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

Chapter 5 Water Infrastructure
most common are the lap-weld, butt-weld, and butt strap joints. Refer to AWWA
Manual M-11 for a photo of each type of joint:
Lap-Weld Joint. In general, SPU prefers a lap-weld joint because they are easy
to install. Lap-welded pipe is a bell and spigot pipe with the bell welded to the
spigot where they overlap. The design engineer can select an interior only weld,
exterior only weld, or double lap weld (interior and exterior). SPU recommends
the double lap weld. It provides an added safety factor at each joint, but can
only be done in larger diameters because it requires a welder to make an
interior weld. Also, each joint can be checked for leakage with an air test. With a
lap-weld joint, small deflections can be made at each joint before welding.
Given the geometry of the double welded lap joint, it experiences twice as much
strain as the pipe wall when post-construction forces (settlement) cause pipe
movement.
Butt-Weld Joint. Butt-weld joints are made by aligning the ends of two pipe
sections and welding at the point of “near-contact” of the two ends. To
complete a butt-weld joint, both ends of the pipe must be the same size. A buttweld
joint is more difficult in the field because the pipes must be near perfectly
aligned. This style weld eliminates the geometric strain that can be induced at a
lap-weld joint. SPU has used butt-welded joints for pipelines designed for higher
levels of seismic loading. The butt-weld joint is also used in horizontal
directional drilling (HDD) applications, where having a bell shape on the pipe is
not recommended. A full penetration butt-welded steel pipe is one of the best
choices for seismic protection.
Butt-Strap Joint. A butt-strap joint consists of a strip of steel that overlaps two
plain end pieces of pipe by several inches. The butt strap is joined to each pipe
by an exterior weld (and an interior weld if the pipes have sufficient diameter).
The two pipe ends do not have to have identical outside diameters, but they
must be relatively similar. Typically, a butt-strap joint is used to join a new steel
pipe to an existing steel pipeline.
SPU follows the recommendations of the American Welding Society (AWS) Structural
Welding Code.
D. Flexible Restrained Joints
Flexible restrained joints allow some deflection and movement of the joint, but restrain
the joint while under pressure. These joints are the standard restrained joints used by
SPU. All ductile iron pipe manufacturers make a boltless restrained joint ductile iron
pipe that uses a restraining ring and locking lugs to restrain joints yet provide flexibility.
Check the manufacturer’s literature because products vary.
The use of flexible restrained joint pipe systems should be used when special site or
system conditions are present and the use of concrete thrust blocks is not appropriate.
Flexible restrained joint pipe systems are required when site/project needs have the
following characteristics:
The water main is to be located in an area of liquefiable soils.
The area is defined to have soils with a poor bearing capacity.
SPU Design Standards and Guidelines
5-27