Transmission And Substation Foundations - Technical Design Manual (TD06088E)

and a horizontal shear load at the ground line. These three loads can be dealt with in a number of ways. Typically one or more helical piles are used for each leg of the tower and may be installed at a batter to better resist the horizontal shear loads. Steel grillages and reinforced concrete caps have been used to facilitate load transfer from the structure to the helical piles. This type design will not be covered in this design example since the intent is to focus on the guyed mast tower structure. Figure 8-18 shows the tower that will be used for these sample calculations. It will be noted that the four upper guywires come to the ground at a single guywire point and that the three lower guywires come to ground at a different guywire point. There must be at least a single helical anchor installed at each of these points to provide restraint for the guywires which in turn stabilize the tower by resisting lateral loads on the structure. For this tower, the vertical and horizontal components of the guywire loads are given and must be resolved into the tension load the helical guywire anchor is to resist. Upper Guywire Loads

• Vertical load component = 16.6 kip • Horizontal load component = 17.9 kip

• Tension in the upper guywire anchor = T ug = (16.6 2 + 17.9 2 ) 0.5 = 24.4 k • Helical guywire anchor installation angle = IA ug = tan -1 (16.6/17.9) = 43° Lower Guywire Loads

DESIGN EXAMPLES

• Vertical load component: 7.9 kip • Horizontal load component: 9.7 kip

• Tension in the lower guywire anchor = T lg = (7.9 2 + 9.7 2 ) 0.5 = 12.5 k • Helical guywire anchor installation angle = IA lg = tan -1 (7.9/9.7) = 39° Mast Foundation Loads

• Compression (C) = 68.0 kip • Horizontal shear (V) = 0.3 kip Selecting Helical Guywire Anchors

Hubbell Power Systems, Inc. HeliCAP ® engineering software will be utilized to determine the appropriate helical anchor/pile sizes for this tower. Soil conditions are shown in the sample boring log in Figure 8-19. The soil data and guywire anchor data was input into the HeliCAP ® engineering software to get an appropriate output. The minimum acceptable Factor of Safety (FS) = 2. Upper Guywire Helical Anchor The HeliCAP ® Summary Report for the upper guywire helical anchor is shown in Figure 7-2. This report provides the following information: • Helical Anchor: SS5 (1.5” square shaft, 5500 ft-lbs torque rating, 70 kips ultimate tension rating) • 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 (R uc ): 50.2t (kips tension) The Factor of Safety for this tension anchor is R uc /T lg = 50.2 / 24.4 = 2.05 > 2 (OK). Use this helical anchor at each of three upper guywire anchor locations per tower.

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