p = S
PR + kt
in which S = the strength of the band; p = the distance between bands; P = the intensity of bursting pressure in the pipe; R = the radius of the inside of the pipe; t = the thickness of the staves; k = the swelling strength of wood, usually taken at 100 pounds per sq. in.
Fig. 79.—Shoe for Wood Stave Pipe.
Transverse joints between staves are closed by inserting metal strips between them, or by shaping the edges irregularly so that they fit closely together with an irregular joint. Transverse joints between all staves at any one point are avoided by splitting the joints between staves. Longitudinal joints between staves are usually made smooth and are closed by steel bands which are drawn tight about the pipe by inserting the ends in coupling shoes as shown in Fig. 79.
Fig. 80.—B in Formula W = CwB2
102. External Loads on Buried Pipe.—Prof. Anston Marston and H. C. Anderson published[[68]] the results of a series of experiments on the loads on buried pipes which are of extreme value in the design of sewer pipe. The load on the pipe is given by the empirical expression W = CwB2, in which w is the weight of the backfilling material in pounds per cubic foot, B is the width of the trench in feet at the elevation of the end of a radius making an angle of 45 degrees upwards with the horizontal diameter of the pipe as illustrated in Fig. 80, and C is a coefficient dependent on the character of the backfill and the ratio of the width to the depth of the trench. Values of C are given in Table 42. The weights of various classes of backfilling are given in Table 43.
| TABLE 42 | ||||
|---|---|---|---|---|
| Approximate Safe Working Values of C in the Expression W = CwB2 | ||||
| From Bulletin No. 31 of the Engineering Experiment Station, Iowa State College of Agriculture. | ||||
| Ratio of Depth to Width | Approximate Values of C | |||
| Damp Top Soil and Dry and Wet Sand | Saturated Top Soil | Damp Yellow Clay | Saturated Yellow Clay | |
| 0.5 | 0.46 | 0.47 | 0.47 | 0.48 |
| 1.0 | 0.35 | 0.86 | 0.88 | 0.90 |
| 1.6 | 1.16 | 1.21 | 1.25 | 1.27 |
| 3.0 | 1.47 | 1.51 | 1.56 | 1.62 |
| 2.6 | 1.70 | 1.77 | 1.83 | 1.91 |
| 3.0 | 1.90 | 1.99 | 2.08 | 2.19 |
| 3.6 | 2.08 | 2.18 | 2.28 | 2.43 |
| 4.0 | 2.22 | 2.35 | 2.47 | 2.65 |
| 4.6 | 2.34 | 2.49 | 2.63 | 2.85 |
| 6.0 | 2.45 | 2.61 | 2.78 | 3.02 |
| 6.5 | 2.54 | 2.72 | 2.90 | 3.18 |
| 6.0 | 2.61 | 2.81 | 3.01 | 3.32 |
| 6.6 | 2.68 | 2.89 | 3.11 | 3.44 |
| 7.0 | 2.73 | 2.95 | 3.19 | 3.55 |
| 7.5 | 2.78 | 3.01 | 3.27 | 3.66 |
| 8.0 | 2.82 | 3.06 | 3.33 | 3.74 |
| 8.5 | 2.85 | 3.10 | 3.39 | 3.82 |
| 9.0 | 2.88 | 3.14 | 3.44 | 3.89 |
| 9.5 | 2.90 | 3.18 | 3.48 | 3.96 |
| 10.0 | 2.92 | 3.20 | 3.52 | 4.01 |
| 11.0 | 2.95 | 3.25 | 3.58 | 4.11 |
| 12.0 | 2.97 | 3.28 | 3.63 | 4.19 |
| 13.0 | 2.99 | 3.31 | 3.67 | 4.25 |
| 14.0 | 3.00 | 3.33 | 3.70 | 4.30 |
| 15.0 | 3.01 | 3.34 | 3.72 | 4.34 |
| ∞ | 3.03 | 3.38 | 3.79 | 4.50 |
| TABLE 43 | |
|---|---|
| Approximate Weights of Ditch Filling Material to be Used in the Expression W = CwB2[[69]] | |
| Ditch Filling | Pounds per Cubic Foot |
| Partly compacted top soil (damp) | 90 |
| Saturated top soil | 110 |
| Partly compacted damp yellow clay | 100 |
| Saturated yellow clay | 130 |
| Dry sand | 100 |
| Wet sand | 120 |