Page 298 - Buried Pipe Design

Rigid Pipe Products 269

5. If required, provide additional external load capacity by increasing
the effective moment of inertia of the longitudinal pipe section or by
improving the bedding material or compaction requirements. The
effective moment of inertia may be increased by increasing the area
or diameter of the bar reinforcement or by increasing the coating
thickness to a maximum of 1.25 in.

Example 5.5 A 15-in-diameter sanitary sewer line is to be installed 14 ft
deep in native sand. The trench width at the top of the pipe is to be 3.0 ft.
For class B, class C, and class D bedding, select the required strength for
nonreinforced concrete pipe and the required strength for a reinforced con-
crete pipe.
From Example 5.4,
W d 2592 lb/ft

Nonreinforced:

SF SF
Strength W (3-edge) W d 2952
LF LF
where SF safety factor 1.5 and LF load factor for particular bedding
class (see Example 5.4).
Reinforced: Reinforced concrete pipe is designed using the D load. The D
load is the required three-edge strength divided by the pipe diameter.

W d /D 2592/D 2592 SF
Strength W (D load) SF SF
LF LF D LF

For this material, the strength for each class is based on a 0.01-in crack,
not failure. Actual failure load (ultimate) will be approximately 1.5 times
the load which causes a 0.01-in crack. Therefore, a safety factor of 1.0 is rec-
ommended based on D load or 1.5 based on ultimate load.
Note: The values in Table 5.10 were calculated, and the required classes
were selected, from Tables 5.7 and 5.8. Also note that a high enough
strength for nonreinforced concrete is not available to withstand loads
imposed if bedding is only class D.

Example 5.6—Transmission pipe A 24-in-diameter transmission line will
deliver water at 7000 gal/m and 5.0 ft/s from a reservoir to a treatment
plant 10 mi away. The pipe will be buried 5.0 ft deep in a 4.0-ft-wide trench
in sand carefully compacted or bedded with a coarse granular material up
to the spring line. Surge and valve control equipment will allow maximum
2
surge pressures of 50 lb/in . The system will operate at maximum pressure
2
of 150 lb/in . Determine the appropriate prestressed concrete transmission
pipe (see Example 5.3).