W21×122 Steel Beam
A W21×122 is a wide flange steel beam 21.7 inches deep with 12.4-inch flanges, weighing 122 pounds per foot. In ASTM A992 steel (Fy = 50 ksi) it has a design bending strength of 1,151 kip-ft (LRFD) when its compression flange is braced, and a design shear strength of 391 kips.
Design a W21×122 beam Opens a 33 ft span in the free beam designer.
W21×122 dimensions
| Dimension | Value |
|---|---|
| Depth, d | 21.7 in |
| Flange width, bf | 12.4 in |
| Flange thickness, tf | 0.960 in |
| Web thickness, tw | 0.600 in |
| Distance to web toe of fillet, kdes | 1.46 in |
| Distance between flange centroids, ho | 20.74 in |
| Cross-sectional area, A | 35.9 in2 |
| Weight | 122 lb/ft |
Dimensions are AISC design values. Detailing dimensions are rounded to the nearest 1/16 in.
W21×122 section properties
| Property | Strong axis (x) | Weak axis (y) |
|---|---|---|
| Moment of inertia, I | 2960 in4 | 305 in4 |
| Elastic section modulus, S | 273 in3 | 49.2 in3 |
| Plastic section modulus, Z | 307 in3 | 75.6 in3 |
| Radius of gyration, r | 9.09 in | 2.92 in |
| Torsion and slenderness | Value |
|---|---|
| Torsional constant, J | 8.98 in4 |
| Warping constant, Cw | 32700 in6 |
| Effective radius of gyration, rts | 3.40 in |
| Flange slenderness, bf/2tf | 6.45 |
| Web slenderness, h/tw | 31.3 |
W21×122 bending and shear strength
For ASTM A992 steel (Fy = 50 ksi, E = 29,000 ksi) per AISC 360-16. Its flanges and web are compact for bending, so with its compression flange braced it reaches its full plastic moment.
| Strength | LRFD | ASD |
|---|---|---|
| Bending, compression flange braced | φMn = 1,151 kip-ft | Mn/Ω = 766 kip-ft |
| Shear | φVn = 391 kips | Vn/Ω = 260 kips |
If the compression flange isn't braced, the beam can buckle sideways before it yields. Up to Lp = 10.3 ft between braces it keeps its full strength; past Lr = 32.7 ft it buckles elastically. With uniform moment between braces (Cb = 1.0):
| Unbraced length, Lb | φMn (LRFD) | Mn/Ω (ASD) |
|---|---|---|
| Braced | 1,151 kip-ft | 766 kip-ft |
| 10 ft | 1,151 kip-ft | 766 kip-ft |
| 10.3 ft (Lp) | 1,151 kip-ft | 766 kip-ft |
| 20 ft | 963 kip-ft | 641 kip-ft |
| 30 ft | 769 kip-ft | 512 kip-ft |
| 32.7 ft (Lr) | 717 kip-ft | 477 kip-ft |
| 40 ft | 548 kip-ft | 364 kip-ft |
| 50 ft | 415 kip-ft | 276 kip-ft |
| 60 ft | 335 kip-ft | 223 kip-ft |
| 70 ft | 281 kip-ft | 187 kip-ft |
A beam with a uniform load between braces has Cb above 1.0 (1.14 for a simple span braced only at its ends), so these values are conservative. The beam designer works out Cb for every segment between braces.
W21×122 load table by span
The largest uniform load a simply supported W21×122 can carry, in kips per foot, with its compression flange braced along its length (by a floor or roof deck, for example). The loads include the beam's own weight of 0.122 kips per foot.
| Span | Factored load (LRFD) | Allowable load (ASD) | Load at span/360 | Load at span/240 |
|---|---|---|---|---|
| 11 ft | 71.0* | 47.3* | 95.5 | 143.3 |
| 13 ft | 54.5 | 36.3 | 57.9 | 86.8 |
| 15 ft | 40.9 | 27.2 | 37.7 | 56.5 |
| 17 ft | 31.9 | 21.2 | 25.9 | 38.8 |
| 19 ft | 25.5 | 17.0 | 18.5 | 27.8 |
| 21 ft | 20.9 | 13.9 | 13.7 | 20.6 |
| 23 ft | 17.4 | 11.6 | 10.5 | 15.7 |
| 25 ft | 14.7 | 9.80 | 8.14 | 12.2 |
| 27 ft | 12.6 | 8.41 | 6.46 | 9.69 |
| 29 ft | 11.0 | 7.29 | 5.21 | 7.82 |
| 31 ft | 9.58 | 6.38 | 4.27 | 6.40 |
| 33 ft | 8.46 | 5.63 | 3.54 | 5.31 |
| 35 ft | 7.52 | 5.00 | 2.97 | 4.45 |
| 37 ft | 6.73 | 4.48 | 2.51 | 3.77 |
| 39 ft | 6.06 | 4.03 | 2.14 | 3.22 |
| 41 ft | 5.48 | 3.65 | 1.85 | 2.77 |
| 43 ft | 4.98 | 3.31 | 1.60 | 2.40 |
| 45 ft | 4.55 | 3.03 | 1.40 | 2.09 |
| 47 ft | 4.17 | 2.77 | 1.22 | 1.84 |
| 49 ft | 3.84 | 2.55 | 1.08 | 1.62 |
| 51 ft | 3.54 | 2.36 | 0.96 | 1.44 |
| 53 ft | 3.28 | 2.18 | 0.85 | 1.28 |
- Factored and allowable loads are limited by bending, or by shear where marked *.
- The deflection columns are the service loads that make the beam sag span/360 or span/240. Compare span/360 with the live load alone and span/240 with dead plus live load, the usual floor limits in IBC Table 1604.3.
Reading the table
At a 33 ft span, a W21×122 carries a factored load of 8.46 kips per foot (LRFD) or an allowable load of 5.63 kips per foot (ASD). A live load of 3.54 kips per foot would deflect it span/360, which is 1.10 in. The beam works if the factored load is under the first number and the live and total loads are under the deflection numbers. For other spans, supports or loads, open it in the beam designer, or follow the steel beam design guide to check one by hand.
W21×122 questions
What does W21×122 mean?
W means a wide flange shape. 21 is its nominal depth in inches (the actual depth is 21.7 in) and 122 is its weight in pounds per foot.
How much does a W21×122 weigh?
122 pounds per foot, so a 20 ft W21×122 weighs about 2,440 pounds.
How far can a W21×122 span?
It depends on the load. Braced along its length, it carries 71.0 kips per foot of factored load at 11 ft and 3.28 kips per foot at 53 ft; the load table has the spans in between. Floor beams are often 33 to 43 ft for this depth (span-to-depth ratios of about 18 to 24), where deflection usually governs.
Is a W21×122 a compact section?
Yes. Its flanges (bf/2tf = 6.45) and web (h/tw = 31.3) are both compact for bending in 50 ksi steel.