Page 105 - Buried Pipe Design

Design of Gravity Flow Pipes 81

3. Select the bedding requirement.
4. Determine the load factor.
5. Apply the safety factor.
6. Select the appropriate pipe strength.

The following example will illustrate the use of the six design steps
and basic rigid pipe principles in selecting the appropriate pipe. It is
not within the scope of this text to discuss pipe material design [i.e.,
how much material (reinforcing steel, asbestos fiber, cement, and so
forth) is required to meet a specific crush strength is not included].
Since ASTM specifies minimum crush strengths, these will be used as
a beginning point for design in the discussion here.

Example Problem 3.1 A 15-in-diameter sanitary sewer line is to be installed
14 ft deep in native material, which is sand. If the trench width is 3.0 ft,
what pipe and bedding classes should be selected?
1. Determine earth load.
H
14/3 4.67
B d
K 0.165 sand
From Fig. 2.2, C d 2.4, so
2
2
W d C d B d 2.4 (120) (3.0) 2592 lb/ft
2. Determine live load (assume H-20 highway loading). From Fig. 2.19, note
that the live load is negligible for 14 ft of cover.
3. Select bedding. Economic and practical engineering judgment is
required. Compare classes D, C, and B (Fig. 3.5).
4. Determine the load factor. From Table 3.2
Class D BF 1.1
Class C BF 1.5
Class B BF 1.9
5. Apply the safety factor.
Recommendations:
Concrete (ACPA) SF 1.25–1.5
Reinforced concrete (ACPA) SF 1.0 based on 0.01-in crack
Clay (CPI) SF 1.0–1.5
Asbestos cement (ACPPA) SF 1.0–1.5
6. Select pipe strength.
SF
W 3-edge W c
BF
SF 1
W D load W c
BF D