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hearwalls applies the overturning factor Jx for each level from NBC 4.1.8.11.(6) to the calculation of hold-down forces and compression forces. In effect this reduces the hold-down force magnitude, as NDS 2015 Commentary J-165 says that J is intended as a corrective to the overestimation of higher mode effects Mv as regards overturning.
Base Overturning Reduction Factor J
The values of the base J used to calculate Jx are listed in Table 4.1.8.11 and depend on the period Ta and the ratio S(0.2)/S(5.0).
The values of J are taken from the section for Walls, Wall Frames SRFS in Table 4.1.8.11. Within the range of structures allowed in Shearwalls, these values are identical to those for Other system.
A separate J is determined for each orthogonal force direction, using period Ta and the ratio S(0.2)/S(5.0) to index the table.
As per to Notes 1 and 3 below the table, linear interpolation is done on Sa(0.2)/Sa(5.0) first, then on Ta.
Relevance to Wood Structures
Non-unity values for base J occur for T = 0.5 only when S(0.2)/S(5.0) > 20. For T = 1.0, they occur for all S(0.2)/S(5.0) ratios.
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 J for any seismic ratio. That height is 8.5 meters, well below the highest allowable structure in Shearwalls. Furthermore ratios of S(0.2)/S(5.0) > 20 .occur in numerous locations in Canada. So either range of J values is possible for structures designed in Shearwalls. In practice taller structures or those in high seismic zones will be affected.
Storey Overturning Reduction Factor Jx
For each level, and in each direction, a story factor Jx is determined using NBC 4.1.8.11.(8):
Jx = 1.0 when hx >= 0.6 hn
Jx = J + (1 – J) hx / 0.6 hn when hx < 0.6 hn
hx is the height at the top of level x, and hn is the mean roof height.
Hold-down Forces
All hold-down forces on a particular level and direction are multiplied by the factor Jx for that level and direction. This is because hold-down forces in Shearwalls are derived from the formula Mx / L, where Mx is defined as in 4.1.8.11.(8) and L is the wall segment length.
Output
Definitions, equations, and values of J and Jx appear in the Seismic Load Generation Details.
Jx is shown for each shear line in the Hold-down Design table. A definition appears in the legend beneath the table.
Jx is shown in the Factors section of the legend in Elevation view.