Structural Steel Issues & Answers
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Development of Alternate Acceptance Criteria Based
on Engineering Analysis

The Issue

Visual verification inspections performed on a sampling of moment-resisting connections revealed numerous weld and base metal discontinuities (e.g., cracks, crack-like indications, incomplete fusion, undercut, under-filled transitions, insufficient weld length and weld throat). In approximately seven percent (7%) of the connections examined, the severity of these discontinuities exceeded the limits of the applicable visual inspection criteria.1 The presence of rejectable indications in this sample suggested that similar indications were present throughout the hundreds of connections. Subsequent construction (e.g., fireproofing, interior framing, ductwork, mechanical sub-assemblies, etc.) resulted in limiting access to some of these connections. Therefore, inspection of the remaining connections and remediation of these discontinuities would have proved to be very costly in terms of repair costs, construction delays, and the inevitable ensuing disputes. These factors, combined with the low number of connections exhibiting rejectable discontinuities, made implementation of an alternate acceptance criteria an attractive approach to resolving this issue.

The Answer

Many of the acceptance criteria specified by codes are based on standards of good workmanship rather than structural integrity. The American Welding Society (AWS) Structural Steel Welding Code (AWS D1.1) addresses this fact by allowing alternative acceptance criteria which "can be based upon evaluation of suitability-for-service using past experience, experimental evidence, or engineering analysis considering material type, service load effects, and environmental factors."2

APTECH applied a rigorous engineering analysis to determine a realistic tolerance for each of these discontinuities. First, detailed measurements of flaw lengths and depths were performed to establish statistical worst case flaw geometries. Inputs for the engineering analysis included material properties, actual loads anticipated under design conditions (including seismic loading), and weld residual and restraint stresses. Using an elastic-plastic fracture mechanics model, failure assessment diagrams were then developed for each of the discontinuities.3 Results of this analysis showed that none of the discontinuities would initiate a brittle fracture, tear, or plastic instability under the postulated worst case loading conditions.

These alternative flaw acceptance criteria were accepted by the engineer which allowed construction of the building to continue and eliminated unnecessary weld repairs.

  1. Structural Welding Code-Steel, ANSI/AWS D1.1, American Welding Society, 15th Edition, February 2, 1996, pp. 4, 10, 169, 176.
  2. Structural Welding Code-Steel, ANSI/AWS D1.1, American Welding Society, 15th Edition, February 2, 1996, para. 6.8, p. 175.
  3. "Guidance on Methods for Assessing the Acceptability of Flaws in Fusion Welded Structures," British Standards Institution, PD6493:1991, August 1991.

Services Offered

  • Design, Fabrication, and Erection Oversight Services
  • Inspection Reviews/Audits
  • Dispute Resolution Technical Support
  • Problem Solving (welding, engineering, stress analysis, flaw acceptance, fitness-for-service)
  • Failure and Root Cause Analysis

Facilities, Technologies, Software

Personnel Experienced in Welding Engineering, Metallurgy, Applied Mechanics, and Expert Testimony

  • Finite Element Stress Analysis Tools Capable of Analytically Predicting Applied, Residual, and Restraint Stresses
  • Fracture Mechanics and Plastic Instability Models for a Wide Variety of Flaws and Design Details
  • Complete Metallurgical Laboratory, Including Mechanical Testing Equipment

Recent Experience

  • USC Norris Cancer Center Building -- Root Cause Analysis and Dispute Resolution
  • Santa Clara Valley Medical Center -- Root Cause Analysis/Design and Fabrication Review
  • Yerba Buena/Sony Entertainment Center -- Flaw Acceptance/Fitness-For-Service Review/ Construction Oversight Services
  • U.C. Davis Medical Center -- Weld Procedure Specification Review

Structural Steel Issues & Answers is a periodic newsletter designed to provide you with timely information about important issues affecting all aspects of the structural steel industry. 

For more information, contact Geoff Egan or Mike Cronin at (408) 745-7000 or 408-745-7000.

 

Aptech Engineering Services, Inc.

601 W. California Ave., Sunnyvale, California (408) 745-7000
© 2005, APTECH Engineering Services, Inc. All rights reserved.
Reproduction in whole or in part in any form without the express written permission of
an officer of the company is strictly prohibited.

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