= 18. CHAPTER 18 — EARTHQUAKE-RESISTANT STRUCTURES
= 18.2 — General
== 18.2.1 Structural systems
=== 18.2.1.1 All structures shall be assigned to a SDC in accordance
with 4.4.6.1.
= R18.2 — General
Structures assigned to SDC A need not satisfy requirements
of Chapter 18 but must satisfy all other applicable
requirements of this Code. Structures assigned to Seismic
Design Categories B through F must satisfy requirements of
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CHAPTER 18—EARTHQUAKE-RESISTANT STRUCTURES
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COMMENTARY
Chapter 18 in addition to all other applicable requirements
of this Code.
Sections 18.2.1.3 through 18.2.1.5 identify those parts of
Chapter 18 that apply to the building based on its assigned
SDC, regardless of the vertical elements of the seismic-force-
resisting system. ASCE/SEI 7 defines the permissible
vertical elements of the seismic-force-resisting system and
applies where adopted. The remaining commentary of R18.2
summarizes the intent of ACI 318 regarding which vertical
elements should be permissible in a building considering
its SDC. Section 18.2.1.6 defines the requirements for the
vertical elements of the seismic-force-resisting system.
The design and detailing requirements should be compatible
with the level of inelastic response assumed in the calculation
of the design earthquake forces. The terms “ordinary,”
“intermediate,” and “special” are used to facilitate this
compatibility. For any given structural element or system,
the terms “ordinary,” “intermediate,” and “special,” refer
to increasing requirements for detailing and proportioning,
with expectations of increased deformation capacity. Structures
assigned to SDC B are not expected to be subjected
to strong ground motion, but instead are expected to experience
low levels of ground motion at long time intervals.
This Code provides some requirements for beam-column
ordinary moment frames to improve deformation capacity.
Structures assigned to SDC C may be subjected to moderately
strong ground motion. The designated seismic-force-resisting
system typically comprises some combination of
ordinary cast-in-place structural walls, intermediate precast
structural walls, and intermediate moment frames. The
general building code also may contain provisions for use
of other seismic-force-resisting systems in SDC C. Provision
18.2.1.6 defines requirements for whatever system is
selected.
Structures assigned to SDC D, E, or F may be subjected to
strong ground motion. It is the intent of ACI Committee 318
that the seismic-force-resisting system of structural concrete
buildings assigned to SDC D, E, or F be provided by special
moment frames, special structural walls, or a combination
of the two. In addition to 18.2.2 through 18.2.8, these structures
also are required to satisfy requirements for continuous
inspection (26.13.1.3), diaphragms and trusses (18.12), foundations
(18.13), and gravity-load-resisting elements that are
not designated as part of the seismic-force-resisting system
(18.14). These provisions have been developed to provide
the structure with adequate deformation capacity for the
high demands expected for these seismic design categories.
The general building code may also permit the use of intermediate
moment frames as part of dual systems for some
buildings assigned to SDC D, E, or F. It is not the intent
of ACI Committee 318 to recommend the use of intermediate
moment frames as part of moment-resisting frame or
dual systems in SDC D, E, or F. The general building code
may also permit substantiated alternative or nonprescriptive
designs or, with various supplementary provisions, the use
=== 18.2.1.2 All members shall satisfy Chapters 1 to 17 and
19 to 26. Structures assigned to SDC B, C, D, E, or F also
shall satisfy 18.2.1.3 through 18.2.1.7, as applicable. Where
Chapter 18 conflicts with other chapters of this Code,
Chapter 18 shall govern.
=== 18.2.1.3 Structures assigned to SDC B shall satisfy 18.2.2 .
=== 18.2.1.4 Structures assigned to SDC C shall satisfy 18.2.2,
18.2.3, and 18.13.
=== 18.2.1.5 Structures assigned to SDC D, E, or F shall satisfy
18.2.2 through 18.2.8 and 18.12 through 18.14.
=== 18.2.1.6 Structural systems designated as part of the
seismic-force-resisting system shall be restricted to those
designated by the general building code, or determined by
other authority having jurisdiction in areas without a legally
adopted building code. Except for SDC A, for which Chapter
18 does not apply, (a) through (h) shall be satisfied for each
structural system designated as part of the seismic-forceresisting
system, in addition to 18.2.1.3 through 18.2.1.5:
(a) Ordinary moment frames shall satisfy 18.3
(b) Ordinary reinforced concrete structural walls need
not satisfy any detailing provisions in Chapter 18, unless
required by 18.2.1.3 or 18.2.1.4
(c) Intermediate moment frames shall satisfy 18.4
(d) Intermediate precast walls shall satisfy 18.5
(e) Special moment frames shall satisfy 18.2.3 through
18.2.8 and 18.6 through 18.8
(f) Special moment frames constructed using precast
concrete shall satisfy 18.2.3 through 18.2.8 and 18.9
(g) Special structural walls shall satisfy 18.2.3 through
18.2.8 and 18.10
(h) Special structural walls constructed using precast
concrete shall satisfy 18.2.3 through 18.2.8 and 18.11
=== 18.2.1.7 A reinforced concrete structural system not satisfying
this chapter shall be permitted if it is demonstrated by
experimental evidence and analysis that the proposed system
will have strength and toughness equal to or exceeding those
provided by a comparable reinforced concrete structure
satisfying this chapter.
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COMMENTARY
of ordinary or intermediate systems for nonbuilding structures
in the higher seismic design categories. These are not
the typical applications that were considered in the writing
of this chapter, but wherever the term “ordinary or intermediate
moment frame” is used in reference to reinforced
concrete, 18.3 or 18.4 apply.
Table 18.2 summarizes the applicability of the provisions
of Chapter 18 as they are typically applied when using
the minimum requirements in the various seismic design
categories. Where special systems are used for structures in
SDC B or C, it is not required to satisfy the requirements
of 18.14, although it should be verified that members not
designated as part of the seismic-force-resisting system will
be stable under design displacements.
Table R18.2 — Sections of Chapter 18 to be
satisfied in typical applications[1]
The proportioning and detailing requirements in Chapter
18 are based predominantly on field and laboratory experience
with monolithic reinforced concrete building structures
and precast concrete building structures designed
and detailed to behave like monolithic building structures.
Extrapolation of these requirements to other types of castin-
place or precast concrete structures should be based on
evidence provided by field experience, tests, or analysis. The
acceptance criteria for moment frames given in ACI 374.1 can
be used in conjunction with Chapter 18 to demonstrate that the
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18 Seismic
No further reproduction or distribution is permitted.
COMMENTARY
strength, energy dissipation capacity, and deformation capacity
of a proposed frame system equals or exceeds that provided
by a comparable monolithic concrete system. ACI ITG-5.1M
provides similar information for precast wall systems.
The toughness requirement in 18.2.1.7 refers to the
requirement to maintain structural integrity of the entire
seismic-force-resisting system at lateral displacements
anticipated for the maximum considered earthquake motion.
Depending on the energy-dissipation characteristics of the
structural system used, such displacements may be larger
than for a monolithic reinforced concrete structure satisfying
the prescriptive provisions of other parts of this Code.
CODE
== 18.2.2 Analysis and proportioning of structural members
=== 18.2.2.1 The interaction of all structural and nonstructural
members that affect the linear and nonlinear response of the
structure to earthquake motions shall be considered in the
analysis.
=== 18.2.2.2 Rigid members assumed not to be a part of the
seismic-force-resisting system shall be permitted provided
their effect on the response of the system is considered in
the structural design. Consequences of failure of structural
and nonstructural members that are not a part of the seismicforce-
resisting system shall be considered.
=== 18.2.2.3 Structural members extending below the base of
structure that are required to transmit forces resulting from
earthquake effects to the foundation shall comply with the
requirements of Chapter 18 that are consistent with the
seismic-force-resisting system above the base of structure.
== R18.2.2 Analysis and proportioning of structural members
It is assumed that the distribution of required strength to the
various components of a seismic-force-resisting system will
be determined from the analysis of a linearly elastic model of
the system acted upon by the factored forces, as required by
the general building code. If nonlinear response history analyses
are to be used, base motions should be selected after a
detailed study of the site conditions and local seismic history.
Because the basis for earthquake-resistant design admits
nonlinear response, it is necessary to investigate the stability of
the seismic-force-resisting system, as well as its interaction with
other structural and nonstructural members, under expected
lateral displacements corresponding to maximum considered
earthquake ground motion. For lateral displacement calculations,
assuming all the structural members to be fully cracked is
likely to lead to better estimates of the possible drift than using
uncracked stiffness for all members. The analysis assumptions
described in 6.6.3.1 may be used to estimate lateral deflections
of reinforced concrete building systems.
The main objective of Chapter 18 is the safety of the structure.
The intent of 18.2.2.1 and 18.2.2.2 is to draw attention
to the influence of nonstructural members on structural
response and to hazards from falling objects.
Section 18.2.2.3 serves as an alert that the base of structure as
defined in analysis may not necessarily correspond to the foundation
or ground level. Details of columns and walls extending
below the base of structure to the foundation are required to be
consistent with those above the base of structure.
In selecting member sizes for earthquake-resistant structures,
it is important to consider constructibility problems
related to congestion of reinforcement. The design should
be such that all reinforcement can be assembled and placed
in the proper location and that concrete can be cast and
consolidated properly. Using the upper limits of permitted
reinforcement ratios may lead to construction problems.
== 18.2.3 Anchoring to concrete
=== 18.2.3.1 Anchors resisting earthquake-induced forces in
structures assigned to SDC C, D, E, or F shall be in accordance
with 17.10.
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== 18.2.4 Strength reduction factors
=== 18.2.4.1 Strength reduction factors shall be in accordance
with Chapter 21.
== R18.2.4 Strength reduction factors
=== R18.2.4.1 Chapter 21 contains strength reduction factors
for all members, joints, and connections of earthquake-resistant
structures, including specific provisions in 21.2.4 for
buildings that use special moment frames, special structural
walls, and intermediate precast walls.
== 18.2.5 Concrete in special moment frames and special
structural walls
=== 18.2.5.1 Specified compressive strength of concrete in
special moment frames and special structural walls shall be
in accordance with the special seismic systems requirements
of Table 9.2.1.1 .
== R18.2.5 Concrete in special moment frames and special
structural walls
Requirements of this section refer to concrete quality in
frames and walls that resist earthquake-induced forces. The
maximum specified compressive strength of lightweight
concrete to be used in structural design calculations is limited
to 35 MPa, primarily because of paucity of experimental and
field data on the behavior of members made with lightweight
concrete subjected to displacement reversals in the nonlinear
range. If convincing evidence is developed for a specific
application, the limit on maximum specified compressive
strength of lightweight concrete may be increased to a level
justified by the evidence.
== 18.2.6 Reinforcement in special moment frames and
special structural walls
=== 18.2.6.1 Reinforcement in special moment frames and
special structural walls shall be in accordance with the
special seismic systems requirements of 20.2.2.
== R18.2.6 Reinforcement in special moment frames and
special structural walls
=== R18.2.6.1 Nonprestressed reinforcement for seismic
systems is required to meet 20.2.2.4 and 20.2.2.5. Starting
with ACI 318-19, ASTM A706 Grades 550 and 690 reinforcement
is permitted to resist moments, axial, and shear
forces in special structural walls and all components of
special structural walls, including coupling beams and
wall piers. ASTM A706 Grade 550 reinforcement is also
permitted in special moment frames. Results of tests and
analytical studies presented in NIST (2014) and Sokoli and
Ghannoum (2016) indicate that properly detailed beams and
columns of special moment frames with ASTM A706 Grade
550 reinforcement exhibit strength and deformation capacities
similar to those of members reinforced with Grade 420
reinforcement. The use of Grade 690 reinforcement is not
allowed in special moment frames because there is insufficient
data to demonstrate satisfactory seismic performance.
To allow the use of ASTM A706 Grades 550 and 690
reinforcement, the 2019 Code includes limits for spacing of
transverse reinforcement to provide adequate longitudinal
bar support to control longitudinal bar buckling. In special
moment frames, the use of Grade 550 reinforcement requires
increased joint depths to prevent excessive slip of beam bars
passing through beam-column joints (18.8.2.3).
The requirement for a tensile strength greater than the yield
strength of the reinforcement ( 20.2.2.5 , Table 0.2.1.3 (b)) is
based on the assumption that the capability of a structural
member to develop inelastic rotation capacity is a function
of the length of the yield region along the axis of the
member. In interpreting experimental results, the length of
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18 Seismic
No further reproduction or distribution is permitted.
COMMENTARY
the yield region has been related to the relative magnitudes
of nominal and yield moments (ACI 352R). According to
this interpretation, the greater the ratio of nominal to yield
moment, the longer the yield region. Chapter 20 requires
that the ratio of actual tensile strength to actual yield strength
be at least 1.25 for ASTM A615 Grade 420.
The restrictions on the value of fyt apply to all types of
transverse reinforcement, including spirals, circular hoops,
rectilinear hoops, and crossties. Research results (Budek
et al. 2002; Muguruma and Watanabe 1990; Sugano et al.
1990) indicate that higher yield strengths can be used effectively
as confinement reinforcement as specified in 18.7.5.4.
The increases to 550 and 690 MPa for shear design of some
special seismic system members is based on research indicating
the design shear strength can be developed (Wallace
1998; Aoyama 2001; Budek et al. 2002; Sokoli and Ghannoum
2016; Cheng et al. 2016; Huq et al. 2018; Weber-
Kamin et al. 2019). The 420 MPa restriction on the value of
fyt for deformed bar in 20.2.2.4 for calculating nominal shear
strength is intended to limit the width of shear cracks at
service-level loads. Service-level cracking is not a concern
in members of the seismic-force-resisting system subjected
to design-level earthquake forces.
CODE
== 18.2.7 Mechanical splices in special moment frames and
special structural walls
== R18.2.7 Mechanical splices in special moment frames and
special structural walls
In a structure undergoing inelastic deformations during
an earthquake, the tensile stresses in reinforcement may
approach the tensile strength of the reinforcement. The
requirements for Type 2 mechanical splices are intended to
avoid a splice failure when the reinforcement is subjected to
expected stress levels in yielding regions. Type 1 mechanical
splices on any grade of reinforcement and Type 2 mechanical
splices on Grade 550 and Grade 690 reinforcement
may not be capable of resisting the stress levels expected in
yielding regions. The locations of these mechanical splices
are restricted because tensile stresses in reinforcement in
yielding regions can exceed the strength requirements of
18.2.7.1. The restriction on all Type 1 mechanical splices
and on Type 2 mechanical splices on Grade 550 and Grade
690 reinforcement applies to all reinforcement resisting
earthquake effects, including transverse reinforcement.
Recommended detailing practice would preclude the
use of splices in regions of potential yielding in members
resisting earthquake effects. If use of mechanical splices in
regions of potential yielding cannot be avoided, there should
be documentation on the actual strength characteristics of the
bars to be spliced, on the force-deformation characteristics
of the spliced bar, and on the ability of the mechanical splice
to be used to meet the specified performance requirements.
Although mechanical splices as defined by 18.2.7 need not
be staggered, staggering is encouraged and may be necessary
for constructibility or provide enough space around the splice
for installation or to meet the clear spacing requirements.
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=== 18.2.7.1 Mechanical splices shall be classified as (a) or (b):
(a) Type 1 – Mechanical splice conforming to 25.5.7
(b) Type 2 – Mechanical splice conforming to 25.5.7 and
capable of developing the specified tensile strength of the
spliced bars
=== 18.2.7.2 Except for Type 2 mechanical splices on Grade
420 reinforcement, mechanical splices shall not be located
within a distance equal to twice the member depth from the
column or beam face for special moment frames or from
critical sections where yielding of the reinforcement is likely
to occur as a result of lateral displacements beyond the linear
range of behavior. Type 2 mechanical splices on Grade 420
reinforcement shall be permitted at any location, except as
noted in 18.9.2.1(c).
== 18.2.8 Welded splices in special moment frames and
special structural walls
=== 18.2.8.1 Welded splices in reinforcement resisting earthquake-
induced forces shall conform to 25.5.7 and shall not
be located within a distance equal to twice the member depth
from the column or beam face for special moment frames or
from critical sections where yielding of the reinforcement is
likely to occur as a result of lateral displacements beyond the
linear range of behavior.
=== 18.2.8.2 Welding of stirrups, ties, inserts, or other similar
elements to longitudinal reinforcement required by design
shall not be permitted.
== R18.2.8 Welded splices in special moment frames and
special structural walls
=== R18.2.8.1 Welding of reinforcement should be in accordance
with AWS D1.4 as required in Chapter 26. The locations
of welded splices are restricted because reinforcement
tension stresses in yielding regions can exceed the strength
requirements of 25.5.7. The restriction on welded splices
applies to all reinforcement resisting earthquake effects,
including transverse reinforcement.
=== R18.2.8.2 Welding of crossing reinforcing bars can lead
to local embrittlement of the steel. If welding of crossing
bars is used to facilitate fabrication or placement of reinforcement,
it should be done only on bars added for such
purposes. The prohibition of welding crossing reinforcing
bars does not apply to bars that are welded with welding
operations under continuous, competent control, as in the
manufacture of welded-wire reinforcement.
[ Lanjut Ke 18.3—Ordinary moment frames ... ]

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