Effect
of various factors on spectral values |
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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. |
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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. | |||||||||||||||||||||||||
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 B_{S} for the constant acceleration region and B_{1} for the constant velocity region are given in the table below.
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. |