Structural Design Codes of Australia and New Zealand: Seismic Actions
Synonyms
Introduction
The 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 near-fault factor in NZS 1170.5:2004 to account for the different seismotectonic 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 Hawke’s 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 February 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 entry 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 than on the provisions about the performance of equivalent static, modal, and time-domain analyses, which can be found elsewhere, including the Standards themselves.
Elastic Response Spectra
Spectral Shape Factor for Different Subsoil Classes
Spectral shape factor (5 % damping) for different site subsoil classes
Site subsoil class | Equivalent static method | Dynamic analyses methods | ||||
---|---|---|---|---|---|---|
Structure period, T (sec) | AS 1170.4:2007 | NZS 1170.5:2004 | Structure period, T (sec) | AS 1170.4:2007 | NZS 1170.5:2004 | |
A | 0 < T ≤ 0.1 | 2.35 | 1.89 | 0 < T ≤ 0.1 | 0.8 + 15.5 T | 1.0 + 1.35(T/0.1) |
0.1 < T < 0.3 | 0.704/T ≤ 2.35 | 0.1 < T < 0.3 | 0.704/T ≤ 2.35 | 2.35 | ||
0.3 ≤ T < 0.4 | 0.3 ≤ T ≤ 1.5 | 1.60(0.5/T)^{0.75} | ||||
0.4 ≤ T ≤ 1.5 | 1.60(0.5/T)^{0.75} | |||||
1.5 < T ≤ 3.0 | 1.056/T ^{2} | 1.05/T | 1.5 < T ≤ 3.0 | 1.056/T ^{2} | 1.05/T | |
3 < T | 3.15/T ^{2} | 3 < T | 3.15/T ^{2} | |||
B | 0 < T ≤ 0.1 | 2.94 | 1.89 | 0 < T ≤ 0.1 | 1.0 + 19.4 T | 1.0 + 1.35(T/0.1) |
0.1 < T < 0.3 | 0.88/T ≤ 2.94 | 0.1 < T < 0.3 | 0.88/T ≤ 2.94 | 2.35 | ||
0.3 ≤ T < 0.4 | ||||||
0.4 ≤ T ≤ 1.5 | 1.60(0.5/T)^{0.75} | 0.3 ≤ T ≤ 1.5 | 1.60(0.5/T)^{0.75} | |||
1.5 < T ≤ 3.0 | 1.32/T ^{2} | 1.05/T | 1.5 < T ≤ 3.0 | 1.32/T ^{2} | 1.05/T | |
3 < T | 3.15/T ^{2} | 3 < T | 3.15/T ^{2} | |||
C | 0 < T ≤ 0.1 | 3.68 | 2.36 | 0 < T ≤ 0.1 | 1.3 + 23.8 T | 1.33 + 1.60(T/0.1) |
0.1 < T < 0.3 | 1.25/T ≤ 3.68 | 0.1 < T < 0.3 | 1.25/T ≤ 3.68 | 2.93 | ||
0.3 ≤ T < 0.4 | ||||||
0.4 ≤ T ≤ 1.5 | 2.0(0.5/T)^{0.75} | 0.3 ≤ T ≤ 1.5 | 2.0(0.5/T)^{0.75} | |||
1.5 < T ≤ 3.0 | 1.874/T ^{2} | 1.32/T | 1.5 < T ≤ 3.0 | 1.874/T ^{2} | 1.32/T | |
3 < T | 3.96/T ^{2} | 3 < T | 3.96/T ^{2} | |||
D | 0 < T ≤ 0.1 | 3.68 | 3.0 | 0 < T ≤ 0.1 | 1.1 + 25.8 T | 1.12 + 1.88(T/0.1) |
0.1 < T < 0.3 | 1.98/T ≤ 3.68 | 0.1 < T < 0.3 | 1.98/T ≤ 3.68 | 3.0 | ||
0.3 ≤ T < 0.56 | 0.3 ≤ T < 0.56 | |||||
0.56 ≤ T ≤ 1.5 | 2.4(0.75/T)^{0.75} | 0.56 ≤ T ≤ 1.5 | 2.4(0.75/T)^{0.75} | |||
1.5 < T ≤ 3.0 | 2.97/T ^{2} | 2.14/T | 1.5 < T ≤ 3.0 | 2.97/T ^{2} | 2.14/T | |
3 < T | 6.42/T ^{2} | 3 < T | 6.42/T ^{2} | |||
E | 0 < T ≤ 0.1 | 3.68 | 3.0 | 0 < T ≤ 0.1 | 1.1 + 25.8 T | 1.12 + 1.88(T/0.1) |
0.1 < T < 0.3 | 3.08/T ≤ 3.68 | 0.1 < T < 0.3 | 3.08/T ≤ 3.68 | 3.0 | ||
0.3 ≤ T < 1.0 | 0.3 ≤ T < 1.0 | |||||
1.0 ≤ T ≤ 1.5 | 3.0/T ^{0.75} | 1.0 ≤ T ≤ 1.5 | 3.0/T ^{0.75} | |||
1.5 < T ≤ 3.0 | 4.62/T ^{2} | 3.32/T | 1.5 < T ≤ 3.0 | 4.62/T ^{2} | 3.32/T | |
3 < T | 9.96/T ^{2} | 3 < T | 9.96/T ^{2} |
- Class A: Strong rock. Strong to extremely strong rock, with:
- (a)
Unconfined compressive strength greater than 50 MPa, and
- (b)
An average shear wave velocity over the top 30 m V _{ s,30} > 1500 m/s, and
- (c)
Not underlain by materials having a compressive strength less than 18 MPa or a shear wave velocity less than 600 m/s
- (a)
- Class B: Rock. Rock, with:
- (a)
Unconfined compressive strength between 1 MPa and 50 MPa, and
- (b)
An average shear wave velocity over the top 30 m V _{ s,30} > 360 m/s, and
- (c)
Not underlain by materials having a compressive strength less than 0.8 MPa or a shear wave velocity less than 300 m/s
A surface layer of no more than 3 m 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 30 m criterion, other seismic codes such as EN-1998-1 or ASCE/SEI 7-10 would classify a site with V _{ s,30} > 360 m/s to dense/stiff soil deposits, rather than rock (class B according to EN-1998-1, which corresponds to 360 < V _{ s,30} < 800 m/s, and class C according to ASCE/SEI 7-10, which corresponds to 366 < V _{ s,30} < 762 m/s). 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 (Bouckovalas and kouretzis 2001).
- (a)
- Class C: Shallow soil sites. Sites that are not classified as class A, class B, or class E:
- (a)
The predominant site period estimated as above is T _{ s } ≤ 0.6 s, 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
Representative undrained shear strength, S _{ u } (KPa)
Representative SPT N-values
Maximum depth of soil (m)
Cohesive soils
Very soft
<12.5
–
0
Soft
12.5–25
–
20
Firm
25–50
–
25
Stiff
50–100
–
40
Very stiff to hard
100–200
–
60
Cohesionless soils
Very loose
–
<6
0
Loose dry
–
6–10
40
Medium dense
–
10–30
45
Dense
–
30–50
55
Very dense
–
>50
60
Gravels
–
>50
100
- (a)
- 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 T _{ s } > 0.6 s, or
- (b)
Soil depth exceeds the maximum values listed in Table 2, or
- (c)
Sites that are underlain by less than 10 m of cohesive soil with undrained shear strength S _{ u } < 12.5 KPa or cohesionless soil soils with SPT values N < 6.
- (a)
- Class E: Very soft soil sites:
- (a)
More than 10 m of very soft cohesive soil with undrained shear strength S _{ u } < 12.5 KPa, or
- (b)
More than 10 m of very loose cohesionless soils with SPT values N < 6, or
- (c)
More than 10 m depth of soft-loose soils with shear wave velocity values V _{ s } < 150 m/s, or
- (d)
More than 10 m combined depth of soils with properties described in (a), (b), and (c) above.
- (a)
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 and 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 Organisation 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 halfway 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 = 0.5 s and soil subclass C, considering a design earthquake with a return period of 500 years (Fig. 1c, d). 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 20 km, a seismic scenario corresponding to the low-seismicity areas of New Zealand. The hazard factor at the high-seismicity 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.
Probability factor (AS 1170.4:2007) or return period factor (NZS 1170.5:2004)
Annual probability of exceedance | R |
---|---|
1/2,500 | 1.8 |
1/2,000 | 1.7 |
1/1,500 | 1.5 |
1/1,000 | 1.3 |
1/800 | 1.25 |
1/500 | 1.0 |
1/250 | 0.75 |
1/200 | 0.70 |
1/100 | 0.50 |
1/50 | 0.35 |
1/25 | 0.25 |
1/20 | 0.20 |
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 nonstructural 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 need not 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 1/2,500 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.
Near-Fault Factor
Maximum values of the near-fault factor N _{max}(T) for different structure periods
NZS 1170.5:2004 | Linear interpolation functions | ||
---|---|---|---|
Structure period, T (sec) | N _{max}(T) | Structure period, T (sec) | N _{max}(T) |
≤1.5 | 1.0 | ≤1.5 | 1.0 |
2 | 1.12 | 1.5 < T < 4 | 0.24 T + 0.64 |
3 | 1.36 | ||
4 | 1.60 | 4 ≤ T < 5 | 0.12 T + 1.12 |
≥5 | 1.72 | ≥5 | 1.72 |
Vertical Design Actions
Structural Ductility and Structural Performance Factor-Inelastic Design Spectrum
- (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 nonstructural 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 nonstructural elements and soil-foundation interaction effects.
Ductility factor μ and structural performance factor S _{ p } for different structural systems and specific structure types (After AS 1170.4:2007)
Structural system | Description | μ | S _{ p } |
---|---|---|---|
Steel structures | Special moment-resisting frames (fully ductile)^{a} | 4 | 0.67 |
Intermediate moment-resisting frames (moderately ductile) | 3 | 0.67 | |
Ordinary moment-resisting frames (limited ductile) | 2 | 0.77 | |
Moderately ductile concentrically braced frames | 3 | 0.67 | |
Limited ductile concentrically braced frames | 2 | 0.77 | |
Fully ductile concentrically braced frames^{a} | 4 | 0.67 | |
Other steel structures not defined above | 2 | 0.77 | |
Concrete structures | Special moment-resisting frames (fully ductile)^{a} | 4 | 0.67 |
Intermediate moment-resisting frames (moderately ductile) | 3 | 0.67 | |
Ordinary moment-resisting frames | 2 | 0.77 | |
Ductile coupled walls (fully ductile)^{a} | 4 | 0.67 | |
Ductile partially coupled walls^{a} | 4 | 0.67 | |
Ductile shear walls | 3 | 0.67 | |
Limited ductile shear walls | 2 | 0.77 | |
Ordinary moment-resisting frames in a combination with limited ductile shear walls | 2 | 0.77 | |
Other concrete structures not listed above | 2 | 0.77 | |
Timber structures | Shear walls | 3 | 0.67 |
Braced frames (with ductile connections) | 2 | 0.77 | |
Moment-resisting frames | 2 | 0.77 | |
Other wood- or gypsum-based seismic-force-resisting systems not listed above | 2 | 0.77 | |
Masonry structures | Close-spaced-reinforced masonry^{b} | 2 | 0.77 |
Wide-spaced-reinforced masonry^{b} | 1.5 | 0.77 | |
Unreinforced masonry^{b} | 1.25 | 0.77 | |
Other masonry structures not complying with AS 3700:2001 | 1 | 0.77 | |
Specific structure types | Tanks, vessels, or pressurized spheres on braced on unbraced legs | 2 | 1.0 |
Cast-in-place concrete silos and chimneys having walls continuous to the foundation | 3 | 1.0 | |
Distributed mass cantilever structures, such as stacks, chimneys, silos, and skirt-supported vertical vessels | 3 | 1.0 | |
Trussed towers (freestanding or guyed), guyed stacks, and chimneys | 3 | 1.0 | |
Inverted pendulum-type structures | 2 | 1.0 | |
Cooling towers | 3 | 1.0 | |
Bins and hoppers on braced or unbraced walls | 3 | 1.0 | |
Storage racking | 3 | 1.0 | |
Signs and billboards | 3 | 1.0 | |
Amusement structures and monuments | 2 | 1.0 | |
All other self-supporting structures not otherwise covered | 3 | 1.0 |
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 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) and equal energy theory (valid for shorter structure periods), a transition point is defined at T _{1} = 0.7 s for soil subclasses A to D and at T _{1} = 1.0 s for soil subclass E, where T _{1} is the largest translation period of vibration along the direction under consideration. So, the factor to derive the inelastic response spectra is estimated as:
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 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.
Characteristics of the typical strong motion recordings from the Christchurch 22 February 2011 M = 6.3 earthquake (Source: www.geonet.org.nz)
Site code | Subsoil class acc. to NZS 1170.5:2004 | Epicentral distance | PGA-horiz.1 (g) | PGA-horiz.2 (g) | PGA-vert. (g) |
---|---|---|---|---|---|
CBGS | D | 7 | 0.553 | 0.452 | 0.360 |
PRPC | E | 6 | 0.669 | 0.595 | 1.88 |
CHHC | D | 6 | 0.345 | 0.364 | 0.601 |
The elastic response spectra of the horizontal components of these three recordings are plotted in Fig. 4a, in comparison with the design response spectra of NZS 1170.5:2004. A band-pass filter in the frequency ranges of 0.10–0.25 Hz and 24.5–25.5 Hz 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.
Scaling factors for the considered typical strong motion records
Site code | Component | k _{1} | D _{1} | k _{2} |
---|---|---|---|---|
CBGS | N89W (principal) | 0.608 | 0.070 | 1.05 |
S01W | 0.789 | 0.105 | ||
PRPC | W (principal) | 0.672 | 0.081 | |
S | 0.785 | 0.097 | ||
CHHC | N01W | 0.683 | 0.064 | |
S89W (principal) | 0.598 | 0.058 |
The record family scale factor, k _{2}, ensures that the principal component of at least one record spectrum (after being scaled by k _{1}) exceeds the target spectrum over the period range of interest. It is estimated as the maximum value of the ratio SA _{target}/max(SA _{principal}) ≥ 1.0 within the period range of interest, where max(SA _{principal}) 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 < k _{2} ≤ 1.3. If k _{2} > 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 k _{1} k _{2}. 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 k _{1} may be greater for that particular strong motion component, the product k _{1} k _{2} may be lower. In the case at hand however, the family factor k _{2} is within the desirable range (Table 7), a fact that can be confirmed visually from Fig. 4.
Finally, note that NZS 1170.5:2004 (as also ASCE/SEI 7-10) bases the accelerograph scaling procedure on spectral values within the range of interest only and does not impose restrictions on the zero period spectral response acceleration values of the strong motion, as, e.g., EN-1998-1.
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 =0.22 g to Z = 0.30 g in the latest compliance document of the New Zealand Building Code (Department of Building and Housing 2011).
Cross-References
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