Design Example - Drilled Shafts.

Table 2-7 describes evaluation of the shaft cross section and percent reinforcement steel required for adequate shafi strength under design loads.....

(1) The maximum bending moment, Mmax is required to determine the amount of reinforcement steel to resist bending. The maximum factored vertical working load, Qw, and the estimate of the maximum applied lateral load, Rmax, are used to calculate Mmax. The full amount of reinforcing steel is not required near the bottom of the pile because bending moments are usually negligible near the pile bottorn.

(2) Load factors are applied to the design live and dead loads to ensure adequate safety against structural failure of the shaft. An example is worked out in Table 2-7c for FDL =  1.35 and FLL = 2.25 for a shaft supporting a bridge column.

(3) The minimum reinforcement steel, normally recommended, is 1 percent of the total cross-sectional area of drilled shaft expected to be exposed along their length by scour or excavation. Reinforcement steel should be full length for shafts constructed in expansive soi1 and for shafts requiring casing while the hole is excavated. Shaft diameter should be increased if the reinforcement steel required to resist bending such that adequate voids through the reinforcement cage will be provided to accommodate the maximum aggregate size of the concrete.

(4) The maximum applied axial load should also include maxirnum downdrag forces for a shaft in compressibie soil and the maximum uplift thrust for a shaft in expansive soil. Uplift thrust may develop before the full structural load is applied to the shaft. Under such conditions, smaller amounts of reinforcement may be used if justified on the basis of relevant and appropriate computations.

Table 2-7 Minimum Requirements for Drilled Shaft Design.