W18×55 vs W21×55
A side-by-side comparison of two AISC wide-flange beams, 16th Edition. Detailing dimensions to the nearest 1/16 in.
W18×55 and W21×55 both weigh 55 lb per foot, so the choice comes down to depth and stiffness rather than weight.
W21×55 is the deeper section at a nominal 21 in., versus 18 in.
W21×55 has the greater bending strength: section modulus Sx of 110 in³ versus 98.3, about 12% more.
| Property | W18×55 | W21×55 |
|---|---|---|
| Weight | 55 lb/ft | 55 lb/ft |
| Depth (d) | 18 1/8" | 20 3/4" |
| Flange width (bf) | 7 1/2" | 8 1/4" |
| Web thickness (tw) | 3/8" | 3/8" |
| Flange thickness (tf) | 5/8" | 1/2" |
| Area (A) | 16.2 in² | 16.2 in² |
| Moment of inertia (Ix) | 890 in⁴ | 1140 in⁴ |
| Section modulus (Sx) | 98.3 in³ | 110 in³ |
| Plastic modulus (Zx) | 112 in³ | 126 in³ |
Open W18×55 in the interactive size chart →
Full detail: W18×55 · W21×55. Verify critical dimensions against the current AISC Steel Construction Manual before fabrication.
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