DESIGN - Prestressed Concrete Transmission Pole Structures - page 40

be designed for the ultimate strength at all sections of the pole to exceed
the required strength calculated from the appropriate factored loads
applied to the structure. Factored loads are to be specifi ed by the Pur-
chaser or specifi ed in applicable codes and other loading guidelines (e.g.,
ASCE [2010] or other similar documents).
Cracking Strength
A prestressed concrete pole should typically be designed for the cracking
strength to exceed the moments calculated from serviceability require-
ments. Service loads are those that might occur with suffi cient frequency
and/or duration such that the structure's condition could become a concern
(i.e., from defl ection or visible cracking, general appearance, corrosion,
structural behavior, creep, defl ection, and others). If the pole remains
cracked under frequent or sustained loading, the reinforcement could be
exposed and may be susceptible to corrosion. Serviceability requirements
should be specifi ed by the Purchaser for structures that are subjected to
permanent lateral load—such as unguyed dead-end or angle structures—
and for structures controlled by defl ection. These types of structures should
be designed to have the crack reopen or zero tension strength to exceed the
moments calculated from the service loading.
Different service load conditions and design philosophies may dictate
the specifi cation of different cracking strength levels as discussed in the
following sections. The Purchaser should clearly specify the service loads
anticipated and the design philosophy to be incorporated. If the design
philosophy is left unspecifi ed, the engineer should use good engineering
judgment and standard industry practice.
First Cracking Strength: This is the pole capacity at which the fi rst
circumferential crack will occur. Under this condition, the moment in the
pole causes the tensile strength of the concrete to be exceeded on the tension
face of the pole. The crack strength is a function of the concrete modulus
of rupture and prestress level. Axial load may be a contributing factor in
the structure design. These cracks will close upon release of the load.
Crack Reopen Strength: This is the pole capacity at which a crack
that was previously created (by exceeding the fi rst cracking strength) will
open again. Under this condition, an applied moment will not cause any
tensile stress in the surfaces of the crack. The frictional effects of the
rupture surfaces can contribute to additional capacity. This strength will
always be less than the fi rst cracking strength.
Zero Tension Strength: This is the calculated capacity at which the
extreme fi ber of the concrete surface is subjected to zero stress due to the