Reliability of dynamic identification techniques connected to structural monitoring of monumental buildings

The impossibility of adjusting the monumental buildings to the protection levels adopted for newly conceived buildings brings about the need to accept lower safety levels. In such case, the potentiality of dynamic monitoring by means of periodic checks on the dynamic characteristics of the structure (its own frequencies and vibration modes) would allow, at least theoretically, one to check globally that there are no changes to the boundary conditions that would further diminish the structural safety level. To this end, the dynamic analyses of a masonry triumphal arch of a medium-sized church are reported in this paper. The dynamic identification campaign was carried out at the ELSA Laboratory, studying the in-plane behaviour of the structure in undamaged and damaged conditions. The dynamic identification campaign, which lasted more than 6 months, allowed us to test the reliability of different acquisition systems, of different typologies of excitation and above all of evaluating the influence of environmental conditions on the dynamic parameters that may be identified. Indeed, variation of the environmental factors (e.g. temperature, insolation, humidity) may determine a change of the structure's own frequencies that must be taken into consideration just so as to be able to recognize the variations that may be correlated to a structural change due to the presence of damage. Copyright © 2007 John Wiley & Sons, Ltd

Seismic displacement-demand and urban damage distribution: the impact of different methods on vulnerability assessment

This paper addresses seismic vulnerability assessment at urban scale. Particularly, it focuses on the differences in damage distribution obtained from the application of several simplified methods for displacement demand determination. The results obtained for two cities of Switzerland (Sion and Martigny) highlight the related impact. Displacement demands predicted using three simplified methods are compared with “reference” seismic demands obtained from non-linear time-history analysis (NLTHA). Comparing the urban seismic damage distributions from the three simplified methods with the one from NLTHA helps in understanding the reliability of displacement demand determination. The following three methods are compared: the usual N2 method, the Lin & Miranda proposal and an optimized version of the N2 method. These methods are evaluated based on the real seismic hazard (microzones), showing furthermore the importance of considering the real soil conditions in the general damage assessment

Seismic displacement-demand and urban damage distribution: the impact of different methods on vulnerability assessment

This paper addresses seismic vulnerability assessment at urban scale. Particularly, it focuses on the differences in damage distribution obtained from the application of several simplified methods for displacement demand determination. The results obtained for two cities of Switzerland (Sion and Martigny) highlight the related impact. Displacement demands predicted using three simplified methods are compared with “reference” seismic demands obtained from non-linear time-history analysis (NLTHA). Comparing the urban seismic damage distributions from the three simplified methods with the one from NLTHA helps in understanding the reliability of displacement demand determination. The following three methods are compared: the usual N2 method, the Lin & Miranda proposal and an optimized version of the N2 method. These methods are evaluated based on the real seismic hazard (microzones), showing furthermore the importance of considering the real soil conditions in the general damage assessment

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

Microzonazione sismica di un centro abitato di piccole dimensioni: il caso studio di Sant’Agata Fossili (AL)

In questo lavoro sono descritte le attività svolte nell’ambito del Progetto Europeo Strategico RISKNAT riguardanti l’analisi di microzonazione di Sant’Agata Fossili (AL). Sono in particolare descritte tutte le indagini ed analisi numeriche condotte al fine di ottenere una microzonazione sismica di livello 3 secondo quanto previsto nelle recenti linee guida nazionali degli. Al fine inoltre di valutare le ricadute di tipo applicativo di una corretta microzonazione sismica del territorio, sono state realizzate delle simulazioni di scenario adottando come riferimento gli spettri di risposta ottenuti dall’analisi di microzonazione. Le valutazioni di scenario ottenute sono state infine confrontate con le previsioni di scenario realizzabili a priori sulla base dell’adozione degli spettri di risposta definiti nelle Norme Tecniche per le Costruzioni

Actigraphic Sensors Describe Stroke Severity in the Acute Phase: Implementing Multi-Parametric Monitoring in Stroke Unit

: Actigraphy is a tool used to describe limb motor activity. Some actigraphic parameters, namely Motor Activity (MA) and Asymmetry Index (AR), correlate with stroke severity. However, a long-lasting actigraphic monitoring was never performed previously. We hypothesized that MA and AR can describe different clinical conditions during the evolution of the acute phase of stroke. We conducted a multicenter study and enrolled 69 stroke patients. NIHSS was assessed every hour and upper limbs' motor activity was continuously recorded. We calculated MA and AR in the first hour after admission, after a significant clinical change (NIHSS ± 4) or at discharge. In a control group of 17 subjects, we calculated MA and AR normative values. We defined the best model to predict clinical status with multiple linear regression and identified actigraphic cut-off values to discriminate minor from major stroke (NIHSS ≥ 5) and NIHSS 5-9 from NIHSS ≥ 10. The AR cut-off value to discriminate between minor and major stroke (namely NIHSS ≥ 5) is 27% (sensitivity = 83%, specificity = 76% (AUC 0.86 p < 0.001), PPV = 89%, NPV = 42%). However, the combination of AR and MA of the non-paretic arm is the best model to predict NIHSS score (R2: 0.482, F: 54.13), discriminating minor from major stroke (sensitivity = 89%, specificity = 82%, PPV = 92%, NPV = 75%). The AR cut-off value of 53% identifies very severe stroke patients (NIHSS ≥ 10) (sensitivity = 82%, specificity = 74% (AUC 0.86 p < 0.001), PPV = 73%, NPV = 82%). Actigraphic parameters can reliably describe the overall severity of stroke patients with motor symptoms, supporting the addition of a wearable actigraphic system to the multi-parametric monitoring in stroke units

Evaluation and control of the in-plane stiffness of timber floors for the performance-based retrofit of URM buildings

The seismic response of existing un-reinforced masonry (URM) buildings is strongly dependent on the characteristics of wooden floors and, in particular, on their in-plane stiffness and on the quality of connection between the floors and the URM elements. It is generally well-recognized that an adequate inplane-stiffness and proper connections can significantly improve the three-dimensional response of these buildings, obtaining a better distribution and transfer of forces to the lateral load resisting walls. However, the extensive damage observed during past earthquakes on URM buildings of different types have highlighted serious shortcomings in typical retrofit interventions adopted in the past and based on stiffening the diaphragm. Recent numerical investigations have also confirmed that increasing the stiffness of the diaphragm is not necessarily going to lead to an improved response, but could actually result to detrimental effects. The evaluation of the in-plane stiffness of timber floors in their as-built and retrofitted configuration is still an open question and a delicate issue, with design guidelines and previous research results providing incomplete and sometimes controversial suggestions to practicing engineers involved in the assessment and/or retrofit of these type of structures. In this contribution, the role of the in-plane stiffness of timber floors in the seismic response of URM buildings is critically discussed, based on the relatively limited available experimental and numerical evidences. A framework for a performance-based assessment and retrofit strategy of URM buildings, capable of accounting for the effects of a flexible diaphragm on the response prior to and after the retrofit intervention, is then proposed. By controlling the in-plane stiffness of the diaphragm, adopting a specific strengthening (or weakening) intervention, the displacements, accelerations and internal force demands can be maintained within targeted levels. This will protect undesired local mechanisms and aim for a more appropriate hierarchy of strength within the whole system

Preliminary Studies on the Structural Behaviour of Traditional Timber Roof in Gjirokastra, Albania

The safety evaluation of an existing timber structure is a key task for maintaining it in service or not, by means of the definition and prescription of conservation measures and interventions. The knowledge acquiring process requires information on both geometrical and mechanical properties of timber members, as well as on the loads that the structure was and will be submitted to. Furthermore, because historic roofs are load-bearing structures made of timber according to a heuristic and an intuitive design without structural engineering theory support, analysis of their real structural behaviour are needed. Traditional roofs in Gjirokastra (Albanian UNESCO World Heritage), seem to be very peculiar in the Southern Western Balkans. The structures consist of a single main warping covered by timber planks and stone slabs of different sizes and thickness, placed without mortar or metal hook connections. Elements are connected to each other with nails in a very simple heel joint, considering the connection between horizontal beams and rafters, and an half-lap joint, concerning the connection between rafters and posts. Horizontal beams are nailed to the masonry's timber ties system, which horizontally reinforces the walls ensuring the overall box-like behavior of the building. The paper proposes a FEM analysis of a first case study of these peculiar timber roof structures, prelimi-narily analyzed during the author’s Master Thesis’ degree in Architectural Engineering at the University of Genoa, which is now part of the author's on - going PhD research titled "Traditional timber roofs in Gjirokastra, Albania. From knowledge to preservation"