Ground motion selection for seismic risk analysis of civil infrastructure - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 81

ogy, whose technical details have been previously elaborated elsewhere
(Bradley, 2010a, 2012a, 2012b).

4.3 Case study: bridge foundation soil system

In order to illustrate the ground motion selection procedure discussed
herein, a particular case study is considered as discussed below.
The seismic response model considered is a bridge-foundation-soil
system representing the transverse direction of the Fitzgerald Avenue
bridge in Christchurch, New Zealand. Several previous seismic response
analysis studies have been performed for this structure, in both the trans-
verse (Bowen and Cubrinovski, 2008) and longitudinal (Bradley et al. ,
2009a) directions. Figure 4.2 illustrates a schematic view of the transverse
direction of the bridge structure and foundation soil. It can be seen that the
soil stratigraphy is composed of primarily four layers with normalized Stan-
dard Penetration Test (SPT) blowcounts of N 1 10, 15, 10 and 30. As a

result, the fi rst three soil layers are potentially liquefi able when subjected
to even moderate ground shaking. The bridge structure, comprising a deck,
pier, and footing, is supported on both existing reinforced concrete (RC)
piles of 0.3 m diameter which are founded within the liquefi able deposit,
and proposed new steel encased RC piles of 1.5 m diameter which are
founded in competent soils at greater depth. Details on the constitutive
modelling, and other numerical details are elaborated in Bowen and
Cubrinovski (2008) and Bradley (2012a).

=

Peak deck
acceleration, a D

Peak free-field
displacement, U FF

Peak pile head
displacement, U PH

( )

z = 2.5 m
z = 4.5 m

N 2 = 10
V s = 128 m/s
N 2 = 15
V s = 157 m/s

( )

Peak pore pressure
ratio, EPWPR Z = 6.2
EPWPR Z = 9.5

z = 11.0 m

( )

Existing RC D 0 = 0.3 m piles
New steel encased concrete
D 0 = 1.5 m piles

N 2 = 10
V s = 128 m/s

EPWPR Z = 12.5
EPWPR Z = 15.8

z = 17.5 m

Peak pile
curvature, f P

( )

N 2 = 30
V s = 220 m/s

EPWPR Z = 20.0

Non-
liquefiable

z = 22.5 m

4.2 Schematic illustration of transverse direction of the bridge-
foundation-soil system (Bradley et al. , 2009a); and seismic response
parameters of interest (after Bradley, 2012a).