Page 36 - Reliability and Maintainability of In service Pipelines

Introduction 25

F 5 impact factor
B c 5 the width of the sewer pipe (m)
C s 5 the load coefficient, which is a function of D and M , and D and M are
2H 2H
width and length, respectively, of the area over which the distributed load acts
H 5 height from the top of the sewer to ground surface (m)

1.5.2 STRESS IN BURIED PIPELINES

Rajani et al. (2000) developed a formulation for total external stresses including
all circumferential and axial stresses in buried pipelines. σ θ is hoop or circumfer-
ential stress:

σ θ 5 σ F 1 σ S 1 σ L 1 σ V ð1:15Þ
where σ F is hoop stress due to internal fluid pressure, σ S is soil pressure, σ L is
frost pressure, and σ V is traffic stress.
Similarly axial stress, σ x , would be:
ð
σ x 5 σ Te 1 σ F 1 σ S 1 σ L 1 σ V Þν p ð1:16Þ
is axial stress due to
is stress related to temperature difference, σ F
where σ T e
internal fluid pressure, ν p is pipe material Poisson’s ratio, and other parameters
have already mentioned. Equations and references used for the abovementioned
stresses have been presented in Table 1.3.
The factors that impact the structural integrity of pipes, mentioned above, are
also in relation to performance limits. Once these factors exceed the structural

TABLE 1.3 Stresses on Buried Pipes
Stress Type Model a References
σ F , hoop stress due to internal fluid pD Rajani et al. (2000)
pressure 2d
2
σ S , soil pressure 3K m γB C d E P dD Ahammed and Melchers
d
3
E P d 1 3K d pD 3 (1994)
σ L , frost pressure f frost :σ S Rajani et al. (2000)
σ V , traffic stress 3K m I c C t FE P dD Ahammed and Melchers

3
AE P d 1 3K d pD 3 (1994)
σ T e , thermal stress 2 E P α P ΔT e Rajani et al. (2000)
σ P , axial stress due to internal fluid p D Rajani et al. (2000)

pressure 2 d 2 1 ν p
a Notations introduced in Table 5.6, Chapter 5.