bending moments are usually negligible near the pile bottom.

Chapter 4 discusses procedures for calculating the

Most drilled shaft foundations will be subject to lateral loads,

distribution of bending moments to determine where steel

bending moments, and shear stresses in addition to

will be placed in the pile.

compressive stresses from vertical loads. Eccentrically

vertical applied loads can generate additional bending

(2) Load factors are applied to the design live and dead

moments.

loads to ensure adequate safety against structural failure of

the shaft. An example is worked out in Table 2-7c for *F*DL =

1.35 and *F*LL = 2.25 for a shaft supporting a bridge column.

specified, the minimum eccentricity should be the larger of

2 inches or 0.1*B*s, where *B*s is the shaft diameter, when tied

(3) The minimum reinforcement steel, normally

cages of reinforcement steel are used and 1 inch or 0.05*B*s

recommended, is 1 percent of the total cross-sectional area of

when spiral cages are used. The minimum eccentricity

drilled shaft expected to be exposed along their length by

should be the maximum permitted deviation of the shaft out

scour or excavation. Reinforcement steel should be full

of its plan alignment that does not require special

length for shafts constructed in expansive soil and for shafts

computations to calculate the needed reinforcement if larger

requiring casing while the hole is excavated. Shaft diameter

eccentricities are allowed.

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

the shaft cross section and percent reinforcement steel

maximum aggregate size of the concrete.

required for adequate shaft strength under design loads.

(4) The maximum applied axial load should also include

(1) The maximum bending moment, *M*max, is required to

maximum downdrag forces for a shaft in compressible soil

determine the amount of reinforcement steel to resist

and the maximum uplift thrust for a shaft in expansive soil.

bending. The maximum factored vertical working load, *Q*w,

Uplift thrust may develop before the full structural load is

and the estimate of the maximum applied lateral load, *T*max,

applied to the shaft. Under such conditions, smaller amounts

are used to calculate *M*max . The full amount of reinforcing

of reinforcement may be used if justified on the basis of

steel is not required near the bottom of the pile because

relevant and appropriate computations.

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