W21X122 Steel Beam — Section Properties
Dimensions
| Property | Value | Unit |
|---|---|---|
| Depth (d) | 21.70 | in (551.2 mm) |
| Flange Width (bf) | 12.40 | in (315.0 mm) |
| Flange Thickness (tf) | 0.96 | in |
| Web Thickness (tw) | 0.60 | in |
| Area (A) | 35.90 | in² (231.6 cmÃÂò) |
| Weight | 122 | lb/ft (182 kg/m) |
Elastic Section Properties
| Property | Strong Axis (X-X) | Weak Axis (Y-Y) | Unit |
|---|---|---|---|
| Moment of Inertia (I) | 2,960 | 305 | in⁴ |
| Elastic Section Modulus (S) | 273 | 49.20 | in³ |
| Plastic Section Modulus (Z) | 307 | 75.60 | in³ |
| Radius of Gyration (r) | 9.09 | 2.92 | in |
Torsional Properties
| Property | Value | Unit |
|---|---|---|
| Torsional Constant (J) | 8.98 | in⁴ |
| Warping Constant (Cw) | 32,700 | in⁶ |
| Distance Between Flange Centroids (ho) | 20.70 | in |
| Effective Radius (rts) | 3.40 | in |
Section Profile Summary
Hot-rolled W-shape with Sx = 273 in³. The section is commonly specified for primary framing in multi-storey buildings, crane girders, and bridge stringers.
At 21.7" deep, this section is frequently specified for primary floor beams, girders, and moment frame columns in commercial and institutional buildings.
Key Design Checks (AISC 360)
| Check | Formula | This Section |
|---|---|---|
| Plastic moment | Mp = Zx × Fy | 15,350 kip-in |
| Deflection | Δ = 5wLâÃÂô/(384EIx) | Use Ix = 2,960 in⁴ |
| Torsion | St. Venant = GJ/L | J = 8.98 in⁴ |
| Column buckling | KL/r âÃÂàFcr | r_x = 9.09 in |
Design Notes
- Compactness: Most W-shapes in ASTM A992 satisfy the compact limits in AISC 360 Table B4.1b for flexure (ÃÂû âÃÂä ÃÂûp). This allows full plastic moment Mp development before local buckling occurs.
- Flange local buckling (FLB): The flange slenderness ratio bf/2tf is compared against ÃÂûpf = 0.38âÃÂÃÂ(E/Fy) âÃÂà9.15 for compact sections.
- Web local buckling (WLB): h/tw is checked against ÃÂûpw = 3.76âÃÂÃÂ(E/Fy) âÃÂà90.5 for flexure, and a more restrictive limit for uniform compression.
- Lateral-torsional buckling (LTB): Lp âÃÂà123.8 in (plastic limit). For unbraced lengths âÃÂä Lp, the full plastic moment is available. Between Lp and Lr, linear interpolation per AISC 360 Eq. F2-2 applies.
Verification (AISC 360): All designs using this section must be verified by a licensed Professional Engineer. Before finalizing member selection, check beam-column interaction (P-M), lateral-torsional buckling, serviceability deflections, and all connection limit states. See Engineering Disclaimer.
Worked Example: Light commercial floor beam — w21x122
Scenario: Retail space floor — higher live load for public occupancy. Simply supported wide flange, 43 ft span, 8 ft tributary width. Service loads: 55 psf dead + 60 psf live (920 plf total).
Given:
- Span L = 43 ft (516 in)
- Total service load w = 920 plf
- Live load portion w_L = 480 plf
- Section w21x122: Ix = 2,960 in⁴, Sx = 273 in³, Zx = 307 in³
- Steel: ASTM A992, Fy = 50 ksi, E = 29,000 ksi
- Design moment strength: φMn = 0.9 × Zx × Fy = 1,151 kip-ft
Step 1 — Live load deflection check:
Δ_LL = 5 w_L LâÃÂô / (384 E Ix)
Δ_LL = 5 × (480/12) × (516)âÃÂô / (384 × 29,000,000 × 2,960)
Δ_LL = 0.43 in
Step 2 — Allowable deflection (IBC Table 1604.3, L/240):
Δ_allow = L / 240 = 516 / 240 = 2.15 in
Step 3 — Dead + live deflection:
Δ_total = 0.82 in
Step 4 — Design check:
âÃÂàLive load deflection is within the L/240 limit.
The required flexural strength at ultimate is approximately w_u LÃÂò/8. With factored loads 1.2D + 1.6L, verify φMn = 1,151 kip-ft is adequate per AISC 360 Chapter F. Also check shear capacity (φVn = 0.6 Fy Aw × 0.9) and bearing at supports.
Related Resources
- Steel Beam Capacity Calculator
- Column Capacity Calculator
- Beam Deflection Calculator
- Section Properties Calculator
- Steel Grades Reference
- Section Comparison Tool
Design Resources
- Section Properties Lookup — Compare with similar sections
- Steel Beam Sizes Reference — Standard beam dimensions
Educational reference only. Verify all section properties against the current AISC 360 Manual and mill certificates before design. Results are PRELIMINARY — NOT FOR CONSTRUCTION.