Calculating the load capacity of an H beam is essential for structural design and material procurement. This article provides a practical methodology for estimating bending moment capacity, shear capacity, and deflection of H beams under common loading conditions, using standard structural steel design principles.
Every standard H beam section is defined by a set of geometric properties that determine its load-carrying capacity:
These properties are tabulated in steel section handbooks for every standard profile. TXD Steel provides section property tables for the full HW, HM, and HN series on request.
For an H beam loaded in bending about its major axis, the elastic moment capacity is:
Mc = Wx × fb
Where Wx is the elastic section modulus (cm³) and fb is the allowable bending stress (MPa). For Q235B steel, the allowable bending stress is typically 0.6 × Fy = 0.6 × 235 ≈ 140 MPa under ASD (Allowable Stress Design). For Q345B, fb ≈ 0.6 × 345 ≈ 207 MPa.
Example: HW200×200 H beam with Wx = 472 cm³ in Q235B: Mc = 472 × 140 = 66,080 N·m ≈ 66 kN·m
Under LRFD (Load and Resistance Factor Design), the design bending strength uses the plastic section modulus: φMn = 0.9 × Fy × Sx
For Q345B HW200×200 (Sx ≈ 530 cm³): φMn = 0.9 × 345 × 530,000 mm³ × 10⁻⁶ = 164.6 kN·m
The most common case in building structures: an H beam simply supported at both ends with a uniformly distributed load (UDL) along its length. The maximum bending moment:
Mmax = w × L² / 8
Where w = uniform load (kN/m) and L = span (m). The maximum deflection:
δmax = 5 × w × L⁴ / (384 × E × I)
Where E = 200,000 MPa (steel elastic modulus) and I = moment of inertia in mm⁴.
Example: HW200×200 (Ix = 4,770 cm⁴ = 47,700,000 mm⁴), span 6.0 m, uniform load 15 kN/m: Mmax = 15 × 6² / 8 = 67.5 kN·m → Compare to Mc = 66 kN·m → marginally over the elastic limit; consider HW250×250 or reduce load. δmax = 5 × 15 × 6,000⁴ / (384 × 200,000 × 47,700,000) = 5 × 15 × 1.296×10¹⁵ / 3.66×10¹⁵ ≈ 26.5 mm
Deflection limit for floor beams is typically L/360 = 16.7 mm, so this beam exceeds the limit. A larger section or reduced span is required.
For simply supported beams at 6.0 m span, Q235B grade, limiting deflection to L/360:
| H Beam Section | Approx. UDL Capacity (kN/m) | Max Moment (kN·m) | |---|---|---| | HW100×100 | ~1.5 | ~6.8 | | HW150×150 | ~5.0 | ~22.5 | | HW200×200 | ~12.0 | ~54 | | HW250×250 | ~22.0 | ~99 | | HW300×300 | ~38.0 | ~171 | | HW350×350 | ~58.0 | ~261 | | HN400×200 | ~30.0 | ~135 | | HN500×200 | ~45.0 | ~203 | | HN600×200 | ~65.0 | ~293 | | HM300×200 | ~18.0 | ~81 |
Values are approximate for preliminary sizing only. Final design must consider actual loading conditions, lateral bracing, and applicable design codes.
H beams loaded in bending about the major axis can fail by lateral-torsional buckling (LTB) if the compression flange is not adequately braced. The buckling resistance reduces as the unbraced length increases. Building codes require that the unbraced length not exceed a limiting value (Lp for plastic design, Lr for inelastic buckling).
For preliminary sizing, assume the compression flange is braced at 2–3 m intervals by floor decking or secondary framing. For beams without continuous lateral restraint (e.g., crane runway beams without a floor slab), a stability check using the applicable design code is mandatory.
Step 1: Determine loading (dead load + live load + any point loads). Step 2: Calculate maximum bending moment and shear for the span and support conditions. Step 3: Select a trial section from the standard H beam range, referencing Wx and Ix values. Step 4: Check bending capacity (Mmax ≤ Mc), shear capacity, and deflection (δmax ≤ L/360 or L/240). Step 5: Verify lateral bracing conditions. If unbraced length is long, check LTB capacity. Step 6: Confirm steel grade and section availability with your supplier.
TXD Steel Profile provides section property data sheets for all H beam sizes in HW, HM, and HN series. Contact our technical sales team for assistance with section selection and current availability.
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