Adopts With Amendments:

ACI 318 2019

Part 1: General

Part 2: Loads & Analysis

Part 3: Members

Part 4: Joints/Connections/Anchors

Part 5: Earthquake Resistance

Part 6: Materials & Durability

Part 7: Strength & Serviceability

Part 8: Reinforcement

Part 9: Construction

Part 10: Evaluation

REFERENCES & Appendices

Heads up: There are no amended sections in this chapter.
This chapter shall apply to the design of nonprestressed and prestressed foundations, including shallow foundations (a) through (e), deep foundations (f) through (i), and retaining walls (j) and (k):
(a) Strip footings
(b) Isolated footings
(c) Combined footings
(d) Mat foundations
(e) Grade beams
(f) Pile caps
(g) Piles
(h) Drilled piers
(j) Cantilever retaining walls
(k) Counterfort and buttressed cantilever retaining walls
Foundations excluded by 1.4.7 are excluded from this chapter.
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Design properties for steel reinforcement shall be selected to be in accordance with Chapter 20.
Materials, design, and detailing requirements for embedments in concrete shall be in accordance with 20.6.
Design and detailing of cast-in-place and precast column, pedestal, and wall connections to foundations shall be in accordance with 16.3.
Structural members extending below the base of the structure that are required to transmit forces resulting from earthquake effects to the foundation shall be designed in accordance with 18.2.2.3.
For structures assigned to Seismic Design Category (SDC) C, D, E, or F, foundations resisting earthquake-induced forces or transferring earthquake-induced forces between structure and ground shall be designed in accordance with 18.13.
Slabs-on-ground that transmit vertical loads or lateral forces from other parts of the structure to the ground shall be designed and detailed in accordance with applicable provisions of this Code.
Slabs-on-ground that transmit lateral forces as part of the seismic-force-resisting system shall be designed in accordance with 18.13.
Plain concrete foundations shall be designed in accordance with Chapter 14.
Foundations shall be proportioned for bearing effects, stability against overturning and sliding at the soil-foundation interface in accordance with the general building code.
For one-way shallow foundations, two-way isolated footings, or two-way combined footings and mat foundations, it is permissible to neglect the size effect factor specified in 22.5 for one-way shear strength and 22.6 for two-way shear strength.
Foundation members shall be designed to resist factored loads and corresponding induced reactions except as permitted by 13.4.2.
Foundation systems shall be permitted to be designed by any procedure satisfying equilibrium and geometric compatibility.
Foundation design in accordance with the strut-and-tie method, Chapter 23, shall be permitted.
External moment on any section of a strip footing, isolated footing, or pile cap shall be calculated by passing a vertical plane through the member and calculating the moment of the forces acting over the entire area of member on one side of that vertical plane.
Mu at the supported member shall be permitted to be calculated at the critical section defined in accordance with Table 13.2.7.1.
Table 13.2.7.1—Location of critical section for Mu
Supported member Location of critical section
Column or pedestal Face of column or pedestal
Column with steel base plate Halfway between face of column and edge of steel base plate
Concrete wall Face of wall
Masonry wall Halfway between center and face of masonry wall
The location of critical section for factored shear in accordance with 7.4.3 and 8.4.3 for one-way shear or 8.4.4.1 for two-way shear shall be measured from the location of the critical section for Mu in 13.2.7.1.
Circular or regular polygon-shaped concrete columns or pedestals shall be permitted to be treated as square members of equivalent area when locating critical sections for moment, shear, and development of reinforcement.
Development of reinforcement shall be in accordance with Chapter 25.
Calculated tensile or compressive force in reinforcement at each section shall be developed on each side of that section.
Critical sections for development of reinforcement shall be assumed at the same locations as given in 13.2.7.1 for maximum factored moment and at all other vertical planes where changes of section or reinforcement occur.
Adequate anchorage shall be provided for tension reinforcement where reinforcement stress is not directly proportional to moment, such as in sloped, stepped, or tapered foundations; or where tension reinforcement is not parallel to the compression face.
Minimum base area of foundation shall be proportioned to not exceed the permissible bearing pressure when subjected to forces and moments applied to the foundation. Permissible bearing pressures shall be determined through principles of soil or rock mechanics in accordance with the general building code, or other requirements as determined by the building official.
Overall depth of foundation shall be selected such that the effective depth of bottom reinforcement is at least 6 in.

13.3.1.3

Diagram
In sloped, stepped, or tapered foundations, depth and location of steps or angle of slope shall be such that design requirements are satisfied at every section.
The design and detailing of one-way shallow foundations, including strip footings, combined footings, and grade beams, shall be in accordance with this section and the applicable provisions of Chapter 7 and Chapter 9.
Reinforcement shall be distributed uniformly across entire width of one-way footings.
The design and detailing of two-way isolated footings shall be in accordance with this section and the applicable provisions of Chapter 7 and Chapter 8.
In square two-way footings, reinforcement shall be distributed uniformly across entire width of footing in both directions.
In rectangular footings, reinforcement shall be distributed in accordance with (a) and (b):
(a) Reinforcement in the long direction shall be distributed uniformly across entire width of footing.
(b) For reinforcement in the short direction, a portion of the total reinforcement, γsAs, shall be distributed uniformly over a band width equal to the length of short side of footing, centered on centerline of column or pedestal. Remainder of reinforcement required in the short direction, (1 — γs)As, shall be distributed uniformly outside the center band width of footing, where γs is calculated by:
(13.3.3.3)
where β is the ratio of long to short side of footing.
The design and detailing of combined footings and mat foundations shall be in accordance with this section and the applicable provisions of Chapter 8.
The direct design method shall not be used to design combined footings and mat foundations.
Distribution of bearing pressure under combined footings and mat foundations shall be consistent with properties of the soil or rock and the structure, and with established principles of soil or rock mechanics.
Minimum reinforcement in nonprestressed mat foundations shall be in accordance with 8.6.1.1.
The design of walls as grade beams shall be in accordance with the applicable provisions of Chapter 9.
If a grade beam wall is considered a deep beam in accordance with 9.9.1.1, design shall satisfy the requirements of 9.9.
Grade beam walls shall satisfy the minimum reinforcement requirements of 11.6.
The stem of a cantilever retaining wall shall be designed as a one-way slab in accordance with the applicable provisions of Chapter 7.
The stem of a counterfort or buttressed cantilever retaining wall shall be designed as a two-way slab in accordance with the applicable provisions of Chapter 8.
For walls of uniform thickness, the critical section for shear and flexure shall be at the interface between the stem and the footing. For walls with a tapered or varied thickness, shear and moment shall be investigated throughout the height of the wall.
Number and arrangement of deep foundation members shall be determined such that forces and moments applied to the foundation do not exceed the permissible deep foundation strength. Permissible deep foundation strength shall be determined through principles of soil or rock mechanics in accordance with the general building code, or other requirements as determined by the building official.
Design of deep foundation members shall be in accordance with 13.4.2 or 13.4.3.
It shall be permitted to design a deep foundation member using load combinations for allowable stress design in ASCE/SEI 7, Section 2.4, and the allowable strength specified in Table 13.4.2.1 if (a) and (b) are satisfied:
(a) The deep foundation member is laterally supported for its entire height
(b) The applied forces cause bending moments in the deep foundation member less than the moment due to an accidental eccentricity of 5 percent of the member diameter or width
Table 13.4.2.1—Maximum allowable compressive strength for deep foundation members
Deep foundation member type Maximum allowable compressive strength[1]
Uncased cast-in-place concrete drilled or augered pile Pa = 0.3fc'Ag + 0.4fyAs (a)
Cast-in-place concrete pile in rock or within a pipe, tube, or other permanent metal casing that does not satisfy 13.4.2.3 Pa = 0.33fc'Ag + 0.4fyAs [2] (b)
Metal cased concrete pile confined in accordance with 13.4.2.3 Pa = 0.4fc'Ag (c)
Precast nonprestressed concrete pile Pa = 0.33fc'Ag + 0.4fyAs (d)
Precast prestressed concrete pile Pa = (0.33fc' —0.27fpc)Ag (e)
[1]Ag applies to the gross cross-sectional area. If a temporary or permanent casing is used, the inside face of the casing shall be considered the concrete surface.
[2]As does not include the steel casing, pipe, or tube.
If 13.4.2.1(a) or 13.4.2.1(b) is not satisfied, a deep foundation member shall be designed using strength design in accordance with 13.4.3.
Metal cased cast-in-place concrete deep foundation members shall be considered to be confined if (a) through (f) are satisfied:
(a) Design shall not use the casing to resist any portion of the axial load imposed.
(b) Casing shall have a sealed tip and shall be mandrel-driven.
(c) Thickness of the casing shall not be less than manufacturer's standard gauge No. 14 (0.068 in.).
(d) Casing shall be seamless, or provided with seams of strength equal to the basic material, and be of a configuration that will provide confinement to the cast-in-place concrete.
(e) Ratio of yield strength of the steel casing to fc' shall be at least 6, and yield strength shall be at least 30,000 psi.
(f) Nominal diameter of the member shall be less than or equal to 16 in.
The use of allowable strengths greater than those specified in Table 13.4.2.1 shall be permitted if accepted by the building official in accordance with 1.10 and justified by load tests.
Strength design in accordance with this section is permitted for all deep foundation members.
The strength design of deep foundation members shall be in accordance with 10.5 using the compressive strength reduction factors of Table 13.4.3.2 for axial load without moment, and the strength reduction factors of Table 21.2.1 for tension, shear, and combined axial force and moment. The provisions of 22.4.2.4 and 22.4.2.5 shall not apply to deep foundations.
Table 13.4.3.2—Compressive strength reduction factors ϕ for deep foundation members
Deep foundation member type Compressive strength reduction factors ϕ
Uncased cast-in-place concrete drilled or augered pile[1] 0.55 (a)
Cast-in-place concrete pile in rock or within a pipe, tube,[2] or other permanent casing that does not satisfy 13.4.2.3 0.60 (b)
Cast-in-place concrete-filled steel pipe pile[3] 0.70 (c)
Metal cased concrete pile confined in accordance with 13.4.2.3 0.65 (d)
Precast-nonprestressed concrete pile 0.65 (e)
Precast-prestressed concrete pile 0.65 (f)
[1]The factor of 0.55 represents an upper bound for well understood soil conditions with quality workmanship. A lower value for the strength reduction factor may be appropriate, depending on soil conditions and the construction and quality control procedures used.
[2]For wall thickness of the steel pipe or tube less than 0.25 in.
[3]Wall thickness of the steel pipe shall be at least 0.25 in.
Cast-in-place deep foundations that are subject to uplift or where Mu is greater than 0.4Mcr shall be reinforced, unless enclosed by a structural steel pipe or tube.
Portions of deep foundation members in air, water, or soils not capable of providing adequate restraint throughout the member length to prevent lateral buckling shall be designed as columns in accordance with the applicable provisions of Chapter 10.
Precast concrete piles supporting buildings assigned to SDC A or B shall satisfy the requirements of 13.4.5.2 through 13.4.5.6.
Longitudinal reinforcement shall be arranged in a symmetrical pattern.
For precast nonprestressed piles, longitudinal reinforcement shall be provided according to (a) and (b):
(a) Minimum of 4 bars
(b) Minimum area of 0.008Ag
For precast prestressed piles, the effective prestress in the pile shall provide a minimum average compressive stress in the concrete in accordance with Table 13.4.5.4.
Table 13.4.5.4—Minimum compressive stress in precast prestressed piles
Pile length, ft Minimum compressive stress, psi
Pile length ≤ 30 400
30 < Pile length ≤ 50 550
Pile length > 50 700
For precast prestressed piles, the effective prestress in the pile shall be calculated based on an assumed total loss of 30,000 psi in the prestressed reinforcement.
The longitudinal reinforcement shall be enclosed by transverse reinforcement according to Table 13.4.5.6(a) and shall be spaced according to Table 13.4.5.6(b):
Table 13.4.5.6(a)—Minimum transverse reinforcement size
Least horizontal pile dimension h, in. Minimum wire size transverse reinforcement[1]
h ≤ 16 W4, D4
16 < h < 20 W4.5, D5
h ≥ 20 W5.5, D6
[1]If bars are used, minimum of No. 3 bar applies to all values of h.
Table 13.4.5.6(b)—Maximum transverse reinforcement spacing
Reinforcement location in the pile Maximum center-to-center spacing, in.
First five ties or spirals at each end of pile 1
24 in. from each end of pile 4
Remainder of pile 6
Overall depth of pile cap shall be selected such that the effective depth of bottom reinforcement is at least 12 in.
Factored moments and shears shall be permitted to be calculated with the reaction from any pile assumed to be concentrated at the centroid of the pile section.
Except for pile caps designed in accordance with 13.2.6.5, the pile cap shall be designed such that (a) is satisfied for one-way foundations and (a) and (b) are satisfied for two-way foundations.
(a) ϕVnVu, where Vn shall be calculated in accordance with 22.5 for one-way shear, Vu shall be calculated in accordance with 13.4.6.5, and ϕ shall be in accordance with 21.2
(b) ϕvnvu, where vn shall be calculated in accordance with 22.6 for two-way shear, vu shall be calculated in accordance with 13.4.6.5, and ϕ shall be in accordance with 21.2
If the pile cap is designed in accordance with the strut-and-tie method as permitted in 13.2.6.5, the effective concrete compressive strength of the struts, fce, shall be calculated in accordance with 23.4.3.
Calculation of factored shear on any section through a pile cap shall be in accordance with (a) through (c):
(a) Entire reaction from any pile with its center located dpile/2 or more outside the section shall be considered as producing shear on that section.
(b) Reaction from any pile with its center located dpile/2 or more inside the section shall be considered as producing no shear on that section.
(c) 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 dpile/2 inside the section.
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