NBC 2005 Seismic Load Generation

Seismic Design Not Required

A design note indicates seismic design not required if f S(0.2) as defined in sentence 4.1.8.4(6) less than or equal to 0.12

Maximum Building Height

Seismic design is not allowed for buildings over a maximum mean roof height h_n of 20 metres or 60 feet, in order that the design provisions that apply for T_a <= 0.5 are used. This height also roughly corresponds to the maximum number of stories allowed for wood-frame construction. A warning message appears when seismic design is attempted for taller buildings.

Site Classes

The Soil Category input has been changed to Site Class, with the classes and abbreviated descriptions from Table 4.1.8.4A.

Spectral Response Acceleration Input

Damped spectral response accelerations: Sa(0.2), Sa(0.5), Sa(1.0), Sa(2.0) edit boxes have been added to the Site Information input form.

Default Spectral Response Acceleration

The default values of the spectral response accelerations that appear for new files, and are specified in the Design Settings via a dropdown list giving these values for dozens of Canadian cities, taken from Appendix C of the NBC and Table J-2 of the structural commentaries.

F_a and F_v Values

Values of acceleration- and velocity-based site co-efficients F_a and F_v have been implemented in accordance with Tables 4.1.8.4B and 4.1.8.4C. Linear interpolation for is used for intermediate values of Sa(0.2) and Sa(1.0)

F_a and F_v for Site Class F

Acceleration- and velocity-based site co-efficients, Fa and Fv edit boxes have been added to the Building Site dialog for user input. These are activated for site class F to allow for user determination of these values according to 4.1.8.4 (5), which calls for site-specific geotechnical investigations.

Spectral Acceleration Values S(T)

To comply with 4.1.8.4(6), spectral acceleration values S(T) are determined as follows (where T is the period in the direction in question):

• F_aS_a(0.2) for T<=0.2s
• F_vS_a(0.5); F_aS_a(0.2) whichever is smaller for T=0.5s
• F_vS_a(1.0) for T<=1.0s
• F_vS_a(2.0) for T=2.0s

  Force Modification Factors

• To comply with 4.1.8.9, the existing input for R has been changed to input two factors for each direction: ductility- related force modification Factor R_d, and overstrength-related force modification factor R_o.

  Additional System Restrictions

To comply with 4.1.8.10, Additional System Restrictions, the program does not allow seismic design for Post disaster building shall have an SFRS with Rd or 2.0 or less. The user is notified in this case.

Fundamental Period T

The fundamental period T is now calculated according to 4.1.8.11 3 c), T = 0.05 (h_n)^3/4_,where h_n is the mean roof height of the structure.

Earthquake Force V

The earthquake force V is calculated according to 4.1.8.11:

• V = S(T_a) M_vI_EW / (R_dR_o)
• V_min = S(2.0) M_vI_EW/(R_dR_o)
• V_max(Rd >= 1.5) ⁼ 2/3 S(0.2) I_EW/(R_dR_o)

Higher Mode Factor M_v

A higher mode factor of M_v of 1.0 is used, corresponding to T_a < 1.0 from Table 4.1.8.11, which in turn corresponds to all buildings less than the maximum height restriction given above..

Importance Factor

The categories for importance factor have changed, but the seismic co-efficients for the corresponding factors (4.1.8.5 ) are the same as in NBC 1995. Refer to the section on Wind Load Generation for changes to Importance Categories and input of these categories.