01 Jul 97
4. Structural Design of Drilled Shafts
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
loads to ensure adequate safety against structural failure of
the shaft. An example is worked out in Table 2-7c for FDL =
a. Eccentricity. If bending moments and shears are not
1.35 and FLL = 2.25 for a shaft supporting a bridge column.
specified, the minimum eccentricity should be the larger of
2 inches or 0.1Bs, where Bs is the shaft diameter, when tied
(3) The minimum reinforcement steel, normally
cages of reinforcement steel are used and 1 inch or 0.05Bs
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
b. Design example. Table 2-7 describes evaluation of
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, Mmax, 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, Qw,
Uplift thrust may develop before the full structural load is
and the estimate of the maximum applied lateral load, Tmax,
applied to the shaft. Under such conditions, smaller amounts
are used to calculate Mmax . 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