Page 139 - Buried Pipe Design

Design of Gravity Flow Pipes 113

or fiberglass-reinforced plastic pipe. However, one of the previous
equations should be used for design, keeping in mind that predicted
buckling pressure will be on the conservative side for most plastic
pipe.
A more exact approach to buckling follows:
The summation of external loads should be equal to or less than the
allowable buckling pressure. The allowable buckling pressure q a may
be determined by the following:

1 EI 1 ⁄2
q a 32 R w B′ E′
FS D 3
where q a allowable buckling pressure, lb/in 2
FS design factor

2.5 for 2
h
D

h
3.0 for
2
D
h height of ground surface above top of pipe, in
D diameter of pipe, in
R w water bouyancy factor

h w
1 0.33 0 h w h
h
h w height of water surface above top of pipe, in
B′ empirical coefficient of elastic support (dimensionless)

Coefficient B′ was given by Luscher 25 in 1966. The equation is as fol-
lows:

2
4 (h Dh)
B′
2
2
(1 )[(2h D) D (1 2 )]
B′ has some dependence on Poisson’s ratio for the soil. However, this
effect is small as is shown in Fig. 3.23. The above equation simplifies
when the value for Poisson’s ratio is taken as . This equation is con-
1
2
servative and should be used for the calculation of B′.
2
4(h Dh)
B′
1.5(2h D) 2