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TR.31.2.2 Canadian Seismic Code (NRC) - 1995

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- 1995 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

There are two stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE NRC LOAD command.

DEFINE NRC LOAD
nrc-spec
weight-data

Refer to Common Weight Data for information on how to specify structure weight for seismic loads.

Where:

nrc-spec = *{ V f1 ZA f2 ZV f3 RX f4 RZ f5 I f6 F f7 (CT f8) (PX f9) (PZ f10) }
ParameterDescription
V f1 Zonal velocity ratio per Appendix C
ZA f2 Factor for acceleration related seismic zone per Appendix C
ZV f3 Factor for velocity related seismic zone per Appendix C
RX f4 Force modification factor along X-direction that reflects the capability of a structure to dissipate energy through inelastic behavior. Please refer Table 4.1.9.1B
RZ f5 Force modification factor along Z-direction that reflects the capability of a structure to dissipate energy through inelastic behavior. Please refer Table 4.1.9.1B
I f6 Seismic importance factor per sentence 10 section 4.1.9.1
F f7 Foundation factor conforming to Table 4.1.9.1C and sentence 11 section 4.1.9.1
CT f8 Factor to be used to calculate the fundamental period of structure .This is an optional parameter.
PX f9 Period of structure (in seconds) in the X- direction. This is an optional parameter.
PZ f10 Period of structure (in seconds) in the Z- direction. This is an optional parameter.

Used for Y if the SET Z UP command is used.

w joint weight associated with list
v1, v2, v3 Used when specifying a uniformly distributed load with a value of v1 starting at a distance of v2 from the start of the member and ending at a distance of v 3 from the start of the member. If v2and v 3 are omitted, the load is assumed to cover the entire length of the member.
v4, v5 Used when specifying a concentrated force with a value of v4 applied at a distance of v5 from the start of the member. If v5 is omitted, the load is assumed to act at the center of the member.
p1 weight per unit area for the plates selected. Assumed to be uniform over the entire plate.

Element Weight is used if plate elements are part of the model, and uniform pressures on the plates are to be considered in weight calculation.

Floor Weight is used if the pressure is on a region bounded by beams, but the entity which constitutes the region, such as a slab, is not defined as part of the structural model. It is used in the same sort of situation in which one uses FLOOR LOADS (see TR.32.4 Area, One-way, and Floor Load Specifications for details of the Floor Load input).

The weights have to be input in the order shown.

Generation of NRC Load

The load so defined as above is applied on the structure in the NRC loadcases. These loadcases have to be the first loadcases in the input file. Built-in algorithms will automatically distribute the base shear among appropriate levels and the roof per the relevant code specifications.

The following general format should be used to generate loads in a particular direction.

LOAD i
NRC LOAD { X | Y | Z } (f1)

Where:

ParameterDescription
LOAD i load case number
NRC LOAD { X | Y | Z } f1 factor to be used to multiply the NRC Load (default = 1.0). May be negative.

Notes

  1. By providing either PX or PZ or both, you may override the period calculated by STAAD using Rayleigh method. If you do not define PX or PZ, the period for Method 2(b) above will be calculated by the program using Rayleigh method and the stipulations of sentence 7(c) of section 4.1.9.1
  2. Some of the items in the output for the NRC analysis are explained below.

    • Ta  = Time period calculated per sentence 7(a) or 7(b) of section 4.1.9.1
    • Tc  = Time period calculated per sentence 7(c) of section 4.1.9.1

    CALC / USED PERIOD

    The CALC PERIOD is the period calculated using the Rayleigh method. For NRC in the x-direction, the USED PERIOD is PX. For the NRC in the z-direction (or Y direction if SET Z UP is used), the USED PERIOD is PZ. If PX and PZ are not provided, then the used period is the same as the calculated period for that direction. The used period is the one utilized to find out the value of S. 

  3. In the analysis for NRC loads, all the supports of the structure have to be at the same level and have to be at the lowest elevation level of the structure.
Note: For additional details on the application of a seismic load definition used to generate loads, refer to GUID-6D9B5C48-9FFF-4548-BFB4-ACAE1E973743.

Methodology

The minimum lateral seismic force or base shear (V) is automatically calculated by STAAD using the appropriate equation(s); namely sentence 4, section 4.1.9.1 of NRC.

V = 0.6⋅Ve/R

Where:

Ve ,the equivalent lateral seismic force representing elastic response (per sentence 5,section 4.1.9.1) is given by:

Ve = v⋅S⋅I⋅W

Where:

  • v =  Zonal velocity ratio per appendix C
  • S = Seismic Response Factor per table 4.9.1.A
  • I = Seismic importance factor per sentence 10 section 4.1.9.1
  • F = Foundation factor conforming to Table 4.9.1.C and sentence 11 section 4.1.9.1
  • W = Total load lumped as weight per sentence 2 section 4.1.9.1
  • R = Force modification factor conforming to Table 4.9.1.B that reflects the capability of a structure to dissipate energy through inelastic behavior.

STAAD utilizes the following procedure to generate the lateral seismic loads.

  1. User provides seismic zone co-efficient and desired "nrc-spec" (1995) through the DEFINE NRC LOAD command.
  2. The program calculates the fundamental period(T) of the structure by

    1. finding out whether the structure being analysed is a moment resisting frame made primarily of steel or of concrete or it is a structure of any other type. Alternatively, the software uses the optional parameter CT if provided. The calculation is done per sentence 7(a) & 7(b) of section 4.1.9.1.
    2. using the Rayleigh method or using the optional parameters PX , PZ –if provided. The stipulations of sentence 7(c) of section 4.1.9.1 are also considered while calculating.
    3. taking the conservative value of T between those calculated by methods (a) and (b) above.
  3. The program finds out the value of Seismic Response Factor(S) per table 4.9.1.A  utilizing the values of T as calculated above and the values of ZA & ZV input by the user.
  4. The program calculates V per sentence 4 section 4.1.9.1. W is obtained from the weight data (SELFWEIGHT, JOINT WEIGHT(s), etc.) provided by the user through the DEFINE NRC LOAD command. The weight data must be in the order shown.
  5. The total lateral seismic load (base shear) is then distributed by the program among different levels of the structure per applicable NRC guidelines like sentence 13(a) section 4.1.9.1.

Example

DEFINE NRC LOAD
V 0.2 ZA 4 ZV 4 RX 4 RZ 4 I 1.3 F 1.3 CT 0.35 PX 2 PZ 2
SELFWEIGHT
JOINT WEIGHT
17 TO 48 WEIGHT 7
49 TO 64 WEIGHT 3.5
LOAD 1 EARTHQUAKE ALONG X
NRC LOAD X 1.0
PERFORM ANALYSIS PRINT LOAD DATA
CHANGE

See also Example Problem Examp14_NRC.std