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C- Joints of Special Moment Frames

Requirements of ACI 21.7 are applicable for joints of special moment frames.

1- General Requirements: • Forces in longitudinal beam reinforcement at the joint face shall be

determined by assuming that the stress in the flexural tensile reinforcement is yf25.1 .

• Beam longitudinal reinforcement terminated in a column shall be extended to the far face the confined column core and anchored in tension according to 21.7.5 and in compression according to chapter 12.

• Where longitudinal beam reinforcement extends through abeam-column joint, the column dimension parallel to the beam reinforcement shall not be less than 20 times the diameter of the largest longitudinal bar.

1- Transverse Reinforcement:

• Transverse reinforcement as discussed in B shall be provided within the joint, unless the joint is confined by structural members as shown below.

• Within the depth of the shallowest framing member, transverse reinforcement equal to at least ½ the amount shown in B shall be provided where members frame into all four sides of the joint and where each member width is at least ¾ the column width. At these locations spacing is permitted to be increased to 15 cm.

• Transverse reinforcement as required in B shall be provided through the joint to provide confinement for longitudinal beam reinforcement outside the column core if such confinement is not provided by a beam framing into the joint.

Effective area of joint

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2- Shear Strength: • The nominal shear strength of the joint shall not be taken greater

than the values specified below: - For joints confined on all four sides jc Af ′3.5 - For joints confined on three faces or on two opposite faces jc Af ′4 - For others jc Af ′2.3

A member that frames into a face is considered to provide confinement to the joint if at least ¾ of the face of the joint is covered by the framing member. A joint is considered to be confined if such members frame into all faces of the joint.

jA is the effective cross-sectional area within a joint computed from joint depth times effective joint width. Joint depth shall be the overall depth of the column, h. Effective joint width shall be the overall width of the column, except where a beam frames into a wider column, effective joint width shall not exceed the smaller of (a) and (b): (a) Beam width plus joint depth (b) Twice the smaller perpendicular distance from longitudinal axis of beam to column side.

3- Development length of bars in tension:

• The development length dhl for a bar with a standard 90 degree hook shall not be less than the largest of

bd8 , 15 cm, and the length required by the following equation which is applicable to bar diameters ranging from 10 mm to 36 mm.

c

bydh f

dfl

′=

2.17

The 90-degree hook shall be located within the confined core of a column.

• For bar diameters 10 mm through 36 mm, the development length

dl for a straight bar shall not be less than (a) and (b): (a) 2.5 times the length required by the previous equation if the depth of the concrete cast in one lift beneath the bar does not exceed 30 cm, and

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(b) 3.5 times the length provided by the same equation if the depth of the concrete cast in one lift beneath the bar exceeds 30 cm.

Horizontal shear in beam-column connection

237

Example (10): Determine the transverse reinforcement and shear strength requirements for the

interior beam-column connection shown in Example (9).

Solution:

A- ACI 21.7.1 "General Requirements"

Based on ACI 21.7.2.1, forces in longitudinal beam reinforcement at the joint

face shall be determined as assuming that the stress in the flexural tensile

reinforcement is yf25.1 .

• Based on ACI 21.7.2.3, where longitudinal beam reinforcement extends

through a beam-column joint, the column dimension parallel to the beam

reinforcement shall not be less than 20 times the diameter of the larger

longitudinal bar.

( ) cm70cm505.220d20 b <== (O.K)

B- ACI 21.7.4 "Transverse Reinforcement":

Based on ACI 21.7.3.1, transverse reinforcement shall be provided within the joint. 2

sh cm88.2A =

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C- ACI 21.7.4 "Shear Strength":

tons84.266.4

77.5069.72V .col =+

=

.col21intjo,u VCTV −+=

tons83.23084.2606.10361.154 =−+=

cm)7045(cm45x2bb bj +≤=+=

( )( ) 2coljj cm31504570hbA ===

( ) tonsVn 24.2181000/31503004 ==

( ) tonsVn 68.16324.21875.0 ==Φ

nu VV Φ> and column dimension in the direction of shear force needs to be

increased.

For nu VV Φ= , ( ) ( )( ) tonshcol 83.2301000/45300475.0 = and 72.98=colh

Increase column cross sectional dimension to 45 cm x 100 cm.

Plan