Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Cohesive Soil (See Figure 7-31) Assumed values:

• Applied shear load at the groundline (V) = 460 lbs. • Applied moment at the groundline (M) = 8600 ft-lbs. • Foundation diameter is 6” nominal Schedule 40. Use 6.625” as the actual pipe size in calculations. Cableway openings are 2.5” wide by 12” high. The allowable moment capacity of this foundation shaft size and cableway opening is 10,860 ft-lbs. • The required length (L) will be determined using the Broms method. • Cohesion (c) = 1000 psf. • Factor of Safety = 2. V F = V (FS) Equation 7-12

= 460 (2) = 920 lbs

Equation 7-13

V M

= M (FS)

= 8600 (2) = 17,200 ft-lbs

L = 1.5D+q [1+{ 2 + (4H+6D)/q} 0.5 ]

Equation 7-14

= 1.5 (6.625/12) + 0.185157 x [1 + { 2+ ( 4 x 18.69565 + 6 x (6.625/12)) / (0.185157)} 0.5 ] = 4.82 ft

D = Diameter of foundation = 6.625 inches q = V F /9cD = 920 / (9 x 1000 x 6.625/12) = 0.185157ft c = Shear strength of cohesive soil = 1000 psf

where:

DESIGN EXAMPLES

Moment / Shear = M/V = V M / V F = 17200 ft-lbs / 920 lbs = 18.69565 ft Calculated Foundation Length to Provide a SF of 2 Against Soil Failure.

H =

L =

The length required to provide a Factor of Safety of 2 against soil failure is 4.82 ft. Since SLF lengths are provided in even foot lengths, use L = 5 ft. For the required embedment length, the maximum moment in the shaft is: M MAX = V ( H + 1.5D + 0.5q) Equation 7-15 = 460 (18.69565 + (1.5 x 6.625/12) + (0.5 x 0.185157) = 9023.5 ft-lbs Maximum moment can be compared with the allowable moment capacity of the foundation shaft to determine adequacy. For this example the allowable moment in the 6” pipe shaft is given as 10,860 ft-lbs, which is greater than the applied moment. Therefore, the 6” diameter by 5’ long SLF is adequate for the applied loads in the clay soil.

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