Pierino Lestuzzi

Simplified non-linear seismic displacement demand prediction for low period structures

The prediction of non-linear seismic demand using linear elastic behavior for the determination of peak non-linear response is widely used for seismic design as well as for vulnerability assessment. Existing methods use either linear response based on initial period and damping ratio, eventually corrected with factors, or linear response based on increased equivalent period and damping ratio. Improvements to the original EC8 procedure for displacement demand prediction are proposed in this study. Both propositions may be graphically approximated, which is a significant advantage for practical application. A comparison with several other methods (equal displacement rule, EC8 procedure, secant stiffness and empirical equivalent period methods) is performed. The study is based on non-linear SDOF systems subjected to recorded earthquakes, modified to match design response spectra of different ground types, and focuses on the low frequency range that is of interest for most European buildings. All results are represented in the spectral displacement/fundamental period plane that highlights the predominant effect of the fundamental period on the displacement demand. This study shows that linearized methods perform well at low strength reduction factors but may strongly underestimate the displacement demand at strength reduction factors greater than 2. This underestimation is an important issue, especially for assessment of existing buildings, which are often related with low lateral strength. In such cases, the corresponding strength reduction factors are therefore much larger than 2. The new proposals significantly improve the reliability of displacement demand prediction for values of strength reduction factors greater than 2 compared to the original EC8 procedure. As a consequence, for the seismic assessment of existing structures, such as unreinforced masonry low-rise buildings, the current procedure of EC8 should be modified in order to provide accurate predictions of the displacement demand in the domain of the response spectrum plateau.

Seismic vulnerability assessment at urban scale for two typical Swiss cities using Risk-UE methodology

This paper contains a seismic assessment at urban scale of the cities of Sion and Martigny in Switzerland. These two cities have been identified for the present research based on their importance regarding size and the characteristics of the building stock for which information was available. Moreover, microzonation investigations are available for both cities. This results in a more accurate characterization of local expected ground shaking, which is expressed through specific response spectra. Sion and Martigny represent, respectively, the capital and second largest city of the canton of Valais. This region is characterized by the highest seismicity within Switzerland. The paper focuses on the assessment using Risk-UE methodology, namely the empirical method LM1 and the mechanical method LM2. The obtained results are compared in order to assess the related accuracy. Firstly, buildings of the two cities were surveyed in order to collect main structural characteristics in a database. Building stock is typical of that region and can be found similar to many other medium-sized Swiss cities. Around half of the buildings are unreinforced masonry buildings, while several others are reinforced concrete buildings with shear walls. Results show the most vulnerable part of the cities regarding earthquake. There are significant differences in global results between LM1 and LM2 methods. The mechanical LM2 method is more pessimistic since it predicts damage grades of about one degree higher than LM1 method. However, the main drawback of the empirical LM1 method is that an a priori determination of an adequate value of the macroseismic intensity is required. Nevertheless, LM2 method may lead to a global overestimation of damage prediction.

Probabilistic evaluation of the physical impact of future tephra fallout events for the Island of Vulcano, Italy

A first probabilistic scenario-based hazard assessment for tephra fallout is presented for La Fossa volcano (Vulcano Island, Italy) and subsequently used to assess the impact on the built environment. Eruption scenarios are based upon the stratigraphy produced by the last 1000 years of activity at Vulcano and include long–lasting Vulcanian and sub-Plinian eruptions. A new method is proposed to quantify the evolution through time of the hazard associated with pulsatory Vulcanian eruptions lasting from weeks to years, and the increase in hazard related to typical rainfall events around Sicily is also accounted for. The impact assessment on the roofs is performed by combining a field characterization of the buildings with the composite European vulnerability curves for typical roofing stocks. Results show that a sub-Plinian eruption of VEI 2 is not likely to affect buildings, whereas a sub-Plinian eruption of VEI 3 results in 90 % of the building stock having a ≥12 % probability of collapse. The hazard related to long-lasting Vulcanian eruptions evolves through time, and our analysis shows that the town of Il Piano, located downwind of the preferential wind patterns, is likely to reach critical tephra accumulations for roof collapse 5–9 months after the onset of the eruption. If no cleaning measures are taken, half of the building stock has a probability >20 % of suffering roof collapse.

Probabilistic mechanics-based loss scenarios for school buildings in Basel (Switzerland)

Developing earthquake scenarios for cities in areas with a moderate seismicity is a challenge due to the limited amount of available data, which is a source of large uncertainties. This concerns both the seismic hazard, for which only recordings for small earthquakes are available and the unknown earthquake resistance of the majority of structures not designed for seismic loading. The goal of the present study is to develop coherent probabilistic mechanics-based scenarios for a mid-size building stock including a comprehensive analysis of the uncertainties. As an application, a loss assessment for the school buildings of the city of Basel is performed for different scenarios of historical significance, such as the 1356 event, and from the deaggregation of the Swiss Probabilistic Seismic Hazard Model of 2015. The hazard part of the computations (i.e. ground motion estimation) is based on this model, a regional microzonation and recordings of small earthquakes on a dense strong motion network to compute site-amplification factors. The school buildings, which are mainly unreinforced masonry or reinforced concrete shear wall buildings, have been classified according to a specifically developed taxonomy. Fragility curves have been developed using non-linear static procedures and subsequently, vulnerability curves in terms of human and financial losses are proposed. The computations have been run with the OpenQuake engine, carefully propagating all the recognized uncertainties. Scenarios before and after retrofitting measures show their impact on the earthquake safety. A sensitivity analysis shows that the largest uncertainties come from the ground motion prediction although an improvement of all parts of the model is necessary to decrease the uncertainties. Although improved data and models are still necessary to be developed, probabilistic mechanics-based models outperform the capabilities of deterministic and/or empirical models for retrieving realistic earthquake loss distributions.

Fragility Analysis of Existing Unreinforced Masonry Buildings through a Numerical-based Methodology

As an approach to the problem of seismic vulnerability evaluation of existing buildings using the predicted vulnerability method, numerical models can be applied to define fragility curves of typical buildings which represent building classes. These curves can be then combined with the seismic hazard to calculate the seismic risk for a building class (or individual buildings). For some buildings types, mainly the unreinforced masonry structures, such fragility analysis is complicated and time consuming if a Finite Element-based method is used. The FEM model has to represent the structural geometry and relationships between different structural elements through element connectivity. Moreover, the FEM can face major challenges to represent large displacements and separations for progressive collapse simulations. Therefore, the Applied Element Method which combines the advantages of FEM with that of the Discrete Element Method in terms of accurately modelling a deformable continuum of discrete materials is used in this paper to perform the fragility analysis for unreinforced masonry buildings. To this end, a series of nonlinear dynamic analyses using the AEM has been per-formed for two unreinforced masonry buildings (a 6-storey stone masonry and a 4-storey brick masonry) using more than 50 ground motion records. Both in-plane and out-of-plane failure have been considered in the damage analysis. The distribution of the structural responses and inter-storey drifts are used to develop spectral-based fragility curves for the five European Macroseismic Scale damage grades.

Data-Interpretation Methodologies for Non-Linear Earthquake Response Predictions of Damaged Structures

Seismic exposure of buildings presents difficult engineering challenges. The principles of seismic design involve structures that sustain damage and still protect inhabitants. Precise and accurate knowledge of the residual capacity of damaged structures is essential for informed decision-making regarding clearance for occupancy after major seismic events. Unless structures are permanently monitored, modal properties derived from ambient vibrations are most likely the only source of measurement data that is available. However, such measurement data is linearly elastic and limited to a low number of vibration modes. Structural identification using hysteretic behavior models that exclusively relies on linear measurement data is a complex inverse engineering task that is further complicated by modeling uncertainty. Three structural identification methodologies that involve probabilistic approaches to data interpretation are compared: error-domain model falsification, Bayesian model updating with traditional assumptions as well as modified Bayesian model updating. While noting the assumptions regarding uncertainty definitions, the accuracy and robustness of identification and subsequent predictions are compared. A case study demonstrates limits on non-linear parameter identification performance and identification of potentially wrong prediction ranges for inappropriate model uncertainty distributions.

Validation and improvement of Risk-UE LM2 capacity curves for URM buildings with stiff floors and RC shear walls buildings

This paper addresses seismic vulnerability assessment at an urban scale and more specifically the capacity curves involved for building damage prediction. Standard capacity curves are a function of predefined building typology and are proposed in the Risk-UE LM2 method for computation of the corresponding damage grades. However, these capacity curves have been mainly developed for building stock of southern European cities and the accuracy of their application with different building features, such as the ones of cities of northern Europe should be assessed. A recent research project of seismic scenarios for the cities of Sion and Martigny in Switzerland provided the opportunity to check the capacity curves of Risk-UE LM2 method. Within the framework of this project, a detailed analysis was achieved for more than 500 buildings. These buildings were typical Swiss buildings and were composed of both unreinforced masonry buildings with stiff floors and reinforced concrete buildings. The construction drawings of each building were collected in order to have the most accurate information about their main structural characteristics. The typological classification that has been adopted was developed in a recent research project. Based on the individual features of the buildings, individual capacity curves were defined. Results of the seismic assessment applied to the 500 buildings compare very well with those obtained by using Risk-UE LM2 method for unreinforced masonry buildings with stiff floors. A slight improvement may be proposed for buildings with three stories through their introduction to the category of low-rise instead of mid-rise buildings. By contrast, accuracy for reinforced concrete buildings with shear walls is very poor. Damage prediction using related capacity curves of Risk-UE LM2 method does not correspond to reality. Prediction is too pessimistic and moreover damage grades increase with the height category (low-rise, mid-rise and high-rise) of these buildings which is in contradiction with the observed damages for this type of buildings. Improvements are proposed to increase the accuracy of the seismic vulnerability assessment for northern European building stock. For unreinforced masonry buildings, a slight modification of the limits of the height category of buildings using the ones defined for RC buildings improves the damage prediction. For reinforced concrete buildings with shear walls improved capacity curves derived from the typological curves of the specific typology C are proposed.

Seismic Assessment of a Historical Masonry Building in Switzerland: The “Ancien Hôpital De Sion”

ABSTRACT This article presents the results of the evaluation of the seismic safety of the Ancien Hôpital de Sion, an important Swiss architectural heritage building, situated in the Canton of Valais, the region with the highest seismic hazard in Switzerland. Three-dimensional Applied Element (AEM) modeling of the whole structure has been performed and validated. The adopted modeling strategy, together with nonlinear dynamic analysis, was able to represent the actual behavior and failure mechanisms typical of complex masonry structures, in addition to a good computational efficiency compared to other available numerical approaches. The local collapse mechanisms have been also studied through a kinematic limit analysis based on rigid block rotation. Both linear and nonlinear approaches have been followed together with the capacity spectrum method. The results provided by the different methodologies have been compared with the aim to provide possible insights concerning a general procedure for the assessment of the safety of such type of structures.

Displacement ducility demand and strength reduction factors for rocking structures

This paper reports the main results of an extensive parametric study using numerical simulations and computing displacement ductility demand of nonlinear single-degree of freedom (SDOF) systems and multi-degree of freedom (MDOF) systems for a set of 164 registered ground motions. The objective of this study is to propose values of strength reduction factors for rocking behavior for seismic analysis. In the first part focused on SDOF systems, non-linear seismic responses obtained with a hysteretic model simulating rocking are statistically compared with the ones related to established hysteretic models for ductile structures. Similar to established hysteretic models, results confirm that the frequency has little influence on the ductility demand if it is below 2 Hz and a substantial influence if it is above 2 Hz. Moreover, they show that the other parameters, especially the hysteretic behavior model, have only little influence on the displacement ductility demand. Surprisingly, displacement ductility demand is found to be practically independent of the additional viscous damping ratio. Finally, a relationship between displacement ductility demand and strength reduction factor for rocking systems is proposed. The second part shows that the results obtained for SDOF systems are also valid for MDOF systems.

Deriving fragility functions from bilinearized capacity curves for earthquake scenario modelling using the conditional spectrum

The development of fragility curves to perform seismic scenario-based risk assessment requires a fully probabilistic procedure in order to account for uncertainties at each step of the computation. This is especially true when developing fragility curves conditional on an Intensity Measure that is directly available from a ground-motion prediction equation. In this study, we propose a new derivation method that uses realistic spectra instead of design spectral shapes or uniform hazard spectra and allows one to easily account for the features of the site-specific hazard that influences the fragility, without using non-linear dynamic analysis. The proposed method has been applied to typical school building types in the city of Basel (Switzerland) and the results have been compared to the standard practice in Europe. The results confirm that fragility curves are scenario dependent and are particularly sensitive to the magnitude of the earthquake scenario. The same background theory used for the derivation of the fragility curves has allowed an innovative method to be proposed for the conversion of fragility curves to a common IM (i.e. spectral acceleration or PGA). This conversion is the only way direct comparisons of fragility curves can be made and is useful when inter-period correlation cannot be used in scenario loss assessment. Moreover, such conversion is necessary to compare and verify newly developed curves against those from previous studies. Conversion to macroseismic intensity is also relevant for the comparison between mechanical-based and empirical fragility curves, in order to detect possible biases.