Ground motion selection for seismic response analysis

Poster 47The rigorous selection of ground motions is an important consideration in a seismic risk assessment as it provides the link between seismic hazard (seismology) and seismic response (earthquake engineering). Despite the fact that many studies have highlighted the differences between the uniform hazard spectrum (UHS) and individual earthquake scenarios, the UHS is still the primary method by which ground motion records are selected and scaled. The conditional mean spectrum (CMS) is one alternative to the UHS for ground motion selection which provides the mean response spectral ordinates conditioned on the occurrence of a specific value of a single spectral period, and is directly linked to probabilistic seismic hazard analysis (PSHA). There are however several limitations in the use of the CMS for ground motion selection, which primarily stem from the fact that spectral accelerations provide only a partial picture of the true character of a ground motion. Based on the identified limitations of the CMS the objective of this work was to develop what is referred to as a generalised conditional intensity measure (GCIM) approach, which allows for the construction of the conditional distribution of any ground motion intensity measure. A holistic method of ground motion selection was also developed based on the comparison of the empirical distribution of a ground motion suite and the GCIM distributions

OpenSHA implementation of the GCIM approach for ground motion selection

Ground motion selection is known to be an important step in seismic hazard and risk assessment. There have been numerous procedures proposed for selecting ground motions ranging from somewhat ad-hoc guidelines specified in seismic design codes to more rigorous approaches which have found favour in the research-community, but are not yet applied routinely in earthquake engineering practice. The most common method (often specified in seismic design codes) for selecting ground motion records for use in seismic response analysis is based on their "fit" to a Uniform Hazard Spectrum (UHS). This is despite the fact that many studies have highlighted the differences between the UHS and individual earthquake scenarios, and therefore its inappropriateness for use in ground motion selection. The reluctance of the earthquake engineering profession to depart from UHS-based selection of ground motions is arguably because of its simplicity to implement relative to methodologies with sounder theoretical bases. To this end, the aim of the present work was to implement a recently developed Generalised Conditional Intensity Measure (GCIM) approach for ground motion selection (Bradley, 2010) into the open-source seismic hazard analysis software OpenSHA (Field et al. 2003)

Galactic Archaeology and Minimum Spanning Trees

Chemical tagging of stellar debris from disrupted open clusters and associations underpins the science cases for next-generation multi-object spectroscopic surveys. As part of the Galactic Archaeology project TraCD (Tracking Cluster Debris), a preliminary attempt at reconstructing the birth clouds of now phase-mixed thin disk debris is undertaken using a parametric minimum spanning tree (MST) approach. Empirically-motivated chemical abundance pattern uncertainties (for a 10-dimensional chemistry-space) are applied to NBODY6-realised stellar associations dissolved into a background sea of field stars, all evolving in a Milky Way potential. We demonstrate that significant population reconstruction degeneracies appear when the abundance uncertainties approach 0.1 dex and the parameterised MST approach is employed; more sophisticated methodologies will be required to ameliorate these degeneracies

A generalized conditional intensity measure approach and holistic ground motion selection

The rigorous selection of ground motions is an important consideration in a seismic risk assessment as it provides the link between seismic hazard (seismology) and seismic response (earthquake engineering). Despite the fact that many studies have highlighted the differences between the uniform hazard spectrum (UHS) and individual earthquake scenarios, the UHS is still the primary method by which ground motion records are selected and scaled. The conditional mean spectrum (CMS) is one alternative to the UHS for ground motion selection which provides the mean response spectral ordinates conditioned on the occurrence of a specific value of a single spectral period, and is directly linked to probabilistic seismic hazard analysis (PSHA). There are however several limitations in the use of the CMS for ground motion selection, which primarily stem from the fact that spectral accelerations provide only a partial picture of the true character of a ground motion. Based on the identified limitations of the CMS the objective of this work was to develop what is referred to as a generalised conditional intensity measure (GCIM) approach, which allows for the construction of the conditional distribution of any ground motion intensity measure. A holistic method of ground motion selection was also developed based on the comparison of the empirical distribution of a ground motion suite and the GCIM distributions

Error estimation of closed-form solution for annual rate of structural collapse

With the increasing emphasis of performance-based earthquake engineering (PBEE) in the engineering community, several investigations have been presented outlining simplified approaches suitable for performance-based seismic design (PBSD). Central to most of these PBSD approaches is the use of closed-form analytical solutions to the probabilistic integral equations representing the rate of exceedance of key performance measures. Situations where such closed-form solutions are not appropriate primarily relate to the problem of extrapolation outside of the region in which parameters of the closed-form solution are fit. This study presents a critical review of the closed form solution for the annual rate of structural collapse. The closed form solution requires the assumptions of lognormality of the collapse fragility and power model form of the ground motion hazard, of which the latter is more significant regarding the error of the closed-form solution. Via a parametric study, the key variables contributing to the error between the closed-form solution and solution via numerical integration are illustrated. As these key variables can not be easily measured it casts doubt on the use of such closed-form solutions in future PBSD, especially considering the simple and efficient nature of using direct numerical integration to obtain the solution

Simulation of systematic site amplification effects observed at Heathcote Valley during the 2010-2011 Canterbury Earthquake sequence

he strong motion station at Heathcote Valley School (HVSC) recorded unusually high peak ground accelerations (2.21g vertical and 1.41g horizontal) during the February 2011 Christchurch earthquake. Ground motions recorded at HVSC in numerous other events also exhibited consistently higher intensities compared with nearby strong motion stations. We investigated the underlying causes of such high intensity ground motions at HVSC by means of 2D dynamic finite element analyses, using recorded ground motions during the 2010-2011 Canterbury earthquake sequence. The model takes advantage of a LiDAR-based digital elevation model (DEM) to account for the surface topography, while the geometry and dynamic properties of the surficial soils are characterized by seismic cone penetration tests (sCPT) and Multi-Channel Analyses of Surface Waves (MASW). Comparisons of simulated and recorded ground motions suggests that our model performs well for distant events, while for near-field events, ground motions recorded at the adopted reference station at Lyttelton Port are not reasonable input motions for the simulation. The simulations suggest that Rayleigh waves generated at the inclined interface of the surficial colluvium and underlying volcanic rock strongly affect the ground motions recorded at HVSC, in particular, being the dominant contributor to the recorded vertical motions

Displacement-based fragility functions for New Zealand buildings subject to ground motion hazard

Building fragility functions provide a probabilistic representation of a building damage potential due to a hazard of a varying intensity. A simplified probabilistic displacement-based framework is adopted to develop fragility functions for NZ building inventory subject to ground motion hazard. To account for the diversity of building characteristics within a given building class, a Monte-Carlo procedure is adopted to simulate geometrical and material property variables of buildings. The adopted displacement-based approach uses mechanically derived formulae to describe displacement capacities of classes of buildings for four different damage states. A practical and simplified approach is suggested to consider the uncertainty associated with spectral displacement demands. The probability of damage state failure is then determined by comparing the spectral displacement demand with spectral displacement capacity computed from building characteristics

Prediction of spatially distributed seismic demands in specific structures: Ground motion and structural response

The efficacy of various ground motion intensity measures (IM’s) in the prediction of spatially distributed seismic demands (Engineering Demand Parameters, EDP’s) within a structure is investigated. This has direct implications to building-specific seismic loss estimation, where the seismic demand on different components is dependent on the location of the component in the structure. Several common intensity measures are investigated in terms of their ability to predict the spatially distributed demands in a 10-storey office building, which is measured in terms of maximum interstorey drift ratios and maximum floor accelerations. It is found that the ability of an IM to efficiently predict a specific EDP depends on the similarity between the frequency range of the ground motion which controls the IM and that of the EDP. An IM’s predictability has a direct effect on the median response demands for ground motions scaled to a specified probability of exceedance from a ground motion hazard curve. All of the IM’s investigated were found to be insufficient with respect to at least one of magnitude, source-to-site distance, or epsilon when predicting all peak interstorey drifts and peak floor accelerations in a 10-storey RC frame structure. Careful ground motion selection and/or seismic demand modification is therefore required to predict such spatially distributed demands without significant bias

Prediction of spatially distributed seismic demands in specific structures: Structural response to loss estimation

A companion paper has investigated the effects of intensity measure (IM) selection in the prediction of spatially distributed response in a multi-degree-of-freedom structure. This paper extends from structural response prediction to performance assessment metrics such as: probability of structural collapse; probability of exceeding a specified level of demand or direct repair cost; and the distribution of direct repair loss for a given level of ground motion. In addition, a method is proposed to account for the effect of varying seismological properties of ground motions on seismic demand that does not require different ground motion records to be used for each intensity level. Results illustrate that the conventional IM, spectral displacement at the first mode, Sde(T1), produces higher risk estimates than alternative velocity-based IM’s, namely spectrum intensity, SI, and peak ground velocity, PGV, because of its high uncertainty in ground motion prediction and poor efficiency in predicting peak acceleration demands

Probabilistic seismic indoor injury estimation

Most injury models in existence either estimate injuries at a regional level and/or focus only on fatalities. In regions with good engineering practice, the likelihood of building collapse is rare and hence fatality risk is also correspondingly low. Research has shown that in such situations non-fatal injuries are likely to result in larger economic loss than fatalities due to their higher incidence, despite non-fatalities having lower consequence. A new building-specific method of indoor injury estimation is proposed in this paper. Injuries are considered due to: (i) occupants being struck by toppling contents; and (ii) occupants losing balance and falling. This model considers the spatial distribution of occupants in the building, time-occupancy relationships, and the severity of injury to occupants. A simple room layout is used to demonstrate the application of the model