Helen Crowley - Academia.edu (original) (raw)

Papers by Helen Crowley

Earthquake Engineering & Structural Dynamics, 2009

The period of vibration is a fundamental parameter in the force-based design of structures as thi... more The period of vibration is a fundamental parameter in the force-based design of structures as this parameter defines the spectral acceleration and thus the base shear force to which the building should be designed. This paper takes a critical look at the way in which seismic design codes around the world have allowed the designer to estimate the period of vibration for use in both linear static and dynamic analysis. Based on this review, some preliminary suggestions are made for updating the clauses related to the estimation of the periods of vibration in Eurocode 8.

Journal of Earthquake Engineering, 2004

Simple empirical relationships are available in many design codes to relate the height of a build... more Simple empirical relationships are available in many design codes to relate the height of a building to its fundamental period of vibration. These relationships have been realised for force-based design and so produce conservative estimates of period such that the lateral shear force will be conservatively predicted from an acceleration spectrum. Where assessment of a structure is concerned, however, it is the displacement demand that gives an indication of the damage that can be expected; this displacement would be underestimated with the use of the aforementioned period-height formulae. Furthermore, the period of vibration of interest in assessment is the yield period, which is calculated using the yield stiffness, also often referred to as the cracked or elastic stiffness. The derivation of a yield period-height formula for use in displacement-based assessment of European buildings is thus the focus of this work. Analytical fibre element models of RC frames of varying height have been developed and the yield period has been sought using eigenvalue, pushover and dynamic analyses.

Journal of Earthquake Engineering, 2008

The occurrence of a damaging earthquake provides an opportunity to compare observed and estimated... more The occurrence of a damaging earthquake provides an opportunity to compare observed and estimated damage, provided that detailed observations of the earthquake effects are made in the field. A question that arises is whether such comparisons can provide the basis for validation of an earthquake loss model. In order to explore this issue, a case study loss model for the northern Marmara region has been set up and the losses have been calculated for various ground-motion fields that arise when different assumptions are made about the ground-motion variability. In particular, the influence of removing the inter-event variability for a scenario earthquake and modeling spatial correlation among ground motions is studied. Further analyses are conducted assuming that a number of accelerograms are available within the region and that knowledge of spatial correlations among ground motions can therefore be used to better predict the motions at sites in the vicinity of the recording stations. The results demonstrate that unless one has a dense network of accelerographs (commensurate with the geographical resolution of exposure), then the variability in the losses cannot be sufficiently reduced to allow validation of the loss model.

Soil Dynamics and Earthquake Engineering, 2008

Assessment of the seismic vulnerability of the building stock in the earthquake-prone Marmara reg... more Assessment of the seismic vulnerability of the building stock in the earthquake-prone Marmara region of Turkey is of growing importance since such information is needed for reliable estimation of the losses that possible future earthquakes are likely to induce. The outcome of such loss assessment exercises can be used in planning of urban/regional-scale earthquake protection strategies; this is a priority in Turkey, particularly following the destructive earthquakes of 1999. Considering the size of the building inventory, Istanbul and its surrounding area is a case for which it is not easy to determine the structural properties and characteristics of the building stock. In this paper, geometrical, functional and material properties of the building stock in the northern Marmara Region, particularly around Istanbul, have been investigated and evaluated for use in loss estimation models and other types of statistic-or probability-based studies. In order to do that, the existing reinforced concrete (RC) stock has been classified as 'compliant' or 'non-compliant' buildings, dual (frame-wall) or frame structures and emergent or embedded-beam systems. In addition to the statistical parameters such as mean values, standard deviations, etc., probability density functions and their goodness-of-fit have also been investigated for all types of parameters. Functionalities such as purpose of use and floor area properties have been defined. Concrete properties of existing and recently constructed buildings and also characteristics of 220 and 420 MPa types of steel have been documented. Finally, the financial effects of retrofitting operations and damage repair have been investigated. r

Advances in Civil Engineering, 2008

Analytical vulnerability assessment methods should ideally be validated or verified by comparing ... more Analytical vulnerability assessment methods should ideally be validated or verified by comparing their damage predictions with actual observed damage data. However, there are a number of difficulties related to the comparison of analytical damage predictions with observed damage; for example, there are large uncertainties related to the prediction of the ground motions to which the damaged buildings have been subjected. Until such problems can be resolved, it is worthwhile considering the mechanics of simplified analytical vulnerability assessment methods and validating this part of the methodology through comparisons with detailed structural models. This paper looks at two mechanics-based vulnerability assessment methods (DBELA and SP-BELA) and compares the nonlinear static response predicted with these methods with finite elements-based nonlinear analyses of prototype buildings. A comparison of the predicted response of urban populations of buildings using the two methods is then carried out, and the influence of these differences on vulnerability curves is studied.

Earthquake Engineering & Structural Dynamics, 2005

Models capable of estimating losses in future earthquakes are of fundamental importance for emerg... more Models capable of estimating losses in future earthquakes are of fundamental importance for emergency planners, for the insurance and reinsurance industries, and for code drafters. Constructing a loss model for a city, region or country involves compiling databases of earthquake activity, ground conditions, attenuation equations, building stock and infrastructure exposure, and vulnerability characteristics of the exposed inventory, all of which have large associated uncertainties. Many of these uncertainties can be classified as epistemic, implying—at least in theory—that they can be reduced by acquiring additional data or improved understanding of the physical processes. The effort and cost involved in refining the definition of each component of a loss model can be very large, for which reason it is useful to identify the relative impact on the calculated losses due to variations in these components. A mechanically sound displacement-based approach to loss estimation is applied to a test case of buildings along the northern side of the Sea of Marmara in Turkey. Systematic variations of the parameters defining the demand (ground motion) and the capacity (vulnerability) are used to identify the relative impacts on the resulting losses, from which it is found that the influence of the epistemic uncertainty in the capacity is larger than that of the demand for a single earthquake scenario. Thus, the importance of earthquake loss models which allow the capacity parameters to be customized to the study area under consideration is highlighted. Copyright © 2005 John Wiley & Sons, Ltd.

Engineering Structures, 2008

Analytical methods for large-scale assessment of the seismic vulnerability of RC buildings have o... more Analytical methods for large-scale assessment of the seismic vulnerability of RC buildings have only recently become feasible due to a combination of advancements in the field of seismic hazard assessment and structural response analysis. In many of the original procedures to define analytical vulnerability curves, nonlinear time-history analyses of prototype structures with randomly varying structural characteristics were carried out for a set of representative earthquakes. However, running nonlinear dynamic analyses for a large number of structures is extremely time consuming and alternative methods have thus been sought. The method presented in this paper defines the nonlinear behaviour of a random population of buildings through a simplified pushover and displacement-based procedure. Displacement capacity limits are identified on the pushover curve and these limits are compared with the displacement demand from a response spectrum for each building in the random population, thus leading to the generation of vulnerability curves.

Journal of Earthquake Engineering, 2007

A new hazard model for Italy has recently been proposed; hazard maps have been produced for vario... more A new hazard model for Italy has recently been proposed; hazard maps have been produced for various return periods, allowing the values of peak ground acceleration (PGA) and spectral accelerations for response periods up to 2 s to be interpolated for each of the 8,101 Italian municipalities. The new model allows for a more refined definition of the hazard in each municipality as compared to the current use of a fixed spectral shape anchored to upper bound 475-year PGA values and scaling factors for different return periods. The aim of this work is to investigate, in a preliminary fashion, the implications that the adoption of the new return-period dependent hazard maps would have on design and assessment of structures. To this end, the seismic performance of reinforced concrete frames of varying height is evaluated assuming they were located in each of the 8,101 municipalities in Italy and the results obtained with the current and the new hazard model are compared. The new model is shown to result in lower seismic risk in the majority of the municipalities.

International Journal of Architectural Heritage, 2008

A simplified pushover-based earthquake loss assessment (SP-BELA) method, which was originally dev... more A simplified pushover-based earthquake loss assessment (SP-BELA) method, which was originally developed to study the vulnerability of reinforced concrete buildings has been adapted in the current work to produce vulnerability curves for unreinforced masonry buildings. The main target of the current article is to adopt various components of existing methodologies, which define the capacity of masonry buildings, within the probabilistic framework of SP-BELA to generate vulnerability curves. In the current application, the curves have been calibrated using data related to the structural characteristics of Italian buildings. Although more data on the characteristics of masonry buildings is necessary to increase the confidence in the results presented herein, a validation exercise has nevertheless been carried out to compare the vulnerability curves with independent studies related to the vulnerability of masonry buildings. These preliminary results show that there is a good agreement between the vulnerability predictions, especially for those which apply to the Italian building stock.

Journal of Earthquake Engineering, 2008

DBELA is a Displacement-Based Earthquake Loss Assessment methodology for urban areas which relate... more DBELA is a Displacement-Based Earthquake Loss Assessment methodology for urban areas which relates the displacement capacity of the building stock to the displacement demand from earthquake scenarios. The building stock is modeled as a random population of building classes with varying geometrical and material properties. The period of vibration of each building in the random population is calculated using a simplified equation based on the height of the building and building type, while the displacement capacity at different limit states is predicted using simple equations which are a function of the randomly simulated geometrical and material properties. The displacement capacity of each building is then compared to the displacement demand obtained from an over-damped displacement spectrum, using its period of vibration; the proportion of buildings where damage exceeds each specified threshold value can thus be estimated. DBELA has been applied using the Turkish building stock following the collection of a large database of structural characteristics of buildings from the northern Marmara region. The probabilistic distributions for each of the structural characteristics (e.g., story height, steel properties, etc.) have been defined using the aforementioned database. The methodology has then been applied to predict preliminary damage distributions and social losses for the Istanbul Metropolitan Municipality for a Mw 7.5 scenario earthquake.

Bulletin of Earthquake Engineering, 2008

The concerted effort to collect earthquake damage data in Italy over the past 30 years has led to... more The concerted effort to collect earthquake damage data in Italy over the past 30 years has led to the development of an extensive database from which vulnerability predictions for the Italian building stock can be derived. A methodology to derive empirical vulnerability curves with the aforementioned data is presented herein and the resulting curves have been directly compared with mechanics-based vulnerability curves. However, it has been found that a valid comparison between the empirical and analytical vulnerability curves is not possible mainly due to a number of shortcomings in the database of surveyed buildings. A detailed discussion of the difficulties in deriving vulnerability curves from the current observed damage database is thus also presented.

Bulletin of Earthquake Engineering, 2006

The prediction of possible future losses from earthquakes, which in many cases affect structures ... more The prediction of possible future losses from earthquakes, which in many cases affect structures that are spatially distributed over a wide area, is of importance to national authorities, local governments, and the insurance and reinsurance industries. Generally, it is necessary to estimate the effects of many, or even all, potential earthquake scenarios that could impact upon these urban areas. In such cases, the purpose of the loss calculations is to estimate the annual frequency of exceedance (or the return period) of different levels of loss due to earthquakes: so-called loss exceedance curves. An attractive option for generating loss exceedance curves is to perform independent probabilistic seismic hazard assessment calculations at several locations simultaneously and to combine the losses at each site for each annual frequency of exceedance. An alternative method involves the use of multiple earthquake scenarios to generate ground motions at all sites of interest, defined through Monte–Carlo simulations based on the seismicity model. The latter procedure is conceptually sounder but considerably more time-consuming. Both procedures are applied to a case study loss model and the loss exceedance curves and average annual losses are compared to ascertain the influence of using a more theoretically robust, though computationally intensive, procedure to represent the seismic hazard in loss modelling.

Soil Dynamics and Earthquake Engineering, 2006

The assessment of building damage caused by liquefaction-induced ground deformations requires the... more The assessment of building damage caused by liquefaction-induced ground deformations requires the definition of building capacity and vulnerability as a function of the demand, as well as damage scales to describe the state of the damaged building. This paper presents a framework for resolving these issues within the context of earthquake loss estimations, where large variations in building stock and ground conditions must be considered. The principal modes of building response to both uniform and differential ground movements are discussed and the uncertainties in their evaluation are highlighted. A unified damage scale is proposed for use in both reconnaissance and assessment of all modes of building damage, including 'rigid body' response of structures on stiff foundations to uniform or differential ground movements. The interaction of ground shaking and liquefaction in the context of induced structural damage is also briefly considered. The paper raises important aspects of earthquake loss estimations in regions of liquefaction potential, which remain relatively poorly defined at present. q

Bulletin of Earthquake Engineering, 2006

Earthquake loss models are subject to many large uncertainties associated with the input paramete... more Earthquake loss models are subject to many large uncertainties associated with the input parameters that define the seismicity, the ground motion, the exposure and the vulnerability characteristics of the building stock. In order to obtain useful results from a loss model, it is necessary to correctly identify and characterise these uncertainties, incorporate them into the calculations, and then interpret the results taking account of the influence of the uncertainties. An important element of the uncertainty will always be the aleatory variability in the ground-motion prediction. Options for handling this variability include following the traditional approach used in site-specific probabilistic seismic hazard assessment or embedding the variability within the vulnerability calculations at each location. The physical interpretation of both of these approaches, when applied to many sites throughout an urban area to assess the overall effects of single or multiple earthquake events, casts doubts on their validity. The only approach that is consistent with the real nature of ground-motion variability is to model the shaking component of the loss model by triggering large numbers of earthquake scenarios that sample the magnitude and spatial distributions of the seismicity, and also the distribution of ground motions for each event as defined by the aleatory variability.

A displacement-based earthquake loss assessment procedure currently under development is presente... more A displacement-based earthquake loss assessment procedure currently under development is presented. Predictions of the degree of damage to buildings under both ground shaking and liquefaction-induced ground failure can be carried out with this method. Earthquake actions and structural reactions are represented by displacements following the observed correlation between building damage and lateral displacements. The main concept is to compare the mechanics-derived displacement capacity of the building stock and the imposed displacement demand from the earthquake. A probabilistic framework has been incorporated to account for the epistemic (knowledge-based) uncertainty in the capacity parameters. Options for treating the aleatory variability in the ground motion are presented for estimates of losses from single and multiple earthquake scenarios. A discussion of the influence of the epistemic uncertainty on the results of a loss model is also presented. The method can be calibrated for different locations and building practices and this advantage forms the base for a proposed new approach for calibrating seismic design codes.

This paper presents the case for the significant elongation of the period of vibration of reinfor... more This paper presents the case for the significant elongation of the period of vibration of reinforced concrete (RC) buildings during strong ground shaking due to earthquakes. This viewpoint is substantiated by the results of experimental tests on RC structures and the strong ground-motion measurements obtained from damaged RC buildings during earthquakes, wherein a large increase in the period of vibration is observed during ground shaking. The increase in the fundamental period is obviously dependent on the level of shaking and the associated extent of non-linearity that is attained within the structure and/or foundation; this behaviour has been more frequently observed in experimental tests than in the field due to the lack of instrumented buildings that have been subject to large strong ground shaking. Analytical models which replicate the results of the experimental tests are introduced and additional studies on the elongation of the period during seismic action are presented.

Bulletin of Earthquake Engineering, 2004

Earthquake loss estimation studies require predictions to be made of the proportion of a building... more Earthquake loss estimation studies require predictions to be made of the proportion of a building class falling within discrete damage bands from a specified earthquake demand. These predictions should be made using methods that incorporate both computational efficiency and accuracy such that studies on regional or national levels can be effectively carried out, even when the triggering of multiple earthquake scenarios, as opposed to the use of probabilistic hazard maps and uniform hazard spectra, is employed to realistically assess seismic demand and its consequences on the built environment. Earthquake actions should be represented by a parameter that shows good correlation to damage and that accounts for the relationship between the frequency content of the ground motion and the fundamental period of the building; hence recent proposals to use displacement response spectra. A rational method is proposed herein that defines the capacity of a building class by relating its deformation potential to its fundamental period of vibration at different limit states and comparing this with a displacement response spectrum. The uncertainty in the geometrical, material and limit state properties of a building class is considered and the first-order reliability method, FORM, is used to produce an approximate joint probability density function (JPDF) of displacement capacity and period. The JPDF of capacity may be used in conjunction with the lognormal cumulative distribution function of demand in the classical reliability formula to calculate the probability of failing a given limit state. Vulnerability curves may be produced which, although not directly used in the methodology, serve to illustrate the conceptual soundness of the method and make comparisons with other methods.

Currently, seismic design of new European buildings follows a force-based approach, whilst the as... more Currently, seismic design of new European buildings follows a force-based approach, whilst the assessment of existing buildings is moving towards a displacement-based philosophy. In forcebased design, conservative estimates of the period of vibration should be produced such that the base shear force will be conservatively predicted from an acceleration spectrum, and thus the use of gross section (uncracked) stiffness in analytical calculations is perhaps acceptable. For the assessment of buildings, the use of the uncracked stiffness in the determination of the period is certainly inappropriate considering cracking of critical elements such as beams generally occurs under gravity loading alone. Even if cracking is not found to have occurred before the design seismic level of excitation (considered unlikely as this level of excitation would with all probability have been preceded by a number of lower intensity events), it will occur early on in the response to excitation and thereafter the stiffness will reduce rapidly leading to the loss of the tension stiffening effect of the concrete. Thus, the reliable stiffness of the members of an existing RC frame can only be confidently taken as the yield/cracked stiffness. The uncracked and yield period of existing European reinforced concrete buildings of varying height is analytically calculated herein using eigenvalue analysis. A simplified equation is proposed to relate the yield period of vibration of existing buildings to their height for use in large-scale vulnerability assessment applications.

Earthquake Engineering & Structural Dynamics, 2009

Journal of Earthquake Engineering, 2004

Journal of Earthquake Engineering, 2008

Soil Dynamics and Earthquake Engineering, 2008

Advances in Civil Engineering, 2008

Earthquake Engineering & Structural Dynamics, 2005

Engineering Structures, 2008

Journal of Earthquake Engineering, 2007

International Journal of Architectural Heritage, 2008

Journal of Earthquake Engineering, 2008

Bulletin of Earthquake Engineering, 2008

Bulletin of Earthquake Engineering, 2006

Soil Dynamics and Earthquake Engineering, 2006

Bulletin of Earthquake Engineering, 2006

Bulletin of Earthquake Engineering, 2004