What is a weld access hole and when is it required?

A weld access hole (sometimes called a rat hole) is a cutout in the web of a beam at the flange-to-web intersection, sized to allow the welder to complete the flange groove weld from both the inside (backing bar side) and outside without the web blocking access. Access holes are standard in moment connections and splices where full-penetration flange welds are required. The geometry is governed by the engineer of record and referenced in AWS D1.1:2025.

What surface roughness is required for thermally cut access hole surfaces under AWS D1.1:2025?

AWS D1.1:2025 requires that thermally cut surfaces in structural applications meet a surface roughness not exceeding 1,000 µin (25 µm) Ra as measured per ASME B46.1. For access holes in demand-critical connections or in material with yield strength above 50 ksi, smoother finish requirements (500 µin or better) are often specified by the engineer. Notches, gouges, or sharp re-entrant angles in the access hole must be ground smooth.

Does an access hole affect the WPS for the flange weld?

Indirectly, yes. The access hole geometry determines the root clearance, backing bar positioning, and the weld runoff tab location for the flange groove weld. If the access hole is undersized, the welder cannot achieve proper electrode access for the root pass, which leads to incomplete fusion at the web-flange junction — a common cause of connection fracture. The WPS must specify the qualified joint detail including the access hole geometry as part of the joint design.

Is preheat required when thermally cutting weld access holes in high-strength steel?

AWS D1.1:2025 does not mandate preheat specifically for thermal cutting operations, but for steels with carbon equivalent above 0.40 — including A913, A514, A709 Gr. 70W, and many A572 Gr. 50 heats — preheat before thermal cutting reduces the risk of heat-affected zone hardening and cold cracking at the cut edge. Project specifications and AWS D1.1 commentary recommend preheat practices consistent with the base metal group, particularly for thick flanges above 1.5 in (38 mm).

Weld access holes in structural fabrication: AWS D1.1:2025

AWS D1.1:2025 geometry and surface finish requirements for weld access holes and cope cuts. What your WPS, shop drawings, and CWI inspection must address.

Weld access holes are a small detail with large consequences. Undersized, miscut, or rough-surfaced access holes have been directly linked to connection fractures in moment frames going back to the 1994 Northridge earthquake. AWS D1.1:2025, in combination with AISC seismic and connection design standards, specifies minimum geometry and surface quality requirements for access holes. For CWIs and QC managers, knowing what to inspect — and when to reject — is not optional.

What a weld access hole does

In a beam-to-column moment connection with a full-penetration flange weld, the web stands between the welder and the inside face of the flange at the flange-web intersection. Without an access hole, the welder cannot:

Get the electrode or filler into the root of the CJP groove at the web-flange junction
Achieve proper fusion across the full flange width at that corner
Complete the back-side weld pass if the joint uses a backing bar

The access hole removes a section of the web at the flange intersection, creating an opening that allows electrode access and complete weld coverage. A backing bar for the flange weld typically extends through the access hole opening and is supported before welding begins.

The consequence of an inadequate access hole is incomplete fusion at the web-flange junction — which is precisely the mode of failure observed in Northridge. The geometry requirements in current standards reflect the engineering response to that failure experience.

Geometry requirements

AWS D1.1:2025 specifies minimum access hole dimensions for structural applications. The basic requirements:

Minimum depth (into the web). The access hole must be deep enough to clear the flange-web fillet and provide room for the electrode. AWS D1.1:2025 specifies minimum depth requirements; the AISC Prequalified Connections (AISC 358) standard, when applicable to seismic moment frames, specifies deeper holes with specific proportions for demand-critical connections.

Minimum length along the web. The hole extends along the web axis far enough to position the backing bar and weld runoff tabs clear of the web. A typical minimum is 1.5 times the flange thickness or a fixed minimum (whichever governs).

Transition geometry. The end of the access hole away from the flange must not have a sharp re-entrant corner. The reentrant radius (the radius at the intersection of the straight web cut and the bottom of the access hole) must meet a minimum radius requirement. Sharp corners at this location are stress concentrations that can initiate fatigue cracks.

Fabrication tolerance. Actual cut dimensions must meet the specified minimums. An access hole that is too small is a fabrication nonconformance and must be corrected by enlarging — not by forcing the electrode into a tight space.

The shop drawings should show the access hole geometry on connection details. The CWI should verify dimensions before welding begins, not after. Once the flange weld is deposited, an undersized access hole cannot be corrected without removing weld metal.

Thermal cutting surface requirements

Most access holes are cut with an automated plasma or oxyfuel torch. Manual thermal cutting is used when machines cannot reach or for repair situations. AWS D1.1:2025 surface quality requirements for thermally cut edges:

Surface roughness. The thermally cut surface must not exceed 1,000 µin (25 µm) Ra per ASME B46.1. This is a relatively coarse finish compared to machined surfaces, but it still requires that obvious drag lines, gouges, or dross be removed by grinding. In practice, a properly set automated torch on A36 or A572 material typically produces a surface in the 250–500 µin range without additional finishing.

Notches and gouges. Any notch or gouge deeper than 3/16 in (5 mm) must be repaired by welding and grinding flush. Smaller notches may be ground smooth with the cross-section maintained. The CWI should reject surfaces with visible notching along the access hole edge, particularly at the re-entrant radius.

Edge hardness. For high-strength or alloy steels, the thermal cut edge may harden due to rapid cooling. On A514, A709 Gr. 70W, and similar steels, this hardened edge layer can be a hydrogen cracking initiation site if not addressed. The remediation is preheating the area before thermal cutting to slow the cooling rate, which softens the heat-affected zone at the cut edge.

Preheat for thermal cutting

AWS D1.1:2025 addresses preheat for welding; it does not set mandatory preheat requirements for the thermal cutting operation itself. However, the engineering guidance is consistent: for steels with carbon equivalent above 0.40 — particularly heavy flanges above 1.5 in (38 mm) thick — preheating before thermal cutting reduces HAZ hardness at the cut edge and lowers the risk of cold cracking.

Specific guidance:

For A36 and low-CE A572 Gr. 50 material at standard thicknesses: thermal cutting without preheat is standard practice.
For A992 wide flanges at flange thicknesses above 1 in: consider preheat to 150°F (66°C) when cutting access holes, particularly in cold shop conditions.
For A913, A514, A709 Gr. 70W, or similar high-strength steels: consult the WPS for the applicable base metal group and use the same preheat as specified for welding, or as directed by the welding engineer.

Document preheat requirements for thermal cutting in the quality plan if project specifications require it. Some AISC Category 1 certified shops include thermal cutting preheat in their written quality procedures for fracture-critical material.

Inspection hold point: before the flange weld

Access hole inspection is a CWI hold point for moment connections in seismic applications and for all connections subject to fatigue provisions. The inspection sequence:

Dimensional verification — measure access hole depth and length against shop drawings. Record dimensions on the inspection traveler.
Surface quality check — visually inspect the cut surface for gouges, notches, and drag lines. Check the re-entrant radius geometry.
Re-entrant radius check — use a radius gauge or template at the bottom of the access hole to verify minimum radius compliance.
Edge condition at backing bar location — verify that the backing bar seats flat against the flange root at the access hole perimeter. Gap at the backing-to-flange interface must be within the WPS joint detail tolerance.
Document before welding begins — sign the hold point and confirm that welding shall not proceed until inspection is complete.

For seismic demand-critical connections (per AISC 341 and AISC 358), the inspector must also verify that the access hole geometry matches the prequalified connection detail in AISC 358, since deviations from the prequalified detail require special inspection and engineer approval.

Access holes and WPS documentation

The connection detail — including access hole geometry — is part of the joint design that the WPS references. A prequalified WPS under AWS D1.1:2025 Clause 5 must reference a prequalified joint detail from Annex B or a project-specific detail shown on shop drawings. The WPS should not leave the access hole geometry undefined, because it affects:

Root opening at the backing bar
Electrode accessibility for root pass
Weld runoff tab placement
Amount of base metal consumed by the access hole (relevant to design)

For connections designed under AISC 358 (seismic prequalified connections), the WPS must be specifically qualified for the connection type and follow the supplementary WPS requirements in AISC 358 Section 3.3. These requirements go beyond AWS D1.1:2025 alone.

See joint design and prequalified geometries under Annex B for how joint details flow from the code into the WPS. For information on CJP groove welds and backing removal requirements that apply at the same locations as access holes, see CJP groove weld backing removal requirements.

If you manage multiple moment connection procedures across multiple projects, welding procedure library software can track which WPS covers which connection detail and which access hole geometry, linking each to the applicable prequalified standard or PQR.

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