W18×311 Steel Beam
A W18×311 is a wide flange steel beam 22.3 inches deep with 12-inch flanges, weighing 311 pounds per foot. In ASTM A992 steel (Fy = 50 ksi) it has a design bending strength of 2,828 kip-ft (LRFD) when its compression flange is braced, and a design shear strength of 1,017 kips.
Design a W18×311 beam Opens a 33 ft span in the free beam designer.
W18×311 dimensions
| Dimension | Value |
|---|---|
| Depth, d | 22.3 in |
| Flange width, bf | 12.0 in |
| Flange thickness, tf | 2.74 in |
| Web thickness, tw | 1.52 in |
| Distance to web toe of fillet, kdes | 3.24 in |
| Distance between flange centroids, ho | 19.56 in |
| Cross-sectional area, A | 91.6 in2 |
| Weight | 311 lb/ft |
Dimensions are AISC design values. Detailing dimensions are rounded to the nearest 1/16 in.
W18×311 section properties
| Property | Strong axis (x) | Weak axis (y) |
|---|---|---|
| Moment of inertia, I | 6970 in4 | 795 in4 |
| Elastic section modulus, S | 624 in3 | 132 in3 |
| Plastic section modulus, Z | 754 in3 | 207 in3 |
| Radius of gyration, r | 8.72 in | 2.95 in |
| Torsion and slenderness | Value |
|---|---|
| Torsional constant, J | 176 in4 |
| Warping constant, Cw | 76200 in6 |
| Effective radius of gyration, rts | 3.53 in |
| Flange slenderness, bf/2tf | 2.19 |
| Web slenderness, h/tw | 10.4 |
W18×311 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 = 2,828 kip-ft | Mn/Ω = 1,881 kip-ft |
| Shear | φVn = 1,017 kips | Vn/Ω = 678 kips |
If the compression flange isn't braced, the beam can buckle sideways before it yields. Up to Lp = 10.4 ft between braces it keeps its full strength; past Lr = 81.2 ft it buckles elastically. With uniform moment between braces (Cb = 1.0):
| Unbraced length, Lb | φMn (LRFD) | Mn/Ω (ASD) |
|---|---|---|
| Braced | 2,828 kip-ft | 1,881 kip-ft |
| 10 ft | 2,828 kip-ft | 1,881 kip-ft |
| 10.4 ft (Lp) | 2,827 kip-ft | 1,881 kip-ft |
| 20 ft | 2,667 kip-ft | 1,774 kip-ft |
| 30 ft | 2,499 kip-ft | 1,662 kip-ft |
| 40 ft | 2,331 kip-ft | 1,551 kip-ft |
| 50 ft | 2,163 kip-ft | 1,439 kip-ft |
| 60 ft | 1,995 kip-ft | 1,327 kip-ft |
| 70 ft | 1,827 kip-ft | 1,215 kip-ft |
| 80 ft | 1,659 kip-ft | 1,104 kip-ft |
| 81.2 ft (Lr) | 1,637 kip-ft | 1,089 kip-ft |
| 90 ft | 1,476 kip-ft | 982 kip-ft |
| 100 ft | 1,327 kip-ft | 883 kip-ft |
| 110 ft | 1,206 kip-ft | 802 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.
W18×311 load table by span
The largest uniform load a simply supported W18×311 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 0.311 kips per foot.
| Span | Factored load (LRFD) | Allowable load (ASD) | Load at span/360 | Load at span/240 |
|---|---|---|---|---|
| 11 ft | 184.9* | 123.3* | 225.0 | 337.5 |
| 13 ft | 133.8 | 89.1 | 136.3 | 204.5 |
| 15 ft | 100.5 | 66.9 | 88.7 | 133.1 |
| 17 ft | 78.3 | 52.1 | 61.0 | 91.4 |
| 19 ft | 62.7 | 41.7 | 43.7 | 65.5 |
| 21 ft | 51.3 | 34.1 | 32.3 | 48.5 |
| 23 ft | 42.8 | 28.4 | 24.6 | 36.9 |
| 25 ft | 36.2 | 24.1 | 19.2 | 28.7 |
| 27 ft | 31.0 | 20.6 | 15.2 | 22.8 |
| 29 ft | 26.9 | 17.9 | 12.3 | 18.4 |
| 31 ft | 23.5 | 15.7 | 10.1 | 15.1 |
| 33 ft | 20.8 | 13.8 | 8.33 | 12.5 |
| 35 ft | 18.5 | 12.3 | 6.98 | 10.5 |
| 37 ft | 16.5 | 11.0 | 5.91 | 8.87 |
| 39 ft | 14.9 | 9.89 | 5.05 | 7.57 |
| 41 ft | 13.5 | 8.95 | 4.34 | 6.52 |
| 43 ft | 12.2 | 8.14 | 3.77 | 5.65 |
| 45 ft | 11.2 | 7.43 | 3.29 | 4.93 |
| 47 ft | 10.2 | 6.81 | 2.88 | 4.33 |
| 49 ft | 9.42 | 6.27 | 2.55 | 3.82 |
| 51 ft | 8.70 | 5.79 | 2.26 | 3.39 |
| 53 ft | 8.05 | 5.36 | 2.01 | 3.02 |
| 55 ft | 7.48 | 4.98 | 1.80 | 2.70 |
- 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 W18×311 carries a factored load of 20.8 kips per foot (LRFD) or an allowable load of 13.8 kips per foot (ASD). A live load of 8.33 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.
W18×311 questions
What does W18×311 mean?
W means a wide flange shape. 18 is its nominal depth in inches (the actual depth is 22.3 in) and 311 is its weight in pounds per foot.
How much does a W18×311 weigh?
311 pounds per foot, so a 20 ft W18×311 weighs about 6,220 pounds.
How far can a W18×311 span?
It depends on the load. Braced along its length, it carries 184.9 kips per foot of factored load at 11 ft and 7.48 kips per foot at 55 ft; the load table has the spans in between. Floor beams are often 33 to 45 ft for this depth (span-to-depth ratios of about 18 to 24), where deflection usually governs.
Is a W18×311 a compact section?
Yes. Its flanges (bf/2tf = 2.19) and web (h/tw = 10.4) are both compact for bending in 50 ksi steel.