DESIGN - Prestressed Concrete Transmission Pole Structures - page 48

obtained from classical elastic stability theory or using a nonlinear fi nite
element formulation. Such numerical or fi nite element techniques are not
practical without computers. These methods are described in advanced
analysis textbooks.
Defl ections
For loads less than those causing the fi rst crack, elastic defl ections
can be determined using classical structural analysis methods. For
loads in excess of cracking, an inelastic pole design method should be
used.
Concrete Creep Defl ection: Additional structural defl ection can
occur because of concrete creep. This is the plastic deformation of the
concrete resulting from an application of loads over an extended time
period. This could result in increased defl ections for poles used as strain
poles, self-supporting dead ends, or guyed structures. For most pole
applications, creep is not a major design consideration. However, it can
be of signifi cance for nonuniform stress distribution resulting from the
combined effect of sustained load and prestress.
Determination of Elastic Defl ection: For loading conditions that do
not exceed the cracking capacity of the pole, an elastic method may be
used. This could include virtual work, the conjugate beam method, slope
defl ection, or a fi nite element computer analysis.
Determination of Inelastic Defl ection: Up to the point of cracking,
the defl ection may be computed using elastic methods previously
described. After cracking, the modulus of elasticity E becomes both stress
and time dependent, and the moment of inertia I becomes crack depen-
dent. Because the product EI varies with stress, time, and pole geometry,
computing inelastic defl ections is suffi ciently complicated to warrant
using iterative computer computations.
The inelastic defl ection can be approximated using reduced values of
the elastic product EI . These values may range from E c I g at a level of
moment at cracking to E c I g /3 as the member approaches ultimate strength.
REFERENCES
American Concrete Institute (ACI). (2008). “Building code requirements
for structural concrete.” ACI 318 , Farmington Hills, MI.
American National Standards Institute (ANSI). (2008). “Wood poles-
specifi cations and dimensions.” 05.1-2008 , New York.