Chance Technical Design Manual

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 kip • Helical guywire anchor installation angle = IA ug = tan -1 (16.6/17.9) = 43° LOWER GUYWIRE LOADS • 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 kip • Helical guywire anchor installation angle = IA lg = tan -1 (7.9/9.7) = 39°

DESIGN EXAMPLES

PLAN

TOWER GUY ANCHOR AND FOUNDATION FIGURE 8-18

MAST FOUNDATION LOADS • Compression (C) = 68.0 kip • Horizontal shear (V) = 0.3 kip

• Length: 45 ft (along the shaft at the 43° installation angle) • Recommended Ultimate Capacity (R uc ): 50.2 kip (tension) The Factor of Safety for this tension anchor is R uc /T ug = 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: EQUATION 8-57 T = (T ug x FS) / K t = (24,400 x 2.0) / 10 = 4,900 ft∙lb where K t = Empirical torque factor = 10 (default value for Type SS5 series) T = 4,900 ft∙lb is less than the rated torque (5,700 ft∙lb) of the Type SS5 series. (OK).

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 an chor is shown in Figure 8-20. 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: 43° • Datum Depth (depth below grade where installation starts): 0 ft

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