4.1.8.1 — Analysis

(1) Except as required by Clause (2)(b), structures with a Type 6 irregularity, Discontinuity in Capacity – Weak Storey, as described in Table 4.1.8.6., are not permitted unless IEFaSa(0.2) is less than 0.2 and the forces used for design of the SFRS are multiplied by RdRo. (2) Post-disaster buildings shall, (a) not have any irregularities conforming to Types 1, 3, 4, 5, 7 and 9 as described in Table 4.1.8.6., in cases where IEFaSa(0.2) is equal to or greater than 0.35, (b) not have a Type 6 irregularity as described in Table 4.1.8.6., (c) have an SFRS with an Rd of 2.0 or greater, and (d) have no storey with a lateral stiffness that is less than that of the storey above it. (3) For buildings having fundamental lateral periods, Ta, of 1.0 s or greater and where IEFvSa(1.0) is greater than 0.25, shear walls that are other than wood-based and form part of the SFRS shall be continuous from their top to the foundation and shall not have irregularities of Type 4 or 5 as described in Table 4.1.8.6. (4) For buildings constructed with more than 4 storeys of continuous wood construction and where IEFaSa(0.2) is equal to or greater than 0.35, timber SFRS of shear walls with wood-based panels, braced frames or moment-resisting frames as defined in Table 4.1.8.9. within the continuous wood construction shall not have irregularities of Type 4 or 5 as described in Table 4.1.8.6. (4.1) For buildings where IEFaSa(0.2) is equal to or greater than 0.35 or IEFvSa(1.0) is equal to or greater than 0.2 that are constructed with more than 4 storeys of continuous wood construction, timber SFRSs consisting of moderately ductile or limited ductility cross-laminated timber shear walls, platform-type construction, as defined in Table 4.1.8.9., within the continuous wood construction shall not have Type 4, 5, 6, 8, or 9 irregularities as described in Table 4.1.8.6. (5) The ratio, α, for Type 9 irregularity as described in Table 4.1.8.6. shall be determined independently for each orthogonal direction using the following equation: 𝛼 = QG / Qy where, QG =gravity-induced lateral demand on the SFRS at the critical level of the yielding system, and Qy =the resistance of the yielding mechanism required to resist the minimum earthquake loads, which need not be taken less than Ro multiplied by the minimum lateral earthquake force as determined in Article 4.1.8.11. or 4.1.8.12, as appropriate. (6) For buildings with a Type 9 irregularity as described in Table 4.1.8.6. and where IEFaSa(0.2) is equal to or greater than 0.5, deflections determined in accordance with Article 4.1.8.13. shall be multiplied by 1.2. (7) Structures where the value of α, as determined in accordance with Sentence (5), exceeds twice the limits in Table 4.1.8.6. for a Type 9 irregularity, and where IEFaSa(0.2) is equal to or greater than 0.5 are not permitted unless determined to be acceptable based on non-linear dynamic analysis studies.

(1) The static loading due to earthquake motion shall be determined according to the procedures given in this Article. (2) Except as provided in Sentence (12), the minimum lateral earthquake force, V, shall be calculated using the following formula: V = S (Ta) MvIEW/ (RdRo) except, (a) for walls, coupled walls and wall-frame systems, V shall not be less than, [[FORCE_FORMULA]] S (4.0) Mv IEW/ (RdRo) (b) for moment-resisting frames, braced frames and other systems, V shall not be less than, [[FORCE_FORMULA]] S (2.0) Mv IEW/ (RdRo) (c) for buildings located on a site other than Class F and having an SFRS with an Rd equal to or greater than 1.5, V need not be greater than the larger of, [[FORCE_FORMULA]] (2/3) · S(0.2) · I_E · W / (R_d R_o) and [[FORCE_FORMULA]] S (0.5) IEW/ (RdRo) (3) Except as provided in Sentence (4), the fundamental lateral period, Ta, in the direction under consideration in Sentence (2) shall be determined as, (a) for moment-resisting frames that resist 100% of the required lateral forces and where the frame is not enclosed by or adjoined by more rigid elements that would tend to prevent the frame from resisting lateral forces, and where hn is in metres, (i) 0.085 (hn)3/4 for steel moment frames, (ii) 0.075 (hn)3/4 for concrete moment frames, or (iii) 0.1 N for other moment frames, (b) 0.025 hn for braced frames where hn is in metres, (c) 0.05 (hn)3/4 for shear wall and other structures where hn is in metres, or (d) other established methods of mechanics using a structural model that complies with the requirements of Sentence 4.1.8.3. (8), except that, (i) for moment-resisting frames, Ta shall not be taken greater than 1.5 times that determined in Clause (a), (ii) for braced frames, Ta shall not be taken greater than 2.0 times that determined in Clause (b), (iii) for shear wall structures, Ta shall not be taken greater than 2.0 times that determined in Clause (c), (iv) for other structures, Ta shall not be taken greater than that determined in Clause (c), and (v) for the purpose of calculating the deflections, the period without the upper limit specified in Subclauses (d)(i) to (iv) may be used, except that, for walls, coupled walls and wall-frame systems, Ta shall not exceed 4.0 s, and for moment-resisting frames, braced frames, and other systems, Ta shall not exceed 2.0 s. (4) For single-storey buildings with steel deck or wood roof diaphragms, the fundamental lateral period, Ta, in the direction under consideration is permitted to be taken as, (a) 0.05 (hn)3/4 + 0.004 L for shear walls, (b) 0.035 hn + 0.004 L for steel moment frames and steel braced frames, or (c) the value obtained from methods of mechanics using a structural model that complies with the requirements of Sentence 4.1.8.3. (8), except that Ta shall not be greater than 1.5 times the value determined in Clause (a) or (b), as applicable, where L is the shortest length of the diaphragm, in m, between adjacent vertical elements of the SFRS in the direction perpendicular to the direction under consideration. (5) The weight, W, of the building shall be calculated using the formula, [[FORCE_FORMULA]] W = Σ_{i=1}^{n} W_i (6) The higher mode factor, Mv, and its associated base overturning moment reduction factor, J, shall conform to Tables 4.1.8.11.A. to 4.1.8.11.E. (7) The total lateral seismic force, V, shall be distributed such that a portion, Ft, shall be assumed to be concentrated at the top of the building, where Ft, is equal to 0.07 TaV but need not exceed 0.25 V and may be considered as zero, where the fundamental lateral period, Ta, does not exceed 0.7 s; the remainder, V - Ft, shall be distributed along the height of the building, including the top level, in accordance with the formula, F_x = (V − F_t) · W_x · h_x / (Σ_{i=1}^{n} W_i h_i) (8) The structure shall be designed to resist overturning effects caused by the earthquake forces determined in Sentence (7) and the overturning moment at level x, Mx, shall be determined using the formula, M_x = J_x · Σ_{i=x}^{n} F_i (h_i − h_x) where, Jx =1.0 for hx ≥ 0.6hn, and Jx =J + (1- J)(hx / 0.6hn) for hx,< 0.6hn where, J =base overturning moment reduction factor conforming to Table 4.1.8.11. (9) Torsional effects that are concurrent with the effects of the forces mentioned in Sentence (7) and are caused by the simultaneous actions of the following torsional moments shall be considered in the design of the structure according to Sentence (11): (a) torsional moments introduced by eccentricity between the centres of mass and resistance and their dynamic amplification, and (b) torsional moments due to accidental eccentricities. (10) Torsional sensitivity shall be determined by calculating the ratio Bx for each level x according to the following equation for each orthogonal direction determined independently: [[FORCE_FORMULA]] Bx = δmax / δave where, B =maximum of all values of Bx in both orthogonal directions, except that the Bx for one-storey penthouses with a weight less than 10% of the level below need not be considered, δmax =maximum storey displacement at the extreme points of the structure, at level x in the direction of the earthquake induced by the equivalent static forces acting at distances ± 0.10 Dnx from the centres of mass at each floor, and δave =average of the displacements at the extreme points of the structure at level x produced by the above-mentioned forces. (11) Torsional effects shall be accounted for as follows: (a) for a building with B ≤1.7 or where IEFaSa(0.2) is less than 0.35, by applying torsional moments about a vertical axis at each level throughout the building, derived for each of the following load cases considered separately, (i) Tx = Fx(ex + 0.10 Dnx), and (ii) Tx = Fx(ex – 0.10 Dnx) where Fx is the lateral force at each level determined according to Sentence (6) and where each element of the building is designed for the most severe effect of the above load cases, or (b) for a building with B >1.7, in cases where IEFaSa(0.2) is equal to or greater than 0.35, by a Dynamic Analysis Procedure as specified in Article 4.1.8.12. (12) Where the fundamental lateral period, Ta, is determined in accordance with Clause (3)(d) and the building is constructed with more than 4 storeys of continuous wood construction and has a timber SFRS consisting of shear walls with wood-based panels, braced frames or moment-resisting frames as defined in Table 4.1.8.9., the lateral earthquake force, V, as determined in accordance with Sentence (2) shall be multiplied by 1.2 but need not exceed the value determined by using Clause (2)(c).

(1) Except as provided in Articles 4.1.8.19. and 4.1.8.21., the Dynamic Analysis Procedure shall be in accordance with one of the following methods: (a) Linear Dynamic Analysis by either the Modal Response Spectrum Method or the Numerical Integration Linear Time History Method using a structural model that complies with the requirements of Sentence 4.1.8.3. (8), or (b) Non-linear Dynamic Analysis, in which case a special study shall be performed. (2) The spectral acceleration values used in the Modal Response Spectrum Method shall be the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4. (7). (3) The ground motion histories used in the Numerical Integration Linear Time History Method shall be compatible with a response spectrum constructed from the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4. (7). (4) The effects of accidental torsional moments acting concurrently with the lateral earthquake forces that cause them shall be accounted for by the following methods: (a) the static effects of torsional moments due to (± 0.10 Dnx)Fx at each level x, where Fx is either determined from the elastic dynamic analysis or determined from Sentence 4.1.8.11. (7) multiplied by RdRo/IE, shall be combined with the effects determined by dynamic analysis, or (b) if B, as defined in Sentence 4.1.8.11. (10), is less than 1.7, it is permitted to use a three-dimensional dynamic analysis with the centres of mass shifted by a distance of – 0.05 Dnx and + 0.05 Dnx. (5) Except as provided in Sentence (6), the design elastic base shear, Ved, is equal to the elastic base shear, Ve, obtained from a Linear Dynamic Analysis. (6) For structures located on sites other than Class F that have an SFRS with Rd equal to or greater than 1.5, the elastic base shear obtained from a Linear Dynamic Analysis may be multiplied by the larger of the following factors to obtain the design elastic base shear, Ved: [[FORCE_FORMULA]] 2S(0.2)/3S(Ta) ≤ 1.0 and [[FORCE_FORMULA]] S(0.5) / S(Ta) ≤ 1.0 (7) The design elastic base shear, Ved, shall be multiplied by the importance factor, IE, as determined in Article 4.1.8.5., and shall be divided by RdRo, as determined in Article 4.1.8.9., to obtain the design base shear, Vd. (8) Except as required by Sentence (9) or (12), if the base shear, Vd, obtained in Sentence (7) is less than 80% of the lateral earthquake design force, V, of Article 4.1.8.11., Vd shall be taken as 0.8 V. (9) For irregular structures requiring dynamic analysis in accordance with Article 4.1.8.7., Vd shall be taken as the larger of the Vd determined in Sentence (7) and 100% of V. (10) Except as required by Sentence (11), the values of elastic storey shears, storey forces, member forces, and deflections obtained from the Linear Dynamic Analysis, including the effect of accidental torsion determined in Sentence (4), shall be multiplied by Vd/Ve to determine their design values, where Vd is the base shear. (11) For the purpose of calculating deflections, it is permitted to use a value for V based on the value for Ta determined in Clause 4.1.8.11. (3)(d) to obtain Vd in Sentences (8) and (9). (12) For buildings constructed with more than 4 storeys of continuous wood construction, having a timber SFRS consisting of shear walls with wood-based panels, braced frames or moment-resisting frames as defined in Table 4.1.8.9., and whose fundamental lateral period, Ta, is determined in accordance with Clause 4.1.8.11. (3)(d), the design base shear, Vd, shall be taken as the larger value of Vd determined in accordance with Sentence (7) and 100% of V.

(1) Except as provided in Sentences (5) and (6), lateral deflections of a structure shall be calculated in accordance with the loads and requirements defined in this Subsection. (2) Lateral deflections obtained from a linear elastic analysis using the methods given in Articles 4.1.8.11. and 4.1.8.12. and incorporating the effects of torsion, including accidental torsional moments, shall be multiplied by RdRo/IE and increased as required by Sentences 4.1.8.10. (6) and 4.1.8.16. (1) to give realistic values of anticipated deflections. (3) Based on the lateral deflections calculated in Sentences (2), (5) and (6), the largest interstorey deflection at any level shall be limited to 0.01 hs for post-disaster buildings, 0.02 hs for High Importance Category buildings, and 0.025 hs for all other buildings. (4) The deflections calculated in Sentence (2) shall be used to account for sway effects as required by Sentence 4.1.3.2. (12). (5) The lateral deflections of a seismically isolated structure shall be calculated in accordance with Article 4.1.8.20. (6) The lateral deflections of a structure with supplemental energy dissipation shall be calculated in accordance with Article 4.1.8.22.

(1) Adjacent structures shall be, (a)[[NO_FORMULA]] separated by a distance equal to at least the square root of the sum of the squares of their individual deflections calculated in Sentence 4.1.8.13. (2), or (b) connected to each other. (2) The method of connection required in Sentence (1) shall take into account the mass, stiffness, strength, ductility and anticipated motion of the connected buildings and the character of the connection. (3) Rigidly connected buildings shall be assumed to have the lowest RdRo value of the buildings connected. (4) Buildings with non-rigid or energy-dissipating connections require special studies.

(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 increased displacements of the structure resulting from foundation movement shall be shown to be within acceptable limits for both the SFRS and the structural framing elements not considered to be part of the SFRS. (2) Except as provided in Sentences (3) and (4), foundations shall be designed to have factored shear and overturning resistances greater than the lateral load capacity of the SFRS. (3) The shear and overturning resistances of the foundation determined using a bearing stress equal to 1.5 times the factored bearing strength of the soil or rock and all other resistances equal to 1.3 times the factored resistances need not exceed the design forces determined in Sentence 4.1.8.7. (1) using RdRo = 1.0 except that the factor of 1.3 shall not apply to the portion of the resistance to uplift or overturning resulting from gravity loads. (4) A foundation is permitted to have a factored overturning resistance less than the lateral load overturning capacity of the supported SFRS, provided the following requirements are met: (a) neither the foundation nor the supported SFRS are constrained against rotation, and (b) the design overturning moment of the foundation is, (i) not less than 75% of the overturning capacity of the supported SFRS, and (ii) not less than that determined in Sentence 4.1.8.7. (1) using RdRo = 2.0. (5) The design of foundations shall be such that they are capable of transferring earthquake loads and effects between the building and the ground without exceeding the capacities of the soil and rock. (6) In cases where IEFaSa(0.2) is equal to or greater than 0.35, the following requirements shall be satisfied: (a) piles or pile caps, drilled piers, and caissons shall be interconnected by continuous ties in no fewer than two directions, (b) piles, drilled piers, and caissons shall be embedded a minimum of 100 mm into the pile cap or structure, and (c) piles, drilled piers, and caissons, other than wood piles, shall be connected to the pile cap or structure for a minimum tension force equal to 0.15 times the factored compression load on the pile. (7) At sites where IEFaSa(0.2) is equal to or greater than 0.35, basement walls shall be designed to resist earthquake lateral pressures from backfill or natural ground. (8) At sites where IEFaSa(0.2) is greater than 0.75, the following requirements shall be satisfied: (a) piles, drilled piers, or caissons shall be designed and detailed to accommodate cyclic inelastic behaviour when the design moment in the element due to earthquake effects is greater than 75% of its moment capacity, and (b) spread footings founded on soil defined as Site Class E or F shall be interconnected by continuous ties in no fewer than two directions. (9) Each segment of a tie between elements that is required by Clause (6)(a) or (8)(b) shall be designed to carry by tension or compression a horizontal force at least equal to the greatest factored pile cap or column vertical load in the elements it connects, multiplied by a factor of 0.10 IEFaSa(0.2), unless it can be demonstrated that equivalent restraints can be provided by other means. (10) The potential for liquefaction of the soil and its consequences, such as significant ground displacement and loss of soil strength and stiffness, shall be evaluated based on the ground motion parameters referenced in Subsection 1.1.2., as modified by Article 4.1.8.4., and shall be taken into account in the design of the structure and its foundations.

(1) The potential for slope instability and its consequences, such as slope displacement, shall be evaluated based on site-specific material properties and ground motion parameters referenced in Subsection 1.1.2., as modified by Article 4.1.8.4., and shall be taken into account in the design of the structure and its foundations.

(1) Except as provided in Sentences (2), (7) and (16), elements and components of buildings described in Table 4.1.8.18. and their connections to the structure shall be designed to accommodate the building deflections calculated in accordance with Article 4.1.8.13. and the element or component deflections calculated in accordance with Sentence (9), and shall be designed for a lateral force, VP, applied through the centre of mass of the element or component that is equal to: [[FORCE_FORMULA]] Vp= 0.3FaSa(0.2) IESpWp where, Fa =as defined in Sentence 4.1.8.4. (7), Sa(0.2) =spectral response acceleration value at 0.2 s, as defined in Sentence 4.1.8.4. (1), IE =importance factor for the building, as defined in Article 4.1.8.5., Sp =CpArAx/Rp (the maximum value of Sp shall be taken as 4.0 and the minimum value of Sp shall be taken as 0.7), where, Cp =element or component factor from Table 4.1.8.18., Ar =element or component force amplification factor from Table 4.1.8.18., Ax =height factor (1 + 2 hx / hn), Rp =element or component response modification factor from Table 4.1.8.18., and Wp =weight of the component or element. (2) For buildings other than post-disaster buildings, seismically isolated buildings and buildings with supplemental energy dissipation systems, where IEFaSa(0.2) is less than 0.35, the requirements of Sentence (1) need not apply to Categories 6 through 22 of Table 4.1.8.18. (3) For the purpose of applying Sentence (1) for Categories 11 and 12 of Table 4.1.8.18., elements or components shall be assumed to be flexible or flexibly connected unless it can be shown that the fundamental period of the element or component and its connection is less than or equal to 0.06 s, in which case the element or component is classified as being rigid or rigidly connected. (4) The weight of access floors shall include the dead load of the access floor and the weight of permanent equipment, which shall not be taken as less than 25% of the floor live load. (5) When the mass of a tank plus its contents or the mass of a flexible or flexibly connected piece of machinery, fixture or equipment is greater than 10% of the mass of the supporting floor, the lateral forces shall be determined by rational analysis. (6) Forces shall be applied in the horizontal direction that results in the most critical loading for design, except for Category 6 of Table 4.1.8.18., where the forces shall be applied up and down vertically. (7) Connections to the structure of elements and components listed in Table 4.1.8.18. shall be designed to support the component or element for gravity loads, shall conform to the requirements of Sentence (1), and shall also satisfy these additional requirements: (a) friction due to gravity loads shall not be considered to provide resistance to seismic forces, (b) Rp for non-ductile connections, such as adhesives or power actuated fasteners, shall be taken as 1.0, (c) Rp for anchorage using shallow expansion, chemical, epoxy or cast-in place anchors shall be 1.5, where shallow anchors are those with a ratio of embedment length to diameter of less than 8, (d) power-actuated fasteners and drop-in anchors shall not be used for tension loads, (e) connections for non-structural elements or components of Category 1, 2 or 3 of Table 4.1.8.18. attached to the side of a building and above the first level above grade shall satisfy the following requirements: (i) for connections where the body of the connection is ductile, the body shall be designed for values of CP, Ar and Rp given in Table 4.1.8.18., and all of the other parts of the connection, such as anchors, welds, bolts and inserts, shall be capable of developing 2.0 times the nominal yield resistance of the body of the connection, and (ii) connections where the body of the connection is not ductile shall be designed for values of Cp=2.0, Rp =1.0 and Ar given in Table 4.1.8.18., and (f) a ductile connection is one where the body of the connection is capable of dissipating energy through cyclic inelastic behaviour. (8) Floors and roofs acting as diaphragms shall satisfy the requirements for diaphragms stated in Article 4.1.8.15. (9) Lateral deflections of elements or components shall be based on the loads defined in Sentence (1) and lateral deflections obtained from an elastic analysis shall be multiplied by Rp/IE to give realistic values of the anticipated deflections. (10) The elements or components shall be designed so as not to transfer to the structure any forces unaccounted for in the design, and rigid elements such as walls or panels shall satisfy the requirements of Sentence 4.1.8.3. (6). (11) Seismic restraint for suspended equipment, pipes, ducts, electrical cable trays, etc. shall be designed to meet the force and displacement requirements of this Article and be constructed in a manner that will not subject hanger rods to bending. (12) Isolated suspended equipment and components, such as pendant lights, may be designed as a pendulum system provided that adequate chains or cables capable of supporting 2.0 times the weight of the suspended component are provided and the deflection requirements of Sentence (11) are satisfied. (13) Free-standing steel pallet storage racks are permitted to be designed to resist earthquake effects using rational analysis, provided the design achieves the minimum performance level required by this Subsection. (14) Except as provided in Sentence (15), the relative displacement of glass in glazing systems, Dfallout , shall be equal to the greater of, (a) 13 mm, or (b) Dfallout ≥ 1.25IEDp , where, Dfallout =relative displacement at which glass fallout occurs, and Dp =relative earthquake displacement that the component must be designed to accommodate, calculated in accordance with Article 4.1.8.13. and applied over the height of the glass component. (15) Glass need not comply with Sentence (14), provided at least one of the following conditions is met: (a) IEFaSa(0.2) < 0.35, (b) the glass has sufficient clearance from its frame such that Dclear ≥ 1.25 Dp calculated as follows: Dclear= 2C1(1 +hpC2/(bpC1)) where, Dclear =relative horizontal displacement measured over the height of the glass panel, which causes initial glass-to-frame contact, C1=average of the clearances on both sides between the vertical glass edges and the frame, hp =height of the rectangular glass panel, C2=average of the top and bottom clearances between the horizontal glass edges and the frame, and bp =width of the rectangular glass panel, (c) the glass is fully tempered, monolithic, installed in a building that is not a post-disaster building, and no part of the glass is located more than 3 m above a walking surface, or (d) the glass is annealed or heat-strengthened laminated glass in a single thickness with an interlayer no less than 0.76 mm and captured mechanically in a wall system glazing pocket with the perimeter secured to the frame by a wet, glazed, gunable, curing, elastomeric sealant perimeter bead of 13 mm minimum glass contact width. (16) For a structure with supplemental energy dissipation, the following criteria shall apply: (a) the value of Sa(0.2) used in Sentence (1) shall be determined from the mean 5% damped floor spectral acceleration values at 0.2 s by averaging the individual 5% damped floor spectra at the base of the structure determined using Non-Linear Dynamic Analysis, and (b) the value of Fa used in Sentence (1) shall be 1.

(1) For the purposes of this Article and Article 4.1.8.20., the following terms shall have the meaning stated herein: (a)“seismic isolation” is an alternative seismic design concept that consists of installing an isolation system with low horizontal stiffness, thereby substantially increasing the fundamental period of the structure, (b)“isolation system” is a collection of structural elements at the level of the isolation interface that includes all individual isolator units, all structural elements that transfer force between elements of the isolation system, all connections to other structural elements, and may also include a wind-restraint system, energy-dissipation devices, and a displacement restraint system, (c)“seismically isolated structure” includes the upper portion of the structure above the isolation system, the isolation system, and the portion of the structure below the isolation system, (d)“isolator unit” is a structural element of the isolation system that permits large lateral deformations under lateral earthquake design forces and is characterized by vertical-load-carrying capability combined with increased horizontal flexibility and high vertical stiffness, energy dissipation (hysteretic or viscous), self-centering capability, and lateral restraint (sufficient elastic stiffness) under non-seismic service lateral loads, (e)“isolation interface” is the boundary between the isolated upper portion of the structure above the isolation system and the lower portion of the structure below the isolation system, and (f)“wind-restraint system” is the collection of structural elements of the isolation system that provides restraint of the seismically isolated structure for wind loads and is permitted to be either an integral part of the isolator units or a separate device. (2) Every seismically isolated structure and every portion thereof shall be analyzed and designed in accordance with, (a) the loads and requirements prescribed in this Article and Article 4.1.8.20., (b) other applicable requirements of this Subsection, and (c) appropriate engineering principles and current engineering practice. (3) For the analysis and modeling of the seismically isolated structure, the following criteria shall apply: (a) three dimensional Non-linear Dynamic Analysis of the structure shall be performed in accordance with Article 4.1.8.12, (b) unless verified from rational analysis, the inherent equivalent viscous damping — excluding the hysteretic damping provided by the isolation system or supplemental energy dissipation devices — used in the analysis shall not be taken as more than 2.5% of the critical damping at the significant modes of vibration, (c) all individual isolator units shall be modeled with sufficient detail to account for their non-linear force-deformation characteristics, including effects of the relevant loads, and with consideration of variations in material properties over the design life of the structure, and (d) except for elements of the isolation system, other components of the seismically isolated structure shall be modeled using elastic material properties in accordance with Sentence 4.1.8.3. (8). (4) The ground motion histories used in Sentence (3) shall be, (a) appropriately selected and scaled following good engineering practice, (b) compatible with, (i) a response spectrum derived from the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4. (9) for ground conditions of Site Classes A, B and C, and (ii) a 5% damped response spectrum based on a site-specific evaluation for ground conditions of Site Classes D, E and F, and (c) amplitude-scaled in an appropriate manner over the period range of 0.2 T1 to 1.5 T1, where T1 is the period of the isolated structure determined using the post-yield stiffness of the isolation system in the horizontal direction under consideration, or the period specified in Sentence 4.1.8.20. (1) if the post-yield stiffness of the isolation system is not well defined.

(1) In this Subsection, [[FORMULA_BLOCK]] Ar =response amplification factor to account for type of attachment of mechanical/electrical equipment, as defined in Sentence 4.1.8.18. (1), Ax =amplification factor at level x to account for variation of response of mechanical/electrical equipment with elevation within the building, as defined in Sentence 4.1.8.18. (1), Bx =ratio at level x used to determine torsional sensitivity, as defined in Sentence 4.1.8.11. (9), B =maximum value of Bx, as defined in Sentence 4.1.8.11. (9), Cp =seismic coefficient for mechanical/electrical equipment, as defined in Sentence 4.1.8.18. (1), Dnx =plan dimension of the building at level x perpendicular to the direction of seismic loading being considered, ex =distance measured perpendicular to the direction of earthquake loading between centre of mass and centre of rigidity at the level being considered, Fa =site coefficient, as defined in Sentence 4.1.8.4. (7), F(PGA) =site coefficient for PGA, as defined in Sentence 4.1.8.4. (5), F(PGV) =site coefficient for PGV, as defined in Sentence 4.1.8.4. (5), Fs =site coefficient, as defined in Sentence 4.1.8.1. (2), F(T) =site coefficient for spectral acceleration, as defined in Sentence 4.1.8.4. (5), Ft =portion of V to be concentrated at the top of the structure, as defined in Sentence 4.1.8.11. (6), Fv =site coefficient, as defined in Sentence 4.1.8.4. (7), Fx =lateral force applied to level x, as defined in Sentence 4.1.8.11. (6), hi, hn, hx =the height above the base (i = 0) to level i, n, or x respectively, where the base of the structure is the level at which horizontal earthquake motions are considered to be imparted to the structure, hs =interstorey height (hi - hi-1), IE =earthquake importance factor of the structure, as described in Sentence 4.1.8.5. (1), J =numerical reduction coefficient for base overturning moment, as defined in Sentence 4.1.8.11. (5), JX =numerical reduction coefficient for overturning moment at level x, as defined in Sentence 4.1.8.11. (7), Level i =any level in the building, i =1 for first level above the base, Level n =level that is uppermost in the main portion of the structure, Level x =level that is under design consideration, Mv =factor to account for higher mode effect on base shear, as defined in Sentence 4.1.8.11. (5), Mx =overturning moment at level x, as defined in Sentence 4.1.8.11. (7), N =total number of storeys above exterior grade to level n, =Average Standard Penetration Resistance for the top 30 m, corrected to a rod energy efficiency of 60% of the theoretical maximum, PGA =Peak Ground Acceleration expressed as a ratio to gravitational acceleration, as defined in Sentence 4.1.8.4. (1), PGAref =reference PGA for determining F(T), F(PGA) and F(PGV), as defined in Sentence 4.1.8.4. (4), PGV =Peak Ground Velocity, in m/s, as defined in Sentence 4.1.8.4. (1), PI =plasticity index for clays, Rd =ductility-related force modification factor reflecting the capability of a structure to dissipate energy through reversed cyclic inelastic behaviour, as given in Article 4.1.8.9., Ro =overstrength-related force modification factor accounting for the dependable portion of reserve strength in a structure designed according to these provisions, as defined in Article 4.1.8.9., Rs =combined overstrength and ductility-related modification factor, as defined in Sentence 4.1.8.1. (7), SP =horizontal force factor for part or portion of a building and its anchorage, as given in Sentence 4.1.8.18. (1), S(T) =design spectral response acceleration, expressed as a ratio to gravitational acceleration, for a period of T, as defined in Sentence 4.1.8.4. (7), Sa(T) =5% damped spectral response acceleration, expressed as a ratio to gravitational acceleration, for a period of T, as defined in Sentence 4.1.8.4. (1), SFRS =Seismic Force Resisting System(s) is that part of the structural system that has been considered in the design to provide the required resistance to the earthquake forces and effects defined in Subsection 4.1.8., su =average undrained shear strength in the top 30 m of soil, T =period in seconds, Ta =fundamental lateral period of vibration of the building or structure in seconds in the direction under consideration, as defined in Sentence 4.1.8.11. (3), Ts =fundamental lateral period of vibration of the building or structure in seconds in the direction under consideration, as described in Sentence 4.1.8.1. (7), Tx =floor torque at level x, as defined in Sentence 4.1.8.11. (10), TDD =Total Design Displacement of any point in a seismically isolated structure, within or above the isolation system, obtained by calculating the mean + (IE × the standard deviation) of the peak horizontal displacements from all sets of ground motion histories analyzed, but not less than × the mean, where the peak horizontal displacement is based on the vector sum of the two orthogonal horizontal displacements considered for each time step, V =lateral earthquake design force at the base of the structure, as determined by Article 4.1.8.11., Vd =lateral earthquake design force at the base of the structure, as determined by Article 4.1.8.12., Ve =lateral earthquake elastic force at the base of the structure, as determined by Article 4.1.8.12., Ved =lateral earthquake design elastic force at the base of the structure, as determined by Article 4.1.8.12., VP =lateral force on a part of the structure, as determined by Article 4.1.8.18., Vs =lateral earthquake design force at the base of the structure, as determined by Sentence 4.1.8.1. (7), =average shear wave velocity in the top 30 m of soil or rock, W =dead load, as defined in Article 4.1.4.1., except that the minimum partition load as defined in Sentence 4.1.4.1. (3) need not exceed 0.5 kPa, plus 25% of the design snow load specified in Subsection 4.1.6., plus 60% of the storage load for areas used for storage, except that storage garages need not be considered storage areas, and the full contents of any tanks, Wi, Wx =portion of W that is located at or is assigned to level i or x respectively, WP =weight of a part or portion of a structure, e.g., cladding, partitions and appendages, Wt =sum of Wi over the height of the building, δave =average displacement of the structure at level x, as defined in Sentence 4.1.8.11. (9), and δmax =maximum displacement of the structure at level x, as defined in Sentence 4.1.8.11. (9). [[END_FORMULA_BLOCK]]

(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).

(1) For the purposes of this Article and Article 4.1.8.22., the following terms shall have the meaning stated herein: (a)“supplemental energy dissipation device” is a dedicated structural element of the supplemental energy dissipation system that dissipates energy due to relative motion of each of its ends or by alternative means, and includes all pins, bolts, gusset plates, brace extensions and other components required to connect it to the other elements of the structure; a device may be classified as either displacement-dependent or velocity-dependent, or a combination thereof, and may be configured to act in either a linear or non-linear manner, and (b)“supplemental energy dissipation system” is a collection of energy dissipation devices installed in a structure that supplement the energy dissipation of the SFRS. (2) Every structure with a supplemental energy dissipation system and every portion thereof shall be designed and constructed in accordance with, (a) the loads and requirements prescribed in this Article and Article 4.1.8.22., (b) other applicable requirements of this Subsection, and (c) appropriate engineering principles and current engineering practice. (3) Where supplemental energy dissipation devices are used across the isolation interface of a seismically isolated structure, displacements, velocities, and accelerations shall be determined in accordance with Article 4.1.8.20. (4) For the analysis and modeling of structures with supplemental energy dissipation devices, the following criteria shall apply: (a) a three-dimensional Non-linear Dynamic Analysis of the structure shall be performed in accordance with Article 4.1.8.12., (b) for SFRS with Rd > 1.0, the non-linear hysteretic behaviour of the SFRS shall be explicitly — with sufficient detail — accounted for in the modeling and analysis of the structure, (c) unless verified from rational analysis, the inherent equivalent viscous damping — excluding the damping provided by the supplemental energy dissipation devices — used in the analysis shall not be taken as more than 2.5% of the critical damping at the significant modes of vibration, (d) all supplemental energy dissipation devices shall be modeled with sufficient detail to account for their non-linear force deformation characteristics, including effects of the relevant loads, and with consideration of variations in their properties over the design life of the structure, and (e) except for the SFRS and elements of the supplemental energy dissipation system, other components of the structure shall be modeled using elastic material properties in accordance with Sentence 4.1.8.3. (8). (5) The ground motion histories used in Sentence (4) shall be, (a) appropriately selected and scaled following good engineering practice, (b) compatible with a 5% damped response spectrum derived from the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4. (9), and (c) amplitude-scaled in an appropriate manner over the period range of 0.2 T1 to 1.5 T1, where T1 is the fundamental lateral period of the structure with the supplemental energy dissipation system.

(1) The values of storey shears, storey forces, member forces, and deflections for the design of all structural framing elements and all supplemental energy dissipation devices shall be obtained from analysis conforming to Sentence 4.1.8.21. (4) 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. (2) The largest interstorey deflection at any level of the structure as determined in accordance with Sentence (1) shall conform to the limits stated in Sentence 4.1.8.13. (3). (3) The force-deformation and force-velocity characteristics of the supplemental energy dissipation devices used in the analysis and design of structures with supplemental energy dissipation systems shall be validated by testing at least two full-size specimens of each type of supplemental energy dissipation device. (4) The force-deformation and force-velocity characteristics and damping values of a representative sample of the supplemental energy dissipation devices installed in the building shall be validated by tests prior to their installation. (5) Elements of the supplemental energy dissipation system, except the supplemental energy dissipation devices themselves, shall be designed to remain elastic for the design loads. (6) All structural framing elements shall be designed, (a) for an SFRS with Rd =1.0, using the forces referred to in Sentence (1) with RdRo = 1.0, except that the SFRS shall be detailed in accordance with the requirements for Rd ≥ 1.5 and the applicable referenced design standards, or (b) for an SFRS with Rd > 1.0, using the forces referred to in Sentence (1) with RdRo = 1.0, except that the SFRS shall be detailed in accordance with the requirements for the selected Rd and the applicable referenced design standards. (7) Supplemental energy dissipation devices and other components of the supplemental energy dissipation system shall be designed in accordance with Sentence (1) with consideration of the following: (a) low-cycle, large-displacement degradation due to seismic loads, (b) high-cycle, small-displacement degradation due to wind, thermal, or other cyclic loads, (c) forces or displacements due to gravity loads, (d) adhesion of device parts due to corrosion or abrasion, biodegradation, moisture, or chemical exposure, (e) exposure to environmental conditions, including, but not limited to, temperature, humidity, moisture, radiation (e.g., ultraviolet light), and reactive or corrosive substances (e.g., salt water), (f) devices subject to failure due to low-cycle fatigue must resist wind forces without slip, movement, or inelastic cycling, (g) the range of thermal conditions, device wear, manufacturing tolerances, and other effects that cause device properties to vary during the design life of the device, and (h) connection points of devices must provide sufficient articulation to accommodate simultaneous longitudinal, lateral, and vertical displacements of the supplemental energy dissipation system. (8) Means of access for inspection and removal for replacement of all supplemental energy dissipation devices shall be provided.

(1) The building shall be designed to meet the requirements of this Subsection and of the design standards referenced in Section 4.3. (2) Structures shall be designed with a clearly defined load path, or paths, that will transfer the inertial forces generated in an earthquake to the supporting ground. (3) The structure shall have a clearly defined Seismic Force Resisting System(s) (SFRS), as defined in Article 4.1.8.2. (4) The SFRS shall be designed to resist 100% of the earthquake loads and their effects. (5) All structural framing elements not considered to be part of the SFRS must be investigated and shown to behave elastically or to have sufficient non-linear capacity to support their gravity loads while undergoing earthquake-induced deformations calculated from the deflections determined in Article 4.1.8.13. (6) Stiff elements that are not considered part of the SFRS, such as concrete, masonry, brick or pre-cast walls or panels, shall be, (a) separated from all structural elements of the building such that no interaction takes place as the building undergoes deflections due to earthquake effects as calculated in this Subsection, or (b) made part of the SFRS and satisfy the requirements of this Subsection. (7) Stiffness imparted to the structure from elements not part of the SFRS, other than those described in Sentence (6), shall not be used to resist earthquake deflections but shall be accounted for, (a) in calculating the period of the structure for determining forces if the added stiffness decreases the fundamental lateral period by more than 15%, (b) in determining the irregularity of the structure, except the additional stiffness shall not be used to make an irregular SFRS regular or to reduce the effects of torsion, and (c) in designing the SFRS if inclusion of the elements not part of the SFRS in the analysis has an adverse effect on the SFRS. (8) Structural modelling shall be representative of the magnitude and spatial distribution of the mass of the building and of the stiffness of all elements of the SFRS, including stiff elements that are not separated in accordance with Sentence 4.1.8.3. (6), and shall account for, (a) the effect of cracked sections in reinforced concrete and reinforced masonry elements, (b) the effect of the finite size of members and joints, (c) sway effects arising from the interaction of gravity loads with the displaced configuration of the structure, and (d) other effects that influence the lateral stiffness of the building.

(1) The peak ground acceleration (PGA), peak ground velocity (PGV) and the 5% damped spectral response acceleration values, Sa(T), for the reference ground conditions (Site Class C in Table 4.1.8.4.A.) for periods T of 0.2 s, 0.5 s, 1.0 s, 2.0 s, 5.0 s and 10.0 s, shall be determined in accordance with Subsection 1.1.2. and are based on a 2% probability of exceedance in 50 years. (2) Site classifications for ground shall conform to Table 4.1.8.4.A. and shall be determined using V̄_S30 or, where V̄_S30 is not known, using Sentence (3). (3) If average shear wave velocity, V̄_S30, is not known, Site Class shall be determined from energy-corrected Average Standard Penetration Resistance, N̄_60, or from soil average undrained shear strength, su, as noted in Table 4.1.8.4.A., N̄_60 and su being calculated based on rational analysis. (4) For the purpose of determining the values of F(T) to be used in the calculation of design spectral acceleration, S(T), in Sentence (9), and the values of F(PGA) and F(PGV), the value of PGAref to be used with Tables 4.1.8.4.B. to 4.1.8.4.I. shall be taken as, (a) 0.8 PGA, where the ratio Sa(0.2)/PGA < 2.0, and (b) 1 PGA, in all other cases. (5) The values of the site coefficient for design spectral acceleration at period T, F(T), and of similar coefficients F(PGA) and F(PGV) shall conform to Tables 4.1.8.4.B. to 4.1.8.4.I. using linear interpolation for intermediate values of PGAref. (6) Site-specific evaluation is required to determine F(T), F(PGA) and F(PGV) for Site Class F. (7) For all applications in Subsection 4.1.8., Fa = F(0.2) and Fv = F(1.0). (8) For structures with a fundamental period of vibration equal to or less than 0.5 s that are built on liquefiable soils, Site Class and the corresponding values of F(T) may be determined as described in Tables 4.1.8.4.A., 4.1.8.4.B., and 4.1.8.4.C. by assuming that the soils are not liquefiable. (9) The design spectral acceleration values of S(T) shall be determined as follows, using linear interpolation for intermediate values of T: S(T) =F(0.2)Sa(0.2) or F(0.5)Sa(0.5), whichever is larger, for T ≤ 0.2 s =F(0.5)Sa(0.5) for T = 0.5 s =F(1.0)Sa(1.0) for T = 1.0 s =F(2.0)Sa(2.0) for T = 2.0 s =F(5.0)Sa(5.0) for T = 5.0 s =F(10.0)Sa(10.0) for T ≥ 10.0 s

(1) The earthquake importance factor, IE, shall be determined according to Table 4.1.8.5.

(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 4.1.8.6. (1) may be considered regular. (3) Except as required by Article 4.1.8.10., in cases where IEFaSa(0.2) is equal to or greater than 0.35, structures designated as irregular must satisfy the provisions referenced in Table 4.1.8.6.

(1) Analysis for design earthquake actions shall be carried out in accordance with the Dynamic Analysis Procedure described in Article 4.1.8.12., except that the Equivalent Static Force Procedure described in Article 4.1.8.11. may be used for structures that meet any of the following criteria: (a) in cases where IEFaSa(0.2) is less than 0.35, (b) regular structures that are less than 60 m in height and have a fundamental lateral period, Ta, less than 2 s in each of two orthogonal directions as defined in Article 4.1.8.8., or (c) structures with structural irregularity, of Type 1, 2, 3, 4, 5, 6 or 8 as defined in Table 4.1.8.6., that are less than 20 m in height and have a fundamental lateral period, Ta, less than 0.5 s in each of two orthogonal directions as defined in Article 4.1.8.8.

(1) Earthquake forces shall be assumed to act in any horizontal direction, except that the following shall be considered to provide adequate design force levels in the structure: (a) where components of the SFRS are oriented along a set of orthogonal axes, independent analyses about each of the principal axes of the structure shall be performed, (b) where the components of the SFRS are not oriented along a set of orthogonal axes and IEFaSa(0.2) is less than 0.35, independent analyses about any two orthogonal axes is permitted, or (c) where the components of the SFRS are not oriented along a set of orthogonal axes and IEFaSa(0.2) is equal to or greater than 0.35, analysis of the structure independently in any two orthogonal directions for 100% of the prescribed earthquake loads applied in one direction plus 30% of the prescribed earthquake loads in the perpendicular direction, with the combination requiring the greater element strength being used in the design.

(1) Except as provided in Sentence 4.1.8.20. (7), the values of Rd and Ro and the corresponding system restrictions shall conform to Table 4.1.8.9. and the requirements of this Subsection. (2) When a particular value of Rd is required by this Article, the corresponding Ro shall be used. (3) For combinations of different types of SFRS acting in the same direction in the same storey, RdRo shall be taken as the lowest value of RdRo corresponding to these systems. (4) For vertical variations of RdRo, excluding rooftop structures not exceeding two storeys in height whose weight is less than the greater of 10% of W and 30% of Wi of the level below, the value of RdRo used in the design of any storey shall be less than or equal to the lowest value of RdRo used in the given direction for the storeys above, and the requirements of Sentence 4.1.8.15. (5) must be satisfied. (5) If it can be demonstrated through testing, research and analysis that the seismic performance of a structural system is at least equivalent to one of the types of SFRS mentioned in Table 4.1.8.9., then such a structural system will qualify for values of Rd and Ro corresponding to the equivalent type in that Table.