AISC 360 N5b: What "10% of CJP Groove Welds" Means
AISC 360-22 §N5b requires UT on 10% of certain CJP groove welds for Risk Category II. Commentary §C-N5b clarifies that 10% means full length of 10% of the CJP groove welds, not spot length inside every weld, for QA sampling.
The Short Answer
For a Risk Category II building, AISC 360-22 §N5b does not say to scan 10% of the length of every CJP groove weld. It says QA performs UT on 10% of the qualifying CJP groove welds. Commentary §C-N5b then closes the ambiguity: the full length of 10% of the CJP groove welds is inspected. In the printed specification this is the Section 5b CJP Groove Weld NDT rule within Chapter N.
The qualifier is just as important as the percentage. The N5b 10% rule applies to CJP groove welds in butt, T-, and corner joints that are subjected to transversely applied tension loading, in material 5/16 in. [8 mm] thick or greater, for Risk Category II structures. If a weld is not in that scope, do not force it into the 10% sample just because it is a CJP weld.
Commentary N5b states that the 10% requirement means the full length of 10% of the CJP groove welds is inspected.
Which Welds Count in the 10% Sample
Use this filter before counting anything. A weld belongs in the AISC N5b Risk Category II 10% UT population only when all four conditions are true:
| Condition | N5b requirement |
|---|---|
| Risk category | Risk Category II structure. Risk Category III or IV moves to 100% UT for the same qualifying weld type. |
| Weld and joint type | CJP groove weld in a butt, T-, or corner joint. |
| Loading direction | Transversely applied tension loading. Compression, shear, or non-transverse loading is not the same N5b trigger. |
| Material thickness | 5/16 in. [8 mm] thick or greater. The N5b user note says CJP groove weld NDT is not required below this thickness. |
This is why a drawing note that says "10% CJP UT per AISC N5b" is incomplete by itself. The inspection plan still needs the qualifying weld population: which joints are CJP, which are transverse-tension welds, which are at least 5/16 in. thick, and which are part of a Risk Category II structure.
What Changes for Risk Categories I, III, and IV
AISC N5b sets three practical tiers. Risk Category I has no NDT requirement for CJP groove welds under the N5b user note. Risk Category II has the 10% UT requirement for the qualifying CJP groove weld population. Risk Category III and IV require UT on all qualifying CJP groove welds.
The risk-category jump is not a paperwork detail. It changes the inspection budget from sampling to full coverage for the same joint/loading/thickness class. If the project documents do not identify risk category clearly, the inspection percentage cannot be applied cleanly.
Who Performs the UT and Which Code Controls the Method
AISC Chapter N separates quality control and quality assurance. Section §N1 states that QC is provided by the fabricator and erector, while QA is provided by others when required by the AHJ, applicable building code, purchaser, owner, or EOR. It also states that NDT is performed by the agency or firm responsible for quality assurance, except as permitted by §N6.
Section 5a then assigns UT, MT, PT, and RT, where required, to QA and says those methods are performed in accordance with AWS D1.1/D1.1M. AISC tells you which structural building welds require UT and at what rate; AWS D1.1 supplies the method and acceptance framework for the NDT work.
The Rejection-Rate Escalation Rule
The 10% rate is not guaranteed to stay at 10%. Section 5f says that for Risk Category II and higher structures where the initial UT rate is 10%, the NDT rate for an individual welder or welding operator increases to 100% when that person's rejection rate exceeds 5% of the welds tested. A sampling of at least 20 completed welds on the project is required before implementing the increase.
The same section allows the rate to return to 10% if the welder's or welding operator's rejection rate falls to 5% or less based on at least 40 completed welds. This is an individual performance rule, not a blanket project penalty. The inspection plan needs to track tested welds and rejected welds by welder or welding operator, otherwise the escalation rule cannot be applied defensibly.
Common Misreadings
Misread 1: "Inspect 10% of every weld length." Commentary N5b says full length of 10% of the CJP groove welds, which means selected whole welds, not a spot-length slice inside every weld.
Misread 2: "Inspect 10% per welder from the start." N5b states the population by weld type, risk category, loading, and thickness. N5.5f uses individual welder rejection rate for escalation. Those are related, but they are not the same sentence.
Misread 3: "All CJP welds need 10% UT." The N5b 10% rule is narrower: butt, T-, and corner CJP groove welds under transversely applied tension in material 5/16 in. thick or greater. CJP welds in compression or shear are treated differently in Commentary N5b because they do not carry the same crack-propagation risk.
Misread 4: "Fillet welds are in the same NDT sample." Commentary N5b says fillet welds are designed for shear stresses regardless of load application and therefore do not warrant NDT under this rationale. A project may still require fillet-weld NDT by contract, but that is not the N5b 10% CJP groove weld rule.
Practical inspection note: Build the sample list from the weld schedule, not from field convenience. First mark the Risk Category II transverse-tension CJP groove welds that meet the 5/16 in. thickness threshold. Then choose 10% of those welds and inspect their full length.
Frequently Asked Questions
For Risk Category II structures, AISC 360-22 N5b requires UT on 10% of the qualifying CJP groove welds. Commentary N5b clarifies the sampling basis: the full length of 10% of the CJP groove welds must be inspected. It is not a spot-check of 10% of the length inside every weld.
The N5b 10% sample applies to CJP groove welds in butt, T-, and corner joints that are subjected to transversely applied tension loading in Risk Category II structures, when the material is 5/16 in. [8 mm] thick or greater. CJP welds outside that loading, joint, thickness, or risk-category scope are not part of this specific N5b 10% requirement.
AISC 360-22 N5b assigns the UT to QA. Section N5.5a states that UT, MT, PT, and RT, where required, are performed by QA in accordance with AWS D1.1/D1.1M. Section N1 also states that NDT is performed by the agency or firm responsible for quality assurance, except as permitted by Section N6.
Under AISC 360-22 N5.5f, for Risk Category II and higher structures where the initial UT rate is 10%, the NDT rate for an individual welder or welding operator increases to 100% if the rejection rate exceeds 5% of the welds tested for that person. A sample of at least 20 completed welds on the project is required before implementing the increase.
Yes. AISC 360-22 N5.5f allows the UT rate to decrease back to 10% if the welder's or welding operator's rejection rate falls to 5% or less on the basis of at least 40 completed welds. The reduction is tied to that individual's demonstrated project performance, not a blanket project reset.
Source: ANSI/AISC 360-22 Chapter N and Commentary N5b, verified against the local source PDF and extracted corpus.