See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/262337390
Structural Design Codes of Australia and New Zealand: Seismic Actions
Chapter · January 2015
DOI: 10.1007/978-3-642-36197-5_120-1
CITATIONS
READS
4
18,798
3 authors, including:
George Kouretzis
Clive Allen
The University of Newcastle, Australia
The University of Newcastle, Australia
86 PUBLICATIONS 1,762 CITATIONS
5 PUBLICATIONS 15 CITATIONS
SEE PROFILE
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Soil-Pile-Superstructure Interaction under Combined Horizontal and Vertical Strong Ground Motions View project
Advanced Modelling of Soft Soil Behaviour View project
All content following this page was uploaded by George Kouretzis on 16 May 2014.
The user has requested enhancement of the downloaded file.
STRUCTURAL DESIGN CODES OF AUSTRALIA AND NEW
ZEALAND: SEISMIC ACTIONS
George P. Kouretzis1*, Mark J. Masia1 and Clive Allen1
1
Faculty of Engineering and Built Environment, The University of Newcastle, Australia
*
email: georgios.kouretzis@newcastle.edu.au
Synonyms: Australia; New Zealand; Structural; Design; Seismic Actions; Earthquake
1. Introduction
Design of structures to resist earthquake effects in Australia and New Zealand follows the
pertinent Australian Standard AS 1170.4:2007 and New Zealand Standard NZS 1170.5:2004
provisions. Both Standards are based on the common Australian/New Zealand Standard
AS/NZS 1170:2002 on structural design actions; and although they share elements such as
the site subsoil classification system, they incorporate certain provisions such as the nearfault factor in NZS 1170.5:2004 to account for the different seismo-tectonic regime of the
two countries. Indeed, Australia is an area of generally low seismicity, with the most
catastrophic recent event being the 1989 Newcastle earthquake of magnitude M=5.6, which
resulted in 13 casualties and significant damages in the wider Newcastle area. On the other
hand, New Zealand is located on the boundary between the Indo-Australian and Pacific Plate,
and suffers from frequent strong earthquakes. In fact, New Zealand is one of the first
countries to account for seismic actions in a building standard; as early as 1935, and
following the 1931 Hawkes Bay earthquake that claimed 256 lives, the NZS 95 provided
seismic loads to be considered in design (McRae et al., 2011). Despite constant advances and
amendments in the seismic standards over the years, the recent 22-Feb-2011 Christchurch
earthquake of magnitude M=6.3 resulted in 185 casualties, numerous injuries, and extensive
damages in the central business district of Christchurch and its eastern suburbs.
The purpose of this chapter is to provide an outline of the seismic design actions currently
considered in the abovementioned Standards and a short discussion on the rationale behind
their adaptation, in the light of other widely-used modern seismic codes provisions, such as
the Eurocode 8 EN-1998-1 and the American ASCE/SEI 7-10. It covers the determination of
elastic and inelastic design response spectra to be used together with static and dynamic
analysis methods, as well as scaling of strong motion recordings according to NZS
1170.5:2004 to perform time history analyses. Emphasis is put on the determination of the
input rather on the provisions about the performance of equivalent static, modal and time
domain analyses, which can be found elsewhere, including the Standards themselves.
2. Elastic response spectra
The elastic response spectra for horizontal loading, C(T) can be described by the following
generic equation, compatible with both Standards:
C (T ) = Ch (T ) ⋅( Z ⋅ R) ⋅ N (T , D)
(1)
where Ch(T) is the spectral shape factor; Z·R is the product of the hazard factor Z times the
factor R that depends on the annual probability of exceedance of the design earthquake and
the design limit state (ultimate or serviceability) under consideration; and N(T,D) is the near-
1
fault factor, applicable only to the NZS 1170.5:2004. The parameters comprising the elastic
response spectra equation are presented in the following paragraphs.
2.1. Spectral shape factor for different subsoil classes
The spectral shape factors Ch(T) to be used for equivalent static analyses and for dynamic
(modal and time-history) analyses are plotted separately in Figure 1, while the relevant
functions are provided in Table 1 for the different site subsoil classes. These spectral shape
factors correspond to 5% structural damping, and a correction factor to convert them to other
damping values is not provided as e.g. in EN-1998-1. However, such a provision would be of
value only for special structures such as tanks containing liquids, which are not within the
scope of the specific the Australian and New Zealand Standards.
Table 1. Spectral shape factor (5% damping) for different site subsoil classes.
Site
subsoil
class
A
B
C
D
E
Equivalent static method
Structure
period, T
(sec)
0<T<0.1
0.1<T<0.3
0.3≤T<0.4
0.4≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<0.4
0.4≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<0.4
0.4≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<0.56
0.56≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<1.0
1.0≤T≤1.5
1.5<T≤3.0
3<T
AS
1170.4:2007
NZS
1170.5:2004
2.35
1.89
0.704/T≤2.35
1.056/T2
1.60(0.5/T)0.75
1.05/T
3.15/T2
2.94
1.89
1.60(0.5/T)
1.05/T
3.15/T2
0.75
3.68
2.36
1.25/T≤3.68
1.874/T2
2.0(0.5/T)
1.32/T
3.96/T2
3.0
1.98/T≤3.68
2.4(0.75/T)
2.14/T
6.42/T2
0.75
3.68
3.0
3.08/T≤3.68
4.62/T2
0.3≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.75
3.68
2.97/T2
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.88/T≤2.94
1.32/T2
Dynamic analyses methods
Structure
AS
NZS
period, T
1170.4:2007
1170.5:2004
(sec)
0<T<0.1
0.8+15.5T
1.0+1.35(T/0.1)
0.1<T<0.3
2.35
0.704/T≤2.35
0.3≤T≤1.5
1.60(0.5/T)0.75
0.75
3.0/T
3.32/T
9.96/T2
0.3≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<0.56
0.56≤T≤1.5
1.5<T≤3.0
3<T
0<T<0.1
0.1<T<0.3
0.3≤T<1.0
1.0≤T≤1.5
1.5<T≤3.0
3<T
1.056/T2
1.0+19.4T
0.88/T≤2.94
1.32/T2
1.3+23.8T
1.25/T≤3.68
1.874/T2
1.1+25.8T
1.98/T≤3.68
2.97/T2
1.1+25.8T
3.08/T≤3.68
4.62/T2
1.05/T
3.15/T2
1.0+1.35(T/0.1)
2.35
1.60(0.5/T)0.75
1.05/T
3.15/T2
1.33+1.60(T/0.1)
2.93
2.0(0.5/T)0.75
1.32/T
3.96/T2
1.12+1.88(T/0.1)
3.0
2.4(0.75/T)0.75
2.14/T
6.42/T2
1.12+1.88(T/0.1)
3.0
3.0/T0.75
3.32/T
9.96/T2
As mentioned in the introduction, Australian and New Zealand Standards share a common
site classification scheme, which is preferably based on the predominant site period (or low
amplitude natural period) of the site Ts, estimated as:
Ts = 4H s Vs
(2)
2
where Hs is the depth to the seismic bedrock of the site, and Vs is the shear wave velocity of
the overlying soil layer. In the absence of sufficient data to estimate the predominant site
period, the site can be classified to one of the five subsoil classes (A to E) using (in the order
of most preferred to least preferred):
Figure 1. Spectral shape factor (5% damping) plots for (a) AS 1170.4:2007/Equivalent static
analyses, (b) AS 1170.4:2007/Dynamic analyses, (c) NZS 1170.5:2004/Equivalent analyses,
(d) NZS 1170.5:2004/Dynamic analyses.
Borehole log data together with in situ and laboratory tests; Seismological methods including
strong motion recordings (Nakamura, 1989 Bouckovalas et al., 2002); (qualitative) Borehole
log descriptions; Geological information for the site. For layered sites, the site period can be
estimated via an averaging method described in AS 1170.4:2007 and NZS 1170.5:2004.
The five different subsoil classes are defined in the Standards as follows (AS 1170.4:2007,
NZS 1170.5:2004):
Class A: Strong Rock. Strong to extremely strong rock, with:
(a) Unconfined compressive strength greater than 50MPa, and
(b) An average shear wave velocity over the top 30m Vs,30>1500m/sec, and
(c) Not underlain by materials having a compressive strength less than 18MPa or a shear
wave velocity less than 600m/sec.
Class B: Rock. Rock, with:
(a) Unconfined compressive strength between 1MPa and 50MPa, and
(b) An average shear wave velocity over the top 30m Vs,30>360m/sec, and
3
(c) Not underlain by materials having a compressive strength less than 0.8MPa or a shear
wave velocity less than 300m/sec.
A surface layer of no more than 3m depth of highly weathered or completely weathered rock
or soil material may be present in a Class B site. It should be mentioned here that, based on
the average shear wave velocity over the top 30m criterion, other seismic codes such as EN1998-1 or ASCE/SEI 7-10 would classify a site with Vs,30>360m/sec to dense/stiff soil
deposits, rather than rock (Class B according to EN-1998-1 which corresponds to
360<Vs,30<800m/sec, and Class C according to ASCE/SEI 7-10, which corresponds to
366<Vs,30<762m/sec). This suggests that Class B in AS 1170.4:2007 and NZS 1170.5:2004 is
rather broad, covering a range of subsoil conditions that may not exhibit similar behavior
during an earthquake rich in high- to medium- frequency content.
Class C: Shallow soil sites. Sites that are not classified as class A, class B or class E, and:
(a) The predominant site period estimated as above is Ts≤0.6sec, or
(b) Soil depth does not exceed the maximum values listed in Table 2.
Table 2. Maximum depth limits for site subsoil class C (after AS 1170.4:2007, NZS
1170.5:2004).
Soil type and description
Cohesive soils
Cohesionless soils
Very soft
Soft
Firm
Stiff
Very stiff to hard
Very loose
Loose dry
Medium dense
Dense
Very dense
Gravels
Representative
undrained shear
strength, Su (KPa)
<12.5
12.5-25
25-50
50-100
100-200
-
Representative SPT
N-values
Maximum depth of
soil (m)
<6
6-10
10-30
30-50
>50
>50
0
20
25
40
60
0
40
45
55
60
100
Class D: Deep or soft soil sites. Sites that are not classified as class A, class B or class E,
and:
(a) The predominant site period estimated as above is Ts>0.6sec, or
(b) Soil depth exceeds the maximum values listed in Table 2, or
(c) Are underlain by less than 10m of cohesive soil with undrained shear strength
Su<12.5KPa or cohessionless soil soils with SPT values N<6.
Class E: Very soft soil sites.
(a) More than 10m of very soft cohesive soil with undrained shear strength Su<12.5KPa,
or
(b) More than 10m of very loose cohesionless soils with SPT values N<6, or
(c) More than 10m depth of soft-loose soils with shear wave velocity values Vs<150m/sec
(d) More than 10m combined depth of soils with properties described in (a), (b) and (c)
above.
A comparison between the spectral shape factors of AS 1170.4:2007 and NZS 1170.5:2004 is
attempted in Figure 2, for the two extreme cases of rock (Α&Β) and soft soil (Ε) sites. The
New Zealand Standard, which generally applies to earthquakes with higher magnitudes
(M≥6.5), features lower spectral factor values in the low structural period range (T<1sec), and
4
higher spectral factor values in the high period range (T>1sec). This is compatible with the
fact that soil non-linearity effects alter the spectral content of stronger earthquakes with
magnitude M>5.5; which in addition have a richer low frequency content attributed also to
near-fault effects.
Figure 2. Comparison of AS 1170.4:2007 and NZS 1170.5:2004 dynamic spectral shape
factor plots for rock (A, A&B) and soft soil (E) types (5% damping).
2.2. Hazard factor and annual probability of exceedance
The hazard factor Z of a particular location corresponds to the peak ground acceleration (in
g’s) for site class B (AS 1170.4:2007) or classes A&B (NZS 1170.5:2004), considering a
design earthquake with return period of 500 years i.e. a 10% probability of exceedance
during a 50-year design life. The hazard factors in the Australian Standard have not been
updated since its previous version AS 1170.4:1993, and are based on the work of Gaull et al.
(1990), who used data of the Australian Geological Survey Organization dated back to 1856.
All areas in Australia are assumed to be seismically active for design purposes, with the
hazard factor generally ranging between Z=0.05 and Z=0.13; with the exception of the
Meckering region in Western Australia, where the hazard factor ranges between Z=0.14 and
Z=0.22 and the Macquarie Island, located half-way between New Zealand and Antarctica,
where the hazard factor is equal to Z=0.60.
In NZS 1170.5:2004, the hazard factor Z corresponds to 0.5 times the spectral acceleration
(5% damping) for a structure period T=0.5sec and soil subclass C, considering a design
earthquake with a return period of 500 years (Figure 1c-1d). The minimum value across New
Zealand is Z=0.13, to ensure no-collapse of structures even in areas of low seismicity i.e.
every structure is designed to survive the 84-percentile strong motion of a magnitude M=6.5
normal-faulting earthquake at source-to-site distance of 20km; a seismic scenario
corresponding to the low-seismicity areas of New Zealand. The hazard factor at the highseismicity major cities (e.g. Wellington, Napier, Hastings) is of the order of Z=0.40, with the
maximum value considered in the Standard being Z=0.60.
The probability factor (AS 1170.4:2007) or return period factor (NZS 1170.5:2004) R, listed
in Table 3, is used to scale the hazard factor to the required annual probability of exceedance,
for the design limit state under consideration. In AS 1170.4:2007, the annual probability of
exceedance is correlated with the design working life and the importance level of the
structure, with the latter provided in AS/NZS 1170.0:2002 (Appendix F). The design annual
probability of exceedance applies to the ultimate limit state only: According to AS
1170.4:2007, structures of importance levels 1-3 (i.e. all structures except ones carrying
critical post-disaster functions and exceptional structures) de facto satisfy the serviceability
5
Table 3. Probability factor (AS 1170.4:2007) or Return period factor (NZS 1170.5:2004).
Annual probability of exceedance
1/2500
1/2000
1/1500
1/1000
1/800
1/500
1/250
1/200
1/100
1/50
1/25
1/20
R
1.8
1.7
1.5
1.3
1.25
1.0
0.75
0.70
0.50
0.35
0.25
0.20
limit state requirements when designed in accordance to the Standard. A special study needs
to be carried out for critical post-disaster structures only (importance level 4), to ensure that
they remain operational after a seismic event equivalent to the ultimate limit state design
event for ordinary importance level 2 structures. The design annual probability of exceedance
for structures and buildings (importance level 2) with a working life of 50 years is 1/500, and
corresponds to a 10% probability of exceedance during the design life of the structure. For
major structures affecting crowds and associated with “very great” economic, social and
environmental consequences of failure (importance level 3), the annual probability of
exceedance is reduced to 1/1000 for structures with a design working life of 50 years, and to
1/2500 for structures with a design working life of 100 years.
The required annual probability of exceedance for structures designed according to NZS
1170.5:2004 is provided again in AS/NZS 1170.0:2002 (Table 3.3 of AS/NZS 1170.0:2002);
however in the New Zealand Standard it is correlated with the design (serviceability or
ultimate) limit state: The annual probability of exceedance for the common serviceability
limit state SLS1 (requirement for no repairs on structural and non-structural components) is
1/25 for all structures. The special serviceability limit state SLS2 applies to structures
carrying critical post-disaster functions only (importance level 4), and is associated with an
annual probability of exceedance of 1/500 for design working life of 50 years. A special
hazard study is required to define the SLS2 actions for structures of importance level 4 with
design working life of 100 years or more.
As far as the ultimate limit state is concerned, the design annual probability of exceedance is
the same as in AS 1170.4:2007, for structures classified to importance levels 2 and 3. Note
that the product Z·R in NZS 1170.5:2004 needs not to exceed Z·R=0.70 for ultimate limit
state analyses, but must be higher than Z·R=0.20 when an annual probability of exceedance
1/2500 is considered. These Z·R bounds correspond to the higher and lower seismicity
regions of New Zealand, respectively: The upper bound matches the 84-percentile near-fault
strong motion due to a M=8.1 event from the major Alpine fault that runs along the South
Island, divided by a margin of safety equal to 1.5 likely to result from applying the code
design provisions.
2.3. Near-fault factor
The near fault factor N(T,D) is introduced in NZS 1170.5:2004 to account for near-source
effects on the strong motion during the activation of a strike-slip fault: a) forward directivity,
resulting in high peak velocities and displacements, and b) polarization of the long-period
motions in the near source region, where medium- to long- period pulses tend to be stronger
in the direction perpendicular to the strike of the fault. Near fault effects in NZS 1170.5:2004
6
are considered for eleven (11) major strike-slip faults classified as “Class A” faults according
to the Californian Category A fault criteria (Petersen et al., 2000), capable of producing
earthquakes with magnitude M=7.0 or greater, and having slip rates of 5mm/year of greater.
The near-fault factor N(T,D) must be considered during the derivation of the medium- to
long- period band of the elastic response spectra for sites located within a distance D less
than 20km from the traces of the major strike-slip faults listed in NZS 1170.5:2004, and
applies to annual probability of exceedance less that 1/250:
N(T,D) = Nmax(T)
for
D≤2km
(3.1)
N(T,D) = 1+(Nmax(T)-1)·(20-D)/18
for
2km<D≤20km
(3.2)
N(T,D) = 1.0
for
D>20km
(3.3)
where the factor Nmax(T) is provided as a function of the structure period T in Table 4 and
Figure 3a. Linear interpolation functions are provided for intermediate period values, as
proposed in NZS 1170.5:2004. In addition, a comparison of the effect of near-fault correction
(eqs. 3.1-3.3) on the spectral shape factors for dynamic analyses is illustrated in Figure 3b,
for different distances from the fault.
Table 4. Maximum values of the near-fault factor Nmax(T) for different structure periods.
NZS 1170.5:2004
Structure period, T (sec)
≤1.5
2
3
4
≥5
Linear interpolation functions
Structure period, T (sec)
Nmax(T)
≤1.5
1.0
Nmax(T)
1.0
1.12
1.36
1.60
1.72
1.5<T<4
0.24T+0.64
4≤T<5
≥5
0.12T+1.12
1.72
Figure 3. (a) Variation of the near-fault factor Nmax(T) with the structure period, T, and (b)
Comparison of spectral shape factors corrected for near-fault effects for a rock site (soil
subclass A&B) at a distance D≤2km, D=10km and D>20km from a major active fault (5%
damping).
2.4. Vertical design actions
According to AS 1170.4:2007, vertical earthquake actions generally need not to be
considered in design. For the design of mechanical and electrical components, the vertical
7
earthquake forces are taken equal to 50% of the horizontal forces. However, a provision is
included stating that when the dynamic analysis requires the consideration of vertical
earthquake forces, both upwards and downwards motion need to be considered, and the
vertical spectral shape factor shall be taken equal to:
Cv (T ) = 0.5⋅Ch (Tv ) (AS 1170.4:2007)
(4)
where Tv is the vertical period of vibration. In NZS 1170.5:2007 on the other hand, when
vertical design actions need to be considered in the analyses (e.g. time history analyses), the
elastic response spectrum for vertical loading is taken equal to 70% of the corresponding
horizontal spectrum, as:
Cv (T ) = 0.7 ⋅Ch (Tv ) (NZS 1170.5:2004)
(5)
where Tv=0 for the design of the structure as a whole, or equal to the vertical period of the
element under consideration, for the design of parts and components. Note that the vertical
component of strong earthquake motion is usually rich in high-frequency content, thus peak
ground acceleration and spectral values in the near-source region may exceed the horizontal
values (NZS 1170.5 Supp 1:2004, Niazi and Bozorgnia, 1992 Ambrasseys and Simpson,
1996). In the light of this, NZS 1170.5 Supp 1:2004 recommends that when the distance of
the site from the fault trace is less than D=10km, the vertical design actions be considered
equal to the horizontal design actions for structure period T≤0.30sec. Strong motion
recordings from recent earthquakes, such as the Christchurch 2011 earthquake, provided
further evidence in support of the above, as discussed later in this chapter.
3. Structural ductility and structural performance factor-Inelastic design spectrum
Derivation of the (inelastic) design spectrum requires the correction of the elastic response
spectrum to account for the ability of the particular structure to dissipate seismic energy via
non-linear response. Apart from the structural ductility factor, µ, which is related to the level
of inelastic demand that can be reliably sustained by the structure, AS 1170.4:2007 and NZS
1170.5:2004 introduce the structural performance factor Sp to reduce the elastic design
seismic actions. The structural performance factor is employed to quantify a number of
effects that are not explicitly taken into account in state-of-practice structural analysis
procedures, by simply scaling the design loads. Such effects include (NZS 1170.5 Supp
1:2004):
a) Excitation effects: The estimated seismic loads correspond to a peak ground
acceleration value, which may be reached only during a single loading cycle, and
therefore is unlikely to lead to significant damage (“effective” ground acceleration
concept).
b) Individual structural elements typically feature a higher capacity than modeled during
the analysis of the structure, due to higher material strength, strain hardening, strain
rate effects etc.
c) The total structural capacity is typically higher than predicted, due to redundancy
effects, or the contribution of non-structural elements (e.g. in fill walls) which is not
taken directly into account in typical analysis models.
d) The energy dissipation of the structure is typically higher than assumed, due to
damping introduced from non-structural elements and soil-foundation interaction
effects.
The ductility factor µ for the ultimate limit state is calculated in accordance with the
appropriate material standard, where such data are provided. Contrary to NZS 1170.5:2004,
8
where reference is made to material standards only and a special study is required otherwise,
maximum ductility factors for typical structural systems are provided directly in AS
1170.4:2007 (Table 5). Alternatively, for a specific structure, µ and Sp can be determined via
a non-linear static push-over analysis. Note also that AS 1170.4:2007 applies only to
structures with ductility factor µ≤3, and when a higher ductility factor is considered, the
design must be perfomed in compliance with NZS 1170.5:2004 provisions (Table 5).
Table 5. Ductility factor µ and structural performance factor Sp for different structural
systems and specific structure types (after AS 1170.4:2007).
µ
Sp
Steel structures
Special moment-resisting frames (fully ductile)*
Intermediate moment-resisting frames (moderately ductile)
Ordinary moment-resisting frames (limited ductile)
Moderately ductile concentrically braced frames
Limited ductile concentrically braced frames
Fully ductile concentrically braced frames*
Other steel structures not defined above
4
3
2
3
2
4
2
0.67
0.67
0.77
0.67
0.77
0.67
0.77
Concrete structures
Special moment-resisting frames (fully ductile)*
Intermediate moment-resisting frames (moderately ductile)
Ordinary moment-resisting frames
Ductile coupled walls (fully ductile)*
Ductile partially coupled walls*
Ductile shear walls
Limited ductile shear walls
Ordinary moment-resisting frames in a combination with limited ductile shear
walls
Other concrete structures not listed above
4
3
2
4
4
3
2
2
0.67
0.67
0.77
0.67
0.67
0.67
0.77
0.77
2
0.77
Timber
structures
Shear walls
Braced frames (with ductile connections)
Moment-resisting frames
Other wood or gypsum based seismic-force-resisting systems not listed above
3
2
2
2
0.67
0.77
0.77
0.77
Close-spaced reinforced masonry**
Wide-spaced reinforced masonry**
Unreinforced masonry**
Other masonry structures not complying with AS 3700:2001
2
1.5
1.25
1
0.77
0.77
0.77
0.77
2
3
1.0
1.0
3
1.0
3
2
3
3
3
3
2
3
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
Specific structure types
Description
Masonry
structures
Structural
system
*
Tanks, vessels or pressurized spheres on braced on unbraced legs
Cast-in-place concrete silos and chimneys having walls continuous to the
foundation
Distributed mass cantilever structures, such as stacks, chimneys, silos and skirtsupported vertical vessels
Trussed towers (freestanding or guyed), guyed stacks and chimneys
Inverted pendulum-type structures
Cooling towers
Bins and hoppers on braced on unbraced walls
Storage racking
Signs and billboards
Amusement structures and monuments
All other self-supporting structures not otherwise covered.
: The design of structures with µ>3 should be in accordance to NZS 1170.5:2004
**
: Values from AS 3700:2001
While NZS 1170.5:2004 does not refer explicitly to ductility factor values for the ultimate
limit state, for serviceability limit state analyses of common structures (SLS1) it is
9
provisioned that 1.0≤µ≤1.25, and for critical post-disaster structures (SLS2) it is provisioned
that 1.0≤µ≤2.0. Furthermore, the ductility factor for vertical actions is always considered to
be µ=1.0.
Note that in NZS 1170.5:2004, the inelastic horizontal design spectra are not derived by
directly multiplying the elastic spectra by (1/µ). Instead, to account for the transition between
equal displacement theory (which is valid for longer structure periods) to equal energy theory
(valid for shorter structure periods), a transition point is defined at T1=0.7sec for soil
subclasses A to D and at T1=1.0sec for soil subclass E, where T1 is the largest translation
period of vibration along the direction under consideration. So, the factor to derive the
inelastic response spectra is estimated as:
For subsoil classes A, B, C and D:
kµ = µ
for
Τ1≥0.7sec
(6.1)
kµ = 1+(µ-1)Τ1/0.7
for
Τ1<0.7sec
(6.2)
kµ = µ
for
Τ1≥1.0sec or µ<1.5
(6.3)
kµ = 1.5+(µ-1.5)Τ1
for
Τ1<1.0sec and µ≥1.5
(6.4)
For subsoil class E:
For the purpose of calculating kµ, the largest translation period of vibration T1 is not taken
less than 0.4sec.
The structural performance factor Sp in AS 1170.4:2007 is provided in Table 5. In NZS
1170.5:2004, the structural performance factor is taken equal to Sp=0.7 for ultimate limit state
analyses, except for low-ductility structures with 1.0<µ<2.0, where Sp is calculated as:
Sp = 1.3-0.3µ
(7)
For ultimate limit state verification of structures against sliding or overturning Sp is taken
equal to Sp=1.0. For serviceability limit state SLS1 and SLS2 analyses, Sp is taken equal to
Sp=0.7, except otherwise dictated by the relevant material standard.
In accordance to the above, the inelastic design response spectrum in AS 1170.4:2007 is
defined as:
Cd (T ) =
C (T ) S p
(8)
µ
and in NZS 1170.5:2004 as:
Cd (T ) =
C (T ) S p
kµ
Cd (T ) = Cv (T ) S p
(for horizontal actions)
(9.1)
(for vertical actions)
(9.2)
4. Actions for dynamic time history analyses
In addition to actions applicable to common equivalent static and modal dynamic analyses,
AS 1170.4:2007 and, mainly, NZS 1170.5:2004 include certain provisions for the
determination of the appropriate design actions when dynamic time-domain analyses are to
be performed. While in AS 1170.4:2007 there is a vague requirement that the response
spectra of the actual acceleration time histories used shall "approximate" the design spectrum
10
of eq. (8), NZS 1170.5:2004 prescribes a more elaborate and explicit procedure for the
selection and scaling of ground motion records.
Ground motion accelerographs to be used for time domain analyses according to NZS
1170.5:2004 shall consist of both horizontal components of the recording; the vertical
component may have to be considered too, for the analysis of structures sensitive to vertical
strong ground motion. The above imply that the procedure refers to three-dimensional
analyses, yet an adaption to two-dimensional models is feasible, by considering only one
horizontal component of each ground motion record as e.g. in ASCE/SEI 7-10. A "family" of
no less than three records must be employed in each time history analysis.
The selected acceleration records must be consistent with the local seismo-tectonic regime
(expected magnitude, fault rupture mechanism, source-to-site distance etc.). When the site is
located near a major fault i.e. N(T,D)>1 (eqs. 3), then one set of the records must have a
forward directivity component. Simulated, artificial accelerographs may be used too, when
appropriate real acceleration recordings are not available. Each record shall be scaled by a
"record scale factor" k1 to match the target design spectrum over the period range of interest,
and by a "family scale factor" k2 to ensure that the energy content of at least one record in the
family exceeds that of the design spectrum over the period range of interest. The target
design spectrum is defined as:
⎛1+ S ⎞⎟
p⎟
SAtarget = ⎜⎜⎜
⎟C (T )
⎜⎝ 2 ⎟⎟⎠
(10)
where the spectral shape factor C(T) is calculated from eq. (1) and Sp is the structural
performance factor defined in section 3.
In order to present the procedure of scaling ground motion records according to NZS
1170.5:2004, the horizontal components of three typical strong ground motion recordings
from the 2011 Christchurch M=6.3 earthquake were used. These records were scaled to
match the target ultimate limit state spectrum of NZS 1170.5:2004 for a common structure
(importance level 2) located in Christchurch, founded on subsoil class D (soft soil).
According to NZS 1170.5:2004, for a structure of importance level 2 in Christchurch with a
design life of 50 years, Z·R=0.22g and N(T,D)=1 i.e. no near-fault effects need to be taken
into account due to possible rupture of the Port Hills fault that is believed to have caused the
2011 earthquake. Assuming for simplicity that Sp=1, the target spectrum is plotted in Figure
4.
Strong motion recordings during the Christchurch 2011 earthquake exceeded by far the
design peak ground acceleration at the ground surface of class D soft soils implied by NZS
1170.5:2004 (0.246g), and reached maximum values of 1.67g in the horizontal direction and
2.20g in the vertical direction at the Heathcote Valley Primary School Station HVSC
(geonet.org.nz). Here a family of three near-field recordings on soft to very soft soil are used,
with peak horizontal ground accelerations ranging from 0.34g to 0.67g (Table 6).
Table 6. Characteristics of the typical strong motion recordings from the Christchurch 22Feb-2011 M=6.3 earthquake (source: geonet.org.nz)
Site code
CBGS
PRPC
CHHC
Subsoil class acc. to
NSZ 1170.5:2004
D
E
D
Epicentral
distance
7
6
6
11
PGA-Horiz.1
(g)
0.553
0.669
0.345
PGA-Horiz.2
(g)
0.452
0.595
0.364
PGA-Vert.
(g)
0.360
1.88
0.601
The elastic response spectra of the horizontal components of these three recordings are
plotted in Figure 4a, in comparison with the design response spectra of NZS 1170.5:2004. A
band-pass filter in the frequency ranges of 0.10-0.25Hz and 24.5Hz-25.5Hz was applied to
the time histories used to derive the spectra. It is clear that the spectral values of these typical
records are above the design spectra across practically the whole range of important structural
periods, an indication of the severity of the Christchurch earthquake.
Figure 4. (a) Comparison of the 2011 Christchurch strong motion recordings elastic response
spectra (5% damping, horizontal components) with the design NZS 1170.5:2004 elastic
design spectra for the city of Christchurch and soil subclass D, (b) Scaled elastic response
spectra of the same strong motion recordings to match the NZS 1170.5:2004 design spectra
for largest structure translation period T1=1sec and Sp=1.
Scaling of ground motion records to match the target design spectrum via the scale factor k1
was performed while taking into account a structure period range that depends on the largest
translational period of the structure in the direction of interest, T1. The period range of
interest is defined as 0.4T1≤T≤1.3T1, with the product 0.4T1 not taken less than 0.4sec. Notice
that this band of significant periods is considerably narrower compared to the corresponding
one prescribed in EN-1998-1, which is 0.2T1≤T≤2.0T1, or even ASCE/SEI 7-10, where the
corresponding range is 0.2T1≤T≤1.5T1. For the case at hand it was assumed that T1=1sec, and
the period range of interest is denoted by the yellow band in Figure 4. The best-fit scale
factor k1 of each horizontal ground motion component was determined as the value
minimizing the function log(k1SAcomponent/SAtarget) over the period range of interest, in a least
mean square sense. In other words, the aim was to find via an interactive procedure the k1
value that minimizes the sum:
2
⎡ ⎛ k SA
⎞⎤
⎢ log ⎜⎜ 1 component ⎟⎟⎥ dT = min
⎟⎟⎥
∫ ⎢ ⎜⎜⎝ SA
⎟⎠⎥
target
0.4T1 ⎢⎣
⎦
1.3T1
(9)
The selected recordings must satisfy a "similarity" criterion 0.33<k1<3.0. Moreover, in order
to verify that each selected record reasonably matches the target spectrum over the period
range of interest, the following inequality quantifying the mean (over the period of interest)
square difference of the scaled over the target spectral values must be satisfied:
12
2
1.3T1 ⎡
⎛ k SA
⎞⎟⎤
1
⎜
D1 =
⋅ ∫ ⎢⎢ log ⎜⎜ 1 component ⎟⎟⎟⎥⎥ dT ≤ log (1.5)
(1.3−0.4)T1 0.4T1 ⎢⎣ ⎜⎝ SAtarget ⎟⎠⎥⎦
(10)
Although not a normative criterion, a stricter fit D1≤ log(1.3) is proposed for most cases,
according to NZS 1170.5 Supp 1:2004.
The k1 and D1 factors for the records at hand are listed in Table 7, where it is depicted that the
specific records are compatible with NZS 1170.5:2004 requirements. The component of each
record with the lower k1 value was nominated as "principal" (Figure 4, Table 7) and this
value of k1 was used as the record scale factor.
Table 7. Scaling factors for the considered typical strong motion records.
Site code
CBGS
PRPC
CHHC
Component
N89W (principal)
S01W
W (principal)
S
N01W
S89W (principal)
k1
0.608
0.789
0.672
0.785
0.683
0.598
D1
0.070
0.105
0.081
0.097
0.064
0.058
k2
1.05
The record family scale factor, k2 ensures that the principal component of at least one record
spectrum (after being scaled by k1) exceeds the target spectrum over the period range of
interest. It is estimated as the maximum value of the ratio SAtarget/max(SAprincipal)≥1.0 within
the period range of interest, where max(SAprincipal) is the maximum spectral value between all
the principal components of the family, at each period step considered for the derivation of
the spectra. To confirm the selection of the principal and secondary components, the record
family scale factor must be within the range 1.0<k2<1.3. If k2>1.3 then either a different
record must be selected, or the selection of the principal/secondary components may be
reversed, aiming to minimize the product k1k2. This may be the case when all three principal
components in a family feature low spectral values within a particular period band, while one
of the secondary components is rich in frequency content within the same band; thus
although the scale factor k1 may be greater for that particular strong motion component, the
product k1k2 may be lower. In the case at hand however, the family factor k2 is within the
desirable range (Table 7), a fact that can be confirmed visually from Figure 4.
The above scaling procedure must be followed for all directions of interest, corresponding to
different translational periods of the structure, T1, and thus a different period range of interest.
Although more elaborate than the analogous procedure of EN-1998-1 or ASCE/SEI 7-10 for
the time-history representation of the seismic action, where it is simply required that no value
of the mean elastic spectrum of all time histories used is less than 90% (EN-1998-1) or 100%
(ASCE/SEI 7-10) of the corresponding value of the elastic response spectrum, the procedure
prescribed in NZS 1170.5:2004 results in a more rational scaling of strong motion records;
and in a closer match to the design spectrum, as it does not impose restrictions related only to
lower-than-target spectral acceleration values found in EN-1998-1 and ASCE/SEI 7-10. As a
result of the above, if one embraced the more conservative EN-1998-1 and ASCE/SEI 7-10
provisions regarding the average response spectra from the three principal components, the
specific scaling factors applied would not be acceptable; as their average spectrum is below
the 100% (ASCE/SEI 7-10) and marginally below the 90% (EN-1998-1) of the target
spectrum (Figure 5).
Finally, note that NZS 1170.5:2004 (as also ASCE/SEI 7-10) base the accelerograph scaling
procedure on spectral values within the range of interest only, and does not impose
13
restrictions on the zero period spectral response acceleration values of the strong motion, as
e.g. EN-1998-1.
Figure 5. Comparison of the mean response spectrum (5% damping) of the three principal
components listed in Table 7 with the NZS 1170.5:2004 target spectrum for soil subclass D
and Sp=1. The period of interest band according to NSZ 1170.5:2004 (0.4T1≤T≤1.3T1) and
according to ASCE/SEI 7-10 (0.2T1≤T≤1.5T1) is also drawn.
5. Summary
Seismic design action definitions in the Australian AS 1170.4:2007 and New Zealand NZS
1170.5:2004 Standards were presented in a concise way, focusing on the determination of the
elastic and inelastic design response spectra to be used together with static and dynamic
methods of analysis. A short discussion against provisions of other modern seismic codes,
such as EN-1998-1 and ASCE/SEI 7-10 was attempted, to point out certain key differences in
particular clauses. Although Australian and New Zealand Standards embrace most of the
recent developments on earthquake engineering, such as accounting for near-source effects
on strong motion (but not topography effects as e.g. EN-1998-1), any seismic code is not a
"static" document, but rather a "dynamic" one; and evolves with lessons learned from recent
major earthquakes, that are included in future revisions as e.g. the increase of the hazard
factor Z for the Christchurch area from Z =0.22g to Z=0.30g in the latest compliance
document of the New Zealand Building Code (Department of Building and Housing, 2011).
6. Cross-references
1. Earthquake Response Spectra and Design Spectra-368961
2. Earthquake Return Period and its Incorporation into Seismic Actions-368962
3. European Structural Design Codes: Seismic Actions-368965
4. Near-source Seismic Effects-374553
5. Response-spectrum-compatible ground motion processes-382576
6. Seismic Actions due to Near-Fault Ground Motion-368967
7. Selection of Ground Motions for Time-History Analyses-368960
14
7. References
8. Ambrasseys, N.N. & Simpson, K.A. (1996). Prediction of vertical response spectra in
Europe. Earthquake Engineering and Structural Dynamics, 25, 401-412.
9. AS 1170.4:2007. Structural design actions Part 4: Earthquake actions in Australia.
Standards Australia.
10. AS 3700:2001. Masonry Structures. Standards Australia.
11. AS/NZS 1170:2002. Structural design actions. Standards Australia/Standards New
Zealand.
12. ASCE/SEI 7-10. Minimum design loads for buildings and other structures. ASCE
Standard, American Society of Civil Engineers, Structural Engineering Institute.
13. Bouckovalas, G.D., Kouretzis, G.P., Kalogeras, I.S. (2002). Site-specific analysis of
strong motion data from the September 7, 1999 Athens, Greece Earthquake. Natural
Hazards, 27, 105-131.
14. Department of Building and Housing (2011). Compliance Document for New Zealand
Building Code, Clause B1, Structures, Amendment 11, August. New Zealand
Government.
15. EN-1998-1. Eurocode 8: Design of structures for earthquake resistance - Part 1:
General rules, seismic actions and rules for buildings. European Committee for
Standardization.
16. Gaull, B.A., Michael-Leiba, M.O., Rynn, J.M.W. (1990). Probabilistic earthquake risk
maps of Australia. Australian Journal of Earth Sciences, 37, 169-187.
17. GeoNet-Earthquake Commission-GNS
Accessed September 2013.
Science.
website:
www.geonet.org.nz.
18. McRae, G., Clifton, C., Megget, L. (2011). Review of NZ building codes of practice.
Report to the Royal Commission of Inquiry into the Building Failure Caused by The
Christchurch Earthquakes ENG.ACA.0016.1.
19. Nakamura, Y. (1989). A method for dynamic characteristics estimation of subsurface
using microtremors on the ground surface. RTRI Quarterly Report, Vol. 30, No. 1,
Japan.
20. Niazi, M. & Bozorgnia, Y. (1992). Behaviour of near-source vertical and horizontal
response spectra at SMART-1 array. Earthquake Engineering and Structural
Dynamics, 21, 37-50.
21. NZS 1170.5:2004. Structural Design Actions Part 5: Earthquake actions-New
Zealand. Standards New Zealand.
22. NZS 1170.5 Supp 1:2004. Structural Design Actions Part 5: Earthquake actions-New
Zealand-Commentary. Standards New Zealand.
23. NZS 95. New Zealand Standard Code of Building By-Laws. Wellington, Standards
Association of NZ.
24. Petersen, M.D., Toppozada, T.R., Cao, T., Cramer, C.H., Reichle, M.S., Bryant, W.A.
(2000). Active fault near-source zones within and bordering the state of California for
the 1997 Uniform Building Code. Earthquake Spectra, 16(1), 69-83.
15
View publication stats