Earthquake Loadings Simon Matthews M+G Consulting Designing

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Designing Structures for
Earthquake Loadings
ACEA
ACEA CPD SEMINAR
Sydney, November 2006
Simon Matthews
MG Consulting
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Reasons for New Code

• AS1170.4 - 1993 Thirteen years old.
• Update to current technology and design practices
• Align with the Importance levels of the BCA
  note it is the ABCB whom set societal
  expectations of design recurrence of extreme
  events.
• Originally it was going to be a combined code
  with New Zealand as the other loading codes are.
  In 2004 it was decided to have separate loading
  codes. In 2004 New Zealand issued NZS
  1170.5-2004.

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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Major Changes from AS 1170.4 1993 to AS
1170.4-200?
Importance levels from BCA used defined by the
ABCB.   Soil descriptors changed - harmonised
with NZ 1170.5 2004.   New site sub-soil
spectra, higher for short period structures,
lower for long period structures generally high
EQ loads for most building structures. Higher
structures least effected and wind loads will
likely govern taller structures.   Earthquake
design categories introduced EDCI, EDCII
EDCIII. Minimum or no EQ design or detailing
for low height structures at low risk sites
e.g. structures less then 12m high on rock in
Sydney.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Simplified rules for most structures under 15m
high no seismic analysis required prescribed
loads.   Ordinary moment resisting frames allowed
above 50m - AS 1170.4 1993 disallowed.   Load-bea
ring un-reinforced masonry structures not allowed
more then 12-15m high depending on soil type. EQ
Detailed design moved to AS 3700 This clarifies
the intent of the old code that was no always
observed.   Non-load bearing un-reinforced
masonry (URM) is allowed in buildings over 12m
provided a separate seismic resisting exists and
the URM elements allow the system to effectively
resist the earthquake actions.   Structural
Response factors changes - Sp and ? are used
aligned with NZS 1170.5-2004 and other
international codes.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Structural Performance Factor Sp is a (fudge)
factor to account for the fact that structures
tend to have more capacity then accounted for in
analysis. This is due to many factors and
represents the change in dynamic response of
buildings to undergo plastic deformations. Less
ductile structures tend to have higher Sp values.
  Structural Ductility factor ? is a measure of
the ability of structural system to sustain
inelastic displacements. Consequently, the higher
the design ductility the more onerous the
detailing that is required. The ductility chosen
for the design of a structure must be matched to
the level of detailing. The material standards AS
3700, AS 3600 AS 4100 etc. are expected to be
amended to provide the required detailing to
achieve different ?s. AS 1170.4-200? Has a upper
limit of ?3.   Earthquake engineering is all
about designing structures so that they can
absorb the energy of the EQ by deforming.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Simplified calculation of accidental torsion and
much easier for structural design packages to be
incorporated, only related to centre of mass -
dont have to calculate the centre of rigidity
(shear centre).   Also clarification of the
action of this accidental torsion in multi-storey
buildings this - 0.1b eccentricity shall be
applied in the same direction at all levels and
orientated to produce the most adverse torsional
moment for the 100 and 30 load.   Stability
clause of AS 1170.4 1993 deleted was ambiguous
less design required now. The issue of a
structure being irregular or regular has been
removed from AS1107.4 2000?. This standard
assumes that all structures are irregular as it
is very unusual to find a structure that meets
the requirements of regularity. But it should be
noted that the more irregular the building is,
the worse it will perform when subjected to an
earthquake.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Restriction on the results of a Modal Spectra
Analysis deleted - this is the one that required
you to scale up the results of the to 90 or 100
of the results obtained by the Equivalent Static
Analysis more use of these analysis method will
probably be made as most computer structural
analysis packages have this facility. Vertical
components of the EQ action are generally not
required to be accounted for except for parts
components (Section 8) less design input
required.   New simplified design method for
parts components (Section 8) that will
generally result in lower loads and less complex
design.   Changes to terms and definitions to
align with international codes.   Structural
Alterations Section 8 and Appendix E of AS 1170.4
1993 removed, as the BCA does not cover existing
structures. This is a big issue for us
structural engineers as now its up to us and the
councils!!!
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Scope General (Section 1) - what AS 1170.4
doesnt cover
High risk structures.   Bridges refer to AS
5100.2-2004 which is aligned with AS 1170.4-1993
. Tanks containing liquids use the NZ red
book, a new addition soon to be released, and NZ
codes.   Civil structures i.e. dams, bunds
etc.   Offshore structures special studies
required. Earth-retaining structure Refer to
AS 4678 - note that this code is not referenced
by the BCA.   Structures with 1st mode period
greater than 5 sec i.e. inverted cantilever
structures use NZS 1170.5.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Structures with high ductility demand i.e.
structural ductility factor ? ? 3 use NZ 1170.5
and appropriate material standard for
detailing.   Domestic Structures (Type 1a 1b as
defined in the BCA) which are less then 8.5m high
and located on sites with a Hazard factor (Z) ?
1.0 and a probability factor (kp) of 1.0 (i.e.
annual probability of exceedance 1/500) are
deemed to satisfy this standard (?) - they do not
require any design nor detailing for earthquakes
is this a good thing?   Importance level 1
structures i.e. farm buildings, minor temporary
structures etc no design or detailing
requirements for EQ required.   Structures
constructed on a site with a Hazard (Z) factor ?
0.3 i.e. Macquarie Islands (Z0.60).
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Effect of related phenomena such as settlement,
slides, subsidence, liquefaction or faulting.  
Seismic weight and Building Height
Figure 1.5 (B) Example of determination of the
top of the structure
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Location of base shear reaction fig 1.5(C) note
sloping site but influence of earth pressures
on high side soil structure interaction
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Design Procedures (Section 2)
Simplified clause 5.4.2.3
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Importance Level 1 structures, Domestic
Structures which meet Appendix A are deemed to
satisfy the standard even though if you analysed
them in accordance with the standard they may
not.   The ABCB are very keen that the price of
housing does not increase lower societal
expectation?   For all structures sited on
sub-soil type E (worst sub-soil class) except
Appendix A complying structures (domestic
housing) the design should consider the effects
of subsidence or differential settlement of the
foundation material under EQ actions e.g.
liquefaction. The code doesnt tell you what to
do - hopefully it will be in the commentary.
Note the parts and components including
non-loading bearing walls for out-of-plane EQ
forces (can be substantial) are required to be
designed for EQ loads there was some confusion
with this in the old code AS 1170.4 1993.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Serviceability limit states are deemed to be
satisfied for EQ actions for importance levels,
1, 2 3 designs in accordance with this Standard
and the appropriate materials Standard.   A
Special Study is required for importance level 4
structures to ensure that they remain serviceable
for immediate use following the design event for
importance level 2 structures.reference should
be made to NZS 1170.5-2004 and its
commentary. Importance Level 1 structures,
Domestic Structures which meet Appendix A are
deemed to satisfy the standard.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Site Hazards (Section 3)
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The return periods of 500 800 yrs come from AS
1170.4 1993.   NZ and most of the world design
structures for earthquakes with return periods in
the order of 1000 yrs (130 of 500 yr) and 2500
yrs (180 500 yr) depending on the importance of
the structure note this is not up to the code
committee. ABCB is responsible for determining
the societys expectations (unlike NZ
practice).   It is my understanding that a
recommendation will be made to bring these into
align with world practice but this may have cost
implications which may be unacceptable to the
ABCB.   The Hazard factor is now Z - it used to
be a in AS 1170.4 1993.   It is still the
acceleration co-efficient with an annual
probability of exceedance in 1/500 (i.e. a 10
probability of exceedance in 50 years.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The values have not significantly changed from AS
1170.4 1993 (Sydney is still 0.08) and the maps
are the same.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
It should be noted that Australian Earthquakes
are intra-plate earthquakes and there is some
argument that due to our relatively short period
of EQ records the earthquake hazard of Australia
is similar throughout. The contours concentrate
on locations of recent earthquakes.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Site Sub-Soil Class (Section 4)
Five site sub-soil classes have been introduced
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
These are defined in line with International
practice and a hierarchy for site classification
is given along with a method for evaluating
periods for layered sites.   Normalized Response
Spectra for the sub-soil class
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Note that the upper bound values of the Spectral
Shape Factor for sub-soil classes C, D E are
the same, which means that for single-storey
buildings (low period structures) the earthquake
forces will be the same for structures founded on
these three classes. Site classification for
piled foundations generally the response of
piled structures where the piles extend through
soil to a stronger, less flexible layer. It is
the weaker layers that drive the structural
response (not the stronger layer) unless the
piles are decoupled (isolated) from the weaker
layer or are raked.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Earthquake Design (Section 5)
Ductility, Ductility, Ductility   Basic Design
principles applicable for all structures   Import
ant to have a clearly defined load path or
preferably load paths for earthquake loads to be
transferred to the foundation soils.   All parts
of the structure shall be tied together both in
horizontal and vertical planes.   Footings shall
be tied together where the foundation soils are
low strength (ultimate vertical bearing value
less than 250 kPa, as per AS 1170.4-1993).
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Clause 5.2.3 - Performance under EQ deformations
- a very important clause
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Diaphragms Sometimes these are the weak link,
due to them being too flexible or due to large
penetrations. A good paper on the designing
them is Bull, D.K., Understanding the
Complexities of Designing Diaphragms in Buildings
for Earthquakes, Bulletin of NZSEE, Vol 37, No
2. June 2004. Earthquake Design Category
(EDCs)   Three Design Categories EDC I, EDC II
EDC III. A higher level of analysis then
specified in Table 2.1 for a particular EDC may
be used. This may be used possibly to reduce EQ
loads by say doing a Modal Analysis (Dynamic,
Section 7) instead of an Equivalent Static
Analysis (Section 8).
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
EDC I A simple lateral load applied at each
level (0.1 x seismic weight at that level). No
earthquake load analysis is required.   Only
applicable for structures less than 12m
high.   Vertical EQ actions, except for parts
components need not be considered.   Pounding
need not be considered i.e. no boundary set backs
required.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
EDC 2 Requires an Equivalent Static Analysis to
be performed (a dynamic analysis can be
used).   For structural components which
participate in resisting EQ forces in both the
major axes of the structure, the effects of the
two directions determined separately shall be
added by taking 100 of the horizontal EQ forces
for one direction and 30 in the perpendicular
direction.   Torsion is required to be
considered.   Connections between components of
the structure are to be designed for the
calculated loads from the analysis but not less
that 5 of the vertical reaction arising from the
seismic weight or 5 of the seismic weight of the
component whichever is greater (robustness, load
paths and tying together).
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
For structures that do not exceed a height of 15m
a simplified design method is allowed which
specifies the load to be applied at each level
without doing a detailed seismic
analysis.   Inter-storey drift at the ultimate
state is not to exceed 1.5 of the storey height
for each level. This is deemed to be satisfied if
the primary force-resisting elements are
structural walls that extend to the base of the
structure.   Attachment of cladding etc. should
be designed to accommodate the seismic drifts,
noting that the actual drifts will be larger then
the calculated elastic drifts by a factor of ?/Sp.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Pounding needs to be considered for buildings
over 15 m height. This is deemed to be satisfied
if the primary force-resisting elements are
structural walls that extend to the base of the
structure or the set back is more than 1 of the
structural height. (Note this is 150 mm for a 15m
building).   Parts and components have to be
designed in accordance with Section 8 except
that, for buildings less then 15 m height and
importance level 2 or 3, parts and components of
non-brittle construction may be designed for a
horizontal capacity of 10 of their seismic
weight.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
EDC 3 This design category requires a full
dynamic analysis as given in Section
7.   Connections between components and drift
requirements are as per EDC II.   Pounding is
deemed to be satisfied with a set back of 1 of
the structure height maybe an issue with
clients.   Parts and components to be designed in
accordance with Section 8.     Equivalent Static
Analysis (Section 6)
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Very similar to AS 1170.4-1993 except with
different notation.   EQ base shear note no
requirements to calculate base overturning moment
as was required by AS 1170.4-1993.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The calculations for the seismic weights are
similar to AS 1170.4-1993.   The natural period
of the structure can be obtained from a new
formula (from the Eurocode 8) or by a rigorous
structural analysis (computer programme) but the
base shear obtained from a rigorous analysis
shall not be less than that obtained with the
period as calculated by eg 6.2(7)
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Incorrect definition for kt 0.11 and 0.075
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The vertical distribution of the horizontal EQ
force is also similar to AS 1170.4-1993.   The
spectral shape factor (Ch(T)) is obtained from
Table 6.4 for the soil sub-type.   The Structural
ductility (?) and Structural Performance Factor
(Sp) shall be determined from the appropriate
material standard or as given in Table 6.5 (A) or
6.5 (B)
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Note that a lower structural ductility factor may
always be chosen, also both ? Sp may be
determined by a non-linear static push-over
analysis - hopefully the commentary will give
information on this.   Torsional Effects.   Very
much simplified. Now 0.1 b from the centre of
mass.   Inter-story drift calculated taking into
account ? Sp.   P-delta effects need to be
considered if you have a flexible
structure.   Can be an issue if you are using
large ductilities.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Dynamic Analysis (Section 7) Either a
modal-response spectrum analysis or a time
history analysis can be used.   There is very
little direction given for the time history
analysis, it is expected that this method will
not be used by many practitioners.   Most
structural analysis packages can do a
modal-response spectrum analysis.   The site
hazard spectrum of section 6 can be used or a
site-specific design response spectrum can be
used.   The requirements of the modal-response
spectrum analysis is very similar to AS
1170.4-1993 except that there are no requirements
to scale the results in accordance with the
Equivalent Static Analysis and the calculation of
the effects of torsion are simplified.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings

• Design of Parts and Components (Section 8)
• Three methods may be used
• Established principles of structural dynamics
  you are out on your own!
• The general method of Cl. 8.2
• The simplified method (Cl. 8.3)
• Forces on any component shall be applied
  at the centre of gravity of the component in any
  horizontal direction. Vertical EQ forces of 50
  of the horizontal EQ force shall be used for all
  mechanical electrical equipment.
• Mechanical connectors for curtain walls,
  external walls and walls enclosing stairs, lifts
  and exits etc. are to be designed for 1.5 times
  the design force of the supported element.
•  
• Clause 8.1.4 defines all the parts and
  components and their connections shall be
  designed in accordance with this section.

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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The general method of clause 8.2 requires the
calculation of the effective floor acceleration
at the level where the component is situated.
This means that some knowledge of the structure
is required to calculate it.   The simple method
does not require any specific information of the
building other than the height at which the part
is attached and the structural height of the
building.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
Structural Alterations   Section 8 and Appendix
E, Structural Alterations of AS 1170.4-1993 has
not being repeated, as the BCA does not extend to
existing structures.   It means that there is now
no direction in this regard. Unfortunately many
of use are involved with designing additions and
alterations to existing buildings.   There is the
Australian Standard AS 3826-1998 Strengthening
Existing Buildings for Earthquakes, this has not
been referenced by the BCA nor adopted by many
councils.   It provides a useful guide for
assessing and strengthening an existing building.
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ACEA
ACEA CPD SEMINAR
Sydney, November 2006 Designing
Structures for Earthquake Loadings
The issue of what EQ loads that an existing
building should be able to resist is always a
very difficult question.   Recently New Zealand
passed regulations that required earthquake
prone buildings to be upgraded. The New Zealand
Society for Earthquake Engineering recently
published Assessment and Improvement of the
Structural Performance of Buildings in
Earthquakes, June 2006. It can be downloaded
free at http//www.nzsee.org.nz/PUBS/pubs.shtml .
This lengthy publication has much information on
the topic.
MG Consulting