7.b Analysis/Design of Buildings
Performance Objectives for Low Damage Seismic Design of Buildings
H.John Hare, Stuart J. Oliver & Bruce D. Galloway
Low damage design is emerging as a way forward for building designers to implement damage reduction measures into seismic design. Although some low damage measures can be incorporated into conventional structural systems, most research is concentrated on developing new structural systems or devices which can deliver improved building performance. However, not all of the proposed systems will provide low damage outcomes, in the broadest sense.
The performance of buildings in the Canterbury earthquakes is discussed in relation to desired future outcomes, with an outline of the required limit states to be considered. A set of general performance objectives is proposed that such systems can be measured against. Design methodologies are discussed, noting that current Building Code approaches will generally not provide adequate verification methods. In addition, some consideration of future development criteria is offered, with the aim of ensuring that the credibility of the industry is maintained.
Nonlinear Analysis Acceptance Criteria for the Seismic Performance of Existing Reinforced Concrete Buildings
S.J. Oliver, A.G. Boys & D.J. Marriott
Poor performance of some buildings in the 2011 Christchurch earthquakes has re-emphasised the need for structural resilience. This has been addressed in the Detailed Engineering Evaluation (DEE) procedure developed by the DBH Engineering Advisory Group which requires consideration of building performance at the ultimate limit and collapse limit states.
The importance of providing a full performance-based seismic assessment considering both Life Safety (ultimate limit state) and Collapse Prevention performance objectives has become evident. Such an assessment allows progressive levels of structural damage to be defined and captures critical structural weaknesses. No specific guidance is provided in the DEE procedure for nonlinear analysis procedures.
Furthermore, the Life Safety and Collapse Prevention limit states can be difficult to quantify for older RC structural elements with low transverse reinforcement contents and poor structural detailing. Experimentally-derived acceptance criteria from test data-bases are preferred for structures prevailing modern design codes. A nonlinear analysis performance assessment criteria has been developed using New Zealand and North American research. The criteria provides recommendations on ground motion record scaling and the application of the NZS 1170.5 structural performance factor for existing buildings. Individual building component and overall building system performance is considered. Building component deficiencies have been classified recognising that the failure of individual building components does not always constitute a building system failure.
Inter-storey Drift Limits for Buildings at Ultimate limit States
S.R. Uma, A.B. King & T. Holden
Design engineers often use quasi-static procedures to analyse buildings and to determine the inter-storey drift demands rather than inelastic time history analysis procedures. For elastic procedures such as the modal response spectrum method, the NZS1170.5 loading standard suggests modification (or scaling) factors for simulating inter-storey drift in the post-elastic range corresponding to an ultimate limit state condition. In this study, the consistency between inter-storey drifts obtained from modal response analysis and inelastic time history analysis are verified, and suitable scale factors for design purposes are suggested.
Building models representing reinforced concrete and steel braced frames with various levels of ductility were analysed for a family of earthquake records matched to the design spectrum. The design of buildings was carried out to achieve a target drift ratio of 2.5% which is the maximum limit suggested in the standard. The inter-storey drift profiles for reinforced concrete buildings from inelastic time history analysis basically followed the inter-story drift profiles derived from the elastic method but exceeded them at upper storeys. In steel braced frames, the inelastic displacement was limited to the lower 1/3rd of building height only. Even though the inelastic methods resulted in higher inter-storey drifts compared to elastic methods, all values were within the prescribed code limit of 2.5%.
The influence of pounding on member demands in lowrise buildings
G.L. Cole, R.P. Dhakal, A.J. Carr & D.K Bull
While pounding between insufficiently separated buildings is commonly acknowledged to occur during earthquakes, very little information on pounding's loading effects are currently available. This paper presents a numerical study of two Wellington buildings with various separations that result in pounding. Both buildings are modelled as 1960's, three storey buildings with concrete moment resisting frames. Member shear and ductility demands are recorded and compared to each building's demands when the buildings are sufficiently separated to prevent pounding. Additionally, the collision force at the point of contact is compared between models. The effects of soil structure interaction are also investigated for pounding models. Pounding is found to increase or decrease ductilities and interstorey shears depending on the specific configuration.
Interstorey shears induced by pounding are found to exceed 'no pounding' configurations by up to 35%. The implications and likely loadings due to pounding for low rise Wellington buildings are also discussed and tentatively quantified.
Estimated seismic performance of a standard NZS3101:2006 RC office building during the 22 February 2011 Christchurch earthquake
Y. Ishikawa & B.A. Bradley
This paper discusses the seismic performance of the standard RC office building in Christchurch that is given as a structural design example in NZS3101, the concrete structures seismic standard in New Zealand. Firstly the push-over analysis was carried out to evaluate the lateral load carrying capacity of the RC building and then to compare that carrying capacity with the Japanese standard law. The estimated figures showed that the carrying capacity of the New Zealand standard RC office building of NZS3101:2006 was about one third of Japanese demanded carrying capacity. Secondly, time history analysis of the multi-mass system was performed to estimate the maximum response story drift angle using recorded ground motions. Finally, a three-dimensional analysis was carried out to estimate the response of the building to the 22nd February, 2011 Canterbury earthquake. The following outcomes were obtained. 1) The fundamental period of the example RC building is more than twice that of Japanese simplified calculation, 2) The example building’s maximum storey drift angle reached 2.5% under the recorded ground motions. The main purpose of this work is to provide background information of seismic design practice for the reconstruction of Christchurch.
Influence of Stiffness Variation on the Performance of Houses in Earthquakes
A.Z. Liu & G.J. Beattie
There are about one and half million light timber framed (LTF) buildings nationwide in New Zealand and satisfactory seismic performance of LTF buildings in major earthquakes is crucial in terms of the community's resilience to earthquakes. Observed earthquake damage to light timber framed (LTF) buildings demonstrates that, while LTF buildings generally perform well in major earthquakes, they do suffer extensive damage if there are significant stiffness incompatibilities or there is severe land damage, including liquefaction. There can be various stiffness incompatibilities and the effects of these on the seismic performance of LTF buildings vary. This paper reports on a study of the effects of broad stiffness incompatibility issues on the seismic performance of LTF houses, based on earthquake damage observations and theoretical examinations of critical seismic aspects. The study revealed the following: (a) a timber floor diaphragm plays an important role in allocating seismic actions to different lateral load resisting systems and the floor deformation has to be considered if there is a stiffness incompatibility, (b) timber floor diaphragms need to be adequately designed to meet their required function, (c) a displacement-based design approach is more rational when there is a significant deformation incompatibility issue.
Observations on the Performance of Residential Concrete Slabs under Seismic and Soil Liquefaction Conditions
C.W. Ashby
The humble domestic concrete slab is a very important component in getting things right first time. Because there are so many of them, the cost of poor detailing and construction techniques have had a significant effect on the economy and wellbeing of the community running into billions of dollars worth of damage in Christchurch and causing displacement and trauma to people unnecessarily.
However many habitable houses with unreinforced cracked concrete floors are being written off unnecessarily, due to the wording of insurance policies and ignorance with respect to the reparability of such.
Unreinforced slabs are now no longer permitted for new construction of residential houses, and experience has proved the worth of waffle slabs. There is however still room for improvement if a cost effective base isolation system can be developed to protect occupants, superstructure and contents.