Seismic Analysis Parameters (US Code ASCE 7-16 or IBC 2012)

The parameters of the seismic simplified analysis according to the equivalent lateral force method depend on a seismic code and a method of defining values of the fundamental periods selected in the Seismic Analysis dialog.

Access

1. Click Tools Job Preferences Design Codes.
2. Go to the Loads section, and then select ASCE 7-16 or IBC 2012 from the seismic loads menu.
3. Click AnalysisAnalysis Types.

   The Analysis Types dialog displays.

4. Click New.

   The New Case Definition dialog opens.

5. Select Seismic (Equivalent Lateral Force Method), and click OK.

   The Seismic Analysis dialog opens.

6. Click Seismic Analysis Parameters.

   The Parameters dialog opens.

Dialog elements

Fundamental period (Ta)

The calculation of the fundamental period (Ta) depends on the method selected in the Seismic analysis dialog.

• Approximate method

  The fundamental period Ta is calculated according to the equation 4.6:

  T_a = C_t * h_n ^x

  where:

  C_t - building period coefficient depending on the type of seismic-force-resisting system that is used (given by the code)

  x = building period formula exponent (given by the code)

  h - structure height (m), defined by difference between Structure range and Reference level

• User defined

  Only Ta on both directions is to be defined.

• Precise method

  According to this method you can calculate the exact value for both X and Y directions. If the option Periods with maximal mass participation is off in the Seismic Analysis dialog, the first mode in direction where more masses is active. If this option is on, one of modes, which fulfill limitations and most masses is active in the direction. The maximal period is calculated according to the formula:

  T_max = Cu * Ta,

  where:

  Cu - Upper Limit Coefficient (Table 12.8-1),

  T_a - Approximative Period

Code parameters

To complete the seismic analysis according to the rules given in this code, define the following parameters:

• Site class

  (A, B, C, D, E or F) based on the site soil properties, classified according to Chapter 20 of ASCE 7.

• Spectral response accelerations

  as specified in ASCE 7-10 Section 1613.3.1, shown in Figures 1613.3.1(1) through 1613.3.1(6) or ASCE 7-16 Section 11.4.4:

  • S₁ - Acceleration parameter for 1-second period determined according to Chapter 22 of ASCE 7
  • S_s - Acceleration parameter for short periods determined according to Chapter 22 of ASCE 7.
• T_L - long-period transition period, specified according to Chapter 22 of ASCE 7.
• R - response modification coefficient, as specified in ASCE 7, Tables 12.2-1
• Ie - importance factor, as specified in ASCE 7, Section 11.5.1.

Base shear

Base shear force is calculated according to the formula of the seismic code ASCE 7-05 Section 12.8:

The lateral seismic force induced at any level is determined from:

F_x = V * w_x * h^k _x / (Σ w_i * h^k_i) (eq. 12.8-11 and 12.8-12)

where:

w_x and w_i - floor weight

h - height from base level to appropriate floor level

V - base shear force (this value is already calculated in Robot)

k - exponent depending on the period value:

Period	k
< 0,5 sec	1
0,5 - 2,5 sec	Linear interpolation
> 2,5 sec	2

Disregard density

Excludes the density of the structure element (ρ=0) during the estimation of effective seismic weight applied to the stories of the structural model during analysis.