Deep Foundation

Notation

A_g = gross area of concrete section, in²
A_s = area of nonprestressed longitudinal tension reinforcement, in²
d_pile = diameter of pile at footing base, in.
D = dead load
E = earthquake load
E_h = effect of horizontal seismic forces
E_m = seismic load effect including overstrength
E_mh = effect of horizontal seismic forces, including overstrength
E_v = vertical seismic effect applied in the vertical downward directio
f_c^' = specified compressive strength of concrete, psi
f_y = specified yield strength for nonprestressed reinforcement, psi
F = load due to fluids with well-defined pressures and maximum heights;
L = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials;
L_r = roof live load
P_a = maximum allowable compressive strength of a deep foundation member, lb
R = rain load
S = snow load
V_n = nominal shear strength, lb
V_u = factored shear force at section, lb
W = wind load
ϕ = strength reduction factor

Allowable Axial Strength

According to ACI 13.4.2.1, ASCE/SEI Section 2.4 and allowable strength specified in ACI Table 13.4.2.1 can be used to design a depp foundation member.

Basic Combinations given in ASCE/SEI Section 2.4 are shown below;

It shall be permitted to design a deep foundation member using allowable strength specified in ACI Table 13.4.2.1 if the two conditions given below are satisfied. If one of these two conditions is not satisfied, a depp foundation member should be designed using strength design in accordance with ACI 13.4.3.

• The deep foundation member is laterally supported for its entire height

• The applied forces cause bending moments in the deep foundation member less than the moment due to accidental eccentricity of %5 of the member diameter or width

Strength Design

Strength design in accordance with this section can be used for all deep foundation members.

According to ACI 13.4.2.2, the strength design of deep foundation members should be in accordance with Column Required and Design Strength title using the compressive strength reduction factors of ACI Table 13.4.3.2 for axial load with no moment, and the strength reduction factors of ACI Table 21.2.1 for tension, shear and combined axial force and moment.

Pile Caps

According to ACI 13.4.6.1, overall depth of pile cap should be selected such that the effective depth of bottom reinforcement is at least 12 inches.

According to ACI 13.4.6.3, pile caps should be designed such that condition 1 is satisfied for one-way foundations and condition 1 and condition 2 are satisfied for two-way foundations. These two conditions are given below,

1. ϕV_n ≥ V_u , where V_n should be calculated in accordance with One-Way Shear Strength title.

2. ϕv_n ≥ v_u , where v_n should be calculated in accordance with Two-Way Shear Strength title.

According to ACI 13.4.6.5, calculation of factored shear force one any pile cap section should be in accordance with conditions given below;

• Entire reaction from any pile with its center located d_pile/2 or more outside the section shall be considered as producing shear on that section.

• Reaction from any pile with its center located d_pile/2 or more inside the section shall be considered as producing no shear on that section.

• For intermediate positions of pile center, the portion of the pile reaction to be considered as producing shear on the section shall be based on a linear interpolation between full value at dpile/2 outside the section and zero value at d_pile/2 inside the section.