Civil Engineering Reference
In-Depth Information
Table 3.18
Northridge records possessing a strong vertical component (
after
Broderick
et al.
, 1994 ).
Station
d
(km)
HPGA (g)
VPGA (g)
VPGA/HPGA
Tarzana, Cedar Hill Nursery
5
1.82
1.18
0.65
Arleta, Nordhoff Avenue Fire Station
10
0.35
0.59
1.69
Sylmar, County Hospital
16
0.91
0.60
0.66
Newhall, LA County Fire Station
20
0.63
0.62
0.98
Key
: d = source distance; HPGA = horizontal peak ground acceleration; VPGA = vertical peak ground
acceleration.
Table 3.19
Kobe records possessing a strong vertical component (
after
Elnashai
et al.
, 1995 ).
Station
d
(km)
HPGA (g)
VPGA (g)
VPGA/HPGA
JMA Station
18
0.84
0.34
0.41
Port Island Array
20
0.35
0.57
1.63
Kobe University
25
0.31
0.43
1.39
Key
: d = source distance; HPGA = horizontal peak ground acceleration; VPGA = vertical peak ground
acceleration.
in the horizontal ground shaking, but the vertical motion continued to be amplifi ed through the liquefi ed
layer. The vertical-to-horizontal peak ground acceleration ratio was 1.63 (Table 3.19).
Vertically propagating dilatational waves are amplifi ed in a manner identical to that of vertically
propagating shear waves. Consequently, the vertical component of motion can be linearly amplifi ed
from bedrock to the surface up to very high levels, leading to the widely observed high v/h ratios near
the source. For example, Table 3.20 shows the results of the study by Ambraseys and Simpson (1995 )
that involved worldwide records generated at source distances
d
≤
15 km by relatively large inter - plate
earthquakes with magnitude
M
S
≥
6.0 and signifi cant vertical accelerations (
a
v
≥
0.10 g). The identifi ed
data set consists of 104 records in the magnitude range 6.0
7.6. A simple linear regression
analysis, which is fully justifi able in the small distance range considered, was carried out. The results
in Table 3.20 show the 84.1% confi dence limit.
The computed values clearly show that the assumption that the vertical peak is 2/3 of the horizontal
component suggested by Newmark
et al.
(1973) can be a serious underestimate, especially for short
distances from the source, e.g. less than 15 to 20 km.
≤
M
S
≤
3.4.7 Vertical Motion Spectra
The commonly used approach of taking the vertical spectrum as 2/3 of the horizontal, without a change
in frequency content, has been superseded (Elnashai and Papazoglou, 1997; Collier and Elnashai, 2001 ).
The effect of vertical motion is currently subject to re-evaluation and independent vertical spectra have
been proposed for implementation within codes of practice, for example in Europe. Below, two alterna-
tives are given for obtaining a more realistic vertical spectrum than seismic codes have hitherto
employed with the exception of recent European seismic standards (Eurocode 8, 2004).
Ambraseys
et al.
( 2005b ) proposed vertical ground -motion parameters attenuation relationships of
the same form as those given in Section 3.4.3.1. The relationship for 5% damping spectral acceleration
S
a
can be expressed as follows:
