286 CHAPTER ELEVEN
Conduit deflectionis 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
(11.29)
where (^) xhorizontal deflection of pipe, in (mm)
D 1 deflection lag factor
Kbedding constant (dependent on bedding angle)
Wcvertical load per unit length of pipe, lb per linear in (N/mm)
rmean radius of pipe, in (mm)
Emodulus of elasticity of pipe material, lb/in^2 (MPa)
Imoment of inertia per unit length of cross section of pipe wall, in^4 /in
(mm^4 /mm)
Emodulus of passive resistance of enveloping soil, lb/in^2 (MPa)
Soil modulus Ehas 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 Projectsis 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 K0.86. The total load is multiplied by K
to obtain the design pressure P acting on the steel. If the height of cover is less
(^) x
D 1 KWcr^3
EI0.061Er^3