 for total load on floors, L/180 for roof members supporting non-plaster ceilings, and L/240 for roof members supporting plaster ceilings. Stricter limits (L/480, L/600) are common for floors supporting brittle finishes, masonry partitions, or vibration-sensitive equipment. Cantilevers usually use L/180 live or L/120 total (and remember L is the cantilever length, not span/2). When in doubt, check the project specifications or consult the structural engineer of record.
Allowable Stress Design (ASD) vs Load and Resistance Factor Design (LRFD)?
AISC permits both methods. ASD compares service-level loads against an allowable stress (or capacity divided by a safety factor Ω); LRFD compares factored loads against a reduced nominal capacity (φRn). For flexure of compact sections, ASD uses Ω = 1.67 (giving an allowable stress around 0.6Fy for elastic design or about 0.66Fy when plastic capacity is invoked), while LRFD uses φ = 0.9. Final designs can differ in member size depending on the dead-to-live ratio. This tool uses a simplified ASD-style 0.66Fy factor — for LRFD or non-compact/unbraced cases, lower the factor manually or use a full design tool. Always have the final design reviewed by a PE.
When should I consult a structural engineer?
Every time — for any occupied structure. This estimator is for rough budgeting and feasibility only. A licensed structural engineer must check load combinations per ASCE 7, lateral-torsional buckling (which depends on unbraced length Lb vs Lp and Lr), shear capacity, web crippling and yielding under concentrated loads, lateral bracing requirements, vibration for floor systems, connections (bolted or welded), bearing details, and deflection under all service load cases. Many jurisdictions require PE-stamped structural drawings for permitting on any non-trivial framing change. Use this tool to scope material weights and budget, then engage a PE before you order steel or cut anything.