= 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): | |
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(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 | |
No further reproduction or distribution is permitted. | |
== 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 . | |
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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 | |
No further reproduction or distribution is permitted. | |
== 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. | |
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ℓdh = fy.db/(5.4λ.sqrt(fc') ) ..(18.8.5.1) | |
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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. | |
American Concrete Institute – Copyrighted © Material – www.concrete.org | |
PART 5: EARTHQUAKE RESISTANCE 313 | |
18 Seismic | |
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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