Page 355 - Civil Engineering Formulas

286 CHAPTER ELEVEN

Conduit deflection is given by the Iowa formula. This formula gives the rel-
ative influence on the deflection of the pipe strength and the passive side pres-
sure resisting horizontal movement of the pipe wall, or

D 1 KW c r 3
x (11.29)
EI 0.061E r 3
where horizontal deflection of pipe, in (mm)
x
D deflection lag factor
1
K bedding constant (dependent on bedding angle)
W vertical load per unit length of pipe, lb per linear in (N/mm)
c
r mean radius of pipe, in (mm)
2
E modulus of elasticity of pipe material, lb/in (MPa)
4
I moment of inertia per unit length of cross section of pipe wall, in /in
4
(mm /mm)
2
E modulus of passive resistance of enveloping soil, lb/in (MPa)
Soil modulus E has not been correlated with the types of backfill and com-
paction. This limits the usefulness of the formula to analysis of installed struc-
tures that are under observation.
AMERICAN IRON AND STEEL
INSTITUTE (AISI) DESIGN PROCEDURE

The design procedure for corrugated steel structures recommended in their Hand-
book of Steel Drainage and Highway Construction Projects is given below.

Backfill Density
Select a percentage of compaction of pipe backfill for design. The value chosen
should reflect the importance and size of the structure and the quality that can
reasonably be expected. The recommended value for routine use is 85 percent.
This value usually applies to ordinary installations for which most specifica-
tions call for compaction to 90 percent. However, for more important structures
in higher fill situations, consideration must be given to selecting higher quality
backfill and requiring this quality for construction.

Design Pressure
When the height of cover is equal to, or greater than, the span or diameter of
the structure, enter the load-factor chart (Fig. 11.11) to determine the percent-
age of the total load acting on the steel. For routine use, the 85 percent soil
compaction provides a load factor K 0.86. The total load is multiplied by K
to obtain the design pressure P acting on the steel. If the height of cover is less