Castellated & Cellular Beam Design — Geometry, Limit States & Sizing

Castellated beams are fabricated by cutting a rolled W-shape along a zigzag pattern, then re-welding the two halves offset to create a deeper section with hexagonal web openings. Cellular beams use a similar process but produce circular openings. Both types increase the beam depth by roughly 50% without adding steel weight, dramatically improving flexural stiffness for long-span floor and roof applications.

How castellated beams are made

A standard W-shape is flame-cut along the web in a zigzag (castellated) or sinusoidal (cellular) pattern. The two halves are separated and re-welded with the peaks aligned, producing a beam approximately 1.5d deep (where d is the original section depth). For example, a W18x35 becomes a CB27x35 — same weight, 50% deeper, with significantly higher moment of inertia.

The key geometric parameters per AISC Design Guide 31:

Parameter	Castellated (hexagonal)	Cellular (circular)
Opening height (ho)	typically 0.5 × dt to 0.75 × dt	diameter Do = 0.6 × dt to 0.8 × dt
Web post width (e)	≥ ho/3 for strength	≥ Do/4
Spacing (S)	e + opening width	center-to-center = Do + e
Overall depth (dt)	~1.5 × d_original	~1.5 × d_original

Limit states unique to castellated beams

Standard beam checks (flexure, shear, deflection) still apply, but castellated beams introduce additional failure modes:

1. Vierendeel bending — At each opening, the beam acts like a Vierendeel truss. The tee sections above and below the opening resist the shear force through local bending. The Vierendeel moment in each tee is approximately:

Mv = Vglobal × S / 4

Where Vglobal is the global shear at the opening and S is the opening spacing. Each tee must resist Mv with its own plastic moment capacity (Mp,tee = Fy × Zx,tee). This check typically governs near supports where shear is highest.

2. Web post buckling — The narrow web post between adjacent openings can buckle horizontally under the horizontal shear transferred between tee sections. Web post buckling is checked using the method in AISC Design Guide 31 Section 5.3, which treats the post as an effective strut.

3. Web post flexure (horizontal bending) — The web post bends about its vertical axis due to the difference in axial forces between adjacent tee sections. The interaction of this bending with the vertical shear determines whether the post is adequate.

Worked example — Vierendeel check at opening

Given: CB27x35 (from W18x35), Fy = 50 ksi. Opening height ho = 14 in, spacing S = 18 in, web post e = 4 in. Global shear at the opening Vu = 30 kips.

Step 1 — Vierendeel moment in each tee: Mv = 30 × 18 / 4 = 135 kip-in per tee (top and bottom tees share the shear equally, but each resists the full Vierendeel moment at its root).

Step 2 — Tee section properties: Top tee: flange = 6.00 × 0.425 in, stem depth = (27 − 14)/2 = 6.5 in, tw = 0.300 in. Approximate Zx,tee = 0.425 × 6.00 × (6.5 − 0.425/2) + 0.300 × 6.075² / 4 ≈ 16.0 + 2.77 = 18.8 in³ (simplified).

Step 3 — Tee moment capacity: phi × Mp,tee = 0.90 × 50 × 18.8 / 2 = 423 kip-in >> 135 kip-in — OK

(The factor of 2 approximation accounts for the tee's asymmetric shape reducing the effective plastic modulus; in practice, use the exact Zx from section property tables.)

Span-to-depth ratios

Castellated beams excel at long spans. Typical span-to-depth ratios:

Application	Span/depth (composite)	Span/depth (non-composite)
Office floors	20–24	16–20
Parking garages	18–22	14–18
Roof beams	22–28	18–24

A cellular beam spanning 45 ft with dt = 27 in gives span/depth = 20 — efficient for a composite office floor.

Code comparison

AISC Design Guide 31 (USA): The primary US reference for castellated and cellular beam design. Provides detailed procedures for Vierendeel bending, web post buckling, and horizontal shear. Uses LRFD with phi = 0.90 for flexure and phi = 0.90 for the web post checks. DG31 superseded the earlier DG2 methods for these sections.

SCI P355 (UK/Europe): The Steel Construction Institute publication P355 covers design of composite beams with large web openings per EN 1993. It uses the Vierendeel mechanism approach similar to AISC but with Eurocode partial safety factors (gamma_M0 = 1.00, gamma_M1 = 1.00). P355 also provides design charts for standard Westok cellular beam profiles.

AS 4100-2020 (Australia): No specific castellated beam provisions exist in AS 4100. Australian practice follows SCI P355 or first-principles Vierendeel analysis using AS 4100 capacity reduction factors (phi = 0.90 for flexure, phi = 0.90 for shear). The OneSteel Design Capacity Tables provide guidance for specific proprietary sections.

Common mistakes engineers make

Placing openings in high-shear zones near supports. Vierendeel bending is proportional to global shear. Openings in the end quarter of the span, where shear is highest, frequently fail the Vierendeel check. Infill the first one or two openings near each support.
Ignoring web post buckling for closely-spaced openings. Reducing the web post width (e) to fit more openings increases the risk of web post buckling. Minimum e ≥ ho/3 (castellated) or Do/4 (cellular) is a practical floor.
Using non-composite section properties for deflection when the slab is composite. Castellated and cellular beams are almost always designed as composite with the concrete slab. Using non-composite stiffness overestimates deflection by 40–60%, leading to unnecessarily heavy sections.
Forgetting to check the net section at openings for axial load. When castellated beams serve as chord members in a truss or carry significant axial force from diaphragm action, the reduced net section at each opening must be checked for the combined axial and bending demand.

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Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.