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Therefore, because of both the aspect ratio and the Vu criteria, diagonal reinforcement must be provided.

Observe that if either of the criteria was not satisfied, we would have had the option of designing the beam CB1 without the diagonal reinforcement. We could have used conventional horizontal reinforcement to resist flexure and vertical stirrups to resist shear. However, research has shown that diagonal reinforcement improves coupling beam performance, even at lower shear stress level. See SEAOC's 1999 Blue Book Commentary, Section C 407.7.

In some buildings it may be impractical to use diagonal reinforcement. Do the designers have any fallback position? Yes, they do. The requirements of Section 21.6.7.3 for diagonal reinforcement may be waived if coupling beams are not used as part of the lateral force resisting system. Such beams are permitted at locations where damage to these elements does not impair vertical load-carrying capacity or egress of the structure, or integrity of nonstructural components and their connections to the structure.

Returning to the example problem, the equation that determines the area of diagonal reinforcement Avd is given by

This can be written as

2ffy sin a 2ffy sin a where a = angle between the diagonal reinforcement and the longitudinal axis of the coupling beam (Fig. 4.47). Avd = area of diagonal reinforcement in each diagonally reinforced beam.

It should be noted that diagonally oriented reinforcement is effective only if the bars are placed with a reasonably large inclination angle a. So, diagonally reinforced coupling beams are restricted to beams having an aspects ratio ln/h < 4.0. This ratio approximately corresponds to a = 13°. Therefore, for beams with a geometry that results in a less than about 13°, ACI 318-02 does not permit diagonal reinforcement.

Each diagonal element is reinforced similar to a column consisting of longitudinal and transverse reinforcement. The column cage must consist of at least four longitudinal

Figure 4.47. Geometry for calculating a, the angle between diagonal reinforcement and the longitudinal axis of the coupling beam.

(solve for a by trial and error.)

cos a l

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