= 18.13 — Foundations
== 18.13.1 Scope
=== 18.13.1.1 This section shall apply to foundations resisting
earthquake-induced forces or transferring earthquake induced
forces between structure and ground.
=== 18.13.1.2 The provisions in this section for piles, drilled
piers, caissons, and slabs-on-ground shall supplement other
applicable Code design and construction criteria, including
1.4.6 and 1.4.7.
== 18.13.2 Footings, foundation mats, and pile caps
=== 18.13.2.1 The provisions of this section shall apply to
structures assigned to SDC D, E, or F.
=== 18.13.2.2 Longitudinal reinforcement of columns and
structural walls resisting forces induced by earthquake
effects shall extend into the footing, mat, or pile cap, and
shall be fully developed for tension at the interface.
=== 18.13.2.3 Columns designed assuming fixed-end conditions
at the foundation shall comply with 18.13.2.2 and,
if hooks are required, longitudinal reinforcement resisting
flexure shall have 90-degree hooks near the bottom of the
foundation with the free end of the bars oriented toward the
center of the column.
= R18.13 — Foundations
== R18.13.1 Scope
Requirements for foundations supporting buildings
assigned to SDC C, D, E, or F represent a consensus of a
minimum level of good practice in designing and detailing
concrete foundations. However, because repairs to foundations
can be extremely difficult and expensive, it may be
desirable that the elements of the foundation remain essentially
elastic during strong ground motions. Methods to
achieve this goal include designing the foundation to include
an overstrength factor or an increased seismic demand level
when compared to the superstructure, or comparing strengths
to demands predicted by nonlinear response history analyses
with appropriate consideration of uncertainty in demands
(Klemencic et al. 2012).
== R18.13.2 Footings, foundation mats, and pile caps
=== R18.13.2.3 Tests (Nilsson and Losberg 1976) have
demonstrated that flexural members terminating in a footing,
slab, or beam (a T-joint or L-joint) should have their hooks
turned inward toward the axis of the member for the joint to
be able to resist the flexure in the member forming the stem
of the T or L.
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PART 5: EARTHQUAKE RESISTANCE 343
18 Seismic
No further reproduction or distribution is permitted.
=== 18.13.2.4 Columns or boundary elements of special structural
walls that have an edge within one-half the footing
depth from an edge of the footing shall have transverse
reinforcement in accordance with 18.7.5.2 through 18.7.5.4
provided below the top of the footing. This reinforcement
shall extend into the footing, mat, or pile cap a length equal
to the development length, calculated for fy in tension, of
the column or boundary element longitudinal reinforcement.
=== 18.13.2.5 Where earthquake effects create uplift forces in
boundary elements of special structural walls or columns,
flexural reinforcement shall be provided in the top of the
footing, mat, or pile cap to resist actions resulting from the
factored load combinations, and shall be at least that required
by 7.6.1 or 9.6.1.
=== 18.13.2.6 Structural plain concrete in footings and basement
walls shall be in accordance with 14.1.4.
=== 18.13.2.7 Pile caps incorporating batter piles shall be
designed to resist the full compressive strength of the batter
piles acting as short columns. The slenderness effects of
batter piles shall be considered for the portion of the piles
in soil that is not capable of providing lateral support, or in
air or water.
=== R18.13.2.4 Columns or boundary members supported
close to the edge of the foundation, as often occurs near
property lines, should be detailed to prevent an edge failure
of the footing, pile cap, or mat.
=== R18.13.2.5 The purpose of this section is to emphasize
that top reinforcement in footings, mats, and pile caps may
be required, in addition to other required reinforcement.
=== R18.13.2.6 Foundation and basement walls should be
reinforced in buildings assigned to SDC D, E, or F.
=== R18.13.2.7 Batter piles typically attract higher lateral
forces during earthquakes than vertical piles. Extensive
structural damage has been observed at the junction of batter
piles and building foundations. The pile cap and surrounding
structure should be designed for the potentially large forces
that can be developed in batter piles.
== 18.13.3 Grade beams and slabs-on-ground
=== 18.13.3.1 For structures assigned to SDC D, E, or F, grade
beams and beams that are part of a mat foundation subjected
to flexure from columns that are part of the seismic-forceresisting
system shall be in accordance with 18.6.
=== 18.13.3.2 For structures assigned to SDC C, D, E, or F,
slabs-on-ground that resist in-plane earthquake forces from
walls or columns that are part of the seismic-force-resisting
system shall be designed as diaphragms in accordance with
18.12. The construction documents shall clearly indicate that
the slab-on-ground is a structural diaphragm and part of the
seismic-force-resisting system.
== R18.13.3 Grade beams and slabs-on-ground
For earthquake conditions, slabs-on-ground (soil-supported
slabs) are often part of the lateral-force-resisting system and
should be designed in accordance with this Code as well as
other appropriate standards or guidelines (refer to 1.4.8).
=== R18.13.3.1 Grade beams resisting flexural stresses from
column moments should have reinforcement details similar
to the beams of the frame above the foundation.
=== R18.13.3.2 Slabs-on-ground often act as a diaphragm to
tie the building together at the ground level and minimize the
effects of out-of-phase ground motion that may occur over
the footprint of the building. The construction documents
should clearly state that these slabs-on-ground are structural
members so as to prohibit saw cutting of the slab.
== 18.13.4 Foundation seismic ties
=== 18.13.4.1 For structures assigned to SDC C, D, E, or F,
individual pile caps, piers, or caissons shall be interconnected
by foundation seismic ties in orthogonal directions,
unless it can be demonstrated that equivalent restraint is
provided by other means.
== R18.13.4 Foundation seismic ties
=== R18.13.4.1 The foundation seismic ties should sufficiently
interconnect foundations to act as a unit and be designed to
minimize the relative movement of an individual column or
tie relative to the foundation. This is essential where surface
soils are soft enough to require deep foundations or where
the site soils are susceptible to liquefaction.
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344 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
=== 18.13.4.2 For structures assigned to SDC D, E, or F, individual
spread footings founded on soil defined in ASCE/SEI
7 as Site Class E or F shall be interconnected by foundation
seismic ties.
=== 18.13.4.3 Where required, foundation seismic ties shall
have a design strength in tension and compression at least
equal to 0.1SDS times the greater of the pile cap or column
factored dead load plus factored live load unless it is demonstrated
that equivalent restraint will be provided by (a), (b),
(c), or (d):
(a) Reinforced concrete beams within the slab-on-ground
(b) Reinforced concrete slabs-on-ground
(c) Confinement by competent rock, hard cohesive soils,
(d) Other means approved by the building official
=== 18.13.4.4 For structures assigned to SDC D, E, or F, grade
beams designed to act as horizontal foundation seismic ties
between pile caps or footings shall have continuous longitudinal
reinforcement that shall be developed within or beyond
the supported column or anchored within the pile cap or
footing at all discontinuities and shall satisfy (a) and (b):
(a) The smallest cross-sectional dimension of the grade
beam shall be at least equal to the clear spacing between
connected columns divided by 20, but need not exceed
(b) Closed tie transverse reinforcement shall be provided at
a spacing not to exceed the lesser of 0.5 times the smallest
orthogonal cross-sectional dimension and 300 mm.
=== R18.13.4.2 The ties between footings should have the same
characteristics as the ties between pile caps in R18.13.4.1.
=== R18.13.4.3 The minimum foundation seismic tie design
strength requirement based on a percentage of the factored
dead plus live load provides a minimum connection between
foundation elements. Other types of restraint can be used
if substantiated as equivalent to the minimum tie design
strength. The required design strength for the tie beam must
be at least equal to 0.1SDS times the larger force on either end
of the tie beam, and that force is from the column or pile cap,
whichever applies.
== 18.13.5 Deep foundations
=== 18.13.5.1 This section shall apply to the following types
(a) Uncased cast-in-place concrete drilled or augered piles
(b) Metal cased concrete piles
(c) Concrete filled pipe piles
(d) Precast concrete piles
=== 18.13.5.2 For structures assigned to SDC C, D, E, or F,
piles, piers, or caissons resisting tension loads shall have
continuous longitudinal reinforcement over their length to
resist design tension forces.
=== 18.13.5.3 For structures assigned to SDC C, D, E, or F, the
minimum longitudinal and transverse reinforcement required
by 18.13.5.7 through 18.13.5.10 shall be extended over the
entire unsupported length for the portion of deep foundation
member in air or water, or in soil that is not capable
of providing adequate lateral restraint to prevent buckling
throughout this length.
== R18.13.5 Deep foundations
Adequate performance of piles and caissons for earthquake
effects requires that these provisions be met in addition
to other applicable standards or guidelines (refer to
R1.4.7).
=== R18.13.5.3 Minimum reinforcement lengths for both
longitudinal and transverse reinforcement are based on
the assumption that soil is capable of providing lateral
support. For portions of the pile above ground, typically in
air or water, or where soil is not capable of providing this
lateral restraint, the minimum reinforced lengths should be
increased, and the member should be designed as a column.
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PART 5: EARTHQUAKE RESISTANCE 345
18 Seismic
No further reproduction or distribution is permitted.
=== 18.13.5.4 For structures assigned to SDC C, D, E, or F,
hoops, spirals, and ties in deep foundation members shall be
terminated with seismic hooks.
=== 18.13.5.5 For structures assigned to SDC D, E, or F or
located in Site Class E or F, concrete deep foundation
members shall have transverse reinforcement in accordance
with 18.7.5.2, 18.7.5.3, and Table 18.7.5.4 Item (e) within
seven member diameters above and below the interfaces
between strata that are hard or stiff and strata that are liquefiable
or soft.
=== 18.13.5.6 For structures assigned to SDC D, E, or F, in
foundations supporting one- and two-story stud bearing wall
construction, concrete piles, piers or caissons, and foundation
ties are exempt from the transverse reinforcement
requirements of 18.13.5.3 through 18.13.5.5.
=== R18.13.5.5 During earthquakes, piles can be subjected to
high flexural and shear demands at points of discontinuity,
such as at interfaces between stiff and soft soil strata. ASCE/
SEI 7 defines limits for soil strata. Transverse reinforcement
is required in these regions to provide ductile behavior.
In determining the portions of a pile with increased transverse
reinforcement, accommodations are often made to the
length of the reinforced zone for transverse reinforcement to
account for variations in the driven pile tip elevations and
variations in the interface elevations between stiff and soft
soil strata.
=== 18.13.5.7 Uncased cast-in-place drilled or augered
==== 18.13.5.7.1 For structures assigned to SDC C, D, E, or
F, reinforcement shall be provided in uncased cast-in-place
drilled or augered concrete piles where required by analysis
and in accordance with the requirements in Table 18.13.5.7.1.
=== R18.13.5.7 Uncased cast-in-place drilled or augered
==== R18.13.5.7.1 Longitudinal and transverse reinforcement
requirements prescribed by this section result in ductility
consistent with the applicable Seismic Design Category
(SDC) to withstand ground deformation that occurs during
earthquakes.
Where piles are subjected to significant uplift forces, the
longitudinal reinforcement length required by analysis may
exceed the minimum reinforcement length requirements.
Transverse reinforcement is required at the top of the pile
to provide ductile performance where flexural yielding can
potentially occur. For SDC D, E, and F and Site Classes A,
B, C, and D, one-half of the transverse reinforcement for
special moment frame columns is acceptable because some
level of confinement is attributed to competent soils. For Site
Class E and F, full column confinement is required because
the soils are either liquefiable or not considered competent
enough to provide confinement.
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346 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
Table 18.13.5.7.1—Minimum reinforcement for uncased cast-in-place or augered concrete piles or piers
Minimum reinforcement_
==== 18.13.5.7.2 Minimum longitudinal and transverse reinforcement
shall be provided along minimum reinforced
lengths measured from the top of the pile in accordance with
Table 18.13.5.7.1.
==== 18.13.5.7.3 Longitudinal reinforcement shall extend at least
the development length in tension beyond the flexural length of
the pile, which is defined in Table 18.13.5.7.1 as the distance
from the bottom of the pile cap to where 0.4Mcr > Mu.
==== R18.13.5.7.3 Reinforcement should extend ℓd beyond the
point where plain concrete is no longer adequate to resist the
factored moment.
=== 18.13.5.8 Metal-cased concrete piles
==== 18.13.5.8.1 For structures assigned to SDC C, D, E, or
F, longitudinal reinforcement requirements and minimum
reinforced lengths for metal-cased concrete piles shall be the
same as for uncased concrete piles in 18.13.5.7.
==== 18.13.5.8.2 Metal-cased concrete piles shall have a spiral-welded
metal casing of a thickness not less than 2 mm that
=== R18.13.5.8 Metal-cased concrete piles
==== R18.13.5.8.2 Spiral-welded metal casing with the specified
wall thickness provides confinement equivalent to
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PART 5: EARTHQUAKE RESISTANCE 347
18 Seismic
No further reproduction or distribution is permitted.
==== R18.13.5.8.2 Continuation
closed ties or spirals required in an uncased concrete pile
and eliminates the need for confinement ties.
=== R18.13.5.9 Concrete-filled pipe piles
==== R18.13.5.9.1 For resistance to uplift forces, concrete bond
to the steel pipe is to be ignored in determining anchorage
of the pile. Concrete shrinkage can be detrimental to bond,
therefore shrinkage should be controlled, or force transfer
via other methods such as headed studs or surface irregularities
on the pipe should be considered. Reinforcement at the
top of the pile is extended into the pile cap to tie the elements
together and assist transfer of force to the pile cap.
=== R18.13.5.10 Precast concrete piles
==== R18.13.5.10.1 The potential for driving precast piles to a
tip elevation different than that specified in the construction
documents should be considered when detailing the pile. If
the pile reaches refusal at a shallower depth, a longer length
of pile will need to be cut off. If this possibility is not foreseen,
the length of transverse reinforcement required by
these provisions may not be provided after the excess pile
length is cut off.
==== R18.13.5.10.4(a) In a study of minimum confinement
reinforcement for prestressed concrete piles (Sritharan et al.
2016), the relationship between curvature ductility demand
==== 18.13.5.8.2 Continuation
is adequately protected from possible deleterious action due
to soil constituents, changing water levels, or other factors
indicated by boring records of site conditions.
=== 18.13.5.9 Concrete-filled pipe piles
==== 18.13.5.9.1 For structures assigned to SDC C, D, E or F,
concrete-filled pipe piles shall have longitudinal reinforcement
in the top of the pile with a total area of at least 0.01Ag
and with a minimum length within the pile equal to two times
the required embedment length into the pile cap, but not less
than the development length in tension of the reinforcement.
=== 18.13.5.10 Precast concrete piles
==== 18.13.5.10.1 For precast concrete driven piles, the length
of transverse reinforcement provided shall be sufficient to
account for potential variations in the elevation of pile tips.
==== 18.13.5.10.2 Precast nonprestressed concrete piles for
structures assigned to SDC C shall satisfy (a) through (d):
(a) Minimum longitudinal steel reinforcement ratio shall
be 0.01.
(b) Longitudinal reinforcement shall be enclosed within a
minimum of No. 10 closed ties or 10 mm diameter spirals,
for up to 500 mm diameter piles, and No. 13 closed ties
or 13 mm diameter spirals, for larger diameter piles.
(c) Spacing of transverse reinforcement within a distance
of 3 times the least cross-sectional dimension of the pile
from the bottom of the pile cap shall not exceed the lesser
of 8 times the diameter of the smallest longitudinal bar
and 150 mm.
(d) Transverse reinforcement shall be provided throughout
the length of the pile at a spacing not exceeding 150 mm.
==== 18.13.5.10.3 For structures assigned to SDC D, E, or
F, precast nonprestressed concrete piles shall satisfy the
requirements of 18.13.5.10.2 and the requirements for
uncased cast-in-place or augered concrete piles in SDC D,
E, or F in Table 18.13.5.7.1.
==== 18.13.5.10.4 For structures assigned to SDC C, precastprestressed
concrete piles shall satisfy (a) and (b):
(a) If the transverse reinforcement consists of spirals or
circular hoops, the volumetric ratio of transverse reinforcement,
ρs, in the upper 6 m shall not be less than that
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348 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
No further reproduction or distribution is permitted.
==== 18.13.5.10.4 Continuation
calculated by Eq. (18.13.5.10.4a) or calculated from a
more detailed analysis by Eq. (18.13.5.10.4b):
0.15 (fc'/fyt) ... (18.13.5.10.4a)
0.04 (fc'/fyt)( 2.8 + (2.3Pu)/(fc' Ag) ) ... 18.13.5.10.4b)
and fyt shall not be taken greater than 690 MPa.
(b) A minimum of one-half of the volumetric ratio of
spiral reinforcement required by Eq. (18.13.5.10.4a) or
Eq. (18.13.5.10.4b) shall be provided for the remaining
length of the pile.
==== 18.13.5.10.5 For structures assigned to SDC D, E, or F,
precast-prestressed concrete piles shall satisfy (a) through
(e) and the ductile pile region shall be defined as the length
of pile measured from the bottom of the pile cap to the point
of zero curvature plus 3 times the least pile dimension, but
not less than 10.5 m. If the total pile length in the soil is 10.5
m or less, the ductile pile region shall be taken as the entire
length of the pile:
(a) In the ductile pile region, the center-to-center spacing
of spirals or hoop reinforcement shall not exceed the least
of 0.2 times the least pile dimension, 6 times the diameter
of the longitudinal strand, and 150 mm.
(b) Spiral reinforcement shall be spliced by lapping one
full turn, by welding, or by the use of a mechanical splice.
If spiral reinforcement is lap spliced, the ends of the spiral
shall terminate in a seismic hook. Mechanical and welded
splices of deformed bars shall comply with 25.5.7.
(c) If the transverse reinforcement consists of spirals, or
circular hoops, the volumetric ratio of transverse reinforcement,
ρs, in the ductile pile region shall not be less
than that calculated by Eq. (18.13.5.10.5a) or calculated
from a more detailed analysis by Eq. (18.13.5.10.5b),
and the required volumetric ratio shall be permitted to be
obtained by providing an inner and outer spiral.
0.2 (fc'/fyt) ... (18.13.5.10.5)
0.06 (fc'/fyt) ( 2.8 + (2.3 Pu)/(fc' Ag) ) ... (18.13.5.10.5b)
and fyt shall not be taken as greater than 690 MPa.
(d) Outside of the ductile pile region, spiral or hoop reinforcement
shall be provided with a volumetric ratio not
less than one-half of that required within the ductile pile
==== R18.13.5.10.4 (a) Continuation
on prestressed piles and overall system ductility demand
was considered in the context of all soil profiles identified
in ASCE/SEI 7. It was concluded that Eq. (18.13.5.10.4b)
results in adequate deformation capacity for structures
assigned to SDC C. The factored axial force on a pile should
be determined from Eq. (5.3.1c) and Eq. (5.3.1g) with 5.3.7
and 5.3.8 as applicable.
==== R18.13.5.10.5 Observed damage from earthquakes and
concerns about the accuracy of calculated pile demands have
led to prescriptive requirements for confinement of potential
yielding regions of piles. The required confinement is
intended to provide adequate ductility capacity for structures
assigned to SDC D, E, and F (Sritharan et al. 2016).
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PART 5: EARTHQUAKE RESISTANCE 349
18 Seismic
No further reproduction or distribution is permitted.
==== 18.13.5.10.5 Continuation
region, and the maximum spacing shall be in accordance
with Table 13.4.5.6(b).
(e) If transverse reinforcement consists of rectangular
hoops and crossties, the total cross-sectional area of lateral
transverse reinforcement in the ductile region shall be the
greater of Eq. (18.13.5.10.5c) and Eq. (18.13.5.10.5d).
The hoops and crossties shall be equivalent to deformed
bars not less than No. 10 in size, and rectangular hoop
ends shall terminate at a corner with seismic hooks.
Ash = 0.3 sbc (fc'/fyt) ( Ag/Ach -1.0 )( 0.5 + (1.4Pu)/(fc' Ag) )
... (18.13.5.10.5c)
Ash = 0.12 sbc (fc'/fyt) ( 0.5 + (1.4 Pu)/(fc'Ag) )
... (18.13.5.10.5d)
and fyt shall not be taken as greater than 690 MPa.
==== 18.13.5.10.6 For structures assigned to SDC C, D, E, or
F, the maximum factored axial load for precast prestressed
piles subjected to a combination of earthquake lateral force
and axial load shall not exceed the following values:
(a) 0.2fc′Ag for square piles
(b) 0.4fc′Ag for circular or octagonal piles
==== R18.13.5.10.6 The axial load in precast prestressed piles is
limited to preclude spalling of the concrete cover prior to the
pile section experiencing flexural cracking, as this will result
in a significant loss in pile resistance (Sritharan et al. 2016).
== 18.13.6 Anchorage of piles, piers, and caissons
=== 18.13.6.1 For structures assigned to SDC C, D, E, or F,
the longitudinal reinforcement in piles, piers, or caissons
resisting tension loads shall be detailed to transfer tension
forces within the pile cap to supported structural members.
=== 18.13.6.2 For structures assigned to SDC C, D, E, or F,
concrete piles and concrete filled pipe piles shall be connected
to the pile cap by embedding the pile reinforcement in the
pile cap a distance equal to the development length or by the
use of field-placed dowels anchored in the concrete pile. For
deformed bars, the compression development length is used
if the pile is in compression. In the case of uplift, the tension
development length is used without reduction in length for
excess reinforcement.
=== 18.13.6.3 For structures assigned to SDC D, E, or F, if
tension forces induced by earthquake effects are transferred
between pile cap or mat foundation and precast pile by reinforcing
bars grouted or post-installed in the top of the pile,
the grouting system shall have been demonstrated by testing
to develop at least 1.25fy of the bar.
== R18.13.6 Anchorage of piles, piers, and caissons
=== R18.13.6.1 A load path is necessary at pile caps to transfer
tension forces from the reinforcing bars in the column or
boundary element through the pile cap to the reinforcement
of the pile or caisson. Examples of different types of pile
connections to pile caps are available in ASCE/COPRI Standard
for the Seismic Design of Piers and Wharves (61-14).
=== R18.13.6.2 Development length is determined according
to requirements of Chapter 25. Reductions in development
length for calculated stresses less than fy are not permitted,
as indicated in 25.4.10.2. Full development of the pile longitudinal
reinforcement into the pile cap is intended to enable
the capacity of the pile to pile cap connection to meet or
exceed the pile section strength.
=== R18.13.6.3 Grouted dowels in a blockout in the top of a
precast concrete pile need to be developed, and testing is
a practical means of demonstrating strength. Alternatively,
reinforcing bars can be cast in the upper portion of the pile,
exposed by chipping of concrete and mechanically spliced
or welded to an extension.
[ Lanjut Ke 18.14—Members not designated as part of the seismic-force-resisting system ... ]

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