Civil Engineering Reference
In-Depth Information
There are several classic methods that are employed to estimate the
burial depth required to resist the lateral forces and overturning of a con-
crete pole. Foundation design is not an exact science because geotechnical
conditions are so variable. Once the soil design parameters have been
determined, the engineer may approach the design by using formulas avail-
able from ASCE, IEEE, or other available publications from similar organi-
zations. There are also commercially available computer programs that use
the load-defl ection curves of the soil properties obtained from actual full-
scale testing of different materials. These programs can provide reasonable
values of lateral groundline movement of the foundation and can thus be
highly useful in estimating the angle of the pole's resulting rotation.
Engineers are cautioned not to readily accept “rules of thumb” burial
depth estimates, such as the classic wood pole formula of 10% of pole
length plus 2 ft, because these values will seldom satisfy the burial require-
ments of heavily loaded transmission poles. Soil conditions are too vari-
able and line reliability is too much at stake to rely on burial depth
determinations made without some actual soil investigation.
Special Pole Considerations for Shallow Embedments
When a concrete pole is installed in a cylinder pile, in other precast
foundations, or in rock, it is imperative that allowances be made for trans-
ferring the shear and developing the bond of the prestressing strand.
Minimum embedments of 8 to 10 ft are commonly used to provide ade-
quate bond lengths and shear resistance to transfer the groundline
moment. Additional spiral reinforcing or structural rings also may be
employed to resist the shear. See Chapter 4 for further information about
this requirement.
Performance and Reliability
Foundation defl ection and rotation should be considered in the design.
Excessive defl ection or rotation of the foundation will create an undesir-
able appearance and may cause unfavorable load redistribution and an
increase in P-Delta effects. Foundation design should incorporate ade-
quate design factors to minimize future plumbing or adjustment of the
structure. In areas containing soils with high plasticity, consideration
should be given to the defl ections and reactions created by normal loading.
Pole weight, as well as confi guration eccentricities, may result in more
soil creep than in structures made of other materials.
Foundations for concrete pole structures typically can be classifi ed as
one of the following three types:
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