Seismic assessment of reinforced concrete buildings in Australia including the response of gravity frames

Abstract

Over recent years there has been a growing need to assess the seismic performance of potentially vulnerable buildings in Australia in order to make informed risk mitigation decisions. The aim of this study has been to assess the seismic performance of in-situ reinforced concrete (RC) buildings under 10-storeys high and constructed prior to 1995. This class of buildings has been identified to be particularly vulnerable to earthquakes due to the relatively low natural periods of buildings in this height range, and lack of consideration given to seismic design and detailing. The buildings assessed have core walls as the lateral load resisting system, also referred to as the primary system, and perimeter moment resisting frames together with band-beam or flat-slab floor systems as the gravity load resisting system, also referred to as the secondary system. There have been concerns about the displacement compatibility between the primary and secondary system and the loss of axial load carrying capacity of the gravity system (resulting in the total collapse of the building) prior to or immediately after the primary system loses its lateral load carrying capacity. Therefore, the assessment of the buildings conducted in this study has involved the seismic performance of both the lateral load resisting system and the gravity load resisting system.

The assessment procedure adopted in this study has been in line with the performance-based earthquake engineering (PBEE) assessment framework. The main outcome of the study has been the development of fragility curves, thus the three key stages of the PBEE assessment process have been undertaken, including: hazard analysis, structural analysis, and damage analysis.

The hazard analysis stage involved conducting a review of the seismic hazard models developed for Australia. The most recent models which have been proposed for the Australian earthquake loading standard, AS 1170.4, have resulted in significant reductions in hazard values in comparison to the current hazard model in AS 1170.4:2007. However, there has been a lack of consensus amongst researchers about the changes and hence the proposed models are still under development. Therefore, in this study the evaluation of the seismic performance of the buildings has been based on the current hazard model in AS 1170.4:2007.

Furthermore, a detailed study was conducted to investigate the validity of the method used in AS 1170.4:2007 for incorporating local site effects. The study demonstrated the importance of considering site period, rather than the average shear wave velocity up to a depth of 30 m, in understanding the seismic site response. Based on the findings, re-classification of the site classes in AS 1170.4:2007 were recommended, with an emphasis on site period as a key criteria in classifying soil/soft rock classes. A new systematic method was also proposed for obtaining the displacement response spectra. This method helps to significantly improve the prediction of displacement response in the short period range up to the second corner period at which maximum spectral displacement response occurs. The structural analysis phase was conducted by developing archetypal buildings for assessment. An investigation was conducted to obtain the history of the design of RC buildings in Australia to provide an understanding of the existing building stock, including typical building configurations and design detailing. This was achieved by speaking to and corresponding with experienced practicing structural engineers and from reviewing older editions of the Australian concrete structures and loading standards. Based on the findings six archetypal buildings were designed. Three buildings heights were investigated; 2-, 5-, and 9-storeys, and for each building height two plan configurations were analysed; one with plan symmetry and the other with plan asymmetry.

The expected governing failure mechanisms of the building components were identified, including the core walls, beam-column joints, columns and beams. This was achieved by reviewing reconnaissance reports and experimental studies for buildings and building components with similar detailing to the archetypal buildings. In general, it was identified that the building components were vulnerable to brittle failures and experiencing significant strength and stiffness degradation after the peak capacity was reached. This is because the buildings which have been assessed have a poor quality of detailing; typically referred to as non-ductile or seismically non-conforming detailing in the literature.

In order to develop the 3-dimensional nonlinear models of the archetypal buildings a critical review of the existing state-of-the-art approach for modelling non-ductile RC building components in a macro-finite element modelling space was conducted. The modelling methods investigated included the incorporation of the inelastic response of beam-column joints, flexural response of members, bar-slip of longitudinal reinforcement bars, flexure-shear behaviour of members, and methods for modelling non-planar walls. The review revealed that there was a lack of consensus amongst researchers about the best method to model the various building components for the purpose of global analysis. Therefore, various modelling approaches were evaluated by examining their performance against three key criteria: accuracy, computational efficiency, and numerical stability and reliability. The accuracy of the various models was predominantly examined at a component level by comparing simulated results with experimental results. The computational efficiency, and the numerical stability and reliability of the various modelling approaches were examined based on running simulations at a component level and then at a system level where the approach was utilised in 2D and 3D nonlinear models. In general, it was evident that the two competing modelling approaches suitable for global analysis could be broadly characterised as the distributed plasticity and the lumped plasticity approaches. Based on the three key criteria, the lumped plasticity approach was selected to be the most suitable and reliable approach for global analysis of non-ductile buildings analysed up to the point of axial load failure of the building components.

Nonlinear dynamic time history analyses were conducted to obtain the seismic performance of the archetypal buildings. The ground motion records were selected such that they were characteristic of Australian earthquakes, they include a combination of: stochastically generated rock records, historical records, and simulated records on various site conditions. Damage analysis of the buildings was achieved by using the cloud analysis method to develop the probabilistic seismic demand model of the buildings at various performance levels. Careful consideration was given to selecting suitable intensity measure and accounting for various uncertainties to develop the fragility curves. Finally, the seismic performance of the buildings was evaluated by comparing the response obtained from fragility curves with respect to performance objectives. The results illustrated that the plan-asymmetric buildings and the 2-storey plan symmetric buildings were particularly vulnerable to exceeding the Life Safety limit state under a 500 year return period event and the Collapse Prevention limit state under a 2500 year return period event if located on soft sites.