Page 50 - Buried Pipe Design
P. 50
28 Chapter Two
h
Figure 2.13 Graphic depiction of the prism load on a pipe.
To obtain load in pounds per foot, multiply the above by the pipe OD in feet:
2
W 1200 ( ) 800 lb/ft
3
The Marston load for this example is 56 percent of the prism load and is
nonconservative for design. Again, for flexible conduits, the prism load
theory represents a realistic estimate of the maximum load and is
slightly conservative.
Trench condition. The Marston-Spangler equation for the load on a
flexible pipe in a trench is given by Eq. (2.10). The load coefficient C d
is obtained from Fig. 2.2. One may ask, Under what conditions, if any,
will the prism load and the ditch (trench) load be equal?
Prism load P H lb/ft 2
Marston load W d C d B d B c
Multiply the prism load by B c (to express in pounds per foot, as in the
Marston load) and set it equal to the Marston load.
PB c HB c W d C d B d B c
