Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 1

Helical Pile Given: • Compression Load = 68.0 kip • Shear Load = 0.3 kip

• Lead Section: 4 helix (8”-10”-12”-14”) • Installation Angle: 43° • Datum Depth (depth below grade where installation starts): 0 ft • Length: 45 ft (along the shaft at the 43° installation angle) • Recommended Ultimate Capacity (Ruc): 50.2 kip (tension) The Factor of Safety for this tension anchor is Ruc /Tug = 50.2 / 24.4 = 2.05 > 2 (OK). Use this helical anchor at each of three upper guywire anchor locations per tower. The required average minimum installation torque (T) is:

Assume three helical piles installed at 120° intervals in plan view with each pile battered away from vertical at a 10° angle: 68/3 piles = 22.67 kip ultimate/pile element. Assume entire shear (0.3 kip) is taken by one battered pile. Therefore, the resultant axial load (DL) to a battered pile is: DL = (22.67 2 + 0.3 2 ) 0.5 = 22.7 kip • The HeliCAP summary report for the helical piles is shown in Figure 8-22. This report provides the following information: • Helical Pile: SS175 (1.75” square shaft, 10,500 ft∙lb torque rating, 100 kip ultimate tension rating) • Lead Section: 4 helix (8”-10”-12”-14”) • Installation Angle: 80° below horizontal (10° away from vertical) • Datum Depth: (depth below grade where installation starts): 0 ft • Length: 34 ft (along the shaft at the 80° installation angle) • Recommended Ultimate Capacity (R uc ): 50.7 kip (compression) The Factor of Safety for this compression pile is R uc / DL = 50.7 / 22.7 = 2.23 > 2 (OK) Use three SS175 helical piles per tower base. The three helical piles must be captured in a “pile cap.” This may be a reinforced concrete cap, the design of which is beyond the scope of this design example. The design of this concrete pile cap is left to the structural engineer.

EQUATION 7-1

T

=

(T ug x FS) / K t

=

(24,400 x 2.0) / 10

=

4,900 ft∙lb

where

=

Empirical torque factor = 10 (default value for Type SS5 series) 4,900 ft∙lb is less than the rated torque (5,700 ft∙lb) of the Type SS5 series. (OK).

K t

T

=

Lower Guywire Helical Anchor The HeliCAP® summary report for the lower guywire helical anchor is shown in Figure 7-4. This report provides the following information: • Helical Anchor: SS5 (1.5” square shaft, 5,700 ft∙lb torque rating, 70 kip ultimate tension rating) • Lead Section: 4 helix (8”-10”-12”-14”) • Installation Angle: 39° • Datum Depth (depth below grade where installation starts): 0 ft • Length: 25 ft (along the shaft at the 39° installation angle) • Recommended Ultimate Capacity (Ruc): 26.6 kip (tension) The Factor of Safety for this tension anchor is Ruc / Tlg = 26.6 / 12.5 = 2.12 > 2 (OK) Use this helical anchor at each of three lower guywire anchor locations per tower.

EQUATION 7-3

T

=

(DL x FS) / K t

=

(22,700 x 2.0) / 10

=

4,500 ft∙lb

where

=

Empirical torque factor = 10 (default value for Type SS175 series) 4,500 ft∙lb is less than the rated torque (10,500 ft∙lb) of the Type SS175 series. (OK).

K t

T

=

EQUATION 7-2

T

=

(T lg x FS) / K t

=

(12,500 x 2.0) / 10

=

2,500 ft∙lb

where

=

Empirical torque factor = 10 (default value for Type SS5 series) 2,500 ft∙lb is less than the rated torque (5,700 ft∙lb) of the Type SS5 series. (OK).

K t

T

=

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