| Actual depth | 310 mm | 303 mm | 300 mm | | Flange width | 165 mm | 165 mm | 150 mm | | Flange thickness | 9.7 mm | 10.2 mm | 10.7 mm | | Web thickness | 5.8 mm | 6.0 mm | 7.1 mm | | Mass per metre | 38.8 kg/m | 40.3 kg/m | 42.2 kg/m | | Ix (10^6 mm^4) | 84.9 | 85.0 | 83.6 | | Sx (10^3 mm^3) | 548 | 561 | 557 | | Zx (10^3 mm^3) | 610 | 641 | 628 | | Iy (10^6 mm^4) | 14.3 | 7.64 | 6.04 | | ry (mm) | 45.2 | 32.0 | 33.5 | | J (10^3 mm^4) | 89.2 | 121 | 201 |

The strong-axis flexural properties (Ix, Sx, Zx) are within 5% of each other — all three sections have essentially the same bending capacity. But the weak-axis stiffness (Iy) and radius of gyration (ry) vary by a factor of 2.4 between the W-shape and the IPE. This difference governs lateral-torsional buckling resistance, connection detailing, and erection stability.

Span-to-Depth Preliminary Selection Rules

The span-to-depth ratio (L/d) is the most reliable starting point for preliminary beam sizing. These ratios are derived from the requirement that a simply supported beam carrying a uniformly distributed load should satisfy both strength (utilisation approximately 0.7-0.85) and the L/360 live-load deflection limit at the preliminary stage without requiring a detailed check.

Simply supported beams (pinned-pinned):

Continuous beams:

Cantilevers:

Important caveats:

• These ratios assume full lateral restraint of the compression flange (composite slab with shear studs at regular spacing). For unbraced beams, start one depth increment deeper.
• For spans beyond 12 m, deflection often governs — compute deflection explicitly rather than relying on L/d alone.
• For point loads or partial-length uniform loads, the ratio may be unconservative — check both strength and deflection in the preliminary stage if the loading pattern is irregular.

Beam Depth Selection by Span — Preliminary Table

The table shows that IPE and W-shapes track depth closely to the L/20 rule, while UB sections often run shallower — in the 12 m span case, UB 530 (528 mm) vs the L/20 target of 600 mm. This reflects the narrower-flange, deeper-web UB design philosophy: the section achieves similar stiffness with less depth because its web is proportionally taller relative to its flange width.

Selection Workflow — Step by Step

Step 1: Establish the span, support conditions, and loading. The support condition determines the effective length for flexure and lateral-torsional buckling. A simply supported beam with a composite slab typically has full lateral restraint at the top flange, making LTB a non-governing limit state. A beam spanning between two girders with only intermittent bridging may be partially restrained — use AISC 360 Chapter F or the equivalent code provision to determine the unbraced length Lb.

Step 2: Compute the required plastic modulus Zx_req. For a simply supported uniformly loaded beam: Mu = wu x L^2 / 8. The required plastic modulus is Zx_req = Mu / (phi x Fy). In imperial: Zx_req (in^3) = Mu (kip-ft) x 12 / (0.9 x 50). In metric: Zx_req (mm^3) = Mu (N.mm) / (0.9 x 345). The phi factor of 0.9 assumes a compact section — if the section is non-compact, phi reduces to 0.9 and the strength is based on the elastic section modulus Sx with a lateral-torsional buckling reduction.

Step 3: Filter sections by minimum Zx. Sort the section table by Zx ascending and select the lightest section (lowest mass per metre) that satisfies Zx >= Zx_req. This initial filter identifies the mass-optimum section for bending strength alone.

Step 4: Check depth against the L/d rule. If the L/d ratio exceeds the recommended range, select a deeper section — deflection is likely to govern even if the section passes the strength check. For preliminary selection, impose a minimum depth d_min = L / 24 for simply supported beams.

Step 5: Verify web classification and shear. Shallow, lightweight sections (W12x14, UB 150x14) have thin webs with high h/tw ratios. Check that the web is compact (AISC 360 Table B4.1b, h/tw <= 2.24 sqrt(E/Fy) = 53.9 for A992) or compact enough that shear buckling does not reduce the available shear strength. For deep, thin-web beams, verify that the web shear capacity phi Vn = phi x 0.6 x Fy x d x tw satisfies the factored shear demand.

Step 6: Check deflection if L/d is marginal. For spans exceeding 10 m, or for beams where the depth was driven down by architectural constraints, compute the live-load deflection delta_LL and check against the applicable limit (L/360 for floors, L/240 for roofs supporting non-structural elements, L/180 for roofs with no occupancy). Use the actual moment of inertia Ix from the section table and the service-level live load.

When to Use Each Section Series

Use W-shapes when:

• The project is in North America (procurement, detailing standards, and connection hardware are W-shape oriented)
• The beam is unbraced for significant lengths and lateral-torsional buckling may govern (the wide flange provides the best LTB resistance of any standard shape)
• The beam also carries axial load (beam-columns) — the wider flange provides better weak-axis buckling resistance
• The connection requires bolting through the flange — wider flanges accommodate standard bolt gauges more easily

Use UB sections when:

• The project is in Australia, New Zealand, or the UK (procurement and standards alignment)
• The beam is a pure flexural element with full lateral restraint (composite slab) — the narrower flange is actually an advantage because more mass is in the web, increasing the section depth for the same weight
• The beam depth is not heavily constrained — UB sections can run deeper for the same weight

Use IPE sections when:

• The project is in continental Europe, the Middle East (many projects specify EN standards), or regions where European sections are the procurement default
• The beam is laterally restrained and depth is not a constraint — IPE sections offer the best strong-axis section modulus per kilogram
• The beam is part of a truss (chord member) where torsional stiffness is less important

Use HEA/HEB sections when:

• The beam depth is severely limited but flexural demand is high — HEA sections are 20-30% deeper for the same nominal label but provide higher Zx at a given depth constraint
• The member functions as a beam-column where weak-axis buckling may govern — HEA sections have wider flanges than IPE sections of the same depth

Worked Example: Floor Beam Selection

A 10 m simply supported composite floor beam in an office building. The beam is fully laterally restrained by the composite slab. Factored uniformly distributed load wu = 35 kN/m (2.4 kip/ft). Live load for deflection: wL = 15 kN/m (1.03 kip/ft). The project is in the United States, specifying ASTM A992 W-shapes.

Step 1: Compute factored moment. Mu = wu x L^2 / 8 = 35 x 10^2 / 8 = 437.5 kN.m (323 kip.ft)

Step 2: Required plastic modulus. Zx_req = Mu / (phi x Fy) = 437.5 x 10^6 / (0.90 x 345) = 1,409 x 10^3 mm^3 (86.0 in^3)

Step 3: Filter W-shapes by Zx >= 1,409 x 10^3 mm^3. Candidates in ascending mass order:

• W18x50: Zx = 1,630 x 10^3 mm^3, mass = 75 kg/m — satisfies Zx, 457 mm deep
• W21x44: Zx = 1,560 x 10^3 mm^3, mass = 65.5 kg/m — satisfies Zx, 525 mm deep (lighter!)
• W21x50: Zx = 1,720 x 10^3 mm^3, mass = 74 kg/m — satisfies Zx, 529 mm deep
• W24x55: Zx = 1,860 x 10^3 mm^3, mass = 82 kg/m — satisfies Zx, 599 mm deep

The W21x44 is the lightest section that satisfies Zx_req. Mass = 65.5 kg/m, depth = 525 mm. Preliminary utilisation = Zx_req / Zx = 1,409 / 1,560 = 0.903 — borderline. The W21x44 is worth considering but the high utilisation suggests checking it carefully.

Step 4: Check L/d ratio. L/d = 10,000 / 525 = 19.0. The recommended L/20 = 500 mm minimum depth. The W21x44 at 525 mm satisfies the L/d guideline — acceptable.

Step 5: Deflection check. delta_LL = 5 x wL x L^4 / (384 x E x Ix) = 5 x 15 x 10 x 10^9 / (384 x 200,000 x 350 x 10^6) = 27.9 mm. L/360 = 10,000 / 360 = 27.8 mm. The deflection is exactly at the limit — this is too close. Select the next deeper section.

Try W21x50: Ix = 410 x 10^6 mm^4, delta_LL = 5 x 15 x 10 x 10^9 / (384 x 200,000 x 410 x 10^6) = 23.8 mm — L/420, acceptable. Depth = 529 mm, L/d = 18.9. Mass = 74 kg/m.

Selected: W21x50. Flexural utilisation = 0.83, deflection well within limit, section is compact (bf/2tf = 7.0 < 9.15 for compact flange per AISC 360 Table B4.1b).

If this were a UK project: The equivalent UB selection would start with UB 457x191x67 (453 mm deep, Zx = 1,470 x 10^3 mm^3) — slightly less depth but heavier than the W21x50. The UB 533x210x82 (528 mm deep, Zx = 2,060 x 10^3 mm^3) provides more capacity at higher mass.

Selection Tips from Practice

• Don't select the absolute lightest section. A section at 95% utilisation leaves no margin for connection eccentricity, fabrication tolerances, or future capacity upgrades. Target 75-85% utilisation for floor beams.
• Check the section is actually available. Not every section in the AISC manual is stocked by every service centre. W21x44 and W21x50 are common; W21x48 and W27x84 are produced but less frequently stocked. The procurement team should confirm availability before the section is locked into the design.
• Consider connection geometry at the selection stage. A W21 beam connected to a W14 column requires the beam flange width to match the column bolt gauge. If the beam flange is too narrow, the bolted connection becomes more complicated (extended shear tab, stiffened seated connection). Select the beam section with the connection in mind.
• Vibration may govern for long-span light beams. A W21x44 spanning 12 m with a composite slab may satisfy strength and static deflection but exhibit perceptible floor vibration under foot traffic. For spans over 10 m with natural frequencies below 8 Hz, consult AISC Design Guide 11 for vibration criteria.

FAQ

Which beam section series is the most efficient for pure bending?

IPE sections are the most mass-efficient for pure strong-axis bending when the compression flange is fully laterally restrained. The narrow-flange, deep-web geometry maximises the elastic section modulus per unit weight. For a given mass, an IPE section typically provides 5-10% more strong-axis stiffness than a UB section and 10-15% more than a W-shape. However, this efficiency comes at the cost of weak-axis stiffness and torsional resistance — the IPE is the least forgiving section when lateral restraint is uncertain.

How do I select a beam when both flexure and axial load are present?

When the beam also carries axial compression (a beam-column), the selection logic changes: the section must satisfy the combined axial-flexure interaction equation (AISC 360 Chapter H, equation H1-1a/b). In this case, W-shapes are almost always the best choice because their wide flanges provide high weak-axis radius of gyration ry, which increases the column buckling capacity Pn. The selection workflow becomes: (1) estimate the required axial capacity Pn_req, (2) filter by ry to keep KL/r below 100-120, (3) check Zx for flexure, (4) verify the interaction equation. A W12 or W14 shape — wider and heavier than a pure flexural optimum — often governs for beam-columns.

What is the difference between W, M, S, and HP shapes in the AISC manual?

W-shapes (wide-flange) are the standard beam and column sections — parallel flange surfaces, wide flanges, optimised for both flexure and compression. M-shapes (miscellaneous) are light, non-standard sections (M8x6.5, M10x9) produced in small quantities — avoid specifying them unless existing stock is confirmed. S-shapes (American standard beams) have tapered flange inner surfaces (slope approximately 16.7%) and are an older standard — still produced but less frequently specified for new construction. HP-shapes (bearing piles) have essentially parallel flange surfaces and equal flange and web thicknesses — suitable for piles and some column applications but rarely economical for beams.

Can I mix beam series in the same project?

Yes — and it is common in international projects. A building in the Middle East might specify IPE sections for secondary beams (procured from European mills), UC sections for columns (Australian supplier with competitive pricing), and W-shapes for the transfer girder (American section that met a specific depth constraint). The key requirement is that each section is designed to its governing standard and that the connection details accommodate the different flange widths, bolt gauges, and section depths. The connection design becomes the limiting factor — a bolted moment connection between a W14 column and an IPE 300 beam requires a custom end plate that accounts for the different flange widths and bolt gauges.

Related pages: Beam Sizes Chart | US Beam Design (AISC 360) | UK Steel Beam Design | Section Properties Database | Beam Capacity Calculator