A castellated beam is made by cutting the web of a rolled beam along a zigzag line, sliding the two halves apart by half a cut, and welding them back together. The result is a deeper beam with a row of hexagonal holes through the web, made from the same steel you started with.
Depth is what makes a beam stiff, so adding depth without adding weight is the whole point. The holes are not a compromise; on most projects they are the second reason to use one.
How One Is Made
Cut the web with circular openings instead of hexagons and the same idea gives you a cellular beam. The Steel Construction Institute’s reference describes cellular beams as formed by splitting two rolled sections longitudinally into Tee sections, then welding them together to form an I-section with web openings, which lets services pass through the beam so that structural and service zones share the same depth (checked September 2026).
Castellated vs Cellular
| Castellated | Cellular | |
|---|---|---|
| Opening shape | Hexagonal | Circular |
| Cut | One zigzag pass | Two curved passes, some web discarded |
| Opening size | Set by the cut geometry | Chosen; commonly 0.6 to 0.8 times final depth |
| Round ducts | Need a rectangle inside a hexagon | Fit the hole directly |
| Appearance | Angular | Regular circles, often left exposed |
On opening geometry, SCI gives the numbers for cellular beams: the opening diameter is typically 0.6 to 0.8 times the final beam depth, with center-to-center spacing about 1.5 times the opening diameter, and notes they are the most common way of designing long spans of roughly 13 to 18 m (checked September 2026).
In everyday US usage “castellated” is often applied loosely to both. On a drawing, look at the openings.
Why the Depth Matters So Much
Bending stiffness scales with the square of the depth for a given amount of flange material, which is why deepening a section is so much more effective than thickening it. You can see the effect in the parent sections themselves, using AISC values for four common beams:
| Shape | Weight (lb/ft) | Depth (in) | Ix (in⁴) | Sx (in³) |
|---|---|---|---|---|
| W16x26 | 26 | 15.7 | 301 | 38.4 |
| W18x35 | 35 | 17.7 | 510 | 57.6 |
| W21x44 | 44 | 20.7 | 843 | 81.6 |
| W24x55 | 55 | 23.6 | 1,350 | 114 |
From W16x26 to W24x55 the weight roughly doubles while Ix goes up more than four times. Castellating exploits the same relationship without buying the extra steel: you pay for the cutting and welding instead of for weight. Full properties for every rolled section are in our wide flange beam sizes chart, and our guide on how to read steel beam sizes explains what the designations mean.
One caution: the properties of a castellated beam are not the parent beam’s properties scaled up. The web has holes in it, so the section changes along the length and the usual formulas for a solid-web beam do not apply.
What Governs the Design
Shear at the web posts
The narrow pieces of web between openings carry the shear. Near supports, where shear is highest, this usually controls, which is why openings are often filled in at the ends.
Vierendeel bending
Across an opening the load has to travel through the tee sections above and below it, which bend locally. This is a check that simply does not exist for a solid beam and is a common failure mode if openings are too large or too close together.
Weld at the post
Each web post contains a welded joint working in a high-shear region. Weld quality is structural here, not cosmetic; our weld symbol guide covers how those welds are specified on drawings.
Lateral stability
A deeper, lighter beam is more slender, so bracing of the compression flange matters more, not less. Deck or joists framing in often provide it, which ties back to how the floor is built; see our steel decking guide.
When They Are Worth It
Castellated and cellular beams earn their keep on long, fairly lightly loaded spans where floor-to-floor height is under pressure: schools, offices, sports halls, warehouses with long clear spans. If ducts have to cross the beams anyway, running them through the web instead of underneath can take a real amount off the overall floor depth, and over several stories that adds up to building height.
They are a poor choice for short heavily loaded spans, where shear rules and the openings are in the way, and for anything with large point loads landing near an opening. They also cost more per pound than a plain rolled beam, so the saving has to come from somewhere else: less steel, shallower floor, fewer transfer details. Compare against a plain rolled section and against a built-up girder before committing; our guide to types of steel beams sets out the options.
Castellated Beam FAQ
Does cutting holes in the web weaken the beam?
It reduces shear capacity and introduces local bending around the openings, but the added depth increases bending capacity by more. That is the trade: better in bending, worse in shear, which is why shear usually governs near the supports.
How much deeper does castellating make a beam?
It depends entirely on the cut geometry, which the fabricator sets. The final depth is a design output, not a fixed multiplier, so take it from the fabricator’s drawings rather than a rule of thumb.
Can I cut extra holes in a castellated beam on site?
No. The web posts and the tees around each opening are sized for the forces at that position. A new hole, or an enlarged one, changes a member that has already been checked opening by opening. Any change goes back to the engineer of record.
Are the end openings usually filled?
Often, yes. Shear peaks at the supports, so the first opening or two is commonly filled with a plate to restore a solid web where it is needed most.
Is a cellular beam better than a castellated one?
Not better, different. Circular openings suit round ducts and give the designer free choice of hole size and spacing; hexagonal openings come from a simpler single cut. Availability and the fabricator’s equipment often decide it.
The Short Version
Castellated beams get depth for free by cutting a rolled beam’s web on a zigzag, offsetting and rewelding it; cellular beams do the same with round holes. You gain bending stiffness and a route for services, and you take on shear at the web posts, Vierendeel bending around the openings and a weld in a high-shear zone. They suit long, lightly loaded spans where floor depth is tight, and they are designed opening by opening by a licensed structural engineer.
Reference information only, not engineering advice. Disclaimer.