W18x55 Section Properties
The W18x55 is a wide-flange steel section weighing 55 lb/ft (81.9 kg/m) with a nominal depth of 18 inches. It is one of the most frequently specified beam sections in commercial and institutional construction, providing an excellent balance of moment capacity, stiffness, and economy for spans in the 25-35 ft range. The W18 group is the go-to depth for typical office building floor framing.
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Complete Section Properties (Imperial)
| Property | Symbol | Value | Unit |
|---|---|---|---|
| Depth | d | 18.1 | in |
| Flange Width | bf | 7.530 | in |
| Flange Thickness | tf | 0.630 | in |
| Web Thickness | tw | 0.390 | in |
| Cross-Sectional Area | A | 16.2 | in² |
| Weight per Foot | wt | 55 | lb/ft |
| Moment of Inertia (strong axis) | Ix | 890 | in⁴ |
| Elastic Section Modulus (strong axis) | Sx | 98.3 | in³ |
| Plastic Section Modulus (strong axis) | Zx | 112 | in³ |
| Radius of Gyration (strong axis) | rx | 7.41 | in |
| Moment of Inertia (weak axis) | Iy | 44.9 | in⁴ |
| Elastic Section Modulus (weak axis) | Sy | 11.9 | in³ |
| Plastic Section Modulus (weak axis) | Zy | 18.5 | in³ |
| Radius of Gyration (weak axis) | ry | 1.67 | in |
| Torsional Constant | J | 1.66 | in⁴ |
| Warping Constant | Cw | 3870 | in⁶ |
Metric Equivalents
| Property | Symbol | Value | Unit |
|---|---|---|---|
| Depth | d | 460 | mm |
| Flange Width | bf | 191 | mm |
| Flange Thickness | tf | 16.0 | mm |
| Web Thickness | tw | 9.91 | mm |
| Cross-Sectional Area | A | 10,452 | mm² |
| Weight per Metre | wt | 81.9 | kg/m |
| Moment of Inertia (strong axis) | Ix | 370 x 10⁶ | mm⁴ |
| Elastic Section Modulus (strong axis) | Sx | 1,611 x 10³ | mm³ |
| Plastic Section Modulus (strong axis) | Zx | 1,835 x 10³ | mm³ |
AISC 360 Compactness Classification
For A992 steel (Fy = 50 ksi, E = 29,000 ksi):
Flange: bf/(2tf) = 7.530 / (2 x 0.630) = 5.98. Compact limit = 9.15. Since 5.98 < 9.15, the flange is compact.
Web: h/tw. For W18x55 with k = 1.19 in: h/tw = (18.1 - 2 x 1.19) / 0.390 = 15.72 / 0.390 = 40.3. Compact limit = 90.6. Since 40.3 < 90.6, the web is compact.
Classification: Compact section -- full plastic moment Mp = Fy x Zx = 50 x 112 = 5,600 in-kips = 467 ft-kips (nominal).
Design Capacity Summary (AISC 360 LRFD, A992 Fy = 50 ksi)
Flexural Capacity (phi = 0.90)
| Unbraced Length Lb | phi x Mn | Governing Limit State |
|---|---|---|
| 0 ft (full bracing) | 420 ft-kips | Yielding (Mp) |
| 6 ft | 420 ft-kips | Yielding (Lb < Lp) |
| 10 ft | 375 ft-kips | Inelastic LTB |
| 15 ft | 281 ft-kips | Inelastic LTB |
| 20 ft | 188 ft-kips | Elastic LTB |
| 25 ft | 126 ft-kips | Elastic LTB |
Lateral bracing limits: Lp = 5.16 ft (based on ry = 1.67 in). Lr = 15.3 ft. In composite floor construction with metal deck welded to the top flange, the compression flange is continuously braced and the full Mp is available.
Shear Capacity (phi = 1.00)
phi x Vn = 1.00 x 0.6 x 50 x (18.1 x 0.390) = 212 kips
Deflection Screening
For a simply supported W18x55 under uniform load (Ix = 890 in⁴):
| Span | Max Service Load for L/360 | Max Service Load for L/240 |
|---|---|---|
| 25 ft | 1.42 kip/ft | 2.13 kip/ft |
| 30 ft | 0.82 kip/ft | 1.23 kip/ft |
| 35 ft | 0.51 kip/ft | 0.77 kip/ft |
Common Applications for W18x55
The W18x55 is a staple of commercial building construction:
- Office building floor beams spanning 28-35 ft with typical composite metal deck. At 30 ft span with 80 psf total load on 10 ft tributary width, the non-composite deflection is approximately L/290, which may require cambering. With composite action, the effective Ix roughly doubles, bringing deflection well within limits.
- Hospital and institutional floor framing where heavier live loads (80-100 psf) and strict deflection limits (L/480 for vibration-sensitive floors) require robust sections.
- Roof girders in wide-bay warehouse and industrial buildings spanning 30-40 ft.
- Composite floor beams -- the W18 depth is ideal for typical composite floor systems: 3 in deck + 3.25 in concrete topping = 6.25 in slab on top of an 18.1 in beam gives a total structural depth of approximately 24.4 in, fitting within standard floor-to-floor heights.
- Moment frame beams in seismic applications. The W18x55 meets the b/t and h/tw limits for moderately ductile members per AISC 341.
The W18x55 is frequently compared to the W18x50 (Ix = 800 in⁴, Zx = 101 in³). The W18x55 provides 11% more Ix and 11% more Zx for a 10% weight premium -- a proportional trade-off that is worthwhile when deflection governs. For lighter loads, the W18x50 is more economical. For heavier loads at the same depth, the W18x60 (Ix = 984 in⁴, Zx = 123 in³) is the next step up.
Available Steel Grades
| Grade | Specification | Fy (ksi) | Fu (ksi) | Status |
|---|---|---|---|---|
| A992 | ASTM A992/A992M | 50 | 65 | Current standard for W-shapes |
| A572 Gr 50 | ASTM A572/A572M | 50 | 65 | Acceptable alternate |
Comparison with Adjacent Sections
| Property | W18x50 | W18x55 | W18x60 | W21x50 |
|---|---|---|---|---|
| Weight (lb/ft) | 50 | 55 | 60 | 50 |
| d (in) | 18.0 | 18.1 | 18.2 | 20.8 |
| Ix (in⁴) | 800 | 890 | 984 | 984 |
| Zx (in³) | 101 | 112 | 123 | 110 |
| ry (in) | 1.65 | 1.67 | 1.69 | 1.30 |
Note: The W21x50 provides the same Ix as the W18x60 at a lower weight, but its smaller ry (1.30 in) means it needs closer lateral bracing when the compression flange is not continuously supported.
Run This Calculation
Beam Capacity Calculator -- check W18x55 bending, shear, and LTB capacity per AISC 360, AS 4100, EN 1993, or CSA S16.
Beam Deflection Calculator -- calculate deflection for W18x55 under any loading pattern.
Beam Span Screener -- find the lightest W-shape for your span and load.
Related Pages
- Section Properties Database
- W-Shape Beam Sizes -- Complete Table
- Steel Beam Load Tables
- Steel Grades Reference
- Beam Formulas Reference
- composite beam calculator
- W30x99 section properties
- W8x31 section properties
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