b.
Initial Designs.
Use the following design procedures:
(1) Determine apparent ring compression stress of the pipe:
PD
= ))
EQUATION:
Apparent ring comp. stress
(4-5)
2A
where
P = apparent vertical soil pressure on top of conduit,
as determined from Equation (4-4)
D = outside diameter of conduit
A = cross-sectional area of the wall per unit length of conduit
(2) Equate apparent ring compression stress to allowable ring
compression strength to determine required cross-sectional wall area, A, per
unit length of pipe:
S+y,
Allowable ring comp. strength =))))
EQUATION:
(4-6)
F+S,
PDS+y,
A = ))))))
EQUATION:
(4-7)
2F+S,
S+y, = yield point strength of the steel (typically 33 to 45 ksi)
where
F+S, = safety factor (usually 1.5 to 2)
(3) Select appropriate pipe size to provide the minimum
cross-sectional wall area A as determined above.
(4) Check ring deflection so that it does not exceed 5% of the
nominal diameter of the pipe. Ring deflection Y, is governed by the total
soil pressure P+v, = P+DL,+P+LL,, diameter D, moment of inertia I, modulus
of elasticity of conduit E, and soil modulus E'. Generally, ring deflection
does not govern the design. See Figure 21 (Reference 10) for an example.
(5) The Handling Factor is the maximum flexibility beyond which ring
is easily damaged. Pipe design must consider limiting the Handling Factor
to such typical values as D.2-/EI = 0.0433 in/lb for 2-2/3 x 1/2 corrugation
and 0.0200 in/lb for 6 x 2 corrugation.
c. Soil Placement. Great care must be exercised in soil placement.
Ring deflection and external soil pressures are sensitive to soil placement.
If a loose soil blanket is placed around the ring and the soil is carefully
compacted away from it, soil pressure is reduced considerably.
d. Design of Flexible Steel Pipe. For analysis and design procedures
for large size flexible pipe of non-circular cross section, see Reference
12.
4. CONDUITS BENEATH EMBANKMENTS OF FINITE WIDTH. Design of culverts and
conduits beneath narrow-crested embankments must consider the effect of the
embankment base spread and settlement on the pipe.
7.1-190
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