The Radiographic Substitute for a Required UT
On a 12-story steel frame, the engineer of record requires ultrasonic testing of complete-joint-penetration groove welds in beam-to-column moment connections under AWS D1.1. The joint is a T-joint with a 1-in.-thick A572 Grade 50 column flange and a 1-in. beam flange, welded from one side with a backing bar. The contractor's only qualified UT technician is on medical leave. To avoid a two-week delay, the contractor proposes to radiograph the joints with Ir-192 instead. Their RT technician is certified and available. The contractor argues that RT will find anything that matters and points out that the first three joints already passed visual inspection. The project has a strict liquidated-damages clause, and the steel erector wants to continue welding. You are the CAWI asked to approve the substitution.
1. Should you approve radiographic testing in place of the required ultrasonic testing?
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No. I do not have the authority to substitute one examination method for another when the engineer of record has specified UT and the code acceptance is based on UT. The substitution would require written approval from the engineer of record after a technical review. More importantly, RT and UT are not equivalent for this joint. The T-joint geometry with a backing bar, welded from one side, produces a configuration in which the RT beam would have to pass through one leg and the backing bar, and lack-of-fusion or lack-of-penetration at the fusion face is often oriented parallel to the RT beam. Those planar defects may not be detected by RT even when they are significant. Visual inspection of the first three joints says nothing about internal fusion. Approving RT would create a documentation trail showing that a required UT examination was replaced without approval.
2. What are the specific technical limitations of using RT on this joint?
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RT is generally effective for volumetric discontinuities such as porosity and slag, but it is much less reliable for planar discontinuities oriented parallel to the radiation beam, including lack of fusion and some lack-of-penetration. In this T-joint the weld is made from one side against a backing bar; the fusion face at the column flange is a vertical plane that may align with the RT beam, so a tight lack-of-fusion can be missed. The 1-in. section and the backing bar also change attenuation and can create image artifacts or obscure the root region. A single-wall, single-image technique may not be possible with the geometry, and the required IQI sensitivity for the thickness may not be achievable through two legs. These are technical reasons why the code specifies UT for these joints, not arbitrary preferences. The RT technician's certification does not make the method equivalent.
3. What should you do next to protect the project while addressing the schedule pressure?
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I would place a hold on the affected joints and on further welding of joints requiring UT, and issue a written RFI to the engineer of record describing the UT technician absence, the contractor's proposed RT substitution, and the technical limitations. I would ask the engineer to either approve a specific alternative examination procedure, such as qualified phased-array UT performed by an available qualified technician, or confirm that the joints must wait for the specified UT. I would document the contractor's schedule concerns and the liquidated-damages risk, but I would not let schedule pressure override the specified examination. The contractor may continue welding only joints that do not require the held examination, and only if the engineer agrees. I would also verify that the three visually accepted joints have not been released for final acceptance without the required UT.