UT Scanning Access for Box Column Welds: CWI Planning Guide
Box columns and closed HSS sections limit UT scanner placement after assembly. Learn how AWS D1.1 handles restricted NDE access and what CWIs must plan ahead.
When a fabricated box column assembly is complete, three or four full-length CJP groove welds run the length of the member — and at least one of those welds is now surrounded on three sides by steel that the UT probe cannot reach. Box columns and closed hollow structural sections (HSS) create NDE access constraints that a CWI must anticipate and plan for before the first piece is tacked, not after the column is bolted to the base plate.
This article covers the UT scanning access problem for closed structural sections, what AWS D1.1:2025 says about restricted NDE, and how to build an inspection plan that survives a third-party audit.
Why Closed Sections Create UT Problems
Conventional shear-wave UT for groove weld inspection relies on scanning from a surface parallel or perpendicular to the weld axis, typically the base metal surface adjacent to the weld. For a T-joint or butt weld in open steel, you can scan from both sides of the weld, giving multiple beam angles and excellent volumetric coverage.
In a completed four-plate fabricated box column, the welds at the four corners are accessible only from the exterior faces — two approximately 90° apart for any given corner weld. The interior of the box is sealed, and any weld backing bars that remain are permanently enclosed. This geometry creates two specific problems:
Near-surface dead zone: The probe cannot be placed directly over the weld root because the probe sits on the exterior flange face, not above the weld crown. Beam angles to reach the root must be steep and long, often pushing the inspection into the probe's near-surface dead zone where small indications are difficult to resolve from the back-wall echo.
Limited scanning directions: With access from only two exterior faces, the number of beam angle and scanning direction combinations is reduced. Discontinuities oriented parallel to the weld axis (which includes longitudinal lack-of-fusion along the weld root) are hardest to detect from limited-angle scans.
These aren't disqualifying problems — UT on closed sections is routinely performed — but they require engineering in advance. See phased array UT (PAUT) applications for structural welds for how PAUT's linear array provides coverage advantages over conventional techniques in restricted-access conditions.
What AWS D1.1:2025 Says About NDE Access
AWS D1.1:2025 does not provide a section that explicitly addresses "restricted access UT" as a special case. The code's UT requirements (Clause 8, Annex S) assume adequate scanning access but do not mandate bilateral access. The practical standard is this: the examination must be capable of detecting the minimum-size discontinuity the code requires you to find, and the procedure must document how that coverage is achieved.
The key documentation requirement is the UT procedure itself — often called a written examination procedure (WEP) — which must describe the calibration block, beam angles, scanning index points, reference sensitivity, and coverage zones. When your scanning access is limited, the WEP must specifically address how coverage of the restricted zones is achieved and what alternative beam angles or techniques compensate for the access limitation.
On projects requiring ASNT SNT-TC-1A or ANSI/ASNT CP-189 UT Level II personnel, the written examination procedure is a formal document reviewed and accepted before production NDE begins. On D1.1 structural work, the procedure requirements are generally less formal, but the principle of documented coverage is the same.
Planning the Inspection Sequence for Box Column Fabrication
The correct time to address UT access is during the box column fabrication sequence, not after it is complete. Here is how experienced CWI and QC programs handle this:
Step 1 — Three-sided assembly NDE. After assembling three plates into a U-channel and completing the two interior corner CJP groove welds (the ones that will be enclosed), perform UT on both welds while interior access is still available. This is the only point in the sequence when the scanner can be placed on the inside face of the web. Document UT results, hold for NDE acceptance, then proceed to closing.
Step 2 — Fit-up and pre-inspection for fourth-side weld. Before closing the box, perform final visual inspection of the third and fourth corner joint fit-up. Measure root opening, root face, and joint alignment; confirm the backing bar (if used) is properly positioned and tacked.
Step 3 — Close and weld. Complete the fourth-side CJP groove welds. These exterior-only welds will require exterior-access UT per your engineered scanning plan.
Step 4 — Post-weld NDE delay. Observe the required diffusion delay before performing final NDE. For higher-carbon-equivalent steels, this delay may extend to 48 hours after completion of all passes. Do not let schedule pressure compress this window. See the note in postweld NDE delay for hydrogen diffusion.
Step 5 — Final UT on fourth-side welds. Execute the documented UT procedure using the engineered beam angles and compensated sensitivity for the restricted-access geometry.
For the box column sequence to work, step 1 must be explicitly listed as a hold point in the welding inspection and test plan (ITP) — if the box gets closed before the interior welds are inspected, you have lost the only chance for adequate coverage. See box column CJP weld sequence and distortion control for a detailed walkthrough of the fabrication sequence.
Closed HSS Sections: Different Problem, Same Principle
Rectangular HSS (ASTM A500, A1085) and round pipe used as structural columns present a simpler version of the same access problem. Welds to end caps, gusset plates, and connection elements at the end of a closed HSS section often have weld roots that face the interior of the tube.
For end cap welds (CJP butt welds closing the end of an HSS section), UT is often impractical entirely — the inspector cannot get a beam angle into the tube interior. In this case, the project engineer may specify:
Radiographic testing (RT) instead of UT, if geometry permits film or detector placement
Magnetic particle testing (MT) for surface and near-surface coverage only, with an engineering acceptance that volumetric UT is not achievable
A design specification that classifies the end cap as a PJP joint with a defined effective throat, avoiding the CJP UT requirement
The substitution must be documented with the engineer of record's approval before the weld is made. Discovering post-fabrication that the required UT cannot be performed is a quality failure even if the weld itself is sound, because you cannot demonstrate acceptance to the code's standard.
Phased Array UT: Practical Gains for Restricted Access
Where conventional single-probe UT struggles with limited scanning angles, phased array UT (PAUT) offers meaningful advantages. A linear array probe can simultaneously fire multiple beam angles and focal laws, substantially expanding the volumetric coverage achievable from a single probe position on the exterior face. TOFD (time-of-flight diffraction) pairing with PAUT provides additional sensitivity to planar indications perpendicular to the fusion line.
The catch: PAUT requires a written procedure, calibration to a representative reference block with geometry matching the production joint, and Level II PAUT-qualified personnel per SNT-TC-1A or equivalent. Not every structural UT subcontractor carries PAUT capability. Specify PAUT in the project requirements early enough that the right subcontractor is selected.
When preparing an AISC certification audit package or an owner submittal for NDE records on box column work, include:
Written UT examination procedure (WEP) noting the restricted-access condition and compensatory technique
Calibration records referencing the applicable calibration block (including any special geometry block used for the restricted-access condition)
Scan index plots or electronic A-scan/B-scan records for each weld
Final UT reports with welder ID, joint ID, weld identification, accept/reject disposition
Evidence that step 1 (pre-closure interior NDE) was completed and accepted before box was closed
Rule library based on AWS D1.1:2025; verify against your governing edition.
For CWI and QC managers who need a system to track NDE hold points, assign weld IDs, and store inspection records in one audit-ready location, wpswelding.com/pricing offers purpose-built welding quality management tools designed for fab shops.
FAQ
Frequently asked.
Can you perform UT on a completed box column CJP groove weld from the outside only?
Yes, but with limitations. When interior access is lost after assembly, UT must be performed from the exterior accessible surface. The scanning plan must demonstrate adequate coverage of the weld cross-section and account for near-surface dead zones. PAUT with a linear array often provides better coverage than conventional single-probe UT in these situations.
When must NDE of box column welds be completed under AWS D1.1?
AWS D1.1:2025 requires that NDE be performed after welding is complete and before the weld is covered or made inaccessible. For box columns, this means the fourth-side CJP groove weld must be inspected before any cladding, encasement, or structural topping is applied. The postweld hydrogen diffusion delay period also applies.
Does AWS D1.1 require interior NDE access for box column welds?
No explicit clause requires interior access. D1.1 requires that NDE achieve the code's acceptance criteria; if UT can demonstrate adequate coverage from the exterior, that satisfies the code. The practical issue is that the coverage must be verifiable, which requires engineering the scanning plan carefully.
What are the NDE timing requirements after welding to avoid false indications?
AWS D1.1:2025 specifies a minimum wait time after welding before performing NDE — 48 hours for steels over certain carbon equivalent levels — to allow hydrogen to diffuse and delayed cracking to manifest. See your project specification for any extensions to this wait period.