How do I calculate the load capacity of a steel beam?

Steel beam load capacity depends on three checks: bending moment (Mr), shear (Vr), and deflection. For a simply supported W shape with uniform load: bending governs when Mr = phi x Zx x Fy (Class 1-2) exceeds Mu = w x L2/8. Deflection governs when actual deflection delta = 5wL4/(384EI) exceeds the limit (L/360 for floors). For typical floor beams, deflection governs at spans greater than 18-22 ft. The maximum uniform load w_max = min(8Mr/L2, 2Vr/L, 384EI x L/360 / 5L4). Always brace the compression flange adequately against lateral-torsional buckling.

What's the difference between LRFD and ASD for beam capacity?

LRFD (Load and Resistance Factor Design) uses factored loads with phi = 0.90 applied to the nominal moment strength: phi Mn = 0.90 x Zx x Fy for compact sections. ASD (Allowable Strength Design) divides the nominal strength by the safety factor Omega = 1.67: Mn/Omega = Zx x Fy / 1.67 = 0.60 x Zx x Fy. The LRFD/ASD ratio is 0.90/0.60 = 1.5 — LRFD allows 50% higher design moment for the same beam. LRFD is the default in AISC 360; ASD is still used for some serviceability checks and is preferred in certain industries (industrial, material handling).

Does beam bracing affect load capacity?

Yes — unbraced length directly affects lateral-torsional buckling (LTB) capacity. A beam continuously braced by a concrete slab achieves its full plastic moment capacity Mr = phi x Zx x Fy. As unbraced length increases beyond Lu (the limiting unbraced length), LTB reduces the moment capacity. For a W16x26 with Lb = 5 ft, Mr = 171 k-ft (fully braced). At Lb = 15 ft, LTB may reduce capacity to 110-130 k-ft depending on moment gradient. The capacity reduction depends on Cb (moment gradient factor) — a beam with uniform moment (Cb = 1.0) is more vulnerable than one with a linear gradient (Cb = 1.67).

How do I convert uniform load capacity to point load capacity?

For a simply supported beam: equivalent uniform load weq = 2P/L for a single point load at midspan, and weq = 3P/L for two equal point loads at third points. Starting from a known uniform load capacity w_max: maximum midspan point load Pmax = w_max x L / 2 (bending). If deflection governs: Pmax = 48EI x L/360 / L3 vs uniform load w_max = 384EI x L/360 / (5L4). Ratio = Pmax / (w_max x L/2) = 0.8 — a point load produces 25% more midspan deflection than an equal-weight uniform load. Always check shear at the point load location.

What safety factors are built into beam load capacity tables?

AISC 360 applies resistance factor phi = 0.90 for flexure (LRFD) or safety factor Omega = 1.67 (ASD). These account for: material strength variability (Fy can exceed specified minimum by 5-15%), section dimension tolerances per ASTM A6, residual stress effects, and modeling uncertainty in the flexural strength formula. For live loads, LRFD applies a load factor of 1.6 (vs 1.0 for ASD service level). The combined LRFD safety margin is approximately 1.6/0.90 = 1.78 on the service load, meaning a beam rated for w = 2.0 k/ft in ASD tables is designed for w_factored = 3.56 k/ft in LRFD, producing similar member sizes.

Steel Beam Load Capacity — Span Tables & Charts

Steel beam load capacity depends on the section properties, span length, support conditions, and loading type. This page provides approximate uniform load capacities for common W-shapes, maximum span guidelines, and the design procedure for determining beam capacity.

Quick Capacity Reference (A992, Simply Supported, Gravity Only)

Approximate total uniform load capacity (kips) for A992 W-shapes, laterally supported, simple span:

Section	10 ft Span	15 ft Span	20 ft Span	25 ft Span	30 ft Span	35 ft Span
W8x31	56	25	14	9	6	4
W10x33	66	29	16	11	7	5
W12x35	79	35	20	13	9	6
W12x40	97	43	24	16	11	8
W14x38	89	39	22	14	10	7
W16x36	104	46	26	17	12	8
W16x45	137	61	34	22	15	11
W18x40	125	55	31	20	14	10
W18x50	164	73	41	26	18	13
W21x44	148	66	37	24	16	12
W21x57	205	91	51	33	23	17
W24x55	201	89	50	32	22	16
W24x68	261	116	65	42	29	21
W27x84	332	147	83	53	37	27
W30x90	362	161	91	58	40	30
W33x118	494	219	123	79	55	40
W36x135	571	254	142	91	63	46

Values are approximate total uniform loads (kips) based on flexural capacity. Assumes full lateral support, compact section, Zx controls, Ω = 1.5 (ASD). Verify with calculations.

Beam Design Procedure

Step 1: Determine Loads

Calculate the total service (ASD) or factored (LRFD) load on the beam:

Dead load: self-weight + superimposed (deck, slab, finishes, MEP)
Live load: occupancy, roof, snow
Load combinations per ASCE 7

Step 2: Calculate Required Moment and Shear

Simply supported, uniform load:

Mmax = wL²/8
Vmax = wL/2

Simply supported, concentrated load at center:

Mmax = PL/4
Vmax = P/2

Cantilever, uniform load:

Mmax = wL²/2
Vmax = wL

Step 3: Select Trial Section

Based on required plastic modulus:

LRFD: Zx,req ≥ Mu / (φ × Fy) where φ = 0.90

ASD: Zx,req ≥ Ma × Ω / Fy where Ω = 1.67

Step 4: Check Capacity

Check	AISC Chapter	Key Parameter
Flexural strength	F	Zx, Sx, Lp, Lr, Cb
Shear strength	G	h/tw, Aw
Deflection (serviceability)	L/360, L/240	Ix, loading
Local buckling	Table B4.1	bf/2tf, h/tw
Connection capacity	J	Bolt/weld checks

Step 5: Check Deflection

Common deflection limits:

Member	Load Type	Limit
Floor beams	Live load	L/360
Floor beams	Total load	L/240
Roof beams	Live load	L/360
Roof beams	Total (gravel roof)	L/180
Crane runway	Crane load	L/800

Simply supported, uniform load: Δ = 5wL⁴ / (384EI)

Typical Beam Selections by Application

Office Building Floors

Span (ft)	Typical Section	Typical Load (psf)	Notes
20	W16x31	80-120 LL	Composite with deck
25	W18x40	80-120 LL	Composite with deck
30	W21x44	80-120 LL	Composite with deck
35	W24x55	80-120 LL	Composite with deck
40	W27x84	80-120 LL	May need camber
45	W30x90	80-120 LL	Long span, deflection governs

Roof Beams (Non-Composite)

Span (ft)	Typical Section	Typical Load (psf)	Notes
20	W12x26	20-30 LL	Light roof
25	W14x30	20-30 LL	Light roof
30	W16x36	20-30 LL	Light roof
35	W18x40	20-30 LL	Light roof
40	W21x44	20-30 LL	Check ponding

Floor Beams (Non-Composite)

Span (ft)	Typical Section	Typical Load (psf)	Notes
15	W12x26	100 LL	Short span
20	W16x31	100 LL	Medium span
25	W18x35	100 LL	Medium span
30	W21x44	100 LL	Check deflection

Self-Weight Reference

Section	Weight (lb/ft)	W12x40 Equivalent
W8x31	31	Light beam
W10x33	33	Medium beam
W12x35	35	Medium beam
W14x38	38	Medium beam
W16x36	36	Medium beam
W18x40	40	Medium beam
W21x44	44	Medium beam
W24x55	55	Medium-heavy beam
W27x84	84	Heavy beam
W30x90	90	Heavy beam
W33x118	118	Very heavy beam
W36x135	135	Very heavy beam

Self-weight must be included in the dead load. For composite beams, the steel weight is typically 5-15% of the total dead load.

Frequently Asked Questions

How much weight can a W8x31 beam hold? A W8x31 spanning 15 feet can support approximately 25 kips total uniform load (about 1,667 lb/ft). At 20 feet, capacity drops to about 14 kips. These are approximate values for A992 steel, laterally supported.

How far can a W12x40 span? A W12x40 can span approximately 25-30 feet for typical office floor loading (100 psf live load). For roof applications with lighter loads, spans of 30-35 feet are feasible.

What size beam do I need for a 20-foot span? For a 20-foot simple span with typical floor loading (100 psf live load, 50 psf dead load, 4 ft tributary width): total load = 150 psf × 4 ft = 600 lb/ft. A W16x31 or W18x35 would typically work.

Does beam deflection affect capacity? Deflection is a serviceability check, not a strength check. A beam can have adequate strength but excessive deflection. Floor beams are typically limited to L/360 for live load deflection, which often governs the selection for spans over 25 feet.

What is the difference between W, S, and M shapes? W shapes (wide flange) are the most common structural beams. S shapes (American Standard) have sloped inner flanges and are less efficient. M shapes (miscellaneous) are non-standard shapes with limited availability. Use W shapes for new design.

Beam Capacity Calculator — Interactive beam design
Beam Sizes — W-shape section properties
Deflection Limits — L/240, L/360 criteria
Beam Formulas — Moment and shear formulas
Steel Weight Calculator — Weight by 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.