Page 78 - Buried Pipe Design

54 Chapter Two

Poisson’s ratio
S h hoop stress due to fluid pressure, lb/in 2
At changes in direction, such as bends and tees, the soil-to-pipe fric-
tion may not be sufficient to prevent expansion of the pipe relative to
the soil. As a result of the pipe movement relative to the soil, the pipe
is subject to bending stresses in addition to the longitudinal stress S L .
In these cases, the current practice would be to account for the thermal
bending stresses in one of two ways:
1. By using formulas such as provided in Appendix VII of ASME B31.1.
2. By a pipe-soil spring model to which the temperature rise is
applied. Special-purpose PC-based computer codes have been devel-
oped to perform these calculations.

Seismic Loads
In certain critical zones, large ground movement associated with an
earthquake may be devastating to a pipeline. These critical zones are
primarily those where high differential movement takes place such as
a fault zone, a soil shear plane, or transition zones where the pipe
enters a structure. Also certain soils will tend to liquefy during the
earthquake vibration, and buried pipelines may rise or tend to float.
On the other hand, most buried flexible pipelines can survive an earth-
quake. Again, a more flexible piping material with a flexible joint will
allow the pipe to conform to the ground movement without failure. In
practice, the design of buried pipe for seismic loads is limited to criti-
cal applications. In earthquake prone areas, seismic design is a con-
sideration for piping that must perform an essential function (such as
providing fire protection water) or prevent the release of toxic or flam-
mable contents (such as from a gas leak). A large body of data on the
behavior of buried pipe during earthquakes has been collected in the
last 20 years. The data point to a few critical characteristics that gov-
ern the seismic integrity of buried pipe (O’Rourke, FEMA): In general,

1. Modern (post-1930s) pipelines constructed with full-penetration
shielded arc welds and proper weld examination performed well.
2. Segmented construction (non-welded segments assembled by
mechanical joints) have experienced damage in large earthquakes.
3. Failures are more often due to soil failures (liquefaction, landslides,
fault movement) than to the transient passage of seismic waves.
4. Seismic damage of storage tanks (sliding, rupture, buckling, or foun-
dation settlement) has caused failures in connected buried pipe.