4.1.8.9 — SFRS Force Reduction Factors, System Overstrength Factors, and General Restrictions

(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 due to loads 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. (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 SFRS, not less than the force corresponding to RdRo = 2.0, and (c) for steel deck roof diaphragms, not less than the force corresponding to RdRo = 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. (4) For single-storey buildings with steel deck or wood roof diaphragms designed with a value of Rd 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 RoRd (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. (5) In cases where IEFaSa(0.2) is equal to or greater than 0.35, 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. (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. (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. (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. (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 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).

(1) The period of the isolated structure, determined using the post-yield stiffness of the isolation system in the horizontal direction under consideration, shall be greater than three times the period of the structure above the isolation interface calculated as a fixed base. (2) The isolation system shall be configured to produce a restoring force such that the lateral force at the TDD at the centre of mass of the isolated structure above the isolation interface is at least 0.025Wb greater than the lateral force at 50% of the TDD at the same location, in each horizontal direction, where Wb is the portion of W above the isolation interface. (3) The values of storey shears, storey forces, member forces, and deflections used in the design of all structural framing elements and components of the isolation system shall be obtained from analysis conforming to Sentence 4.1.8.19. (3) using one of the following values, whichever produces the most critical effect: (a) mean plus IE times the standard deviation of the results of all Non-linear Dynamic Analyses, or (b) √(I_E) times the mean of the results of all Non-linear Dynamic Analyses. (4) The force-deformation and damping characteristics of the isolation system used in the analysis and design of the seismically isolated structures shall be validated by testing at least two full-size specimens of each predominant type and size of isolator unit of the isolation system, which shall include, (a) the individual isolator units, (b) separate supplemental damping devices, if used, and (c) separate sacrificial wind-restraint systems, if used. (5) The force-deformation characteristics and damping value of a representative sample of the isolator units installed in the building shall be validated by tests prior to their installation. (6) A diaphragm or horizontal structural elements shall provide continuity immediately above the isolation interface to transmit forces due to non-uniform ground motions from one part of the structure to another. (7) All structural framing elements shall be designed for the forces described in Sentence (3) with RdRo= 1.0, except, (a) for structures with IE < 1.5, all the SFRS shall be detailed in accordance with the requirements for Rd ≥ 1.5 and the applicable referenced design standards, and (b) for structures with IE = 1.5, all the SFRS shall be detailed in accordance with the requirements for Rd ≥ 2.0 and the applicable referenced design standards. (8) The height restrictions noted in Table 4.1.8.9. need not apply to seismically isolated structures. (9) All isolator units shall be, (a) designed for the forces described in Sentence (3), and (b) able to accommodate the TDD determined at the specific location of each isolator unit. (10) The isolation system, including a separate wind-restraint system if used, shall limit lateral displacement due to wind loads across the isolation interface to a value equal to that required for the least storey height in accordance with Sentence 4.1.3.5. (3).