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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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