What size steel beam do I need for a 20-foot span?

For a typical 20 ft floor beam with 50 psf live load + 20 psf dead load at L/360 deflection limit, a W12x22 or W14x22 typically works for beams spaced 8 ft apart. For a 20 ft simply supported beam carrying a uniform load: total load w = (50+20) x 8 ft = 560 plf, Mmax = wL2/8 = 28.0 k-ft. W12x22 has phi Mn = 87.4 k-ft (A992) which is adequate. Deflection checks at L/360 = 20x12/360 = 0.67 in often govern for shallower sections. Always verify with actual loads and bracing conditions.

What deflection limits apply to steel beams?

Common deflection limits per IBC 1604.3 and industry practice: floor beams — L/360 live load, L/240 total load; roof beams (no ceiling) — L/240 live load, L/180 total load; roof beams (with plaster ceiling) — L/360 live load; floor beams supporting vibration-sensitive equipment — L/480; crane runway girders — L/600 vertical, L/400 lateral. For cantilever beams, limits are L/240 (floor) and L/120 (roof) where L = 2 x cantilever length. Deflection is calculated using unfactored (service) loads with gross section properties.

How does steel grade affect beam span capacity?

Steel grade affects moment and shear capacity but NOT deflection. A W16x26 beam in A36 (Fy = 36 ksi) has Mr = 123 k-ft while the same beam in A992 (Fy = 50 ksi) has Mr = 171 k-ft — a 39% increase in moment capacity. However, deflection is stiffness-governed (E x I), and since E = 29,000 ksi is identical for all carbon steel grades, deflection is unchanged. For most floor beams where deflection (not strength) controls, upgrading from A36 to A992 provides no span benefit. Higher-strength steel benefits cantilevers, heavy point loads, and short spans where flexural strength governs.

How does beam spacing affect beam size selection?

Beam spacing directly sets the tributary width and thus the load per foot. Doubling the spacing doubles the load and doubles the required moment capacity. For deflection: delta is proportional to w x L4 / (E x I), so doubling the spacing doubles the deflection for the same beam. The relationship is linear — at 10 ft spacing a beam carries 2x the load and deflects 2x as much as at 5 ft spacing. This means beam spacing is one of the most cost-sensitive design decisions. Closer spacing = lighter beams but more beams. The economic optimum for steel-framed floors is typically 8-10 ft on center.

Can I use these span tables for composite beams?

No — composite beam span tables are different because the concrete slab acts compositely with the steel beam through shear studs, significantly increasing stiffness and strength. A composite W16x26 with 4.5 in slab at 10 ft spacing can span 30-35 ft, while the same beam non-composite might only span 22 ft. Composite action increases the effective moment of inertia by 50-200% depending on slab thickness, beam size, and stud configuration. For accurate composite beam design, use a dedicated composite beam calculator or the AISC Design Guide 3 tables.

Steel Beam Span Tables — W Shape Max Spans

Steel beam span tables help engineers and contractors quickly select appropriate beam sizes based on span and loading. This page provides preliminary span tables for common W shapes under typical floor and roof loading conditions.

How to Use These Tables

The tables show maximum simple spans based on:

Strength (LRFD factored load capacity)
Deflection (L/360 for floors, L/240 for roofs, L/180 for roof live load)
The smaller value governs

Tables assume simple-span, uniformly loaded, laterally supported beams (full lateral bracing). Actual design must consider specific loading, bracing, and support conditions.

Floor Beam Span Table

Loading: 100 psf total (70 dead + 30 live), tributary width 30 ft (typical office bay)

Beam	Max Span (ft)	Governs	Weight (lb/ft)	Total Weight (lb)
W16x31	22	Deflection	31	682
W16x36	24	Deflection	36	864
W18x35	25	Deflection	35	875
W18x40	27	Deflection	40	1,080
W18x46	29	Deflection	46	1,334
W21x44	31	Strength	44	1,364
W21x50	33	Deflection	50	1,650
W21x57	35	Deflection	57	1,995
W24x55	37	Strength	55	2,035
W24x62	39	Deflection	62	2,418
W24x68	41	Deflection	68	2,788
W27x84	45	Deflection	84	3,780
W30x90	48	Deflection	90	4,320
W30x99	50	Deflection	99	4,950
W33x118	54	Strength	118	6,372
W36x135	58	Strength	135	7,830

Roof Beam Span Table

Loading: 40 psf total (20 dead + 20 live), tributary width 30 ft

Beam	Max Span (ft)	Governs	Weight (lb/ft)
W12x19	22	Deflection	19
W12x22	24	Deflection	22
W14x22	25	Deflection	22
W14x26	27	Deflection	26
W16x26	29	Deflection	26
W16x31	31	Deflection	31
W18x35	34	Deflection	35
W18x40	36	Deflection	40
W21x44	39	Deflection	44
W21x50	41	Deflection	50
W24x55	44	Deflection	55
W24x62	46	Strength	62
W27x84	52	Strength	84

Composite Beam Span Table (With Shear Studs)

Composite beams (steel beam + concrete slab with shear studs) are 30-50% stronger than bare steel beams. Loading: 100 psf total, 30 ft tributary.

Beam	Composite Span (ft)	Bare Steel Span (ft)	% Increase
W16x31	28	22	27%
W18x35	32	25	28%
W18x40	34	27	26%
W21x44	38	31	23%
W21x50	40	33	21%
W24x55	44	37	19%
W24x62	46	39	18%
W27x84	52	45	16%
W30x90	55	48	15%

Composite action provides the biggest benefit for smaller beams. As beams get deeper, the relative improvement decreases.

Typical Beam Selections by Application

Office Buildings

Span (ft)	Typical Beam	Depth (in)	Loading	Notes
20-25	W16x31 to W16x36	16	100 psf, 30 ft trib	Floor framing
25-30	W18x40 to W18x50	18	100 psf, 30 ft trib	Standard office bay
30-35	W21x50 to W21x62	21	100 psf, 30 ft trib	Long-span office
35-40	W24x62 to W24x76	24	80-100 psf	Open plan offices
40-50	W27x84 to W30x99	27-30	80 psf	Atriums, large spaces

Industrial / Warehouse

Span (ft)	Typical Beam	Loading	Notes
20-30	W18x35 to W18x50	150-250 psf	Heavy storage
30-40	W24x55 to W24x76	100-150 psf	Light manufacturing
40-50	W30x90 to W30x116	80-120 psf	Warehouse, clear span
50-60	W33x118 to W36x150	60-100 psf	Heavy industrial

Roof Purlins and Girts

Span (ft)	Typical Beam	Loading	Notes
20-25	W12x16 to W12x22	30-40 psf	Metal building roof
25-30	W14x22 to W14x26	30-40 psf	Standard purlin
20-25	W8x10 to W10x12	20 psf	Wall girt

Deflection Limits

Member Type	Dead Load	Live Load	Total Load	Source
Floor beams	L/360	L/360	L/240	IBC Table 1604.3
Floor beams (sensitive)	L/480	L/360	L/240	Strict criteria
Roof beams	—	L/360	L/240	IBC
Roof beams (LL only)	—	L/180	—	Roof live load
Crane runway	L/800	L/800	L/600	AISE criteria

Deflection is often the governing criterion for floor beams. A beam that satisfies strength may fail the L/360 live load deflection limit.

Quick Weight Estimates

For preliminary estimates, steel beam weight per square foot of floor area:

Span (ft)	Bay Size	Steel Weight (psf)	Beam Depth
25	25 × 30	4-6	W16-W18
30	30 × 30	5-7	W18-W21
35	30 × 35	6-9	W21-W24
40	30 × 40	8-12	W24-W27
45	30 × 45	10-14	W27-W30

These weights are for beams only (not including columns, connections, or bracing). Total structural steel for a typical office building is 8-12 psf.

Frequently Asked Questions

How far can a W21x44 beam span? Under typical office loading (100 psf, 30 ft tributary), a W21x44 can span approximately 31 ft when fully laterally supported. With composite action (shear studs), the span increases to about 38 ft.

What is the longest span for a steel beam? Practically, W-shape beams span up to about 65-70 ft (W36x230 or larger). Beyond 65 ft, trusses, plate girders, or built-up sections become more economical. For spans over 100 ft, space frames or cable-stayed systems are used.

What governs beam span: strength or deflection? For typical floor beams, deflection (L/360 live load) often governs. Deeper beams (W21, W24) are more efficient for deflection control because moment of inertia increases with the cube of depth. Strength may govern for heavily loaded or short-span beams.

How does composite action increase span? Shear studs connect the concrete slab to the steel beam, creating a composite section. The slab acts as additional compression flange area, increasing the effective moment of inertia by 30-50%. This allows either longer spans for the same beam size or smaller beams for the same span.

Beam Capacity Calculator — Design a steel beam
Beam Sizes — W shape section properties
Beam Formulas — Deflection and shear formulas
Composite Slab Design — Steel deck + concrete
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.