Page 296 - Buried Pipe Design

Rigid Pipe Products 267

7. Calculate the required circumferential steel area for the invert
and the side for each of the three combinations of loads shown below.
Combined load design means the pipe is designed to resist the flexural
and axial stresses from each of the following conditions:
■ Condition 1: a combination of working pressure, dead loads
(earth, pipe, and water), and live loads
■ Condition 2: a combination of dead loads (earth, pipe, and water)
and live loads with zero internal pressure
■ Condition 3: a combination of working pressure, surge pressure,
and dead loads (earth, pipe, and water)
8. Select the controlling maximum steel area for the invert (inner)
and the side (outer). The total steel area must be equal to or greater
than the steel area required for the hydrostatic design. Increase the
inner area, outer area, or both sides to meet the required total.
9. Select appropriate bars or fabric to meet the design circumfer-
ential steel areas and spacing. For AWWA C300 type of pipe, ensure
that the area of rod reinforcement is at least 40 percent of the total cir-
cumferential steel area. Check concrete cover over steel.
10. If AWWA C302 type of pipe is to be installed on supports or in
any other condition that would create longitudinal bending, the
required beam strength is provided by adjusting either the laying
length or the wall thickness, or both, so the concrete flexural tensile
stress does not exceed 4.5 lb/in . In the determination of the flexural
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tensile stress, the section modulus of the pipe is calculated about the
centroidal axis of the transverse section with no allowance for longitu-
dinal steel reinforcement.

Design procedure for AWWA C303 type
of pipe
The pipe design procedure for AWWA C303 type of pipe involves the
following steps:

1. Select a steel cylinder thickness equal to or greater than the AWWA
C303 minimum. Calculate the total circumferential steel area
required to resist internal pressure, using the hoop tension for
working pressure and for working pressure plus surge pressure.
2. Calculate the cylinder steel area, and place the remaining required
steel area in the bar by selecting a bar diameter and bar spacing
within the following limits, established in AWWA C303:
■ The area of bar reinforcement shall not exceed 60 percent of the
total area of circumferential reinforcement.
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■ The area of bar reinforcement shall not be less than 0.23 in /lin ft.
■ The center-to-center bar spacing shall not exceed 2 in.