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Important Considerations in Design of Primary Structural Components: Part 2
Organized by: In collaboration with: Supported by:
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SMRF
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Principles for Design of SMRF
• Response Modification Factor of R=8
• Proportioning and detailing requirements are intended to ensure that inelastic response is ductile.
• Three main goals
1) To achieve a strong-column/weak-beam design that spreads inelastic response over several stories
2) To avoid shear failure
3) To provide details that enable ductile flexural response in yielding regions
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Strong-Column/Weak Beam Frame
(b)(a)
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Shear Failure
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Ductile Detailing
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Beams
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Beams in SMRF
• Typical economical beam span, 6 to 9 m
• Clear span > 4 x effective depth (ACI 318-08 § 21.5.1.2)
• ACI allows longitudinal reinforcement ratio up to 0.025 but 0.01 is
a more practical number (ACI 318-08 § 21.5.2.1)
• Beams are allowed to be wider than supporting columns, but beam
width normally does not exceed column width
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Beam Shear
• ACI 318 defines probable moment
strength as moment strength of a
member, determined using the
properties of the member at the joint
faces assuming a tensile stress in
longitudinal bars of “at least 1.25 fy”
and a strength reduction factor φ of 1.0.
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Shear Capacity of Beams
• Shear Capacity
• Assume Vc = 0 when both (a) and (b) occur:
a) The earthquake-induced shear force calculated in accordance with ACI
21.5.4.1 represents one-half or more of the maximum required shear
strength within yielding zones;
b) The factored axial compressive force, Pu, including earthquake effects
is less than Agfc′ /20.
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Beams in SMRF
Hoops and Stirrups Locations and Spacing Requirements
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Beams in SMRF
Hoop Reinforcement Details
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Beam Nonlinear Modeling
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Beam Rotation
Shabir Talpur15Source: UCLA-SGEL Report 2009/06Damage levels of conventional reinforced beam
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Beam Column Joint
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Beam-column Joints
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Joint Shear Strength
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Joint Shear Strength
Effective Joint Width
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Shear Walls
Columns
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Column
• Shear and anchorage requirements for beam-column joints determine the required
joint dimensions and consequently may dictate column dimensions
• Ratio of shortest cross-sectional dimension to perpendicular dimension not less than
0.4 (ACI 318-08 § 21.6.1.2)
• The columns can be designed for the maximum factored gravity loads while limiting
the area of reinforcement to between 1 % and 3 %.
• ACI 318 allows the longitudinal reinforcement to reach 6 % of the gross section area,
but this amount of reinforcement results in very congested splice locations.
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Columns in SMRF
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Column Axial Load
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Column transverse reinforcement
Column transverse reinforcement detail (ACI 318-14 § 18.7.5.2)
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Column transverse reinforcement
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Column transverse reinforcement
Column transverse reinforcement spacing requirements
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Column Shear
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Column Shear
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Column Shear
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Column Rotation
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Column Rotation
Force
Deformation
Vp/Vo ≤ 0.6
Vp/Vo > 1.0
0.6 ≤ Vp/Vo ≤ 1.0
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