Showing papers in "Bulletin of Earthquake Engineering in 2004"

A Probabilistic Displacement-based Vulnerability Assessment Procedure for Earthquake Loss Estimation

Abstract: Earthquake loss estimation studies require predictions to be made of the propor- tion of a building class falling within discrete damage bands from a specified earthquake demand. These predictions should be made using methods that incorporate both computa- tional efficiency and accuracy such that studies on regional or national levels can be effec- tively 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 real- istically 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 classi- cal 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 illus- trate the conceptual soundness of the method and make comparisons with other methods.

The Influence of Strong-Motion Duration on the Seismic Response of Masonry Structures

Abstract: The influence of strong-motion duration on the response of saturated soils is clearly recognised and accounted for in the assessment of liquefaction potential The degree to which duration of shaking influences damage to structures, however, remains a topic of debate, with resolution of the issue complicated by the variety of definitions of duration and the variety of structural behaviours, as well as the difficulty of decoupling the specific effect of duration from other features of the ground motion A suite of seven structural models with strength and stiffness degrading characteristics, designed to reflect the seismic behaviour of masonry structures commonly encountered in many parts of Europe, are analysed using a suite of almost 500 strong-motion accelerograms Correlations are explored between the damage, measured in terms of the strength degradation, and a range of strong-motion parameters, demonstrating that Arias intensity and spectral acceleration at the fundamental initial period of the structure are both reasonably good damage indicators for such structures A significantly improved correlation is obtained by using the elastic spectral accelerations averaged over a period range from the initial period of the structure to a value approximately three times greater, reflecting the stiffness degradation as the shaking progresses The scatter in the correlation is shown to be partially explained by differences in duration

The Bam Earthquake of 26 December 2003

Abstract: The devastating earthquake of 26 December 2003 claimed more than 26,000 lives in the city of Bam and surrounding towns and villages in Southeast Iran, and left the majority of the Bam population homeless. The reason for this tragedy was an unfortunate combination of geological, social and human circumstances. The causative fault practically traversed the city of Bam and the earthquake occurred at a shallow depth. The residential buildings were completely inappropriate for a seismic region, being extremely vulnerable to earthquake shaking, and the earthquake occurred early in the morning when most people were still sleeping. The damage pattern was nearly symmetric about a line 3 km to the west of the surface expression of the Bam fault, and the damage attenuated rapidly with distance from this line. The industrial facilities and the lifelines performed relatively well and experienced slight to moderate damage, but this might have been due to their distance from the earthquake epicentre. However, many of the “qanat” (traditional subterranean irrigation channels) chains that served the twin cities of Bam and Baravat collapsed. Emergency facilities (hospitals, police and fire stations), schools and the university were destroyed and/or heavily damaged during the earthquake. The geotechnical effects of the earthquake were not significant. There was little evidence that site response effects played a major role in the damage pattern in the city. There were no reports of liquefaction and only minor sliding activity took place during the event. A unique set of strong motion acceleration recordings were obtained at the Bam accelerograph station. The highest peak ground acceleration (nearly 1g) was recorded for the vertical component of the motion. However, the longitudinal component (fault-parallel motion in N–S direction) clearly had the largest energy flux, as well as the largest maximum velocity and displacement.

Complex site effects in Thessaloniki (Greece): II. 2D SH modelling and engineering insights

Abstract: This paper presents results of numerical modelling of site response for Thessaloniki, obtained with two different 2D methods; a finite difference and a finite element method Ground motion across a 2D model of the subsoil of the city has been simulated for vertically incident SH waves The predominance of locally generated surface waves is very clear in the synthetic seismograms of a weak event and of stronger ones These results are then compared with the observations in time domain and frequency domain The role of the soil formations with high attenuation in the lateral propagation and the effect of the differential motion close to the lateral variations are also pinpointed The stronger events were finally used to compute strong ground motion in order to reveal and to discuss practical engineering aspects such as peak ground acceleration value, the most familiar indicator in seismic norms, the soil to rock spectral coefficients for the period bandwidth of interest, and the aggravation factor in terms of 2D to 1D response spectra as a useful ruler to account for complex site effects

Comparative Study of the Seismic Hazard Assessments in European National Seismic Codes

Abstract: The European Union has been promoting an homogenisation of the design rules for earthquake resistant structures through the Eurocode 8, which will soon become the official standard (CEN, 2003). However, the zonation for the basic earthquake ground motion will remain in the national authorities competence. Hence, it is important to outline differences and similarities in the `official' seismic hazard assessments (SHA) used by national seismic codes to define seismic zones and levels of seismic actions. The different SHA in 16 European countries were analysed taking into account a selection of comparative ingredients: date of the SHA, earthquake scale, definition of seismogenic zones, maximum earthquake estimation, attenuation relation, hazard calculation and hazard descriptor. Most of the official European SHAs were made more than 5 years ago, in terms of macroseismic intensity, taking into account seismogenic zones, estimating maximum earthquakes from historical records, making use of attenuation relationships for macroseismic intensity and assuming that earthquake occurrences follow a Poisson process. Most of the countries (11/16) depict hazard for a ∼475 year return period; seven of them use peak ground acceleration and four MSK intensity. There is also an important fraction relating the hazard to a different return period (3/16) or expressing it in a deterministic way (2/16). A general updating and homogenisation in many of the national SHA is recommended.

Complex Site Effects in Thessaloniki (Greece): I. Soil Structure and Comparison of Observations with 1D Analysis

Abstract: This paper presents a 2D model of the geological structure of Thessaloniki city and results of empirical and theoretical approaches for the evaluation of site response due to complex site effects. The construction of the 2D model is based on the available geophysical and geotechnical data in terms of the most important parameters needed to model site response. The well-known subsoil structure, despite the existence of some local uncertainties, gave the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from the analysis of one event in frequency and time domains and 1D numerical modelling. The study of site response shows the effect of the lateral variations on ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In this case history, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects.

Comparing Loss Estimation with Observed Damage in a Zone of Ground Failure: A Study of the 1999 Kocaeli Earthquake in Turkey

Abstract: The building losses in Adapazari following the 17 August 1999 Kocaeli earthquake are estimated for both ground-shaking and ground-failure induced damage and the predictions compared with field observations. The ground-shaking damage is estimated using the capacity spectrum approach and accommodating the results of a previously published calibration of this model. These results, which do not incorporate ground-failure damage, are compared to the observed damage patterns in Adapazari to illustrate the significance of not including the more complex ground failure component in a loss model for a region of high liquefaction susceptibility. A preliminary estimation of ground-failure-induced damage is then presented based upon the HAZUS (FEMA, 2003) default methodology. The benefits of these additional calculations to the overall loss model are assessed to provide some quantitative decision-making guidance for those producing loss estimation models. The findings suggest that the benefits of specifically incorporating ground failure into a loss model only start to be obtained if a detailed approach using in situ geotechnical data as well as adequate representation of building foundations is adopted. Otherwise, the additional input data required is not commensurate with the small potential refinement in the estimated losses, particularly considering the compounded uncertainties associated with the simplified approach.

Seismic Performance of RC Frames Designed to Eurocode 8 or to the Greek Codes 2000

Abstract: For the first time after the finalisation of the European Norm for seismic design of buildings (Eurocode 8 – EC8),the performance of RC buildings designed with this code is evaluated through systematic nonlinear analyses. Regular 4-, 8- or 12-storey RC frames are designed for a PGA of 0.2 or 0.4 g and to one of the three alternative ductility classes in EC8. As the Eurocodes are meant to replace soon existing national codes, design and performance is also compared to that of similar frames designed with the 2000 Greek national codes. The performance of alternative designs under the life-safety (475 years) and the damage limitation (95 years) earthquakes is evaluated through nonlinear seismic response analyses. The large difference in material quantities and detailing of the alternative designs does not translate into large differences in performance. Design for either Ductility Class High (H) or Medium (M) of EC8 is much more cost-effective than design for Ductility Class Low (L), even in moderate seismicity. It is also much more cost-effective than design to the 2000 Greek national codes.

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)

Abstract: Pointe-a-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps.

Reliability of Building Inventories in Seismic Prone Regions

Abstract: A study on uncertainties in building inventory and vulnerability assessment for the city of Basel, Switzerland a moderate hazard but high-risk area is presented. Emphasis is put on the special conditions and demands on the data assessment procedure in such an environment. The building information was assessed from the sidewalk using a method similar to that proposed in FEMA-154. With only three persons working for two weeks, about 10% of the city’s building stock was assessed. The quality of the dataset was tested by using pre-existing reference data. These tests revealed patterns of misinterpreted structural information; important features like the floor type in URM buildings were difficult to identify.

On the incorporation of the effect of crustal structure into empirical strong ground motion estimation

Abstract: This article has two purposes. Firstly, a validation exercise of the modal summation technique for the computation of synthetic strong-motion records is performed for two regions of Europe (Umbria-Marche and south Iceland), using a variety of region specific crustal structure models, by comparing the predicted ground motion amplitudes with observed motions. It is found that the rate of decay of ground motions is well predicted by the theoretical decay curves but that the absolute size of the ground motions is underpredicted by the synthetic time-histories. This is thought to be due to the presence of low-velocity surface layers that amplify the ground motions but are not included in the crustal structure models used to compute the synthetic time-histories. Secondly, a new distance metric based on the computed theoretical decay curves is introduced which should have the ability to model the complex decay of strong ground motions. The ability of this new distance metric to reduce the associated scatter in empirically derived equations for the estimation of strong ground motions is tested. It is found that it does not lead to a reduction in the scatter but this is thought to be due to the use of crustal structure models that are not accurate or detailed enough for the regions studied.

Design Earthquakes in the U.K.

Abstract: Three sites in the UK are taken, representative of low, medium and high hazard levels (by UK standards). For each site, the hazard value at 10−4 annual probability is computed using a generic seismic source model, and a variety of ground motion parameters: peak ground acceleration (PGA), spectral acceleration at 10 Hz and 1 Hz, and intensity. Disaggregation is used to determine the nature of the earthquakes most likely to generate these hazard values. It is found (as might be expected) that the populations are quite different according to which ground motion parameter is used. When PGA is used, the result is a rather flat magnitude distribution with a tendency to low magnitude events (\le 4.5 ML) which are probably not really hazardous. Hazard-consistent scenario earthquakes computed using intensity are found to be in the range 5.8–5.9 ML, which is more in accord with the type of earthquake that one expects to be a worst-case event in the UK.

Teaching Structural Hazards Awareness for Preparedness and Community Response

Abstract: Most successful search and rescue is performed by neighbors and household members during the first 24 h after an earthquake, before professional responders arrive on the scene. Many spontaneous responders become casualties in the process. Kandilli Observatory and Earthquake Research Institute’s Disaster Preparedness Education Project with the support of the Earthquake Engineering Department has developed a training module for Community Disaster Responders to help them assess the level of damage for immediate citizen disaster response. Learning the signs of damage in both reinforced concrete and adobe buildings, they will be better able to distinguish the slight and moderately damaged buildings where their light search and rescue efforts will be effective, as opposed to the heavily damaged buildings where professional help is required for safe extrication. Incorporating citizens into the early stages of building collapse rescue is designed to make professional urban search and rescue efforts more effective. The program is being piloted in Turkey by the Civil Defense Directorate, Search and Rescue Brigade. It is expected that this will lead to a new demand for engineers to work in partnership with community-based organizations for planning and prioritizing disaster mitigation efforts, and to participate in immediate post-disaster damage assessment. Engineering recommendations will also be needed for post-earthquake building re-entry and re-habitation risk assessment.