DESIGN - Prestressed Concrete Transmission Pole Structures - page 31

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DESIGN
Prestressed concrete poles generally are designed using classical pre-
stressed concrete theory, as described in ASCE/PCI (1997) and PCI (2010).
Prestressed concrete poles exhibit both linear and nonlinear behavior,
time dependent material behavior, and geometric nonlinear behavior
caused by the change in the section properties relative to the applied
loads.
The section properties of a prestressed concrete pole change when
bending forces applied to the cross section (see Fig. 4-1) cause tensile
forces on the face of the pole that exceed the compressive forces exerted
by the prestressed steel. The bending moment at which this occurs
is called the cracking moment. Prior to reaching the cracking moment,
the pole section has a relatively constant modulus of elasticity and
de fl ects in a linear manner. Above the cracking moment, the pole sec-
tion behaves mostly nonlinearly because of the altered properties of
the cracked section. Both behaviors can be present at the same time
during loading (i.e., the top half of the pole may not yet have exceeded
the cracking moment and thus behaves linearly, while the bottom half of
the pole has exceeded the cracking moment and thus behaves
nonlinearly).
Additional time-dependent consideration in the design of a prestressed
concrete pole includes shrinkage and creep of the concrete and relaxation
of the prestressing steel. These material behaviors should be considered in
determining the section behavior. The magnitude and effects are described
in ACI 318 (ACI 2008) and PCI (2010) .
Defl ection can sometimes cause a signifi cant additional secondary
bending moment because of P-Delta effects. The secondary moments in
the structure are a result of the structure defl ection causing an applied
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