= 18. CHAPTER 18 — EARTHQUAKE-RESISTANT STRUCTURES

= 18.8 — Joints of special moment frames

== 18.8.1 Scope

=== 18.8.1.1 This section shall apply to beam-column joints
of special moment frames forming part of the seismic-force-resisting
system.

= R18.8 — Joints of special moment frames

== 18.8.2 General

=== 18.8.2.1 Forces in longitudinal beam reinforcement at the
joint face shall be calculated assuming that the stress in the
flexural tensile reinforcement is 1.25fy.

=== 18.8.2.2 Longitudinal reinforcement terminated in a
joint shall extend to the far face of the joint core and shall
be developed in tension in accordance with 18.8.5 and in
compression in accordance with 25.4.9.

=== 18.8.2.3 Where longitudinal beam reinforcement extends
through a beam-column joint, the depth h of the joint parallel
to the beam longitudinal reinforcement shall be at least the
greatest of (a) through (c):

(a) (20/λ).db of the largest Grade 420 longitudinal bar, where
λ = 0.75 for lightweight concrete and 1.0 for all other cases;
(b) 26db of the largest Grade 550 longitudinal bar;
(c) h/2 of any beam framing into the joint and generating
joint shear as part of the seismic-force-resisting system in
the direction under consideration.


== R18.8.2 General
 Development of inelastic rotations at the faces of joints
of reinforced concrete frames is associated with strains in
the flexural reinforcement well in excess of the yield strain.
Consequently, joint shear force generated by the flexural
reinforcement is calculated for a stress of 1.25fy in the reinforcement
(refer to 18.8.2.1). A detailed explanation of the
reasons for the possible development of stresses in excess of
the yield strength in beam tensile reinforcement is provided
in ACI 352R.

=== R18.8.2.2 The design provisions for hooked bars are based
mainly on research and experience for joints with standard
90-degree hooks. Therefore, standard 90-degree hooks
generally are preferred to standard 180-degree hooks unless
unusual considerations dictate use of 180-degree hooks. For
bars in compression, the development length corresponds
to the straight portion of a hooked or headed bar measured
from the critical section to the onset of the bend for hooked
bars and from the critical section to the head for headed bars.

=== R18.8.2.3 Depth h of the joint is defined in
Fig. R15.4.2. The column dimension parallel to the beam reinforcement
in joints with circular columns may be taken as that of a
square section of equivalent area. Research (Meinheit and
Jirsa 1977; Briss et al. 1978; Ehsani 1982; Durrani and
Wight 1982; Leon 1989; Aoyama 2001; Lin et al. 2000) has
shown that straight longitudinal beam bars may slip within
the beam-column joint during a series of large moment
reversals. The bond stresses on these straight bars may be
very large. To reduce slip substantially during the formation
of adjacent beam hinging, it would be necessary to have a
ratio of column dimension to bar diameter of approximately
32 for Grade 420 bars, which would result in very large
joints. Tests demonstrate adequate behavior if the ratio of
joint depth to maximum beam longitudinal bar diameter for
Grade 420 reinforcement is at least 20 for normalweight
concrete and 26 for lightweight concrete. A joint depth of
26db for Grade 550 reinforcement is intended to achieve
similar performance to that of a joint depth of 20db for Grade
420 reinforcement and normalweight concrete. The limits on
joint depth provide reasonable control on the amount of slip
of the beam bars in a beam-column joint, considering the
number of anticipated inelastic excursions of the building
frame during a major earthquake. A thorough treatment of
this topic is given in Zhu and Jirsa (1983).
 Requirement (c) on joint aspect ratio applies only to
beams that are designated as part of the seismic-force-resisting
system. Joints having depth less than half the beam
depth require a steep diagonal compression strut across the
joint, which may be less effective in resisting joint shear.
Tests to demonstrate performance of such joints have not
been reported in the literature.

==== 18.8.2.3.1 Concrete used in joints with Grade 550 longitudinal
reinforcement shall be normalweight concrete.

==== R18.8.2.3.1 Test data justifying the combination of lightweight
concrete and Grade 550 longitudinal reinforcement
in joints are not available.
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PART 5: EARTHQUAKE RESISTANCE 311
18 Seismic
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== 18.8.3 Transverse reinforcement

=== 18.8.3.1 Joint transverse reinforcement shall satisfy
18.7.5.2, 18.7.5.3, 18.7.5.4, and 18.7.5.7, except as permitted
in 18.8.3.2.

=== 18.8.3.2 Where beams frame into all four sides of the
joint and where each beam width is at least three-fourths
the column width, the amount of reinforcement required by
18.7.5.4 shall be permitted to be reduced by one-half, and
the spacing required by 18.7.5.3 shall be permitted to be
increased to 150 mm within the overall depth h of the shallowest
framing beam.

=== 18.8.3.3 Longitudinal beam reinforcement outside the
column core shall be confined by transverse reinforcement
passing through the column that satisfies spacing requirements
of 18.6.4.4, and requirements of 18.6.4.2, and 18.6.4.3,
if such confinement is not provided by a beam framing into
the joint.

== R18.8.3 Transverse reinforcement
The Code requires transverse reinforcement in a joint
regardless of the magnitude of the calculated shear force.

=== R18.8.3.2 The amount of confining reinforcement may
be reduced and the spacing may be increased if beams of
adequate dimensions frame into all four sides of the joint.

=== R18.8.3.3 The required transverse reinforcement, or
transverse beam if present, is intended to confine the beam
longitudinal reinforcement and improve force transfer to the
beam-column joint.
 An example of transverse reinforcement through the
column provided to confine the beam reinforcement passing
outside the column core is shown in Fig. R18.6.2 . Additional
detailing guidance and design recommendations for both
interior and exterior wide-beam connections with beam reinforcement
passing outside the column core may be found in
ACI 352R.

== 18.8.4 Shear strength

=== 18.8.4.1 Joint shear force Vu shall be calculated on a plane
at mid-height of the joint from calculated forces at the joint
faces using tensile and compressive beam forces determined
in accordance with 18.8.2.1 and column shear consistent
with beam probable flexural strengths Mpr.

=== 18.8.4.2 ϕ shall be in accordance with 21.2.4.4 .

=== 18.8.4.3 Vn of the joint shall be in accordance with
Table 18.8.4.3 .


Table 18.8.4.3—Nominal joint shear strength Vn

== R18.8.4 Shear strength
The shear strength values given in 18.8.4.3 are based on
the recommendation in ACI 352R for joints with members
that are expected to undergo reversals of deformation into
the inelastic range, although the ACI 352R definition of
effective cross-sectional joint area is sometimes different.
The given nominal joint shear strengths do not explicitly
consider transverse reinforcement in the joint because tests
of joints (Meinheit and Jirsa 1977) and deep beams (Hirosawa
1977) have indicated that joint shear strength is not
sensitive to transverse reinforcement if at least the required
minimum amount is provided in the joint.
 Cyclic loading tests of joints with extensions of beams
with lengths at least equal to their depths have indicated
similar joint shear strengths to those of joints with continuous
beams. These findings suggest that extensions of beams and
columns, when properly dimensioned and reinforced with
longitudinal and transverse bars, provide effective confinement
to the joint faces, thus delaying joint strength deterioration
at large deformations (Meinheit and Jirsa 1981).
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312 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE
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== 18.8.5 Development length of bars in tension

=== 18.8.5.1 For bar sizes No. 10 through No. 36 terminating
in a standard hook, ℓdh shall be calculated by Eq. (18.8.5.1),
but ℓdh shall be at least the greater of 8db and 150 mm for
normalweight concrete and at least the greater of 10db and
190 mm for lightweight concrete.

 ℓdh = fy.db/(5.4λ.sqrt(fc') ) ..(18.8.5.1)

 The value of λ shall be 0.75 for concrete containing lightweight
aggregate and 1.0 otherwise.
 The hook shall be located within the confined core of a
column or of a boundary element, with the hook bent into
the joint.

== R18.8.5 Development length of bars in tension

=== R18.8.5.1 Minimum embedment length in tension for
deformed bars with standard hooks is determined using Eq.
(18.8.5.1), which is based on the requirements of 25.4.3.
The embedment length of a bar with a standard hook is the
distance, parallel to the bar, from the critical section (where
the bar is to be developed) to a tangent drawn to the outside
edge of the hook. The tangent is to be drawn perpendicular
to the axis of the bar (refer to Table !!25.3.1 ).
 Because Chapter 18 stipulates that the hook is to be
embedded in confined concrete, the coefficients 0.7 (for
concrete cover) and 0.8 (for ties) have been incorporated in
the constant used in Eq. (18.8.5.1). The development length
that would be derived directly from 25.4.3 is increased to
reflect the effect of load reversals. Factors such as the actual
stress in the reinforcement being more than the yield strength
and the effective development length not necessarily starting
at the face of the joint were implicitly considered in the
formulation of the expression for basic development length
that has been used as the basis for Eq. (18.8.5.1).
 The requirement for the hook to project into the joint is to
improve development of a diagonal compression strut across
the joint. The requirement applies to beam and column bars
terminated at a joint with a standard hook.

=== 18.8.5.2 For headed deformed bars satisfying 20.2.1.6,
development in tension shall be in accordance with 25.4.4,
by substituting a bar stress of 1.25fy for fy.

=== R18.8.5.2 The factor 1.25 is intended to represent the potential
increase in stresses due to inelastic response, including strain
hardening that may occur in beams of special moment frames.
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PART 5: EARTHQUAKE RESISTANCE 313
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No further reproduction or distribution is permitted.

=== 18.8.5.3 For bar sizes No. 10 through No. 36, ℓd, the development
length in tension for a straight bar, shall be at least
the greater of (a) and (b):
(a) 2.5 times the length in accordance with 18.8.5.1 if the
depth of the concrete cast in one lift beneath the bar does
not exceed 300 mm;
(b) 3.25 times the length in accordance with 18.8.5.1 if
the depth of the concrete cast in one lift beneath the bar
exceeds 300 mm.

=== R18.8.5.3 Minimum development length in tension for
straight bars is a multiple of the length indicated by 18.8.5.1.
Section 18.8.5.3(b) refers to top bars. Lack of reference to
No. 43 and No. 57 bars in 18.8.5 is due to the paucity of
information on anchorage of such bars subjected to load
reversals simulating earthquake effects.

=== 18.8.5.4 Straight bars terminated at a joint shall pass
through the confined core of a column or a boundary
element. Any portion of ℓd not within the confined core shall
be increased by a factor of 1.6.

=== R18.8.5.4 If the required straight embedment length
of a reinforcing bar extends beyond the confined volume
of concrete (as defined in 18.6.4, 18.7.5, or 18.8.3), the
required development length is increased on the premise that
the limiting bond stress outside the confined region is less
than that inside.

ℓdm = 1.6(ℓd – ℓdc) + ℓdc
or :
ℓdm = 1.6ℓd – 0.6ℓdc

where ℓdm is the required development length if bar is not entirely
embedded in confined concrete; ℓd is the required development
length in tension for straight bar as defined in 18.8.5.3; and ℓdc
is the length of bar embedded in confined concrete.

=== 18.8.5.5 If epoxy-coated reinforcement is used, the development
lengths in 18.8.5.1, 18.8.5.3, and 18.8.5.4 shall be
multiplied by applicable factors in 25.4.2.5 or 25.4.3.2.


[ Lanjut Ke 18.9—Special moment frames constructed using
precast concrete ... ]






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