2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

Chapter 5 Water Infrastructure
properly design pipes on supports. The Ductile Iron Pipe Research Association (DIPRA) also
publishes a computer program for selecting supports and spacing of ductile iron pipes.
A. Pile Supports
Pile supported pipelines are rare in the SPU water system. However, in some locations,
such as crossing a wetland or in loose soils, pile-supported pipelines may be necessary.
A licensed civil or structural engineer must design the pile support and calculate pipeline
thickness. Because pipelines installed on piles are typically not continuously supported,
they present unique design challenges. Among the issues are additional stresses placed
on the pipeline due to the lack of support. Such design issues must be investigated and
modeled by a licensed structural engineer.
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Above Grade Pile Support
For an above-grade exposed pile support, the design engineer should consider the
pipeline and pile coating system. In most cases, both the pile and pipe will require a
coating, and cathodic protection must be considered. Additionally, pipeline insulation
may be needed to protect the line from freezing temperatures and in no-flow situations.
Buried Support
If the pipeline is on piles and buried, a qualified licensed civil or structural engineer must
carefully review the connection to the piles to ensure the pipe and piles operate as one
entity during seismic and uplift conditions.
B. Aerial/Bridge Supports
SPU owns and operates a few aerial (aboveground) pipelines in its water system. A
structural engineer licensed in Washington State should be involved in aerial pipeline
design. Like pipelines on piles, aerial pipelines are not continuously supported, and
therefore present unique design challenges.
Aerial pipelines can either be supported from above, by hanging the pipe, or cradled in a
utility corridor under the bridge. In either case, the pipe supports place additional
loadings on the pipe wall.
The following are special considerations for aerial design:
Where possible, aerial pipelines should be avoided for security and vibration
concerns.
When pipes are hung under existing bridges, roadway clearance design must
consider the potential for damage from trucks traveling above the legal height limit.
Additional protection should be considered such as line valves or structural
modifications to the bridge.
With an exposed pipe design, the design engineer must consider the pipeline
coating system. Additionally, pipeline insulation may be needed to help control
thermal expansion of the pipeline, and keep the line from freezing temperatures and
no flow situations. AWWA Manual M11 provides an analysis method to determine if
freezing is a concern.
SPU Design Standards and Guidelines
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