CIVIL ENGINEERING FORMULAS

HIGHWAY AND ROAD FORMULAS 289

A safety factor of 2 is applied to the ultimate wall stress to obtain the design
stressFc, ksi (MPa):

(11.36)

Wall Thickness

Required wall area A, in^2 /ft (mm^2 /m), of width, is computed from the calculated
compressionCin the pipe wall and the allowable stress Fc:

(11.37)

From the AISI table for underground conduits, select the wall thickness that
provides the required area with the same corrugation used for selection of the
allowable stress.

Check Handling Stiffness

Minimum pipe stiffness requirements for practical handling and installation,
without undue care or bracing, have been established through experience. The
resulting flexibility factor FF limits the size of each combination of corrugation
pitch and metal thickness:

(11.38)

whereEmodulus of elasticity, ksi (MPa), of steel 30,000 ksi (206,850 MPa);
andImoment of inertia of wall, in^4 /in (mm^4 /mm).
The following maximum values of FF are recommended for ordinary
installations:

FF0.0433 for factory-made pipe with less than a 120-in (30.48-cm) diam-

eter and with riveted, welded, or helical seams

FF0.0200 for field-assembled pipe with over a 120-in (30.48-cm) diameter

or with bolted seams

Higher values can be used with special care or where experienceindicates.
Trench condition, as in sewer design, can be one such case; use of aluminum
pipe is another. For example, the flexibility factor permitted for aluminum
pipe in some national specifications is more than twice that recommended here
for steel because aluminum has only one-third the stiffness of steel, the modu-
lus for aluminum being about 10,000 versus 30,000 ksi (68,950 versus
206,850 MPa) for steel. Where a high degree of flexibility is acceptable for
aluminum, it is equally acceptable for steel.

FF

D^2

EI

A

C

Fc

Fc

Fb

2