Response spectrum generator according to ASCE/SEI 7-10 (IBC)

Procedure to input a new IBC seismic spectrum

Expand branch Libraries of the main tree menu.
Expand sub-branch Loads.
Start function Seismic spectrum to open the Seismic Spectrum Manager.
Click button New.
In combo box Input type select option IBC.
Review and, if required, adjust the spectrum parameters.
Click button Code parameter to review or modify the code-defined parameters of the spectrum.
Confirm the code parameters with OK.
Click OK to confirm the spectrum.
Close the Seismic Spectrum Manager.

Spectrum definition and code parameters

A response spectrum can be defined according to ASCE/SEI 7-10 by selecting IBC as Input type .

Max frequency & Step

The values of the spectrum are generated between a maximum period of 4 seconds (=0.25 Hz) and the input maximum frequency, using the input Step value.

Code parameters

This button opens the dialogue defining the code-dependent parameters.

Type spectrum

DCE: Design Considered Earthquake (default)
MCE: Maximum Considered Earthquake (= 1.5 * DCE)

Input. Acceleration of gravity, defined in the project settings

Site class

Input. Site class according to Section 11.4.2 and Chapter 20

Input. Response modification coefficient according to Table 12.2-1 Design coefficients and Factors for seismic force-resisting systems

Seismic risk category

Input. Risk category according to Table 1.5-1 Risk category of buildings and other structures for flood, wind, snow, earthquake and ice loads

I_e

Calculated. Importance factor according to Table 1.5-2

Seismic design category

Calculated. Seismic design category according to Section 11.6

S_s

Input. Mapped MCE_R, 5 percent damped, spectral response acceleration parameter at short periods as defined in Section 11.4.1

S₁

Input. Mapped MCE_R, 5 percent damped, spectral response acceleration parameter at period 1s as defined in Section 11.4.1

T_L

Input. Long-period transition period as defined in Section 11.4.5

T₀

Calculated. 0.2 * S_D1 / S_DS

T_s

Calculated. S_D1 / S_DS

F_a

Calculated. Short-period site coefficient (at 0.2s period) according to Section 11.4.3

F_v

Calculated. Long-period site coefficient (at 1.0s period) according to Section 11.4.3

S_MS

Calculated. MCE_R, 5 percent damped, spectral response acceleration parameter at short periods adjusted for site class effects as defined in Section 11.4.3

S_M1

Calculated. MCE_R, 5 percent damped, spectral response acceleration parameter at a period of 1s adjusted for site class effects as defined in Section 11.4.3

S_DS

Calculated. Design, 5 percent damped, spectral response acceleration parameter at short periods as defined in Section 11.4.4

S_D1

Calculated. Design, 5 percent damped, spectral response acceleration parameter at a period of 1s as defined in Section 11.4.4

Theoretical background

F_a is obtained from Table 11.4-1. Use linear interpolation for intermediate values of S_s.

Site Class	S_s≤0.25	S_s=0.5	S_s=0.75	S_s=1.0	S_s≥1.25
A	0.8	0.8	0.8	0.8	0.8
B	1.0	1.0	1.0	1.0	1.0
C	1.2	1.2	1.1	1.0	1.0
D	1.6	1.4	1.2	1.1	1.0
E	2.5	1.7	1.2	0.9	0.9
F	see Section 11.4.7; the corresponding values of F_a must be defined by the user

F_v is obtained from Table 11.4-2. Use linear interpolation for intermediate values of S₁.

Site Class	S₁≤0.1	S₁=0.2	S₁=0.3	S₁=0.4	S₁≥0.5
A	0.8	0.8	0.8	0.8	0.8
B	1.0	1.0	1.0	1.0	1.0
C	1.7	1.6	1.5	1.4	1.3
D	2.4	2.0	1.8	1.6	1.5
E	3.5	3.2	2.8	2.4	2.4
F	see Section 11.4.7; the corresponding values of F_v must be defined by the user

Final acceleration value:

Notes about ELF calculation

In Equivalent Lateral Forces calculation, some additional criteria are applied. Those criteria are not implemented in the definition of the IBC spectrum generator. They are applied only in the calculation of horizontal forces in the ELF procedure.

Minimum value of C_s

The parameter C_s shall not be taken less than the following:

The Cs,min criterion is applied separately to each component (X, Y) of ELF load cases

Maximum value of T

The fundamental period T shall not be taken greater than

where T_a is the approximate fundamental period as defined in Section 12.8.2.1

C_u is taken from Table 12.8-1 (Coefficient for upper limit on calculated period):

S_D1	C_u
≥ 0.4	1.4
0.3	1.4
0.2	1.5
0.15	1.6
≤0.1	1.7

Approximate fundamental period according to ASCE 7-10 12.8.2.1

The fundamental period of buildings may be approximated are follows:

where

T_a = approximate fundamental period

h_n = height of the top mass of the building from the reference level, in [m]

C_t and x are taken from Table 12.8-2:

Structure type	C_t	x
Moment-resisting frame systems in which the frames resist 100% of the required seismic force and are not enclosed or adjoined by components that are more rigid and will prevent the frames from deflecting where subjected to seismic forces:
- Steel moment-resisting frames	0.0724	0.8
- Concrete moment-resisting frames	0.0466	0.9
Steel eccentrically braced frames in accordance with Table 12.2-1 lines B1 or D1	0.0731	0.75
Steel buckling-restrained braced frames	0.0731	0.75
All other structural systems	0.0488	0.75

Vertical distribution of accelerations according to ASCE 7-10 12.8.3

The total horizontal seismic force (base shear) is distributed to the storeys according to a distribution assumption. Additionally to the standard options (linear, modal shape), the ASCE 7-10 defines a polynomial distribution as follows: