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R21.8 — Special moment frames constructed using precast concrete

The detailing provisions in 21.8.2 and 21.8.3 are intended to produce frames that respond to
design displacements essentially like monolithic special moment frames.
Precast frame systems composed of concrete elements with ductile connections are expected to experience flexural yielding in connection regions. Reinforcement inductile connections can be made continuous by using Type 2 mechanical splices or any other technique that provides development in tension or compression of at least 125 percent of the specified yield strength f y of bars and the specified tensile strength of bars.21.37-21.40 Requirements for mechanical splices are in addition to those in 21.1.6 and are intended to avoid strain concentrations over ashort length of reinforcement adjacent to a splice device. Additional requirements for shear strength are provided in 21.8.2 to prevent sliding on connection faces. Precast frames composed of
elements with ductile connections may be designed to promote yielding at locations not adjacent
to the joints. Therefore, design shear, e V , as computed according to 21.5.4.1 or 21.6.5.1, may beconservative.
Precast concrete frame systems composed of elements joined using strong connections are
intended to experience flexural yielding outside the connections. Strong connections include the
length of the coupler hardware as shown in Fig. R21.8.3. Capacity-design techniques are used in
21.8.3(b) to ensure the strong connection remains elastic following formation of plastic hinges.
Additionalcolumn requirements are provided to avoid hinging and strength deterioration of
column-to-column connections.
Strain concentrations have been observed to cause brittle fracture of reinforcing bars at the face
of mechanical splices in laboratory tests of precast beam-column connections.21.41 Locations of
strong connections should be selected carefully or other measures should be taken, such asdebonding of reinforcing bars in highly stressed regions, to avoid strain concentrations that can
result in premature fracture of reinforcement.
R21.8.4 — Precast frame systems not satisfying the prescriptive requirements of Chapter 21
have been demonstrated in experimental studies to provide satisfactory seismic performance
characteristics.21.42,21.43 ACI 374.1 defines a protocol forestablishing a design procedure,
validated by analysis and laboratory tests, for such frames. The design procedure should identify
the load path or mechanism by which the frame resists gravity and earthquake effects. The tests
should be configured to test critical behaviors, and the measured quantities should establish
upper-bound acceptance values for components of the load path, which may be interms of
limiting stresses, forces, strains, or other quantities. The design procedure used for the structure
should not deviate from that used to design the test specimens, and acceptance values should not
exceed values that were demonstrated by the tests to be acceptable. Materials and components
used in the structure should be similar to those used in the tests. Deviations may be acceptable ifthe licensed design professional can demonstrate that those deviations do not adversely affect the
behavior of the framing system.
ACI ITG 1.221.44 defines design requirements for one type of special precast concrete moment
frame for use in accordance with 21.8.4. [105]
R21.9 — Special structural walls and coupling beams
R21.9.1 — Scope
This section contains requirements for the dimensionsand details of special structural walls and
coupling beams.
R21.9.2 — Reinforcement
201
Minimum reinforcement requirements in 21.9.2.1 follow from preceding codes. The uniform
distribution requirement of the shear reinforcement is related to the intent to control the width of
inclined cracks. The requirement for two layers of reinforcement in walls carrying substantial
design shears in...
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