Koi Koi Grace
by on March 2, 2019
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Each structure must be securely embedded or anchored into the ground and facilitate safe transfer of structure loads to the ground strata below. To determine foundation requirements, the engineer must first evaluate the nature and condition of the soil in the vicinity of the structure. The choice of eventual foundation type will further depend on geotechnical characteristics of strata underneath, structure material, configuration, loads, constructability and economy. A majority of tangent poles are directly-embedded into ground; but systems defined by large lateral forces and moments require concrete drilled shafts whose design is a bit more labor-intensive.

Design criteria for foundations depend on type of soil and loads imposed. Design loads are usually factored reactions obtained from structural analysis from computer programs. In lattice towers, the loads transmitted are primarily compression and uplift loads. For single pole structures, the loads transmitted are overturning moment, shear (lateral) and axial loads. Where moment foundations such as drilled shafts are required, it is important to specify allowable deflection or rotation criteria, both elastic and non-recoverable. Chapter 4 will contain more information on this issue.

As with structures, foundations are also designed for given Strength Factors shown in Tables 2.15a and c. However, unlike structures, guidance in the area of foundation design is not well laid out by the codes and standards. Therefore it is common practice to use internal design criteria for foundations. These criteria vary from utility to utility. The structural design of all reinforced concrete systems is in general governed by the ACI 318 (2011). For drilled shafts with a diameter greater than 30 in (0.76 m), ACI- 336-3R (1998) is recommended.

Computer programs such as CAISSONTM (2011), LPILETM (2015) or MFADTM (2015) are used to quickly size a drilled shaft or directly-embedded pole under moment loads. Drilled shafts under moment loads are primarily designed as laterally-loaded piles. Guyed systems require anchors to transmit guy wire loads to the ground. Anchor types range from classical (log) to grouted (rock) to helical (screw) with widely-varying holding strengths depending on site-specific soils. If soil data is available, helical anchors are evaluated using HeliCAPTM (2007).

Geotechnical properties of ground strata are an integral part of the design criteria. In general, properties such as allowable bearing capacity, unit weight (dry, moist and submerged), friction angle, cohesion, subgrade modulus and others are required. Chapter 4 will discuss these issues in greater detail.

 

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