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Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current program behaviour.
The changes described in this topic occur when NBC 2015 is selected as the design code edition in the Design Settings, unless otherwise indicated.
NBC 2010 4.1.8.1.(1) said that the seismic analysis using the Equivalent Static Procedure was not needed for sites with S(0.2) less than 0.12. This provision has been removed, and replaced with a method in 4.1.3.1.(3)-(15) to be used when a set of conditions for low seismic loading in 4.1.8.1.(2) is met. This method tends to create larger seismic loads than the Equivalent Static Procedure; its advantage is simplicity. It is not implemented in Shearwalls.
Upon design, a message box used to appear when S(0.2) was less than 0.12, asking you if you wished to proceed with seismic load generation. This has been replaced by a message that informs you of the simplified procedure, its limitations, and that it produces higher loads. the program proceeds to design with the Equivalent Static Procedure.
The following pertains to the Table C-2 of Appendix C of NBC 2010 and Tables C-2 and C-3 of NBC 2015 that give climatic and seismic data for all significant Canadian towns and cities, which are selected via the default settings in Shearwalls.
The program now includes the values peak ground acceleration (PGA) in the software data as the NBC 2015 Equivalent Static Force Procedure now requires them.
The program now includes the spectral acceleration Sa(5.0) as it is now needed to calculate the ratio S(5.0) / S (0.2) to determine higher order factor Mv and overturning factor J. ). It otherwise corresponds to buildings much higher than those designed in Shearwalls.
The values of Sa(0.2), Sa(0.5), Sa(1.0), Sa(2.0) have been changed to correspond to those in NBC 2015. Virtually every value has changed, many significantly.
This section pertains to the calculation of design spectral acceleration S(T) in 4.1.8.4 Site Properties, which is then used in the Equivalent Static Force Procedure in 4.1.8.11.(2). It is calculated by:
where
PGA, Sa(5.0) and five values of F(T) have been added to the Site Dialog input. The fields Fa and Fv were removed. Like Fa and Fv, F(T)’s are only active for Site class F.
The calculation of PGAref = PGA * 0.8 if Sa(0.2) / PGA < 2.0 from 4.1.8.4.(4) has been added.
As per NBC 4.1.8.4.(7), the program now sets the value of Fa to F(0.2). Although no longer needed in the calculation of S(T), it appears in the expression IEFaSa(0.2) that is still used in many places to classify the structure for the application of various rules, particularly for irregularities.
Fv is no longer needed by the program.
The calculation of S(T) from NBC 2015 4.1.8.4.(9) is S(T) = Sa(T) F(T), whereas for 2010 4.1.8.4.(7) it was S(T) = Sa(T) Fv
NBC 2015 Tables 4.1.8.4.-B to -E for F(T) have been entered into the program. For tables of F(T) for periods on either side of the actual period Ta, the program determines F(T) based on site class and PGAref, interpolating on intermediate values of PGAref.
Linear interpolation is then used to determine the value of F(Ta), and it is also used to determine Sa(Ta). S(Ta) is then determined via F(Ta) Sa(Ta).
For T < 0.2 s, the NBC 2015 now takes the maximum of F(0.2)Sa(0.2) and F(0.5)Sa(0.5), whereas it used to just use F(0.5)Sa(0.5). This has been implemented.
In the Load Generation Details output, when NBC 2015 is selected as the design option,
In the Site Information section which shows the user-input data,
A new provision 4.1.8.10.(4) has been added to NBC 2015 for 5- and 6-storey wood-frame buildings. If the IEFaSa(0.2) product is equal to or greater than 0.35, then Type 4 In Plane Discontinuity and Type 5 Out-of-plane offsets irregularities are not allowed for these structures.
This was already detected and reported by the software following an identical provision in the BC Building Code, so the note under the Seismic Irregularities table has been changed to refer to NBC rather than BCBC.
A new seismic irregularity, Gravity-Induced Lateral Demand, Type 9, has been added to Table 4.1.8.6, with the associated provisions 4.1.8.10.(5)-(7). Note A-4.1.8.10.(5) explains that this corresponds to such elements as inclined columns and floor cantilevers, and involve yielding mechanisms like plastic hinges. This outside the scope of the Shearwalls program and no attempt is made to detect this irregularity.
This irregularity has been included in the Seismic Irregularities of the Design Results output, giving the design code references, an note explaining why it is not applicable, and a description below the table.
The maximum lateral force V calculated in NBC 4.1.8.11.(2)(c) has is now derived from the maximum of two equations as opposed to just one.
For NBC 2010, the equation is
2/3 S(0.2) IE W / (RdRo)
For NBC 2015 it is the larger of this equation and
S(0.5) IE W / (RdRo)
In the Load Generation Details report (formerly part of the Log file), this equation has been added to the Equations section.
For both the NDS 2010 and NDS 2015 options, the program no longer shows the notes indicating that the maximum shear has been used for design, and instead includes three base shear columns V in the Total Design Base Shear table – Vcalc, Vmax, and Vdes, giving the calculated base shear, the maximum base shear, and which of these has been chosen as the design base shear.
A new provision for single-storey wood structures, 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.
4.1.8.11(c) says that Ta computed using other methods but 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).
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 shearwalls on the line, disregarding gaps in between. No attempt is made to determine whether shear resisting element extents overlap on a wall-by wall basis
For one-storey structures, the equation 0.05 hn3/4 + L is used for the approximate period Ta.
Since this provision is "permitted" and not mandatory, the program checks whether the value input in the Site Dialog 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 the Load Generation Details report, for one storey structures,
The program now 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).
In previous editions of the NBC, all Mv = 1 for all values of period T equal to 1.0 or less, so that no building in Shearwalls could have a non-unity Mv. Table 4.1.8.11 of NDS 2015 has non-unity Mv values for T= 1.0 and values of S(0.2)/S(5.0) > 20. Because a new column bas been added with values of 1.0 for T = 0.5, 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. Since that height is 8.5 meters, well below the highest allowable structure in Shearwalls the calculation of Mv in the program is now required. In practice, values of Mv greater than 1.0- will come into play primarily for 5- and 6-storey structures that are newly permitted by NBC.
Note too that ratios of S(0.2)/S(5.0) > 20 for which Mv > 1 occur in numerous locations in Canada.
The values of Mv are taken from the section for 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 and the ratio S(0.2)/S(5.0) to index Table 4.1.8.11.
According to Notes 1 and 2 of Table 4.1.8.11, interpolation should be done on Sa(0.2)/Sa(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 Sa(0.2)/Sa(5.0), for a Ta between 0.5 and 1.0, we
Mv had always been in the Total Design Base Shear table, showing 1.0. The value is now shown to 3 digits precision.
An expression showing the dependencies of Mv has been added to the Equations section, for both NBC 2010 and NBC 2015
Although values of overturning reduction factor Jx less than 1.0 were possible for previous editions of NBC, through interpolating between the value of 1.0 for T=0.5 and the non-unity values for T = 1.0, such structures would largely be 5- and 6-storey buildings not previously permitted by NBC. Furthermore, as per NDS 2015 Commentary J-165, J is intended as a corrective to the overestimation of higher mode effects Mv as regards overturning, and Mv was not in previous versions of Shearwalls.
With the introduction of Mv in this version of the software, the overturning effect factors J and Jx have also been implemented.
As with Mv, non-unity J values occur only for Ta between 0.5 and 1.0, but unlike Mv, the occur for any value of the ratio S(0.2)/S(5.0).
The values of J are taken from the section for 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 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, interpolation is done on Sa(0.2)/Sa(5.0) first, then on Ta.
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.
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.
In the Load Generation Details report:
The value of Jx is shown for each shearline 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.
A new provision in NBC, 4.1.8.11.(12), calls for a 20% increase in base shear for 5 and 6 storey structures for periods determined using 4.1.8.11.(3)(d), i.e. "other established methods of mechanics using a structural model", most commonly Rayleigh analysis.
As an increase in T results in reduced base shear, we assume that whenever you over-ride the period determined using the empirical equation 4.1.8.11.(3)(c) by entering a larger one in the Site Dialog input, 4.1.8.11.(3)(d) is being used and the 20% surcharge is applied.
A decrease in the period is assumed to mean that you are just adjusting the height used in 4.1.8.11.(3)(c), as no advantage is accrued.
For each orthogonal direction independently, if the period Ta is greater than that calculated by the empirical equation 4.1.8.11.(3)(c), a factor 1.2 is applied to the calculation of base shear V in
When applied, the equation for V shown in the Equations section of the Load Generation Details Output includes the 1.2 factor. In the unusual case that it is applied in only one orthogonal direction, two equations are shown for V, one for each direction.
A new provision in NBC, 4.1.8.15.(4) for single-storey structures with a wood roof diaphragm and with Rd greater than 1.5, requires magnification of deflections and/or design forces if the diaphragm deflection exceeds ½ the storey drift of the adjoining shearwalls. This affects only large structures with a large sufficient distance between shearlines to create large diaphragm deflection.
As Shearwalls does not currently calculate diaphragm deflections, a warning is displayed for structures of a size that is susceptible to this condition.
The check for this condition is made before loads are generated. For single-storey structures only, if the Rd value is greater than 1.5, the program will determine, in both directions, the shortest distance between shear-resisting elements, using the same algorithm as given in ) above. For imperial units, if the distance is greater than 100 feet, and for metric, 30 metres, the building is considered susceptible.
If the condition is detected, a Yes, No, Cancel message box asks you whether you want to change Rd to 1.5. The actions taken are