Chance Technical Design Manual

EQUATION 8-78

V F

= V (FS) = 460 (2) = 920 lb

EQUATION 8-79

V M

= M (FS) = 8600 (2) = 17,200 ft∙lb

Broms’ equation for cohesionless soil requires a trial and error solution. For the trial and error solution, start by assuming the foundation diameter (D) is 6.625” and the length (L) is 6 feet: EQUATION 8-82 0 ≤ L 3 - ( 2V F L / K P g D ) – ( 2V M / K P g D ) = 6 3 - [ 2 x 920 x 6) / (3 x 100 {6.625/12})] - [(2 x 17200) / (3 x 100 x {6.625/12})] = - 58.35 where 0 > - 58.35 K P = tan 2 (45 + j /2 ) = 3.0 g = Effective unit weight of soil = 100 lb/ft 3 The 6 foot length is too short so we will try a 7 foot length and repeat the calculation: 0 = 7 3 - [2 x 920 x 7) / (3 x 100 {6.625/12})] - [(2 x 17200) / (3 x 100 x {6.625/12})] = 57.53 0 < 57.53 A 7 foot long SLF will be adequate. The maximum moment in the foundation shaft can be determined with the following equation: EQUATION 8-83 M MAX = V ( H + 0.54 x ( V / g DK P ) 0.5 ) = 460 (18.69565 + 0.54 x ( 460/100 x (6.625/12) x 3) 0.5 ) = 9013.968 ft∙lb This is less than the allowable moment capacity of 10,860 ft∙lb, therefore a 6” diameter by 7’ long SLF is adequate for the ap plied load in the sandy soil.

DESIGN EXAMPLES

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