HSS Seal Weld Requirements Under AWS D1.1:2025

Hollow structural sections bring clean lines and high strength-to-weight ratios to structural steel fabrication. That hollow core also creates a corrosion maintenance problem. Moisture trapped inside an HSS has nowhere to go, and internal corrosion can eat through wall thickness for years before anyone notices. Seal welds — small fillet welds around the perimeter of an end cap or closure plate — are the standard fix.

When a project specification or engineer of record calls for them, your WPS and CWI protocol need to be ready. Here is what AWS D1.1:2025 requires and what CWIs most commonly miss.

When Seal Welds Are Required

AWS D1.1:2025 does not universally mandate seal welds on HSS members. The call belongs to the engineer of record based on exposure conditions, service environment, and long-term maintenance access. Common triggers:

• Exposed exterior HSS columns where open tube ends or cope cuts allow rainwater entry
• Coastal or high-humidity environments where interior corrosion is a recognized design life risk
• End connections that create pockets — cap plates on columns, stub plate connections, slotted base plates
• Project specifications invoking AISC Code of Standard Practice provisions or owner-mandated tube sealing

When the contract documents call for seal welds, those welds must be qualified and inspected under AWS D1.1:2025 like any other structural fillet weld. The sealing function does not exempt them from code compliance.

Venting: The Non-Negotiable Fabrication Step

This is where most field problems originate. Welding a cap plate to an HSS traps air in the hollow section. Heat from the arc expands that air. With no exit path, internal pressure drives gas through the molten weld pool and produces:

• Porosity — distributed gas pockets throughout the weld cross-section
• Blowholes — larger gas voids that open at the weld surface
• Incomplete fusion — the weld blows open before the pass can fill the joint

AWS D1.1:2025 requires that sealed hollow members be provided with vent holes to prevent this pressure buildup. The standard approach is a drilled hole in the cap plate — typically 1/4 in to 3/8 in diameter — positioned to allow gas to escape freely during welding. After the weld cools, the vent may be plug-welded or left open depending on the project specification.

CWI hold point: Verify that the vent hole is present and drilled before the cap plate is tacked in place. Once tacking is complete, retrofitting a vent is difficult and creates unnecessary rework. Catching the miss at fit-up costs nothing; discovering it after a blowhole shows up in visual inspection costs far more.

WPS Selection

Prequalified WPS Path

For most common structural HSS — ASTM A500 Grade B, Grade C, and A1085 — the base metal qualifies under AWS D1.1:2025 Table 6.9 prequalified base metal groups. When the cap plate material is also prequalified, a prequalified fillet weld WPS under Clause 5 is typically the fastest and lowest-cost route to compliance.

Requirements that must be met:

• Filler metal must come from an AWS D1.1:2025 prequalified classification. Note that the 2025 edition removed A5.36 from the GMAW/FCAW allowed list (Table 6.6, row 4) — verify your electrode is still on the current list if you updated from a 2020-era WPS
• Preheat and interpass temperature must meet Clause 5 minimums for the base metal group and thickness combination
• Fillet weld size and number of passes must be documented on the WPS
• Position coverage must match production conditions — 1F for flat, 2F for horizontal, 3F for vertical, 4F for overhead

Tested WPS Path

If the HSS grade is outside Table 6.9 (some high-strength or proprietary HSS grades fall outside prequalified groups), the WPS must be supported by a PQR. The PQR coupon should represent the actual base metal combination — fillet weld on representative base metal — with macro examination of the cross section.

For seal welds specifically, macro examination confirming full fusion to both base metal faces and a proper throat depth is the critical test result. See fillet weld PQR macro examination under AWS D1.1 for what a passing macro section looks like and common failure modes.

Minimum and Maximum Fillet Weld Size

Minimum Size — Table 7.7

AWS D1.1:2025 Table 7.7 sets minimum fillet weld size based on the thickness of the thicker part joined. For HSS seal welds, the thicker part is usually the cap plate if the HSS wall is thin:

Rule library based on AWS D1.1:2025; verify against your governing edition.

The minimum is a code floor, not a design size. When the structural engineer specifies a larger fillet for load-carrying reasons, that larger size governs. For pure seal welds with no calculated load, Table 7.7 minimum is usually sufficient — but document the design basis in the submittal package.

Maximum Size at Edges

For material 3/4 in and thicker, AWS D1.1:2025 limits the fillet weld size at plate edges to the plate thickness minus 1/16 in. For material under 3/4 in, the full plate thickness is the limit.

This catches oversizing on thin HSS walls. A 1/4-in HSS wall can only support a 1/4-in fillet weld at the edge. Running a 5/16-in fillet risks rolling the edge and creating an undercut condition that looks like a larger weld but has reduced effective throat.

Position and Access Challenges

HSS cap plate welds are typically made in the flat (1F) or horizontal (2F) position, but the continuous perimeter fillet around all four sides of a rectangular HSS — or the full circumference of a round HSS — requires the welder to navigate corners without stopping.

CWIs should watch for:

Corner undercutting. Heat concentrates at corners where travel slows. The result is undercut on the base metal adjacent to the weld toe. Measure undercut depth with a weld profile gauge — AWS D1.1:2025 visual acceptance limits apply.

Stop/start porosity. If the welder restarts inside the joint rather than back-stepping onto the previous bead and advancing, gas from the restart crater is trapped. Specify on the WPS that starts must be made at a run-on tab or by back-stepping at least 1/2 in onto the completed bead.

Incomplete fusion at tight corners. Small HSS sections (2×2 or 3×3) have corners with limited electrode access. Electrode angle and position relative to the corner must be specified. Short-arc GMAW or FCAW-G with a small diameter wire is often easier to maneuver than a large-diameter SMAW electrode.

CWI Pre-Weld and Post-Weld Checklist

Before welding:

• Vent hole drilled and correctly positioned in cap plate
• Joint fit-up: gap at root ≤ 1/16 in for prequalified fillet, or within WPS tolerances
• Base metal surface: free of oil, moisture, scale in the weld zone
• Preheat applied and measured — confirm temperature meets WPS minimum before arc starts

During welding:

• Welder qualified for the process and position
• Filler metal classification matches WPS
• Amperage, voltage, and travel speed within WPS-qualified range
• Interpass temperature not exceeding WPS maximum

After welding:

• Visual: fillet leg size meets minimum, no visible cracks, no undercut beyond acceptance limit, no crater cracks at terminations
• Fillet weld gauge: leg measurement and effective throat verified at multiple locations around the perimeter
• Vent hole status: documented as open, plug-welded, or other per project spec
• Welder ID stamp and WPS number recorded in the weld log

For full weld documentation flow from inspection to audit packet, see production weld inspection documentation under AWS D1.1 and weld map and WPS traceability.

Common Nonconformances

Missing vent hole at fit-up. The single most common seal-weld finding. Inspect before tacking, not after.

Undersized fillet. Seal welds are treated as secondary shop welds and often run undersized. Apply fillet weld gauges; do not accept a visual-only pass.

Wrong electrode. SMAW E6013 is not prequalified under AWS D1.1:2025 for structural work. Check the electrode classification against the WPS before every heat.

No preheat on thick cap plates. A 1-in cap plate on A500 Grade C may require preheat depending on carbon equivalent. Check the prequalified preheat requirements for the base metal group and thickness. See preheat verification methods in the field for measurement tools and documentation practice.

If managing WPS libraries and preheat lookups manually is slowing your QC team, see wpswelding.com/pricing for software that automates these checks at the point of welding.

Summary

Seal welds on HSS members are routine, but they carry the same code obligations as any structural weld:

1. The engineer of record or project spec drives the requirement — not the code by itself
2. Vent holes in sealed hollow members must be present before welding begins
3. WPS must cover base metal group, process, filler, position, and minimum fillet size
4. Table 7.7 governs minimum fillet size; edge-thickness limits cap the maximum
5. CWI inspection must record vent hole status, fillet gauge measurements, and full welder/WPS traceability

Running seal welds to the same standard as primary structural welds keeps your QC records audit-ready and keeps moisture out of the HSS for the life of the structure.