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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