Steel Castellated Beam Design: Web Holes, Reinforcement & Deflection

Blue-gray industrial tone—the interior of a plant, a large H-section castellated beam running across the frame with a repeating row of hexagonal web openings, HVAC ducts passing through the holes, a column end-plate connection visible at the beam end, cool industrial lighting, depth of field from a close-up hole to the distant column grid, no text.

A standard I-beam is a solid web. A steel castellated beam design takes that web, cuts it into a repeating zigzag, stretches the beam taller, and re-welds it—opening holes that let ducts and pipes pass through. But every hole is a weak point: it creates Vierendeel bending in the tees above and below the opening, and it changes how shear travels through the section. A steel castellated beam design is about three things: choosing the right hole shape and spacing, reinforcing the opening edges, and controlling the extra deflection that holes introduce.

This guide digs into the web opening itself—how holes change the beam's strength, stiffness, and reinforcement needs. A floor-system article—our steel building floor system piece—covers slab-on-deck selection. A steel castellated beam design article goes deeper: it treats each hole as a localized stress problem, not just a passage for ductwork.

What Is a Castellated Beam & Why Use It?

Start with the manufacturing trick. A standard H-section is cut along a zigzag line through the web, the two halves are offset by one pitch, and they are re-welded along the serrated edges. The result is a beam about 50–60% taller than the parent section, with a repeating row of hexagonal openings—and no additional steel weight. Because bending strength grows with depth cubed, that extra depth buys stiffness and strength almost for free, while the holes become service corridors for ducts, pipes, and cable trays.

Two related products exist. The castellated beam has the classic hexagonal holes produced by the saw-and-weld process described above. A cellular beam is fabricated separately, usually by cutting or drilling circular holes through a standard or fabricated section; it is more expensive but has lower stress concentration and a cleaner architectural look. Elongated (slotted) openings are used where large ductwork must pass through.

Typical spans are 6–12 m (20–40 ft) with depth-to-span ratios of about L/15 to L/20. The holes are sized to roughly 50–70% of the beam depth—large enough to pass ductwork, small enough to keep the post between holes strong. For composite beam parallels, see steel composite beam design; for thin-walled member buckling principles, read steel thin walled member design.

Table 1: Castellated Beam Hole Geometry Comparison

Hole Shape	Depth Increase vs. Parent	Stress Concentration	Relative Cost	Best For
Hexagonal (castellated)	+50–60%	Moderate, even distribution	Low (one cut)	General commercial floors
Circular (cellular)	+30–50%	Lowest, smooth edges	Medium–high	Architecturally exposed work
Elongated / slotted	+40–50%	Local at slot ends	Medium	Large ductwork crossings
Rectangular (rare)	+40–60%	High at corners	Medium–high	Avoid unless necessary

Hexagonal is the default; circular holes are chosen when appearance or fatigue matters.

Where circular drilled openings are preferred over hexagonal cuts—lower stress concentration, cleaner architectural edge, same parent depth—those holes define a steel cellular beam rather than a castellated one; our dedicated guide sizes the circular openings at 0.5–0.7 beam depth, checks the Vierendeel bending in the top and bottom tees around each hole, and specifies collar reinforcement where large openings meet high-shear support zones.

Vierendeel Bending & Stress Distribution

The structural behavior of a steel castellated beam design changes fundamentally once the web is perforated. In a solid-web beam, shear travels diagonally through the web plate. In a castellated beam, that diagonal path is broken at every opening; shear is instead carried by the top and bottom tees (the T-shaped pieces above and below each hole), which act like small horizontal beams spanning from post to post.

That local bending in the tees is called Vierendeel bending. At each hole, the top tee and bottom tee see a local moment that bends them like little beams, and the corners of the opening (the hexagon peaks, or the circle-to-web transition) become stress concentration points with stress concentration factors around 2–3. The short post between adjacent holes carries a large share of the shear and can fail in shear if overloaded.

Three detailing rules follow. First, the end opening must be at least 0.5 beam depth from the support so support shear does not stack onto the first Vierendeel moment. Second, holes should be centered on the beam centerline—offset holes bias stress to one side and raise the peak stress. Third, post width between holes must be checked for shear; too-narrow posts are the most common failure point. Per AISC 360 Specification for Structural Steel Buildings, these checks are part of the opening design. For second-order deflection and moment magnification, see steel structure second order analysis; for post buckling and local stability, read steel member local stability.

Table 2: Castellated Beam Critical Stress Locations

Location	Stress Type	Magnitude Range	Reinforcement Needed	Notes
Hole corners (hex peaks)	Local bending + SCF	~2–3× average	Corner stiffeners if >50% depth	Most critical
Post between holes	Shear + bending	1.2–1.8× web shear	Post stiffeners if overloaded	Check shear rupture
Top / bottom tees at mid-span	Vierendeel moment	0.5–1.0× plastic moment	Edge plates if overstressed	Local tee buckling
End opening near support	Combined shear + moment	1.5–2.5× peak	Move opening ≥ 0.5d from support	Detail rule
Concentrated load point	Local web crippling	Localized	Transverse stiffener	Crane / equipment loads

Stress ranges are indicative; actual values depend on hole size, spacing, and loading.

Web Opening Reinforcement & Stiffeners

Reinforcement in a steel castellated beam design is not always needed. The rule of thumb, consistent with AISC 360 Chapter G, is that openings up to about 50% of the beam depth, centered and away from supports, usually carry the full design load without edge stiffeners. The castellated beam's own tees and posts are sized during fabrication to handle the Vierendeel moments at those standard proportions.

Once an opening exceeds 50% of the beam depth—or sits near a concentrated load such as a crane wheel, a column reaction, or equipment point load—edge reinforcement is required. Two patterns dominate:

• Doubler plates welded to the opening edges, matching or exceeding the web thickness, sized to carry the Vierendeel bending that the removed web would have handled.
• Ring stiffeners around circular cellular openings, used where the hole is large and the surrounding web is thin.

Hexagonal openings get corner stiffeners at the peak points rather than full rings. Transverse stiffeners are added at the posts under concentrated loads to prevent web crippling. Weld detailing matters: stiffener welds must avoid the heat-affected zone of the original castellated cut, which can crack under fatigue. For connection design principles, see steel structure connection design; for weld control, read steel welding distortion control; for bolted alternatives where stiffener welding is impractical, see steel high strength bolt connection deep dive.

Running Ducts Through Beams Without Cutting Strength?

We size castellated beam holes to stay within AISC limits, detail edge reinforcement where openings exceed 50% of depth, and check Vierendeel bending at every hole. Tell us your span and clear height target.

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Deflection & Vibration Control

A perforated web is a weaker web. The effective bending stiffness of a castellated beam is roughly 60–80% of a solid-web beam of the same depth—the holes remove material that would otherwise carry bending and shear. That stiffness loss shows up directly in deflection.

Standard deflection limits apply: L/360 for roof beams, L/240 for floor beams, and L/300 where a ceiling is supported. Because Vierendeel bending adds local deformation at each hole, total deflection is slightly larger than a simple effective-I estimate suggests. Designers typically apply a further 10–15% deflection penalty to account for the localized tee deformation.

Floor vibration is the second serviceability issue. Castellated beams are lighter than solid-web beams of the same depth, so their natural frequency drops. For commercial floors occupied by people, that can push the beam into a vibration-sensitive range that requires a stiffer section or a composite slab. Per ASCE 7 Minimum Design Loads, load combinations include these serviceability checks. For deflection control principles, see steel structure deflection control; for human-induced floor vibration, read steel floor vibration serviceability.

Composite Action & Practical Applications

The most economical steel castellated beam design pairs the beam with a composite concrete slab on metal deck. Shear studs welded through the deck into the top tee make the slab and beam act together, raising effective stiffness by roughly 15–25% and reducing the deflection penalty of the holes. Studs are concentrated at the posts between holes rather than centered under openings, because the post carries the vertical shear transfer best.

Typical applications favor long, straight spans with regular ductwork runs:

• Office buildings: castellated beams let HVAC ducts pass through, dropping floor-to-floor height by 200–300 mm (8–12 in) compared with beams under hung ducts.
• Factories and warehouses: cable trays and air ducts run through the openings, reducing the depth of the ceiling zone.
• Parking structures: longer spans reduce column density and improve parking efficiency.

Castellated beams are not a good choice for crane beams or heavily fatigued members, where repeated loading would drive Vierendeel cracking at the hole corners. For multi-story steel frames that use castellated beams on every floor, see multi story steel building; for composite action details, read steel composite beam design.

Table 3: Castellated Beam Typical Span-Depth Proportions

Span (m / ft)	Typical Depth (mm / in)	Depth / Span Ratio	Hole Size Range	Notes
6 / 20	450 / 18	L/13	230–280 mm / 9–11 in	Light commercial
9 / 30	600 / 24	L/15	300–400 mm / 12–16 in	Office floor
12 / 40	750 / 30	L/16	400–500 mm / 16–20 in	Commercial / factory
15 / 50	900 / 36	L/17	450–600 mm / 18–24 in	Check posts carefully
>15 / 50	—	—	—	Use plate girder or truss

Beyond 12–15 m, Vierendeel moments in posts become large enough that a welded plate girder is usually more economical.

Conclusion

A steel castellated beam design trades solid web for taller depth and open holes. Vierendeel bending at every tee drives corner stresses, openings over 50% of depth need edge reinforcement, and deflection must be checked against the reduced effective stiffness. Hole position, post width, and stiffener layout must be locked during detailed design—you cannot torch an extra hole through an existing castellated beam. Tell our engineers your span and clear-height target, and we will come back with hole geometry and reinforcement details.

Web Holes That Let Ducts Through Without Weakening the Beam.

We size castellated openings to stay within AISC limits, detail edge reinforcement where holes exceed half the depth, and check Vierendeel bending at every tee. Tell us your span and clear height target.

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About the Author

Senior Structural Engineer

With over 20 years of hands-on experience in steel structure design and prefabricated building engineering, our in-house senior structural engineer has personally contributed to more than 500 steel building projects—including warehouses, industrial factories, aircraft hangars, agricultural buildings, and commercial structures. The focus is on translating design codes such as AISC 360, ASCE 7, and Eurocode 3 into buildable, cost-effective steel solutions that balance structural performance, fabrication efficiency, and total project cost.

Learn more about our engineering team

Case Example

A Western European city-center developer needed a two-story steel-framed office of 7,200 m² (77,500 sq ft) on 9 m (30 ft) spans. The architectural brief capped floor-to-floor height at 3.6 m (11.8 ft), which left no room for ducts hung beneath the beams. The design team specified hexagonal castellated beams, 600 mm (24 in) deep, cut from a 400 mm parent section and re-welded 50% taller.

Web openings ran at 50% of beam depth—within the no-stiffener band per AISC 360—with the end opening set 0.6 m (2 ft) clear of the column face. Composite action was achieved by welding shear studs through the metal deck into the top tee at each post, raising effective stiffness by about 20% versus bare steel. The holes let 400 mm (16 in) HVAC ducts pass through, dropping the ceiling zone by 250 mm (10 in) and meeting the floor-to-floor target. As-built mid-span deflection measured L/380 against an L/360 limit, and steel tonnage ran about 12% below a comparable solid-web composite design. The composite action logic is covered in steel composite beam design deep dive, and the floor vibration serviceability check is detailed in steel floor vibration serviceability.

Frequently Asked Questions

Q1: What is the difference between a castellated beam and a cellular beam?

A castellated beam has hexagonal holes cut by sawing a standard I-beam in a zigzag pattern and re-welding it taller. A cellular beam has circular holes cut or drilled after fabrication. The hexagonal shape is cheaper to produce; circular holes have lower stress concentration and look cleaner architecturally.

Q2: When does a web opening need reinforcement?

Per AISC 360, openings up to about 50% of the beam depth usually need no reinforcement if they are centered and away from supports. Larger openings, or openings near concentrated loads, require edge stiffener plates or ring reinforcement sized to carry the Vierendeel bending that the removed web would have handled.

Q3: How much stiffer is a composite castellated beam?

With a concrete slab acting compositely through shear studs, the effective stiffness increases by roughly 15–25% compared to a bare steel castellated beam. This helps control deflection and floor vibration—important because the web holes already reduce stiffness to about 60–80% of a solid-web beam of the same depth.

Q4: What span can a castellated beam cover?

Typical castellated beams span 6–12 m (20–40 ft) with depth-to-span ratios of about L/15 to L/20. Beyond 12 m (40 ft), the Vierendeel bending in the hole posts becomes large enough that a deeper welded plate girder or truss is usually more economical.

Q5: Can I cut extra holes through an existing castellated beam?

No—not without re-analysis. Each hole in a castellated beam is part of a designed pattern: the tees, posts, and Vierendeel moments are sized as a system. An extra torch cut removes more web, changes the local post shear, and introduces a new stress concentration at the cut edge. Any additional opening requires a fresh AISC 360 check and, almost always, edge reinforcement.

Reference Links

• AISC 360 — Specification for Structural Steel Buildings — design provisions for web openings, local buckling, and member strength.
• ASCE 7 — Minimum Design Loads and Associated Criteria for Buildings — load combinations and deflection / vibration serviceability limits.