When someone outside the trade says "I-beam," they usually mean any steel beam with two flanges and a web. Inside the trade, the term has a narrower meaning: the American Standard beam, designated with an S (S12x35, S8x18.4). It is the older of the two I-shaped families in the AISC catalog, and on most jobs it has been replaced by the wide flange (W) shape. It still shows up, though, mostly as monorail and hoist beams, in renovation work, and in misc metals packages. This page lists every S-shape in the AISC database with its weight and dimensions, then covers what changes in your takeoff when a drawing calls for S instead of W.
If you're looking for wide flange sizes, those are in our wide flange beam sizes guide.
How to Read an S-Shape Designation
Format: S[nominal depth]x[weight per foot]
- S12x35: 12 in. deep, 35 lb/ft
- S8x18.4: 8 in. deep, 18.4 lb/ft
- S24x121: 24.5 in. deep (actual), 121 lb/ft
The designation works the same way as a W shape: the first number is depth in inches, the second is weight in pounds per linear foot. One difference is that S-shape depths are much closer to nominal. Within each depth group, most S-shapes have the same actual depth and get heavier by thickening the web, so an S12x31.8 and an S12x35 are both exactly 12.0 in. deep. W shapes of the same nominal depth vary more because heavier W sections grow in both depth and flange thickness. (The 24 and 20 in. groups each split into two depths: S24x106 and S24x121 are 24.5 in. deep, S20x86 and S20x96 are 20.3 in.)
The odd decimals (18.4, 25.4, 31.8, 42.9, 54.7, 17.25) are real weights, not typos. Carry them exactly. Rounding S12x31.8 to 32 lb/ft is harmless on one beam and adds up on a monorail package with a few hundred feet of runway.
S-Shape Sizes Chart (All 28 AISC S Sections)
Weights are from the AISC shapes catalog. Dimensions (depth d, flange width bf, web thickness tw, flange thickness tf, area) are from the AISC Shapes Database v15.0. For S-shapes, tf is the average flange thickness, because the inside face of the flange is sloped (more on that below).
| Designation | Weight (lb/ft) | Depth d (in) | Flange width bf (in) | Web tw (in) | Flange tf (in) | Area (in²) |
|---|---|---|---|---|---|---|
| S24x121 | 121 | 24.5 | 8.05 | 0.8 | 1.09 | 35.5 |
| S24x106 | 106 | 24.5 | 7.87 | 0.62 | 1.09 | 31.1 |
| S24x100 | 100 | 24 | 7.25 | 0.745 | 0.87 | 29.3 |
| S24x90 | 90 | 24 | 7.13 | 0.625 | 0.87 | 26.5 |
| S24x80 | 80 | 24 | 7 | 0.5 | 0.87 | 23.5 |
| S20x96 | 96 | 20.3 | 7.2 | 0.8 | 0.92 | 28.2 |
| S20x86 | 86 | 20.3 | 7.06 | 0.66 | 0.92 | 25.3 |
| S20x75 | 75 | 20 | 6.39 | 0.635 | 0.795 | 22 |
| S20x66 | 66 | 20 | 6.26 | 0.505 | 0.795 | 19.4 |
| S18x70 | 70 | 18 | 6.25 | 0.711 | 0.691 | 20.5 |
| S18x54.7 | 54.7 | 18 | 6 | 0.461 | 0.691 | 16 |
| S15x50 | 50 | 15 | 5.64 | 0.55 | 0.622 | 14.7 |
| S15x42.9 | 42.9 | 15 | 5.5 | 0.411 | 0.622 | 12.6 |
| S12x50 | 50 | 12 | 5.48 | 0.687 | 0.659 | 14.7 |
| S12x40.8 | 40.8 | 12 | 5.25 | 0.462 | 0.659 | 11.9 |
| S12x35 | 35 | 12 | 5.08 | 0.428 | 0.544 | 10.2 |
| S12x31.8 | 31.8 | 12 | 5 | 0.35 | 0.544 | 9.31 |
| S10x35 | 35 | 10 | 4.94 | 0.594 | 0.491 | 10.3 |
| S10x25.4 | 25.4 | 10 | 4.66 | 0.311 | 0.491 | 7.45 |
| S8x23 | 23 | 8 | 4.17 | 0.441 | 0.425 | 6.76 |
| S8x18.4 | 18.4 | 8 | 4 | 0.271 | 0.425 | 5.4 |
| S6x17.25 | 17.25 | 6 | 3.57 | 0.465 | 0.359 | 5.05 |
| S6x12.5 | 12.5 | 6 | 3.33 | 0.232 | 0.359 | 3.66 |
| S5x10 | 10 | 5 | 3 | 0.214 | 0.326 | 2.93 |
| S4x9.5 | 9.5 | 4 | 2.8 | 0.326 | 0.293 | 2.79 |
| S4x7.7 | 7.7 | 4 | 2.66 | 0.193 | 0.293 | 2.26 |
| S3x7.5 | 7.5 | 3 | 2.51 | 0.349 | 0.26 | 2.2 |
| S3x5.7 | 5.7 | 3 | 2.33 | 0.17 | 0.26 | 1.66 |
A quick physics check you can run on any row: area (in²) × 3.4028 should land within about 1% of the weight per foot. S12x35 is 10.2 in² × 3.4028 = 34.7 lb/ft, and S8x23 is 6.76 × 3.4028 = 23.0 lb/ft. If a number on a drawing or a supplier quote fails that check by a wide margin, something is wrong with the designation. The same arithmetic works for any rolled section; our steel beam weight guide walks through it.
S vs W: What Actually Differs
Both are I-shaped, both are hot-rolled, and both carry the same kind of designation. For estimating, the differences that matter are these.
Flange geometry
The inside face of an S-shape flange slopes. AISC lists the slope as 16-2/3%, so the flange is thicker at the web and thinner at the tip. W-shape flanges are parallel (the inner and outer faces are flat and parallel), which is where the "wide flange" family gets its other name, parallel-flange beams.
The sloped flange is the reason S-shapes turn up in monorail work: hoist trolley wheels are often built to ride on a tapered lower flange. Always check the hoist manufacturer's requirement against what the drawings call out before you substitute anything.
Flange width
S-shapes are narrow. Compare sections of about the same weight:
| Section | Weight (lb/ft) | Depth (in) | Flange width (in) | Web tw (in) |
|---|---|---|---|---|
| S12x35 | 35 | 12.0 | 5.08 | 0.428 |
| W12x35 | 35 | 12.5 | 6.56 | 0.300 |
| S10x25.4 | 25.4 | 10.0 | 4.66 | 0.311 |
| W10x26 | 26 | 10.3 | 5.77 | 0.260 |
| S8x18.4 | 18.4 | 8.0 | 4.00 | 0.271 |
| W8x18 | 18 | 8.14 | 5.25 | 0.230 |
| S24x100 | 100 | 24.0 | 7.25 | 0.745 |
| W24x104 | 104 | 24.1 | 12.8 | 0.500 |
(All from the AISC Shapes Database v15.0.) At the same weight, the S-shape puts more steel in the web and less in the flanges. That makes it less efficient in bending than the W, which is the main reason engineers moved to wide flanges. For you, it means an S and a W with similar weights are not interchangeable, and you should never swap one for the other on a bid without the engineer signing off. You can look up the W sizes in the table on the W12x35 page and the W8x18 page.
Bolting through a sloped flange
A bolt through a sloped flange doesn't seat flat. The RCSC bolting specification requires a beveled washer when the bearing surface slopes more than 1:20 from a plane normal to the bolt axis, and the S-shape flange slope is well past that. If you're pricing bolted connections through S-shape flanges (monorail hangers, beam clamps, splice plates), add beveled washers to the hardware list. They are a small cost but they are easy to leave off.
Material and availability
W shapes are typically specified as ASTM A992. For S-shapes, the AISC Manual lists A36 as the preferred material, with A572 Grade 50 also common, so read the general notes and the material spec rather than assuming A992 carries over. Our steel grades guide covers the common grades.
S-shapes are rolled by fewer mills and in fewer sizes than W shapes, and not every service center stocks the full range. The light sizes (S3 through S8) and the common monorail sizes are easier to source than the heavy S20 and S24 sections. On a job with heavy S-shapes, get a price and a lead time from your supplier before you finalize the bid instead of pricing from a per-ton rate.
Where You'll See S-Shapes on Drawings
- Monorail and hoist beams. The most common use today. Look for callouts like "S15x42.9 MONORAIL" or "S12x31.8 HOIST BEAM" on mechanical, equipment, or crane drawings as well as the structural set.
- Renovation and retrofit. Older buildings were framed with S-shapes (and the older "I" designations such as 12 I 31.8). Matching existing framing, or reinforcing it, often means buying S-shapes.
- Misc metals. Lintels, small frames, equipment supports, and trench or pit framing sometimes call for light S-shapes.
- Old drawings with "I" labels. An older set may show "12I35" or "12 I 35". Treat that as an S12x35 for estimating purposes and confirm with the engineer if the drawing is being used for new work. Our section shorthand guide covers this and other legacy notations.
A monorail runway is often shown on a different sheet from the main framing, sometimes on equipment or process drawings. It's one of the items that slips through a takeoff because the estimator only works the S-sheets. A steel estimating checklist that includes "hoist and monorail beams" as its own line helps.
Estimating Notes for S-Shapes
Piece weight is the same math as any beam. Weight per foot × length in feet. An S15x42.9 monorail beam at 32 ft is 42.9 × 32 = 1,372.8 lb, about 0.69 tons.
Price the accessories with the beam. Monorail runways come with hangers, sway bracing, end stops, and sometimes splice plates. Those items are usually detailed on the same sheet. Pick them up with the beam so they don't end up in a "misc" allowance.
Check the spec for runway tolerances. Monorail and crane runway beams often have tighter straightness and alignment requirements than floor framing. That can mean extra shop time or field adjustment. Price it if the spec calls for it.
Watch the substitution note. Some drawings say "S or W, contractor's option" for hoist beams. Others require S because of the trolley. Read the note before you price the cheaper option.
Estimating only, not design. The dimensions on this page are for takeoffs and budgets. Section selection, capacity, and trolley compatibility are the engineer's call.
How to Build a Takeoff That Includes S-Shapes
Tag S-shapes as their own group in your takeoff, even if there are only a few of them. They usually need a separate material quote, a different grade, and sometimes a different supplier than the W shapes, so a combined "beams" line hides information your purchasing person will need later. If you're building a bill of materials by hand, our BOM guide shows a layout that keeps shape families separate.
Steelflo reads S-shape callouts, including the decimal weights, off your PDF drawings and puts each one in the takeoff for you to review, alongside the W, HSS, and angle members on the same sheets. See how it works.
Frequently Asked Questions
Is an I-beam the same as a W beam?
Not strictly. In trade usage, "I-beam" means the American Standard beam, designated S, which has narrow flanges with a sloped inner face. A W beam (wide flange) has wider, parallel flanges. People often use "I-beam" loosely for both, so check the designation on the drawing.
What does S12x35 mean?
S12x35 is an American Standard beam that is 12 in. deep and weighs 35 lb per foot. Per the AISC Shapes Database, its flange is 5.08 in. wide and its web is 0.428 in. thick. A 20 ft piece weighs 700 lb.
Why are S-shapes used for monorails?
The sloped inner flange of an S-shape suits many hoist trolley wheel designs, and S-shapes have been used for monorail beams for a long time. Some trolleys are made for parallel-flange W beams instead. The drawings and the hoist manufacturer's data decide which one to price.
Can I substitute a W beam for an S beam of the same weight?
Not on your own. A W and an S of similar weight have different depths, flange widths, and section properties, and a trolley that fits one may not fit the other. Any substitution needs the engineer's approval.
Do S-shapes need special bolting hardware?
Often, yes. Because the S-shape flange slopes more than 1:20, the RCSC specification requires beveled washers under the bolt head or nut when bolting through the flange. Add them to the hardware list for any bolted connection through an S-shape flange.