TR.31.2.3 Canadian Seismic Code (NRC) – 2005 Volume 1
This set of commands may be used to define the parameters for generation of equivalent static lateral loads for seismic analysis per National Building Code (NRC/CNRC) of Canada 2005 Volume 1 edition. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s).
The seismic load generator can be used to generate lateral loads in the X & Z directions for Y up or X & Y for Z up. Y up or Z up is the vertical axis and the direction of gravity loads (See the SET Z UP command in TR.5 Set Command Specification). All vertical coordinates of the floors above the base must be positive and the vertical axis must be perpendicular to the floors.
General Format
STAAD.Pro utilizes the following format to generate the lateral seismic loads.
DEFINE NRC 2005 (ACCIDENTAL) LOAD
nrcspec
weightdata
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
Where:
nrcspec = { SA1 f1 SA2 f2 SA3 f3 SA4 f4 MVX f5 MVZ f6 JX f7 JZ f8 IE f9 RDX f10 ROX f11 RDZ f12 ROZ f13 SCLASS f14 ( FA f15 ) ( FV f16 ) ( CT f17 ) ( PX f18 ) ( PZ f19) }
where:
If the ACCIDENTAL option is specified, the program calculates the accidental torsion per the NRC 2005 specifications. The value of the accidental torsion is based on the center of mass for each level. The center of mass is calculated from the SELFWEIGHT, JOINT WEIGHT, and MEMBER WEIGHT commands.
The ACC option along with accidental eccentricity factor (default 0.1 as per NRC 2005) needs to be provided in the NRC seismic primary load case (i.e., NRC LOAD X / Z f1 ACC f3). f3 can be negative.
To consider horizontal torsion in cases where a floor diaphragm is present in the model, the ACCIDENTAL option should not be specified. Instead, dynamic eccentricity along with accidental eccentricity should be provided in the NRC seismic primary load case (i.e., NRC LOAD X / Z f1 DEC f2 ACC f3). For equivalent seismic analysis, f2 is 1 and f3 is 0.1 as per NRC 2005 code. f1 is always positive or zero, however f2 can be negative. If f2 is 0.0, only accidental torsion will be considered for this particular load case.
Methodology
The minimum lateral seismic force or base shear (V) is automatically calculated by STAAD.Pro using the appropriate equation (s).
Except that V shall not be less than:
and for an R_{d}= 1.5, V need not be greater than:

T_{a} is the fundamental lateral period in the direction under consideration and is determined as:

For momentresisting frames that resist 100% of the required lateral forces and where the frame is not enclosed by or adjoined by more rigid elements that would tend to prevent the frame from resisting lateral forces, and is calculated by the empirical formulae as described below provided h_{n} is in meter

The period is also calculated in accordance with the Rayleigh method but could be overridden by user specified time period (PX, PZ).
If design spectral acceleration, S(T_{a}),calculated considering structural time period calculated based on method (b) is greater than 0.8 time the same calculated considering structural time period calculated based on method (a), the former is used for further calculation. Otherwise, the later time period is used.

Other established methods of mechanics using a structural model that complies with the requirements of Sentence 4.1.8.3.(8), except that
 for moment resisting frames, Ta shall not be greater than 1.5 times that determined in Clause (a).
 for braced frames, Ta shall not be greater than 2.0 times that determined in Clause (b).
 for shear wall structures, Ta shall not be greater than 2.0 times that determined in Clause (c), and
 for the purpose of calculating the deflections, the period without the upper limit specified is referred from Appendix A.


S(T_{a}) is the design spectral acceleration and is determined as follows, using linear interpolation for intermediate values of T_{a:}
S(T_{a}) = F_{a}S_{a}(0.2) for T_{a} ≤ 0.2s
 = F_{v}S_{a}(0.5) or F_{a}S_{a}(0.2), whichever is smaller for T_{a} = 0.5s
 = F_{v}S_{a}(1.0) for T_{a} = 1.0s
 = F_{v}S_{a}(2.0) for T_{a} = 2.0s
 = F_{v}S_{a}(2.0)/2 for T_{a} ≥ 4.0s
The above terms S_{a}(0.2), S_{a}(0.5), S_{a}(1.0) and S_{a}(2.0) are the Seismic Data and are obtained as user input from the Table C2.
Based on the above values of S_{a}(T_{a}), F_{a }and_{}F_{v,} the acceleration and velocity based site coefficients are determined from the Tables 4.1.8.4.B and 4.1.8.4.C, using linear interpolation for intermediate values of S_{a}(0.2) and S_{a}(1.0). It is to be mentioned that, these are the user inputs based on the site classes from A to E and the desired S_{a}(0.2) and S_{a}(1.0) values as required as per the above equations.

M_{v} is the factor to account for higher mode effect on base shear and the associated base overturning moment reduction factor is J which are obtained as user input from the Table 4.1.8.11. To get this higher mode factor (M_{v}) and numerical reduction coefficient for base overturning moment (J), you must get the ratios of S_{a}(0.2)/S_{a}(2.0) as also the "Type of Lateral Resisting System."
For values of Mv between fundamental lateral periods, Ta of 1.0 and 2.0 s, the product S(Ta). Mv shall be obtained by linear interpolation.
Values of J between fundamental lateral periods, Ta of 0.5 and 2.0 s shall be obtained by linear interpolation.
 I_{E} is the earthquake importance factor of the structure and is determined from the Table 4.1.8.5. This is a user input depending on Importance Category and ULS / SLS

W is the weight of the building and shall be calculated internally using the following formula:
$W=\stackrel{n}{\sum _{i=1}}{W}_{i}$Where W_{i} is the portion of W that is located at or assigned to level i.

R_{d} is the ductilityrelated force modification factor reflecting the capability of a structure to dissipate energy through inelastic behavior as described in article 4.1.8.9.

R_{o} is the overstrengthrelated force modification factor accounting for the dependable portion of reserve strength in a structure designed according to the provision of the Article 4.1.8.9.
These R^{d} and R_{o} values are the user inputs depending on the type of SFRS
As per 4.1.8.11(6), the total lateral seismic force, V, shall be distributed such that a portion, F_{t} shall be concentrated at the top of the building, where,
 F_{t} = 0.07T_{a}V
but F_{t} is not greater than 0.25V and F_{t} = 0 when T_{a} is not greater than 0.7s.
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