Castellated and cellular beams are fabricated from rolled wide-flange or I-beam sections by cutting, shifting, and re-welding the web to produce an expanded depth with periodic openings. They appear in long-span floor systems, parking structures, and roof framing where the structural depth is needed but beam weight must be kept low. The openings also allow mechanical, electrical, and plumbing runs to pass through without floor-to-floor height penalties.
From a structural welding standpoint, these beams introduce weld joints that don't appear on a standard rolled-section connection: web splice groove welds at every apex of a castellated hexagonal pattern, or ring plate fillet welds surrounding circular openings on cellular beams. Knowing which WPS applies, what the inspector verifies, and where AWS D1.1 sets the acceptance floor is essential before work begins.
How These Beams Are Made
Castellated beams are cut along the web with a hexagonal or straight-stepped profile. The cut halves are shifted half a pitch and re-welded at each apex. The result is a beam with a net depth roughly 50% greater than the parent section, with hexagonal openings at regular intervals.
Cellular beams achieve a similar expanded depth by cutting circular arcs from the web of two T-sections, then welding those T-sections back together at the web with the circular openings aligned. Reinforcing ring plates (also called stiffener rings) are sometimes welded around the openings when the opening diameter exceeds the code limit for unreinforced holes, or when point loads or concentrated forces are applied near the openings.
The fabrication welds in both beam types are made in the shop by the beam manufacturer, not at the job site. Still, the structural fabricator who installs connections at the beam ends, and the CWI who inspects the finished beams at receiving or in the field, need to understand what makes those welds acceptable.
The Castellated Beam Apex Weld: WPS Requirements
The apex groove weld is the most structurally critical weld in a castellated beam. It reconnects the two halves of the web at the top and bottom of each hexagonal opening. Under service loads, these welds carry shear and, depending on the loading pattern, bending stresses that are higher than in the web of a comparable unmodified beam.
Joint geometry: The apex weld is typically a butt joint in the web — a square-groove or a V-groove depending on web thickness. For webs thicker than about 3/8 in, a V-groove with a root opening is common. The joint must conform to a prequalified detail in AWS D1.1 Annex B, or it must be proven by PQR.
WPS selection: A prequalified CJP groove WPS under Clause 5 is the most common approach when the joint geometry, process, and material fall within prequalified limits. SMAW with E7018 or FCAW-G with an E71T-1C electrode are the workhorse choices. If the web is ASTM A992 (typical for wide-flange parent stock), it falls into AWS D1.1 base metal grouping with A572 Grade 50, and prequalified preheat requirements apply from Table 3.2.
Backing: Many apex welds use a steel backing bar inside the joint. The backing bar must be the same or a compatible steel, and after welding, removal or leaving it in place follows the contract requirement. Left-in-place backing bars that are parallel to the primary stress direction (as they typically are in a web splice) are generally acceptable under AWS D1.1 unless the contract specifically requires removal.
Preheat: Even though web plates in castellated beams are often 5/16 in to 1/2 in, the carbon equivalent of the parent A992 steel drives preheat. For web thicknesses in the prequalified range and standard A992 composition, minimum preheat of 50°F is common for ambient conditions. For cold-weather work, verify preheat per AWS D1.1 cold-weather welding requirements.
Essential variables: A change in process, electrode classification, or significant heat input change relative to what was documented on the WPS is an essential variable requiring requalification. See AWS D1.1 Table 6.6 essential variables explained for the full scope.
The Cellular Beam Ring Plate Weld: WPS Requirements
When cellular beam openings require reinforcing ring plates, the weld joining the ring plate to the web is a fillet weld — typically on both sides of the plate-to-web interface.
Minimum fillet weld size: AWS D1.1 Table 7-7 sets minimum fillet weld size based on the thicker part joined. For a 3/8 in ring plate to a 5/16 in web, the minimum fillet is 3/16 in. Most ring plate details specify 1/4 in or 5/16 in fillets to provide a comfortable margin above the minimum. Larger fillets are permissible only if the beam manufacturer's engineer has confirmed the additional heat input and distortion won't compromise the opening geometry.
WPS for ring plate fillets: A standard prequalified fillet weld WPS covers these joints. SMAW or FCAW-G in the flat or horizontal position is typical in a shop environment. Matching filler metal to the parent metal classification is straightforward for A992 webs and A36 or A572 ring plates.
Distortion management: Circular ring plates are prone to local distortion from uneven filler passes. A balanced sequence — alternating sides or using a backstep pattern — helps maintain the flatness of the ring plate face, which is often critical for the fit of fire protection or mechanical collars.
What the CWI Verifies Before Arc Strike
Whether reviewing beam-manufacturer shop welds at receiving or witnessing production welds on a castellated beam repair or custom configuration, the inspector's pre-weld checklist covers:
WPS posted: The applicable WPS document is physically available at the welding station. The process, electrode, and preheat minimum stated on the WPS match actual conditions.
Preheat achieved: Contact thermometer or thermal crayon reading confirms the metal is at or above the WPS-specified minimum. On thin webs (3/8 in or less) in ambient temperatures above 50°F, this is often satisfied passively; in cold conditions it requires active heating.
Joint fit-up: The root opening and groove angle (for groove welds) or the fillet weld leg size setup (for ring plates) fall within the WPS and Annex B tolerances. For apex welds with backing, the backing fits snugly with no gaps exceeding 1/16 in.
Base metal identification: Material certified mill test reports (CMTRs) should confirm the web is A992 or the specified grade. Review CMTR verification for structural welding for the CWI's documentation role.
Welder qualification: The welder's WPQ (Welder Performance Qualification) covers the process, position, and joint type being welded. For a vertical apex weld (3G position), the welder must have a qualification test that covers 3G or an all-position qualification.
Visual Acceptance Criteria After Welding
AWS D1.1 Clause 9 visual acceptance criteria apply to all welds, including castellated and cellular beam welds. Key acceptance checks:
- No cracks in the weld or HAZ
- Complete fusion along the weld length — no visible lack of fusion at weld toes or the root
- Undercut not exceeding 1/32 in for primary structural welds subject to tension
- Weld profile meeting convexity and concavity limits per Clause 9 figures
- For fillet welds: actual leg size meeting or exceeding the WPS and drawing requirement
Any weld failing visual acceptance must be repaired per a documented repair weld procedure — documented as a nonconformance and dispositioned before the beam leaves the shop or is accepted in the field.
NDE for Castellated Beam Welds
The contract documents and the engineer of record specify NDE requirements. AWS D1.1 does not mandate NDE for all welds; it sets minimum acceptance criteria when NDE is performed.
For statically loaded castellated beams in typical floor framing, visual inspection is the baseline. For dynamically loaded applications — crane beams, fatigue Category C or higher details at the apex, or any beam in a cyclically loaded structure — the contract typically requires magnetic particle testing (MT) or ultrasonic testing (UT) of the apex welds. MT is the most practical method for the web groove weld geometry. If UT is specified, the inspector must address beam web geometry challenges (curvature, thin plate) in the UT procedure.
Owner-specified NDE requirements for castellated beam shop welds often exceed AWS D1.1 minimums. Confirm those requirements are captured in the ITP (Inspection and Test Plan) before shop fabrication begins.
Getting the WPS Right the First Time
Castellated and cellular beam projects sometimes catch fabricators off guard when they realize the beam manufacturer's WPS may not have been independently reviewed against the project specification. If your contract requires AWS D1.1 compliance on shop welds, verify that the beam manufacturer's WPS library covers the joint types and positions used in production — and that those WPS documents are available for your CWI's review.
WelderWPS helps structural fabricators and CWIs build and review WPS packages for exactly these non-standard joint configurations, including the required essential variable documentation and CMTR traceability.