Direct Answer
Special inspection is independent quality verification performed by a qualified inspector on behalf of the building department — separate from the fabricator's in-house QC. For structural steel, the special inspector verifies: material certifications, welder qualifications, weld procedures, weld visual inspection, bolt installation and pretensioning, and SSPC coating inspection. Their findings are reported directly to the authority having jurisdiction (AHJ).
Full Explanation
AISC 360 and IBC Chapter 17 define the scope of special inspection for structural steel. The inspector is hired by the owner (not the fabricator or GC) to provide an independent check. Special inspection for welding includes: pre-weld inspection (joint geometry, fit-up, preheat), in-process inspection (process compliance, electrode handling), and post-weld inspection (visual inspection per AWS D1.1 acceptance criteria, NDT if specified). Non-destructive testing (MT, PT, UT, RT) is specified for Demand Critical welds and higher-risk connections.
What This Means for Your Shop
A failing special inspection report can shut down your shop or erection operation. The most efficient approach is to treat your in-house QC as if it were special inspection — document everything, and the actual inspection becomes a confirmation rather than a discovery.
Common Mistakes
Welding over defects to hide them before inspection. AWS D1.1 requires that all weld repairs are documented and reinspected. Attempting to hide defects is a code violation and a liability exposure.
Recommended Process
Engage the special inspection agency early. Walk them through your QC process before work begins. Maintain your own inspection records parallel to theirs. When discrepancies arise, resolve them in writing before proceeding.