12.8.1
Special concentrically braced frames (SCBF) should be shown to withstand inelastic deformation
corresponding to a joint rotation of at least 0.04 radians without degradation in strength and stiffness below the full yield value. Special concentrically braced frames meeting the requirements of this section shall be deemed to satisfy the required inelastic deformation.
12.8.1.1
Special concentrically braced frames (SCBF) may be used in any seismic zone [see IS 1893 (Part 1)] and for any building (importance-factor value).
12.8.1.2
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 system to resist earthquake.
12.8.2 Bracing Members
12.8.2.1
Bracing members shall be made of E250E steel of IS 2062 only.
12.8.2.2
The slenderness of bracing members shall not exceed 160 (only hangers).
12.8.2.3
The required compressive strength of bracing member shall not exceed the design strength in axial compression (see 7.1.2).
12.8.2.4
Along any line of bracing, braces shall be provided such that for lateral loading in either direction, the tension braces will resist between 30 to 70 percent of the load.
12.8.2.5
Braced cross-section shall be plastic as defined in 3.7.2.
12.8.2.6
In built-up braces, the spacing of tack connections shall be such that the slenderness ratio of individual element between such connections shall not exceed 0.4 times the governing slenderness ratio of the brace itself. Bolted connection shall be avoided within the middle one-fourth of the clear brace length (0.25 times the length, in the middle).
12.8.2.7
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.
12.8.3 Bracing Connections
12.8.3.1
Bracing end connections shall be designed to withstand the minimum of the following:
- a) A tensile force in the bracing equal to and
- b) Maximum force that can be transferred to the brace by the system.
12.8.3.2
The connection should be checked for tension
rupture and block shear under the load determined in 12.8.3.1.
12.8.3.3
The connection shall be designed to withstand a moment of 1.2 times the full plastic moment of the braced section about the critical buckling axis.
12.8.3.4
Gusset plates shall be checked for bucklin out of their plane.
12.8.4 Column
12.8.4.1
The column sections used in special concentrically braced frames (SCBF) shall be plastic as defined in 3.7.2.
12.8.4.2
Splices shall be located within the middle one-third of the column clear height. Splices shall be designed for the forces that can be transferred to it. In addition, splices in columns shall be designed to develop at least the nominal shear strength of the smaller connected member and 50 percent of the nominal flexural strength of the smaller connected section.