Highly Ductile Concentrically Braced Frame

9.6. CONCENTRICALLY BRACED FRAME WITH HIGH DUCTILITY LEVEL

Concentrically braced steel frames of high ductility level are dimensioned in such a way that no significant loss of strength occurs in the structural system, even if some of the braced members under compression are buckled. The rules to be applied in the dimensioning of these systems are given below:

9.6.1. General Conditions

9.6.1.1 - The width-to-thickness ratios of beams, columns and braces should not exceed the limiting width-to-thickness ratios, λ_hd, from Table 9.3, for members designed with high ductility level.

9.6.1.2 - Along any line of braces, braces shall be deployed in alternate directions such that, for either direction of force parallel to the braces, at least 30% but no more than 70% of the total horizontal force along that line is resisted by braces in compression.

9.6.3. Braces

Braces with high ductility level shall comply with the following requirements:

9.6.3.1 - The slenderness ratio ( effective buckling length / radius of gyration) in brace members must comply with condition KL / i ≤ 200.

9.6.3.2 - Built-up brace section may only be used if built-up sections are joined continuously.

9.6.3.3 - Braces shall be dimensioned by taking into account the load combinations including earthquake effect given in 9.2.5.

9.6.3.4 - The rupture strength is calculated using the effective net area of ​​the brace, while the yield strength is calculated using the gross area of the brace and R_t F_u A_e shall not be less than the R_y F_y A_g. If this condition is not met, the net area cross-section will be reinforced by considering the following conditions:

(a)  The characteristic yield stress of the stiffeners shall be at least equal to the characteristic yield stress of the brace.

(b)  Connection detail of stiffeners to brace shall be dimensioned under the effect of axial force computed by R_y F_y A_fb / 1.5 (GKT) or R_y F_y A_fb (YDKT).