The document discusses the design of coupling beams in three categories based on aspect ratio and shear demand: 1) Coupling beams with an aspect ratio greater than 4 are designed as special moment frame beams with conventional reinforcement. 2) Coupling beams with an aspect ratio less than 2 and shear demand greater than a threshold are designed as diagonally reinforced beams. 3) Other coupling beams can be designed as either special moment frame beams or diagonally reinforced beams.

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2. • Three categories based on Aspect Ratio (ln/h) and Shear Demand (ACI
318, Sect. 21.9.7)
a) Coupling beams (ln/h ≥ 4)
• Design as special moment frame beam (Conventional reinforced)
b) Coupling beams (ln/h < 2 and Vu > 4λ√(fc’) Acw) (Unit: English unit)
• Design as diagonal reinforced beams
c) Other coupling beams (Not falling within above two limits)
• Design as either special moment frame beam or diagonal reinforced
beam
• Right side of dashed line can be designed efficientas moment frame
beam
• Left side are better designed with diagonal reinforcement
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5. • Flexure Design
– Design as special moment frame beams (ACI 318, Sect. 21.5)
– Reinforcement placed horizontally at top and bottom
• Shear Design
– Shear design is based on probable moment strength (use 1.25 fy)
– Within 2h of member ends, shear strength is calculated based on steel
only
• Confinement
– Hoops confine at end regions
– If ln/h is relatively small, longitudinal cannot be lapped
– Easier to use closed hoops over entire beam rather than only 2h at
each end
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6. • Development Length in Tension (ACI 318, Sect. 21.7.5)
– For standard 90 deg. Hook, located within confined core
• ldh = fy db / (65 √fc’), not less than 8 db, 6 in.
– For straight bar
• ld = 2.5 x ldh for 90 deg. Hook, if depth of concrete cast in one lift
beneath the bar does not exceed 12 in.
• ld = 3.5 x ldh for 90 deg. Hook, if depth of concrete cast in one lift
beneath the bar exceeds 12 in.
• Straight bars terminated at a joint shall pass through confined core.
• Any portion of ld not within the confined core shall be increased by a
factor of 1.6.
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9. • Shear Design
Tu = Cu = Ф As fy
Vu = 2 Tu sinα = 2ФAsfy sinα
Vn = 2Avdfy sinα ≤ 10√fc’ Acw (ACI 318, Eq. 21-9)
– Minimum four bars in each group of diagonal
• Confinement (2 Options)
a) Confine individual diagonals
b) Confine entire beam cross section
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12. Confinement of Individual Diagonals
– Diagonal bars shall be enclosed by transverse reinforcement out-to-out
dimensions not smaller than bw/2 in the direction parallel to bw and
bw/5 along the other sides
– Volumetric ratio of confinement,
ρs = max (0.12 fc’ / fyt , 0.45 (Ag/Ach – 1) fc’ / fyt)
– Max. spacing = min (s0, 6db)
– s0 = 4 + (14 – hx) / 3
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13. Confinement of Entire Beam Section
– Volumetric ratio of confinement,
ρs = max (0.12 fc’ / fyt , 0.45 (Ag/Ach – 1) fc’ / fyt)
– Max. spacing = min (6 in., 6db)
• Development Length
– Design for 1.25 fy in tension
• Regardless of the confinement option selected, longitudinal and transverse
reinforcement shall be distributed around the beam perimeter with total
area in each direction not less than 0.002 bw s with min. spacing of 12 in.
• Typically No. 4 or 5 bars should extend only a short distance into the wall
boundary not to develop significant tensile stress due to beam flexure
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-0.01 0 0.01 0.02 0.03 0.04
Story
Coupling Beam Rotation
Coupling Beam Shear Hinge Rotation at V1
LOM_STG
TAB
UNI
KOC
DUZ
STL
LOM_LGP
Average

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Coupling Beam Reinforcement Details