Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 106

3.3.3. Features of hybrid characterizations

Other representative values are used in the field: from a physical point of view
they are more significant than peak values (PGA, PGD and PGV) and simpler than
spectral representations. Many proposals have been made; here we will only refer to
those that have been accepted by the majority of the para-seismic community.

3.3.3.1. Spectral intensity

This value was introduced initially by Housner to characterize the stress average
level of the most “common” structures: he considered the response spectrum integral
on periods ranging from 0.1 to 2.4 seconds:

T=25

| ³ SI (ȟ) Sv(T,ȟ) dT.

T=0.1

As a rule, SI is calculated for high damping values (]: typically 20%), in order to
integrate the already highly smoothed spectrum.

Other values can be defined by changing the integration bounds, altering the
mute variable (period or frequency), or by carrying out the integration on a
logarithmic axis. None really stands out as superior to any other in practice.

However, the notion of spectral intensity has come back in force in Japan over
the last two decades, namely IS SI(0.2)/2.4 s (then associated with the speed
response spectrum average level on the 0.1 - 2.5 s range). The reason for this is the
excellent correlation observed with the intensity of damage, and with a significant
damage apparition threshold for IS t 30 cm/s. The correlation has proved good
enough for Tokyo-Gas to use it in Tokyo and its suburbs, where over 3,000
instruments calculate it directly and transmit it in real time to a monitoring center,
for the purpose of mapping the damage estimated layout to decide which safety
floodgates should be activated.

3.3.3.2. Duration, average quadratic acceleration and the Arias intensity

Many characterizations are based on the increasing monotonous function W(t),
defined by:

t

³

2

W(t) a (IJ) dIJ

0