Field-welded lap-joints were first successfully used in the 1920s to join sections of an all-steel pipe transmission line in Texas. This application was later improved using shielded metal arc-welding technology followed by other semiautomated arc-welding processes (i.e., flux-core arc-welding and gas-metal arc-welding) that currently dominate construction practices for field-welding water transmission lines. Pipeline designers agree that arc-welding processes provide a superior field-welded joint; however, there is considerable uncertainty regarding pipeline design loads and required field-welded joint details necessary to resist these loads. Field-welded lap-joints have proven to be very economical and are contractors' first choice; however, they are also the most vulnerable part of any welded lap-joint steel pipeline. There have been significant failures over the last 80 years since field-welded lap-joints were first introduced. With this in mind, it is unfortunate that design manuals fail to focus on this critical aspect of structural design. The longitudinal forces on buried, continuous, steel water pipelines can be extremely large (e.g., thermal force, Poisson's stress, differential settlement, hydrostatic thrust, seismic force, and soil drag force if pipelines traverse steep terrain) and even more so for large-diameter pipelines. These load requirements are not covered well in the common design standards (AWWA, 2004) and can be hard to quantify. Many designers believe that these longitudinal forces are relatively small. The authors recently reviewed a pipeline design criteria report that stated continuous, buried, steel pipelines are not subject to longitudinal forces greater than hydrostatic thrust. When problems with buried pipe joints develop, fixing or repairing them is expensive and disruptive. Dewatering and accessing a large-diameter main is a major undertaking. Using double-welded lap joints costs more than single-lap welds, but it is better from a corrosion and structural standpoint and allows pressure-testing to ensure that joints are water tight. Further, there are two welds to resist leakage, which are much like the difference between single-gasket and double-gasket joints. It's often a sound investment for infrastructure that you want to serve without problems for many years. There is a place for single lap-welded joints, but they are not a panacea to be used everywhere. When the stakes are high, the designer should consider double lap-welding or even complete joint penetration (CJP) welding. This paper proposes methods for determining welded steel pipeline forces and strategies for resisting these forces with field-welded joints. The application of field-welded lap-joints that are single-welded is compared with double-welded lap-joints.
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