Effect of various factors on spectral values
Modifying the Viscous Damping of Spectra


Since statistically derived spectra have only been generated thus far for 5% viscous damping, it is necessary to modify these spectra if different levels of damping are required. Either Newmark and Hallís response amplification factors or the FEMA 273 procedure can be used to modify statistically derived spectra or other spectra. Note that these factors are period dependent!

Newmark and Hall's Method

For each range of the spectrum, the spectral values are multiplied by the ratio of the response amplification factor for the desired level of damping to the response amplification factor for the current level of damping.

Consider if we have a median spectrum at 5% viscous damping and we would like it at x%. If the 5% Joyner and Boore Sv value is 60 cm/sec on the descending branch, an estimate of the 2% Sv value is 60x(2.03/1.65) = change 60x1.47 = 88 cm/sec.


When using the Change Damping function in Modspec, you can use Newmark and Hall's factors as described above. Try it out for any spectrum of your choice in the following Interactive Example.

Interactive Example


FEMA 273 Procedure

The FEMA 273 procedure, which is based on Newmark and Hall's method, operates in a very similar manner, except that there are only two spectral regions of interest -- constant acceleration and constant velocity. The damping value is changed by simply dividing each region by the correct coefficient. The coefficients BS for the constant acceleration region and B1 for the constant velocity region are given in the table below.

Effective Damping (% critical)

BS
B1

< 2

0.8
0.8
5
1.0
1.0
10
1.3
1.2
20
1.8
1.5
30
2.3
1.7
40
2.7
1.9
> 50
3.0
2.0

The very large values of effective viscous damping are intended for use with structures utilizing seismic isolation or energy-dissipation technology. If values of damping other than those listed in the table are needed, liner interpolotion should be used between table values.

The Change Damping function in Modspec can also use the FEMA 273 procedure described above. Try it out for any spectrum of your choice in the following Interactive Example.

Interactive Example