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Designing structures for earthquake loadings

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					ACEA        ACEA CPD SEMINAR   Sydney, November 2006




       Designing Structures for

        Earthquake Loadings



              Simon Matthews



                                      M+G Consulting
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.




                                                                          M+G Consulting
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.




                                                                           M+G Consulting
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-bearing 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.



                                                                         M+G Consulting
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.



                                                                         M+G Consulting
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 - don’t 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.


                                                                         M+G Consulting
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 it’s up to us
       and the councils!!!

                                                                         M+G Consulting
ACEA                      ACEA CPD SEMINAR                           Sydney, November 2006
                      Designing Structures for Earthquake Loadings



       Scope & General (Section 1) -
       what AS 1170.4 doesn’t 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.

                                                                            M+G Consulting
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 (Z=0.60).




                                                                         M+G Consulting
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



                                                                             M+G Consulting
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 …




                                                             M+G Consulting
ACEA       ACEA CPD SEMINAR                           Sydney, November 2006
       Designing Structures for Earthquake Loadings



                                                       Design
                                                       Procedures
                                                       (Section 2)


                                                       Simplified
                                                       clause
                                                       5.4.2.3




                                                             M+G Consulting
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 doesn’t 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.


                                                                         M+G Consulting
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.




                                                                        M+G Consulting
ACEA                 ACEA CPD SEMINAR                           Sydney, November 2006
                 Designing Structures for Earthquake Loadings


       Site Hazards (Section 3)




                                                                       M+G Consulting
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 society’s 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.



                                                                         M+G Consulting
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.




                                                                        M+G Consulting
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.




                                                                        M+G Consulting
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:




                                                                           M+G Consulting
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:




                                                                         M+G Consulting
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.




                                                                         M+G Consulting
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:

         Important 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).




                                                                            M+G Consulting
ACEA                  ACEA CPD SEMINAR                           Sydney, November 2006
                  Designing Structures for Earthquake Loadings

       Clause 5.2.3 - Performance under EQ deformations - a very
       important clause




                                                                        M+G Consulting
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 (EDC’s)

       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).




                                                                         M+G Consulting
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.




                                                                        M+G Consulting
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).


                                                                         M+G Consulting
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.




                                                                         M+G Consulting
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.




                                                                         M+G Consulting
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)




                                                                         M+G Consulting
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.


                                                                         M+G Consulting
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)




                                                                         M+G Consulting
ACEA                   ACEA CPD SEMINAR                           Sydney, November 2006
                   Designing Structures for Earthquake Loadings


       Incorrect definition for kt = 0.11 and 0.075…




                                                                         M+G Consulting
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)




                                                                         M+G Consulting
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.




                                                                         M+G Consulting
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.

                                                                          M+G Consulting
ACEA                       ACEA CPD SEMINAR                           Sydney, November 2006
                       Designing Structures for Earthquake Loadings


       Design of Parts and Components (Section 8)
       Three methods may be used:

       1.   Established principles of structural dynamics – you are out on
            your own!
       2.   The general method of Cl. 8.2
       3.   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.

                                                                             M+G Consulting
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.




                                                                        M+G Consulting
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.




                                                                        M+G Consulting
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.




                                                                        M+G Consulting

				
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