Response Spectrum Wizard : FEMA 356

The FEMA 356 spectrum is defined by a set of parameters defined in the code.

Site Class

Sites are classed as one of A, B, C, D or E for the soil type as described in Table 1615.1.1. Type F soil requires specialist geotechnical advice and is not included as an option.

$S_S$ and $S_1$

These are the short period response acceleration parameter and the 1s response acceleration parameter as defined in Section 1.6.1.1. Seismic Hazard maps for the USA are available from the U.S. Geological Survey web site by following the USGS hyperlink.

Damping Ratio ($β$)

The damping ratio should be determined according to FEMA 356 1.6.1.5.3

Frame Type and Building Period Coefficient / Exponent

The empirical period of the building (Section 3.3.1.2.2) is calculated from equation (3-7)

$$T=C_t(h_n)^\beta$$

The building period coefficient, $C_t$, and the building period exponent, $β$, depend on the type of building frame. The frame types are:

• Steel moment-resisting
• Reinforced concrete moment-resisting
• Eccentrically braced
• Wood
• Other

And $h_n$ is the height of the building (in feet).

Factor for Vertical Excitation

The spectral values for vertical excitation should be ⅔ of that used for horizontal excitation. (FEMA 356 1.6.1.5.2)

Ignore Short Period Force Reduction

It may be unconservative to reduce spectral values for low periods. This option ignores the reduction and extends the plateau to cover the low periods. FEMA 356 1.6.1.5.1 states that “Use of spectral response accelerations calculated using Equation (1-8) in the extreme short period range ($T<T_0$) shall only be permitted in dynamic analysis procedures and only for modes other that the fundamental mode”.