4.1.8.15.Design Provisions

(1) Except as provided in Sentences (2) and (3), diaphragms, collectors, chords, struts and connections shall be designed so as not to yield, and the design shall account for the shape of the diaphragm, including openings, and for the forces generated in the diaphragm due to the following cases, whichever one governs: (a) forces determined in Article 4.1.8.11. or 4.1.8.12. applied to the diaphragm are increased to reflect the lateral load capacity of the SFRS, plus forces in the diaphragm due to the transfer of forces between elements of the SFRS associated with the lateral load capacity of such elements and accounting for discontinuities and changes in stiffness in these elements, or (b) a minimum force corresponding to the design-based shear divided by N for the diaphragm at level x. (See Note A-4.1.8.15.(1)) (2) Steel deck roof diaphragms in buildings of less than 4 storeys or wood diaphragms that are designed and detailed according to the applicable referenced design standards to exhibit ductile behaviour shall meet the requirements of Sentence (1), except that they may yield and the forces shall be (a) for wood diaphragms acting in combination with vertical wood shear walls, equal to the lateral earthquake design force, (b) for wood diaphragms acting in combination with other SFRSs, not less than the force corresponding to RdRo = 2.0, and (c) for steel deck roof diaphragms, not less than the force corresponding to R dRo = 2.0. (3) Where diaphragms are designed in accordance with Sentence (2), the struts shall be designed in accordance with Clause (1)(a), and the collectors, chords and connections between the diaphragms and the vertical elements of the SFRS shall be designed for forces corresponding to the capacity of the diaphragms in accordance with the applicable CSA standards. (See Note A-4.1.8.15.(3)) (4) For single-storey buildings with steel deck or wood roof diaphragms designed with a value of R d greater than 1.5 and where the calculated maximum relative deflection, ΔD, of the diaphragm under lateral loads exceeds 50% of the average storey drift, ΔB, of the adjoining vertical elements of the SFRS, dynamic magnification of the inelastic response due to the in-plane diaphragm deformations shall be accounted for in the design as follows: (a) the vertical elements of the SFRS shall be designed and detailed to any one of the following: (i) to accommodate the anticipated magnified lateral deformations taken as R oRd(ΔB + ΔD) − RoΔD, (ii) to resist the forces magnified by Rd(1 + ΔD/ΔB)/(Rd + ΔD/ΔB), or (iii) by a special study, and (b) the roof diaphragm and chords shall be designed for in-plane shears and moments determined while taking into consideration the inelastic higher mode response of the structure. (See Note A-4.1.8.15.(4)) (5) Where the Seismic Category is SC3 or SC4, the elements supporting any discontinuous wall, column or braced frame shall be designed for the lateral load capacity of the components of the SFRS they support. (See Note A-4.1.8.15.(5).) (6) Where structures have vertical variations of RdRo satisfying Sentence 4.1.8.9.(4), the elements of the SFRS below the level where the change in RdRo occurs shall be designed for the forces associated with the lateral load capacity of the SFRS above that level. (See Note A-4.1.8.15.(6)) (7) Where earthquake effects can produce forces in a column or wall due to lateral loading along both orthogonal axes, account shall be taken of the effects of potential concurrent yielding of other elements framing into the column or wall from all directions at the level under consideration and as appropriate at other levels. (See Note A-4.1.8.15.(7)) (8) The design forces associated with the lateral capacity of the SFRS need not exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with RdRo taken as 1.0, unless otherwise provided by the applicable referenced design standards for elements, in which case the design forces associated with the lateral capacity of the SFRS need not exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with RdRo taken as less than or equal to 1.3. (See Note A-4.1.8.15.(8)) (9) Foundations need not be designed to resist the lateral load overturning capacity of the SFRS, provided the design and the Rd and Ro for the type of SFRS used conform to Table 4.1.8.9. and that the foundation is designed in accordance with Sentence 4.1.8.16.(4). (10) Foundation displacements and rotations shall be considered as required by Sentence 4.1.8.16.(1).