4.1.8.6.Structural Configuration

(1) Structures having any of the features listed in Table 4.1.8.6. shall be designated irregular. (2) Structures not classified as irregular according to Sentence (1) may be considered regular. (3) Except as required by Article 4.1.8.10., where the Seismic Category is SC3 or SC4, structures designated as irregular must satisfy the provisions referenced in Table 4.1.8.6. Table 4.1.8.6. Structural Irregularities(1)(2) Forming Part of Sentences 4.1.8.6.(1) and (3), Clause 4.1.8.7.(1)(c) and Article 4.1.8.10. Type Irregularity Type and Definition Notes Vertical Stiffness Irregularity For concrete and masonry shear walls, vertical stiffness irregularity shall be considered to exist where the lateral stiffness of the SFRS in 1 any storey is less than 70% of the stiffness in an adjacent storey, or less than 80% of the average stiffness in the three storeys above (3)(4)(5) or below. For all other types of SFRS, vertical stiffness irregularity shall be considered to exist where the interstorey deflection under lateral earthquake forces divided by the interstorey height, hs, of any storey is greater than 130% of that of an adjacent storey. Weight (mass) Irregularity 2 Weight irregularity shall be considered to exist where the weight, W i, of any storey is more than 150% of the weight of an adjacent storey. (3) A roof that is lighter than the floor below need not be considered. Vertical Geometric Irregularity 3 Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the SFRS in any storey is more than 130% (3)(4)(6) of that in an adjacent storey. In-Plane Discontinuity in Vertical Lateral-Force-Resisting Element 4 Except for braced frames and moment-resisting frames, an in-plane discontinuity shall be considered to exist where there is an offset of a (3)(4)(6) lateral-force-resisting element of the SFRS or a reduction in lateral stiffness of the resisting element in the storey below. Out-of-Plane Offsets 5 (3)(4)(6) Discontinuities in a lateral force path, such as out-of-plane offsets of the vertical elements of the SFRS. Discontinuity in Capacity – Weak Storey 6 A weak storey is one in which the storey shear strength is less than that in the storey above. The storey shear strength is the total (3)(4) strength of all seismic-resisting elements of the SFRS sharing the storey shear for the direction under consideration. Torsional Sensitivity (to be considered when diaphragms are not flexible) 7 (3)(4)(7) Torsional sensitivity shall be considered to exist when the ratio B calculated according to Sentence 4.1.8.11.(10) exceeds 1.7. Non-orthogonal Systems 8 (3)(8) A non-orthogonal system irregularity shall be considered to exist when the SFRS is not oriented along a set of orthogonal axes. Gravity-Induced Lateral Demand Irregularity 9 Gravity-induced lateral demand irregularity on the SFRS shall be considered to exist where the ratio calculated in accordance with (3)(4)(8) Sentence 4.1.8.10.(7) exceeds 0.1 for an SFRS with self-centering characteristics and 0.03 for other systems. Sloped Column Irregularity 10 Sloped column irregularity shall be considered to exist where a vertical member that is inclined more than 2° from the vertical supports a (4) portion of the weight of the building in axial compression. Notes to Table 4.1.8.6.: (1) One-storey penthouses with a weight of less than 10% of the level below need not be considered in the application of this Table. (2) See Note A-Table 4.1.8.6. (3) See Article 4.1.8.7. (4) See Article 4.1.8.10. (5) Increased stiffness in storeys below grade need not be considered in the determination of vertical stiffness irregularity. (6) See Article 4.1.8.15. (7) See Sentences 4.1.8.11.(10) and (11), and 4.1.8.12.(4). (8) See Article 4.1.8.8.