|
|
|
|
|
The program includes a calculation of higher mode effects Mv from NBC 4.1.8.11.(6), which is used in the calculation of base shear in 4.1.8.11.(2). The values of Mv are listed in Table 4.1.8.11.(6) and depend on the period Ta and the ratio S(0.2)/S(5.0).
Relevance to Wood Structures
Mv = 1 for all values of of period T equal to 0.5 or less. For T= 1.0 and values of S(0.2)/S(5.0) > 20 have non-unity Mv Due to linear interpolation, non-unity values exist for these ratios whenever the period is greater than 0.5.
Since a period of twice the one calculated by empirical formula 4.1.8.11.(3)(c) is allowed by 4.1.8.11.(3)(d)(iii), any height above that corresponding to T = 0.25 using the formula could have a significant Mv. That height is 8.5 meters, well below the highest allowable structure in Shearwalls, and ratios of S(0.2)/S(5.0) > 20 for which Mv > 1 occur in numerous locations in Canada, so the calculation of Mv is required for some structures. In practice, values of Mv greater than 1.0- will come into play primarily for 5- and 6-storey structures.
Determination of Mv
The values of Mv are taken from the section for the Walls, Wall Frames SRFS from Table 4.1.8.11. Within the range of structures allowed in Shearwalls, these values are identical to those for Other systems.
A separate Mv is determined for each orthogonal force direction, using period Ta
Interpolation
According to Notes 1 and 2 of Table 4.1.8.11, interpolation should be done on S(0.2)/S(5.0) first, then on Ta, and Note 2 says to interpolate on S(Ta) Mv, not just Mv(Ta). So, if we define Sr as the ratio of S(0.2)/S(5.0), for a Ta between 0.5 and 1.0, we
Output
Definitions, equations, and values of Mv appear in the Seismic Load Generation Details.