asset failures
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forces, but weaker than the pipe at high axial forces. Unfortunately, identifying the axial stresses required to induce plastic failures in joints is sensitive to a joint's details. Figure 3 illustrates plastic failure occu rring in the pipe wall at the strength limit state. Joints can, however, be weaker t han the pipe in the axial direction, and still resist the axial forces applied. For example, full-scale tests have shown that some fillet welded joints can resist full axial thrust forces provided local plastic deformat ion is acceptable and the steel has sufficient ductility (Smith, 2006).
Figure 3. Von Mises stresses superimposed on the deflected shape of a ball and socket joint subject to a strength limit state loading combination of internal pressure and axial force (Note: Deformation is exaggerated for clarity. The red zones have yielded). forces and high pressures are likely, and/or pressure transients occur.
The Authors
The design of joints for fatigue is generally not discussed in the standards or literature, although AS 2566.1 req uires fatigue to be considered, without providi ng g uidance. As discussed, high tension stresses are generated at the toe of the fillet welds, and local yieldi ng w ill occur in joints resisting normal operating pressures and moderate to high axial tensions. This suggests that high-stress low-cycle fatigue is a potential failure mechanism that shou ld be considered during design. High-stress lowcycle fatigue is consistent with joints failing years after their initial pressure test (Eberhardt, 1990; Jacob et al., 2007).
formed part of the SRWP Alliance, which included 160 km of large diameter high pressure pipeline. KBR was the design consultant for the alliance.
References American Water Works Association, (2004), Steel Pipe - A Guide for Design and
Ductility and Fatigue Considerations Ductility, and hence the st rength of the joint, may be compromised by factors including: the incorrect specification of the steel for the expected temperatures, the adoption of high strength low ductility steels, the utilisation of too much ductility in the formation of the joint, and inappropriate welding procedures. Care is required to ensure adequate ductility is provided at each joint.
Rob Heywood and Sean Brady from Texcel Expert Services worked jointly w ith t he SRWP Alliance to investigate the behav iour and strength of fillet welded joints. The firm provides expert forensic and investigative structural engineering services to the public, private, and legal sectors. Rob Heywood is the Technical Director of Texcel Expert Services: rob h@texcel.com.au. Sean Brady is Business Manager. Email: seanb@texcel.com.au
Installation: Manual of Water Supply Practices Mt 1, Denver: American Water
Works Association. AS/NZS 2566.1 :1998, Buried flexible pipelines Part 1: Structural design, Standards Australia. AS/NZS 2566. 1 Supp1 :1998, Buried flexible pipelines Part 1: Structural designCommentary, Standards Australia.
Brockenbrough, R. L., (1990), Strength of Bell-and-Spigot Joints, Journal of Structural Engineering, Vol 116, No. 7, July, Pages 1983-1991. Eberhardt, A., (1990), 108-in. Diameter Steel Water Conduit Failure and Assessment of AWWA Practice, Journal of Performance of Constructed Facilities, Volume 4, No. 1, February, Pages 30-50. Jacob B., Sundberg V, Genculu S., and Hunt, L., (2007), Welded Lap Joint Brittle
Sean Bartleet is a principal water and wastewater engineer at the KBR Brisbane office and was the lead pipeline engineer for the Northern Pipeline lnterconnector. The lnterconnector
80 AUGUST 2009 water
Failure: A Structural Assessment of an Atlanta 72-inch Welded Steel Water Pipe Demonstrates Need for Improvement in AWWA Standards, ASCE International
Conference on Pipeline Engineering and Construction, Pipelines 2007: Advances and Experiences with Trenchless Pipeline Projects, ASCE, Page 66. Moser, A.P., (1990), Buried Pipe Design, McGraw-Hill, Page 121. Smith, G., (2006), Steel Water Pipe Joint Testing. Pipelines 2006
Service to the Owner, The Pipeline Division Specialty Conference 2006, July 30-August 2 (p. 42). Chicago, lllonois, USA: ASCE.
Summary In concl usion, t he investigations to date have provided some insights into the behaviour of fillet welded joints, but further discussion and research w ithin the water pipel ine industry is required to not only understand the behaviour and failure mechanisms of fillet welded joints, but also to develop a design approach so that industry guidelines can better encourage the appropriate utilisation of fillet welded joints. Until these knowledge gaps are overcome, care should be taken when specifying fillet welded joints in locations where substantial axial
refereed paper
Tawfik, M.S. and O'Rourke, T.D., (1985), Load-Carrying Capacity of Welded Slip Joints, Journal of Pressure Vessel Technology
(American Society of Mechanical Engineers), Vol. 107, February 1985, Pages 36-43. Tsetseni, S., and Karamanos, S. A., (2007), Axial Compression Capacity of Welded-Slip Pipeline Joints, Journal of Transportation Engineering (ASCE). Vol. 133, No.
5, May 2007, Pages 335-340.
South-East Queensland Water Grid: Installation of piping.
Young, W., C., & Budynas, R., G., (2002), Roark's Formulas for Stress and Strain, McGraw-Hill, Seventh Edition.
technical features