The W1 Issue. I: Extent of Weld Fracturing during Northridge Earthquake

Ultrasonic indications in the roots of complete penetration welds (W1's) were the most common of the unexpected conditions identified after the 1994 Northridge earthquake in welded steel moment frame (WSMF) buildings. Widely treated by the engineering profession as earthquake-caused damage requiring repair, and characterized as such by the SAC, W1's have not previously been subjected to systematic study either to factually establish their ostensible relationship to earthquake ground motions or to debunk it. In this first part of a two-part paper, the conclusions from several studies that were generally directed toward accurately characterizing W1's, identifying their origins, clarifying the extent and distribution of earthquake damage to WSMF buildings caused by the Northridge event, and developing recommendations for cost-efficient post-earthquake damage surveying of WSMF buildings are presented. In a companion paper, the reliability of ultrasonic testing (UT) of WSMF buildings during construction is also investigated as a possible explanation for the widespread presence of W1's in existing WSMF buildings. The studies found that W1's are not related to earthquake ground motion, that the extent of Northridge earthquake damage to WSMF buildings is substantially less than previously published accounts suggest, and that UT as normally implemented by testing lab personnel is an unreliable inspection technique for the roots of complete penetration T-joints with backing.