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:

Step 2: Calculate Required Moment and Shear

Simply supported, uniform load:

Simply supported, concentrated load at center:

Cantilever, uniform load:

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.

Try it now: Check your beam load capacity with our free Steel Beam Capacity calculator →

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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.

Beam Design Methods

Lateral-Torsional Buckling

For beams that are not adequately braced against lateral movement and twist, the nominal moment capacity is governed by lateral-torsional buckling (LTB). The resistance depends on the unbraced length (Lb) relative to limit states:

Shear Design

Web shear strength depends on the panel aspect ratio and stiffener configuration. For unstiffened webs, the nominal shear capacity is:

Compact sections with low web slenderness (h/tw) can develop full shear yielding. Slender webs may require transverse stiffeners to develop adequate shear capacity.

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Frequently Asked Questions

What is the recommended design procedure for this structural element?

The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.

How do different design codes compare for this calculation?

AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.

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