Page 225 - Buried Pipe Design

Design of Pressure Pipes 199

are the thick-wall formula and the thin-walled formula for hoop stress
. Parameters W and P are determined experimentally. With these
t
values, one can determine combinations of internal pressures p and
external crush loads w that are necessary to cause failure. In addi-
tion, the design pressure will require an appropriate safety factor.
Normally, if surges are present, the maximum design (operating)
pressure is one-fourth of the pressure to cause failure. If surges are
not present, the operating design pressure is four-tenths the failure
pressure.
The design crush load is equal to the expected earth load plus live
load times the safety factor (usually 2.5) and divided by a bedding factor
(see Chap. 3 for bedding factors).
(earth load live load) (safety factor)
W
bedding factor

Design curves are given in AWWA C401 and AWWA C403 (see Fig 4.5
for an example). The designer enters the graph by locating the
appropriate design pressure on the vertical axis and the appropriate
external crush load on the horizontal axis. The intersection of these
grid lines locate the appropriate pipe curve. If the intersection is
between curves, choose the next-higher curve and the associated
strength pipe.

Reinforced concrete. Reinforced-concrete pressure pipe is of four basic
types:

1. Reinforced steel cylinder type (AWWA C300)
2. Prestressed steel cylinder type (AWWA C301)
3. Reinforced noncylinder type (AWWA C302)
4. Bar-wrapped, steel cylinder type (AWWA C303)
For rigid pipes discussed up to this point, the performance limits
have been described in terms of rupture of the pipe wall due to either
internal or external loads, or some combination thereof, being greater
than the strength of the pipe. Performance limits for reinforced con-
crete pipe are described in terms of design conditions, such as zero
compression stress and so forth. Generally, the design of reinforced-
concrete pressure pipe requires the consideration of two design cases:

1. A combination of working pressure and transient pressure and
external loads
2. A combination of working pressure and external load (earth plus
live load)