Chance Technical Design Manual
H = Moment / Shear = M/V = V M / V F = 17200 ft∙lb / 920 lb = 18.69565 ft L = Calculated Foundation Length to Provide a SF of 2 Against Soil Failure. 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: EQUATION 8-81 M MAX = V ( H + 1.5D + 0.5q) = 460 (18.69565 + (1.5 x 6.625/12) + (0.5 x 0.185157) = 9023.5 ft∙lb Maximum moment can be compared with the allowable mo ment 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∙lb, 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.
DESIGN EXAMPLES
FOUNDATION IN COHESIVE SOIL FIGURE 8-29
COHESIVE SOIL (SEE FIGURE 8-29) Assumed values:
• Applied shear load at the groundline (V) = 460 lb • Applied moment at the groundline (M) = 8600 ft∙lb • 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 open ing is 10,860 ft∙lb • The required length (L) will be determined using the Broms’ method. • Cohesion (c) = 1000 psf • Factor of Safety = 2 EQUATION 8-78 V F = V (FS) = 460 (2) = 920 lb EQUATION 8-79 V M = M (FS) = 8600 (2) = 17,200 ft∙lb EQUATION 8-80 L = 1.5D+q [1+{ 2 + (4H+6D)/q} 0.5 ] = 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 where D = Diameter of foundation = 6.625 inches q = V F /9cD = 920 / (9 x 1000 x 6.625/12) = 0.185157 ft c = Shear strength of cohesive soil = 1000 psf
3 γ KpDL
FOUNDATION IN COHESIONLESS SOIL FIGURE 8-30
COHESIONLESS SOIL (SEE FIGURE 8-30) Assumed values:
• Applied shear load at the groundline (V) = 460 lb • Applied moment at the groundline (M) = 8600 ft∙lb • 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 open ing is 10,860 ft∙lb. • The required length (L) will be determined using the Broms’ method. • j = 30° • g = 100 lb/ft 3 • Factor of Safety = 2.
Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2023 | Page 8-37
Made with FlippingBook - professional solution for displaying marketing and sales documents online