12.7.1
Ordinary concentrically braced frames (OCBF) should be shown to withstand inelastic deformation corresponding to a joint rotation of at least 0.02 radians without degradation in strength and stiffness below the full yield value. Ordinary concentrically braced frames meeting the requirements of this section shall be deemed to satisfy the required inelastic deformation.
12.7.1.1
Ordinary concentrically braced frames shall not be used in seismic zones IV and V and for buildings
with importance factor greater than unity (I > 1.0) in seismic zone III.
12.7.1.2
The provision in this section apply for diagonal and X-bracing only. Specialist literature may be consulted for V and inverted V-type bracing. K-bracing shall not be permitted in systems to resist earthquake.
12.7.2 Bracing Members
12.7.2.1
The slenderness of bracing members shall not exceed 120.
12.7.2.2
The required compressive strength of bracing member shall not exceed 0.8 times , where is the design strength in axial compression (see 7.1.2).
12.7.2.3
Along any line of bracing, braces shall be provided such that for lateral loading in either direction, the tension braces will have to resist between 30 to 70 percent of the total lateral load.
12.7.2.4
Bracing cross-section can be plastic, compact or semi-compact, but not slender, as defined in 3.7.2
12.7.2.5
For all built-up braces, the spacing of tack fasteners shall be such that the unfavourable slenderness ratio of individual element, between such fasteners, shall not exceed 0.4 times the governing slenderness ratio of the brace itself. Bolted connections shall be avoided within the middle one-fourth of the clear brace length (0.25 times the length in the middle).
12.7.2.6
The bracing members shall be designed so that gross area yielding (see 6.2) and not the net area rupture (see 6.3) would govern the design tensile strength.
12.7.3 Bracing Connections
12.7.3.1
End connections in bracings shall be designed to withstand the minimum of the following:
- a) A tensile force in the bracing equal to
- b) Force in the brace due to load combination in 12.2.3; and
- c) Maximum force that can be transferred to the brace by the system.
12.7.3.2
The connection should be checked for tension rupture and block shear under the load determined in 12.7.3.1.
12.7.3.3
The connection shall be designed to withstand a moment of 1.2 times the full plastic moment of the braced section about the buckling axis.
12.7.3.4
Gusset plates shall be checked for bucklin out of their plane.