Welded Lap Joint Brittle Failure:A Structural Assessment of an Atlanta 72-inch Welded Steel Water Pipe DemonstratesNeed for Improvement in AWWA Standards

摘要

A condition assessment of the City of Atlanta 72-inch buried welded steel raw-water pipelinernwas performed in 2006 by CH2M HILL/Williams Russell & Johnson Joint Venture Inc. andrnATS Inc. on behalf of the City of Atlanta Department of Watershed Management. Thernpipeline is spiral welded with ?-inch wall thickness and double welded bell and spigot laprnjoints. It was fabricated in 1973 and 1974 to the standard of "Mill-Type Steel Water Pipe,"rnAWWA C202-64T, Class B. This assessment focused on structural integrity, as the pipelinernhas a history of numerous failures since entering service in 1975. Past failures have beenrnreported as full-circumferential brittle fractures through the bell at the toe of the internal filletrnweld. In-situ and laboratory structural examinations were performed including extensivernanalyses on a 4.5-foot sample, including an intact bell and spigot lap joint, cut from the 72-rninch welded steel pipe (WSP) alignment. This assessment did not include an investigation ofrnan actual failure site.rnData from this assessment indicates that the steel is extremely brittle within the cold-formedrnbell (Charpy V-Notch < 6 ft-lbs at 60℉). Past failures are likely the result of a highrnconcentration of stress in the bell at the toe of the internal lap (fillet) weld of the fully stabbedrnjoint, where the pipe steel is extremely brittle and where welding-induced notches werernobserved. All failures have occurred during winter months when water temperature is lowest,rnthe steel is most brittle, and thermal induced longitudinal tension is maximized.rnThe authors wish to translate the results of this condition assessment into recommendations—rnfrom the consumer's perspective-for the improvement of the industry standards for weldedrnsteel water pipe. Although AWWA has made improvements to their standards (M11-2005,rnC200-1997, and C206-2003) due to lessons learned from other WSP failures, gaps remainrnwithin these publications that may allow defects in design, fabrication, and installation of arnnew WSP.