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Computed Period
The program computes the period Ta for both orthogonal force directions according to NBC 4.1.8.11.(3).(c)
Ta = 0.05 (hn)3/4
This value appears as the default value in the Site Dialog input for Ta..
User-input Period
You can over-ride the computed period with one you have calculated using "other established methods of mechanics using a structural model", as allowed by 4.1.8.11.(3).(d), most commonly Rayleigh analysis. To do so you uncheck the Calculate T box and enter your value. The program ensures that you respect the limit given in 4.1.8.11.(3).(d).(iii) of twice the calculated Ta
It is possible to specify a different T in each force direction.
Ta for Single-Storey Structures
Shearwalls implements NBC 4.1.8.11.(4)(a), allows for an increase in the period Ta calculated by the empirical formula in 4.1.8.11.(3)(c) by an amount equal to .004L to where L is the shortest distance between shear resisting elements.
L is calculated as the shortest distance between any two shear lines whose extents overlap by one inch or more. The extent of the shear line is defined as the distance between the ends of the extreme shear walls on the line, disregarding gaps in between.
4.1.8.11(c) says that Ta determined using other methods can’t be greater than 1.5 times that from using 4.1.8.11.(4)(a). This compares to a factor of 2.0 using 4.1.8.11.(3)(c). Since this provision is "permitted" and not mandatory, the program checks whether a Ta that you input is greater than the greater of the two restrictions, that is 2.0 Ta using 4.1.8.11.(3) or 1.5 Ta using 4.1.8.11.(4). In practice only in very long, sparse structures will the single-story Ta govern.
Period for Deflection Calculations
The program currently does not allow a different, unrestricted period to be used for deflection calculations as per 4.1.8.11.(3).(d).(v).
5- and 6- Storey Structures
Refer to Base Shear Increase for 5- and 6-Storey Buildings for the 20% increase in base shear for 5- and 6- storey structures that depends in use of 4.1.8.11.(3).(d) to calculate Ta