In lieu of carrying out a special study as stated in Subclause 4.1.8.15.(4)(a)(iii), the anticipated total deformation demand on the vertical elements of the SFRS, including inelastic deformations, may be taken as equal to Ro Rd (ΔB + ΔD) - Ro ΔD, i.e., the difference between the total storey drift including inelastic deformation effects and diaphragm deformations, Ro Rd (ΔB + ΔD), and the diaphragm deformation under Ro times the seismic load, where Ro may be replaced by the actual overstrength of the SFRS vertical elements. The design engineer must verify that the SFRS vertical elements have sufficient deformation capacity to accommodate the computed deformation demand. If the vertical elements of the SFRS do not have sufficient deformation capacity, the design forces for the vertical elements of the SFRS must be magnified by Rd (1 + ΔD/ΔB)/(Rd + ΔD/ΔB). The calculation of the magnified design forces is iterative as the ΔD/ΔB ratio may change when using higher design forces for the vertical elements of the SFRS. Reducing the ΔD/ΔB ratio by increasing the stiffness of the roof diaphragm relative to that of the vertical elements of the SFRS may be considered to reduce the deformation demand on the vertical elements of the SFRS. Additional information can be found in the Commentary entitled “Design for Seismic Effects” in the “Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B)”. [[a-1.4.1.2.#|).]] The dynamic response of the diaphragm with the vertical elements of the SFRS under seismic excitation involves several modes of vibration that affect both the amplitude and distribution of in-plane shears and bending moments in the roof diaphragm. The shape of the fundamental mode of vibration resembles the deflected shape of the diaphragm/vertical SFRS elements under a distributed lateral load while higher modes involve increasing numbers of zero crossings of the deflected shapes along the length of the diaphragm, similar to the modes of a simply supported beam with distributed mass. Shears and bending moments therefore deviate from the values obtained from the equivalent static force procedure essentially due to higher mode response. Modal contributions to shears and bending moments in the diaphragms can be obtained from a Linear Dynamic Analysis. The contribution from the higher modes is generally more pronounced when the ΔD/ΔB ratio, the period in the first mode, or the ratio Sa (0.2)/Sa (2.0) is increased. It also increases when the SFRS is designed with a higher Rd factor as inelastic deformations of the vertical elements of the SFRS attenuate the first mode response. Methods to take into account the inelastic higher mode effects on in-plane diaphragm shears and moments are discussed in the Commentary entitled “Design for Seismic Effects” in the “Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B)”. [[a-1.4.1.2.#|).]]