Response spectrum generator according to SIA 261:2020 (Swiss standard)

Procedure to input a new SIA 261 seismic spectrum

  1. Expand branch Libraries of the main tree menu.
  2. Expand sub-branch Loads.
  3. Start function Seismic spectrum to open the Seismic Spectrum Manager.
  4. Click button New.
  5. In combo box Input type select option Swiss standard.
  6. Review and, if required, adjust the spectrum parameters.
  7. Click button Code parameter to review or modify the code-defined parameters of the spectrum.
  8. Confirm the code parameters with OK.
  9. Click OK to confirm the spectrum.
  10. Close the Seismic Spectrum Manager.

Spectrum definition and code parameters

A response spectrum can be defined according to SIA 261:2003 by selecting Swiss standard 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.

Spectrum type

Seismic zone

Seismic hazard zone: Z1a, Z1b, Z2, Z3a, Z3b, Other

Other allows to define manually the ground acceleration agd

Coeff accel. ag

Ground acceleration divided by g (acceleration of gravity, defined in project settings)

agd

Ground acceleration, depending on the seismic hazard zone

Soil class

Foundation soil class, as defined in SIA 261 Table 25: A, B, C, D, E, Other

Other allows to define manually the soil parameters S, TB, TC, TD

S, TB, TC, TD

Soil parameters, as defined in SIA 261 Table 25

Building class

Structural class of the building: CO I, CO II, CO III, Other

Other allows to define manually the value Gamma f

Gamma f

Importance factor, depending on the selected building class, according to SIA 261 Table 26

Direction

Direction of the seismic action: Horizontal, Vertical, Other

Sets the value of the global factor. Other allows to define manually the value of the global factor.

For Vertical, the value of the behaviour factor q is checked and reduced if greater than 1.5

Global factor

Overall coefficient that multiplies the entire response spectrum. Depends on the direction of the seismic action: 1.0 for horizontal, 0.7 for vertical, user defined for other.

Behaviour factor q

Seismic behaviour factor

Damping xi

Relative damping ratio, for elastic response spectrum only.Default value = 0.05.

Eta

Damping correction factor, calculated from the relative damping ratio xi.

Theoretical background

Elastic response spectrum

Elastic spectra SIA 261:2003Elastic spectra SIA 261:2020

SIA 261 Table 25: Soil Classes

Soil Class Description S TB [s] TC [s] TD [s]
A Rock or other rock-like geological formation,
including at most 5 m of weaker material at the surface		 1.00 0.07 0.25 2.0
B Deposits of very dense sand, gravel, or very stiff clay,
and/or preconsolidated soil, at least 30m in thickness		 1.20 0.08 0.35 2.0
C Deposits of consolidated sand and gravel and/or moraine
material, at least 30m in thickness		 1.45 0.10 0.4 2.0
D Deposits of fine, unconsolidated sand, silt and clay,
at least 30m in thickness		 1.70 0.15 0.5 2.0
E A soil profile consisting of a surface alluvium layer of type
C or D and thickness varying between about 5 m and 30 m,
underlain by stiffer material of type A or B		 1.70 0.10 0.25 2.0
F Deposits of sensitive and/organic material (peat, alm...)
at least 10m in thickness		 detailed investigation required

For vertical component of seismic action, multiply all spectrum values by 0.7.

Design response spectrum

For vertical component of seismic action, multiply all spectrum values by 0.7. The behaviour coefficient q shall be taken not more than 1.5.

SIA 261 Table 26: structural classes

Structural class
CO		 γf
I 1.0
II 1.2
III 1.5

SIA 261 16.2.1.2: Ground acceleration values

Seismic Hazard Zone agd [m/s2]
Z1a 0.6
Z1b 0.8
Z2 1.0
Z3a 1.3
Z3b 1.6