Deliverable 9.4. Report on the use of monitoring as knowledge based assessment and early warning tool

Transcription

1 Deliverable 9.4 Report on the use of monitoring as knowledge based assessment and early warning tool Due date: September 2012 Submission date: November 2012 Issued by: UPC WORKPACKAGE 9: Knowledge based assessment Leader: UPC PROJECT N : ACRONYM: TITLE: COORDINATOR: New integrated knowledge based approaches to the protection of cultural heritage from earthquake-induced risk Università di Padova (Italy) START DATE: 01 January 2010 DURATION: 36 months INSTRUMENT: THEME: Collaborative Project Small or medium scale focused research project Environment (including Climate Change) Dissemination level: PU Rev: FIN

2 INDEX 1 INTRODUCTION Description and objectives of workpackage Objectives and contents of the deliverable OBJECTIVES OF MONITORING Objectives Role of monitoring during the different phases Investigation phase Intervention phase Evaluation phase Maintenance phase REQUIREMENTS GENERAL APPROCHES APPLICATION TO DIFFERENT PROBLEMS Different problem types Application Increase of knowledge as alternative to strengthening Precautionary strengthening Study of vulnerability and evaluation for seismic improvement Study and stabilization of buildings damaged by earthquake Repair and seismic strengthening of structures damaged by earthquake Validation of effectiveness of existing strengthening CONCLUSIONS Knowledge based assessment D9.4 i

3 1 INTRODUCTION 1.1 DESCRIPTION AND OBJECTIVES OF WORKPACKAGE Workpackage 9 is devoted to the development and application of knowledge-based assessment procedures to real buildings selected as case studies for the project. These procedures are applied for the evaluation and validation of the strategies and methods for seismic assessment and seismic improvement that have resulted from the project. In particular, task 9.2, on real case application, aims to the exploitation and validation of the proposed monitoring and intervention techniques and strategies through their application to the selected case studies. The tasks envisaged for the different cases include inspection works, structural analyses and monitoring using the tools developed or evaluated within the project. The general process involves also the proposal of intervention solutions and the selection of optimal ones adequately combining efficiency requirements and satisfactory compliance with conservation principles. Monitoring is considered as an essential tool throughout the entire process comprised by the study and intervention on a building, including the investigation or diagnosis phase, the design and execution of the seismic upgrading and the post-intervention verification and control. In particular, monitoring is regarded as an essential tool in the design and practical implementation of optimal interventions. 1.2 OBJECTIVES AND CONTENTS OF THE DELIVERABLE As part of the activities included in workpackage 9, a set of historical buildings has been selected for the application of the proposed monitoring and intervention strategies and technologies. The selection covers a wide range of conditions regarding geographical location (different countries) local seismicity (low, medium, high), construction material (stone and brick masonry, earth), structural typologies and uses (towers and minarets, churches, large cathedrals, palaces), preservation condition (different levels of damage) and risks involved (i.e., people at risk, valuable artistic contents at risk). Among the buildings considered are a set of churches in L Aquila, Italy (San Agostino, San Marco, San Silvestro, San Bagio and San Giuseppe), a large fortress (Spanish Fortress in l Aquila), a large Roman construction (Arena in Verona, Italy), masonry towers (Civic Tower in L Aquila, Arcisate Bell Tower) and minarets (a set of Ottoman minarets in Bosnia), large Gothic cathedrals (Mallorca Cathedral, Spain and Jerónimos Monastry, in Portugal), masonry buildings (Medersa in Fez, Morocco, the Int. Conservation Centre in Acre, the Israel, Former Casa da Bragança Foundation Headquarter, in Portugal, Mekaad Radwan in Cairo) and a large earthen construction (Ambel Preceptory in Zaragoza, Spain). Deliverable 9.1, on Individuation of proper buildings where applying the new technologies, completed in June 2011, provides a detailed description of the buildings selected. In turn, deliverable 9.2 on Development and calibration of local and global monitoring techniques, also made available in June 2011, includes a detailed description of the inspection non-destructive and monitoring technologies utilized. The proposed knowledge-based methodologies for the study of architectural heritage are largely based on the possibilities of structural monitoring. In this context, monitoring involves the measurement of parameters relative to the environmental actions and the structural response across a certain period of time. As part of these methodologies, monitoring can be utilized for diagnosis, before the intervention, and for verification and control, during and after the intervention. An important aspect of the methodologies proposed lays in the identification of a maintenance period, following the execution of the intervention, during which some sort of monitoring is necessary in order to control the adequate performance of the strengthened structure. The present document includes a detailed description of the proposed use of monitoring as part of the general methodologies emerged from the project and describes its specific application to a wide selection of case studies. The objectives of monitoring and its possible role during the different phases of the study are discussed in section 2. In turn, the application to real buildings is presented in section 5 according to five different categories corresponding to distinct problem Knowledge based assessment D9.4 1

4 types. Sections 3 and 4 refer, respectively, to the requirements and different approaches that can be considering in the laying-out of the systems utilized. The discussion on the case studies focusses largely on the methodological aspects while also mentioning some of the difficulties encountered. A set of final conclusions are presented in section 5 as a synthesis of the experience acquired in the design and development of the monitoring tasks in the different case studies. It must be noted that the present deliverable focuses in the monitoring methods and application to knowledge based seismic assessment. The discussion presented in section 5 is limited to a critical summary of the use of monitoring in different case studies. More specific information in some aspects can be found in companion deliverables. The specific technologies utilized (regarding the type of acquisitions systems and sensors) were already presented and discussed in deliverable D9.3 and are not referred to herein. The buildings were described into detail in deliverable D9.1, and the overall studies carried out on them, showing the inclusion of monitoring in a more general strategy also encompassing inspection and structural analysis, is presented in deliverable D9.5. The specific application of monitoring as part of the design process and verification of interventions is discussed into detail in deliverable D9.4. Knowledge based assessment D9.4 2

5 2 OBJECTIVES OF MONITORING 2.1 OBJECTIVES Knowledge based assessment requires a deep understanding on the condition and the capacity of the structures analyzed. To attain this understanding, knowledge-based methodologies for the study of heritage buildings consist of the exploitation and integration of different approaches including historical research, inspection, monitoring and structural analysis. Monitoring, in particular, is carried out across the entire process and may be utilized not only for diagnosis but also as an auxiliary tool during the intervention and even during the later post-intervention period for control and preventive maintenance purposes. As part of the strategies proposed for the study and protection of heritage buildings, monitoring is defined as any activity allowing the measurement of any parameter related to the environmental actions or the structural response across a certain period of time. Monitoring, in any case, requires the repeated measurement of structural variables (such as stresses, strains, crack openings, elongations or rotations). These repeated measurements can contribute to identify possible variations which may inform on the health and evolution of the structure. These measurements may be used especially to assess the efficiency of a possible intervention and to conclude on the possible need for improvements or corrections. The strategies applied to the different cases studies, as already described in deliverable D9.3, have distinguished among different phases depending on the nature of the activities carried out on the buildings. Monitoring has been considered as a relevant tool in all these phases and, in most of the case studies, some sort of monitoring has been applied (or, in the case of the intervention and long-term survey proposals, is planned to be applied), in all of them. As also indicated in deliverable D9.3, a maintenance period is considered during which the seismic upgrading solution is kept sufficiently efficient by carrying out, in principle, only maintenance works. The different phases considered are: 1) Investigation phase. Oriented to gather all the needed information to obtain the knowledge on the condition and capacity of the structure and to conclude on the repair, stabilization or strengthening needs. This phase involves all the operations concerning inspection, diagnosis and safety evaluation. 2) Intervention phase. It includes the operations leading to the full design of the intervention and its practical implementation. In particular, it may include incremental approaches based on step-by-step procedures. 3) Evaluation phase. Beginning after the execution of the intervention and elapsing during a limited and defined period of time. During this phase the strengthened structure is continuously or periodically assessed and the effectiveness of the intervention is evaluated through a specific inspection and monitoring programme. The phase encompasses normally a short period (up to 2-4 years) and is oriented to the evaluation of the intervention through intensive survey. 4) Maintenance phase. Its purpose is to control the response of the repaired structure and the maintenance of the expected efficiency of the seismic upgrading solution. The response of the structure is continuously or periodically surveyed. One of the aims of this survey is in detecting unexpected responses that may alert of possible problems. It starts after the evaluation phase and lasts up to the end of the maintenance period. It includes the necessary quality control plans, maintenance work, preventive maintenance and corrective Knowledge based assessment D9.4 3

6 measures. When necessary, improvements or corrections are undertaken to secure the required efficiency levels. In spite of the distinction between the different phases, they are intimately connected and any general action on a historical structure should plan all of them according to unified approaches and criteria. Ideally, similar experimental and numerical tools should be utilized in all the phases, and an adequate and balanced technical and budgetary effort should be devoted to all of them. However, it is also recognized that different problems (regarding the condition of the building, the seismicity, the structural technology and the economical or technical constrains) may lead to combine the phases in different ways or to assign them different relative weights. It is also proposed to update the structural models, throughout all the phases, to take into account the changes introduced (such as the effect of the strengthening) and the variations detected by means of the monitoring. If significant variations are observed, a new seismic assessment should be carried out using the updated models in order to analyse the meaningfulness of such variations and their implications for the seismic capacity. Figure 1 schematizes the subsequent phases in the process involving the study, intervention and later control of a cultural heritage structure. In the example, the investigation reveals that structure is not attaining the necessary seismic capacity. The performance target (expected seismic capacity) is attained and even surpassed after the execution of intervention, which may include some distinct increments if an incremental approach is applied. An evaluation phase follows to verify the efficiency of the intervention. The last phase, and longer one, is the maintenance one during which the structure is monitored to verify the maintenance of the expected seismic capacity. Possible correction actions are undertaken if it decreases below the tolerable limit. At the end of the maintenance period a new seismic assessment, with a possible new intervention, may be necessary. Figure 2.1 Different monitoring phases across the study, intervention and maintenance of a cultural heritage building. Knowledge based assessment D9.4 4

7 2.2 ROLE OF MONITORING DURING THE DIFFERENT PHASES Investigation phase This phase is aimed to characterize the condition of the building, its structural reliability and the intervention needs. More specifically, it involves a detailed inspection of the building, a diagnosis on the ultimate causes of damage and deformation, the structural verification (including seismic assessment) and the identification of the need for stabilization, repair or upgrading. During this phase, monitoring has been considered for three different purposes. 1) Identification of dynamic properties and response. Dynamic operational testing or dynamic monitoring can be carried out to measure the main dynamic properties of the structure or specific structural members. These properties include the shapes of the vibration modes, the natural frequencies and the damping of the structure under normallylow amplitude oscillations due ambient vibration. Moreover, a continuous dynamic monitoring can be implemented in order to capture the effect of microtremors or even farepicenter earthquakes during the monitored period, allowing for a characterization for larger amplitude oscillations. In post-earthquake scenarios, a continuous dynamic monitoring can be implemented to characterize the marginal response of damaged (or even severely damaged) structures under the effect of meaningful replicates. 2) Model updating and validation. One of the main purposes of monitoring is found in the validation and, when necessary, improvement of models through modal matching procedures. The experimental response, as measured by either, or both, static and dynamic monitoring campaigns, is compared with the numerical predictions resulting from a numerical structural model. In particular, the modal matching procedure is considered for this purpose. It involves the upgrading of the model until a satisfactory agreement in both frequencies and modal shapes is obtained. As a second possibility, a dynamic analysis in the time domain can be performed by analyzing the response of the numerical model for accelerograms corresponding to earthquakes conveniently captured during the monitoring period. The dynamic response actually displayed by the building during the earthquake, regarding displacements, damage and possible collapse mechanisms, is then compared with the numerical prediction and again the model is improved until satisfactory agreement is obtained. The modal improvements or updating may target to the material properties, the geometry and morphology of structural members, the modeling of the connections, the influence of the soil and possible soil-structure interaction effects, the influence of neighboring buildings and the damage distribution (particularly, the influence of large individual cracks or separations). For obvious reasons, a previous detailed inspection (including the use of NDT) is of upmost importance for a successful modal updating process. 3) Identification of active deterioration processes. Static and dynamic monitoring can be used to identify active processes causing deterioration in the building. a) Static monitoring can be used to detect slow irreversible trends or, in some cases, anomalous or sudden changes, linked to active deterioration processes causing damage and deformation. For that purpose, it is essential to monitor in parallel the main climatic environmental parameters (as especially temperature and humidity) Knowledge based assessment D9.4 5

8 causing periodical cycles. The daily or seasonal cycles will normally have a largely more noticeable effect in the displacements or crack openings than the possible irreversible processes. Adequate mathematical procedures are then needed to accurately decompose the measurements into their reversible and irreversible components, the latter being associated to the active deteriorating processes. b) Dynamic monitoring has been in some cases proposed for also identifying or localizing damage and damage variation with time. However, and more realistically, dynamic monitoring is better presented as a tool allowing some characterization of the influence of major damage (as large individual cracks or separation between structural members) on the overall structural response. This characterization can be carried out through an updating process over a structural model in which this type of damage is conveniently simulated. 4) Identification of the overall deformation trends. It must be noted that due to long-term creep and other related phenomena, a small irreversible deformation (or crack-opening) component can be normally measured. In this case, one of the roles of monitoring can be found in the identification of the deformational stage of the building and the possible distinction among a primary deformational stage, in which the deformation speed decreases with time, a secondary stage, in which the deformation ratio keeps constant with time, and a tertiary stage at which the deformation accelerates, eventually signaling the achievement of a dangerous situation prior to a possible collapse. 5) Need for emergency actions. Finally, both static and dynamic monitoring can be considered to detect anomalous changes alerting of a possible worsening of the performance or the stability condition, especially in post-earthquake scenarios. This evidence can assist in decision taking on emergency stabilization or strengthening actions. For all these purposes, a previous detailed inspection is highly necessary. In the first place, inspection is carried out to provide all the data needed for a characterization of the building regarding geometry, construction features, materials, morphology and damage. This information is used for two different purposes: (1) the preparation of the numerical model input data and (2) the design of the monitoring. Inspection includes visual inspection, different NDT and MDT and in-situ or laboratory tests (chemical, physical or mechanical) oriented to identify the material composition, existing alterations, working stress levels, material properties, soil foundation properties and damage distribution, among other aspects. In turn, an initial structural analysis may assist in taking decisions on the type of monitoring to be implemented (type, accuracy and range of sensors, number of measurements and critical locations) Intervention phase The proposed general strategy also accounts for a strong interaction between monitoring and intervention. Monitoring is necessary for control purposes during and after the implementation of the upgrading solution. Monitoring can be used (and should normally be used) during the execution of the intervention to control and verify its correct implementation. For this purpose, the monitoring implemented during the previous investigation phase must be kept active, with the necessary modifications, during the execution and after. Moreover, monitoring can have a very active role in decision taking on the extent of the intervention through an incremental approach. A step-by step (incremental) approach can be Knowledge based assessment D9.4 6

9 envisaged where the response of the structure after each operation (intervention increment) is closely assessed in order to evaluate the resulting gain in structural performance and the possible need for further upgrading. Depending on the results of monitoring, additional intervention increments may be gradually considered. This procedure can be utilized to produce actually minimized interventions and to obtain an accurate understanding of their effect on the structure. Normally, dynamic monitoring under the effect of micro-tremors will be necessary as a way to allow for the evaluation of the effect of the upgrading increments. The incremental intervention procedure should be carried out during a clearly defined period, during which the necessary monitoring normally a rather intensive one- is kept fully active and the necessary budgetary and technical means for further intervention are made available. During the intervention phase, the monitoring measurements should be oriented to characterize the stiffness of the structure and connectivity between the different parts. Especially, monitoring can be oriented to assess the performance of improved floor-slabs to work as stiff diaphragms and hence produce an enhance box behaviour. The effect of ties in buildings can also be assessed though their ability to improve the connection among walls, or between walls and floor slabs, to also improve the box behaviour. The effect of injections in walls can also be assessed by measuring their influence on the resulting stiffness. In addition, the strengthening devices to be implemented (for instance, possible ties) can be also monitored using strain gauges or load-cells. Even when the intervention is not explicitly designed as an incremental procedure, the possibility of applying further intervention increments may be left open. If the monitoring lectures reveal that the initial intervention is insufficient, additional upgrading operations can be considered in order to further improve the response of the building. The design of the intervention must be based on a detailed simulation of the proposed strengthening technologies using the structural models and computational tools already utilized for the investigation phase. This simulation will contribute to analyse possible alternative solutions, choose the optimal one and numerically validate its efficiency as upgrading solution. Moreover, and still during this phase, the structural models prepared to simulate the upgraded structure may be re-validated or updated based on the results of the new monitoring of the strengthened building and, if necessary, re-used to carry out an updated seismic assessment. As mentioned in the following sections, the intervention project should include control and maintenance plans. It may also include a plan for corrective operations to be considered in the case of anomalous responses detected by the monitoring during the verification period Evaluation phase After the full execution of the intervention, a subsequent monitoring phase is proposed, extended to a limited period of time, during which the upgrading solutions are carefully evaluated. The evaluation period is oriented to control the correct implementation of the upgrading solutions and to verify that the expected improvement has been actually achieved. The response of the strengthened structure is analysed and the performance of the strengthening solutions is carefully investigated regarding their efficiency and actual influence on the response of the structure. The monitoring technologies to be utilized during this phase are similar to those proposed for the investigation and intervention phase; however, their use is extended to a longer period allowing an appreciation of the variation (or maintenance) of the upgrading effect on the structure. Sensible variations observed through the new monitoring during this phase should lead to update the structural models and then re-use them to carry out a new seismic assessment. The duration of the evaluation phase may involve variable periods from a few weeks or months to even one or more years, depending of the nature of the problem, the importance of the building and the strategies applied. Knowledge based assessment D9.4 7

10 2.2.4 Maintenance phase Finally, the adequate performance of the strengthened structure and the strengthening methods and devices is subjected to a long term survey for verification purposes and, if necessary, to ensure their adequate condition and performance. The maintenance phase extends up to the end of the maintenance period. As mentioned, the maintenance period sets up the time during which the upgrading solutions are supposed to keep sufficiently efficient and in satisfactory condition with only (and adequately planned) maintenance works. The attainment of the end of the maintenance period should motivate new studies and a possible substitution or improvement of the intervention. This phase involves a set of parallel activities, including: 1) Periodical inspection works, aimed at identifying changes and alterations in the building and particularly in its damage pattern (such as new cracking or deformation). The periodical inspection, along with the monitoring activity, should be clearly defined in a control plan elaborated as part of the intervention project. 2) Continuous monitoring or periodical (repeated) measurements allowing the detection of possible changes, occurring either slowly or suddenly, which may alert of unexpected responses or problems. Also, monitoring to measure the effect of occurring micro-tremors and earthquakes is a way to gain further evidence on the behavior of the strengthened structure. Depending on the nature of the problem and constrains, the monitoring carried out during the verification phase may be done using very different procedures. In some cases, it may encompass sophisticated and expensive technologies (such as continuous dynamic monitoring). In other cases, it may rely on simpler or more conventional technologies (as periodical dynamic identification or periodic crack measurement). In principle, the objectives and aims should be those already stated in section for the investigation phase, and similar technologies (if not the same) should be utilized. Notwithstanding, and also depending on the problem, in the verification phase the aims and methods can be simplified and, in some cases, simpler technologies may be sufficient. 3) Early warning monitoring oriented to specifically alert of meaningful changes or problems compromising the seismic response of the building. In some cases, early warning is carried out by means of specific devices such as the instrumented anchors developed within WP6. 4) Maintenance. A detailed maintenance plan must be provided as part of the intervention project. The maintenance plan will be carried out mostly during the verification phase, although it may also comprehend the evaluation one when it takes a significant period of time. The maintenance plan must include, in particular, specific operations to grant the material integrity and efficiency of the strengthening solutions. 5) Preventive maintenance. The maintenance plan may include operations intended to avoid the initiation or to control possible deterioration processes potentially affecting the original materials and structure. 6) Structural modeling and analysis. The result of monitoring can be used to update the models and carry updated seismic assessments. In particular, these tasks should be carried out when significant changes are observed in the structural response or after possible earthquakes. Knowledge based assessment D9.4 8

11 7) Corrective operations. When necessary, improvements or corrections are undertaken to secure the required efficiency levels. For that purpose, the possible corrective operations may be foreseen and defined in a specific plan included in the intervention project. However, the need for a major correction, caused by a severe disagreement with the expected behavior, may signal the need for an early termination of the maintenance period. The continuous or periodical inspection and monitoring works foreseen for the different case studies cover different activities, including crack inspection, static monitoring and dynamic monitoring. Crack inspection is in some cases proposed to be carried out periodically (in some cases, only once in a year) in order to assess the evolution of the main cracks identified in the structure, regarding extension and width. The same operation is proposed to be carried out after possible seismic tremors or exceptional wind episodes. The proposed static monitoring includes crackmeters in the main cracks in walls and vaults, base line measurements across arch spans and inclinometers in pillars and buttresses. The static monitoring systems are designed to allow for the identification of the possible progress of deformation and damage in the building. It is also proposed to keep active the dynamic monitoring systems already implemented before and during the execution of the interventions for an additional period of at least 5 years. Dynamic monitoring after intervention is oriented to assess the expected improvement in the response of the building and its maintenance with time. The dynamic system is considered of special interest for the analysis of the response of the building during future seismic tremors. In some cases, it is suggested to carry out a detailed evaluation of the effectiveness and adequacy of the intervention after a given period of time. The need for additional upgrading operations is careful assessed on the base of the information accumulated during the evaluation and verification periods and, if necessary, additional measures are proposed. A similar evaluation is proposed to be carried out if any significant tremors occurs, giving place to a re-consideration of the entire assessment and intervention on the base of the information provided by the monitoring results. In some cases, a detailed analysis of all the information gathered is proposed to be carried at the end of a first period of 5-years period. Based on it, control and maintenance plans can be reviewed to include the activities considered necessary for the continuation of quality assessment. Any new assessment plan should at least include inspection tasks on a yearly basis. Knowledge based assessment D9.4 9

12 3 REQUIREMENTS The design and application of the monitoring systems to the different case studies has been based on a set of requirements oriented to ensure its adequate performance and ability to provide valuable information. These requirements are discussed in the following paragraphs. Some of the requirements are specifically oriented to ensure the adequacy of the systems for cultural heritage buildings. As mentioned, monitoring is carried out during the entire duration of the process and must be undertaken in parallel to the inspection, modelling and intervention design activities. Previous inspection works are necessary for an adequate design of the monitoring systems. Structural analysis is also needed to assist in deciding about the features of the system In turn, a first set of monitoring results (comprising either or both static and dynamic monitoring) can be used to carry out model updating and validation. A detailed research and inspection of the building is needed before undertaking a monitoring program. Historical investigation and geometrical and morphological surveys are needed, including a characterization based on non-destructive or quasi non-destructive tests.. Damage patterns (particularly major cracks) must also be recognized and carefully documented. The foundation (soil and structure) must be characterized carefully since it may significantly influence the motion and deformations to be monitored. Structural analysis is useful for both the design of the monitoring system and for the later interpretation of the collected information. On the one hand, a prior structural analysis may contribute to define relevant aspects of the monitoring system. Simulation using a numerical model can cast light on the most significant variables to be measured, the expectable ranges of variation (which are meaningful for selecting the type of sensors) or the best location for the sensors. On the other hand, numerical modelling and model updating can contribute with a better and deeper interpretation of the monitoring measurements. In order to adequately interpret the results of monitoring, it is important to characterize the main actions affecting the structure during the period monitored, including the environmental climatic ones. Wind (force and direction), temperature, humidity and accelerations caused by earthquakes micro-tremors should be measured in order to allow for an adequate interpretation of the structural response. The impact of temperature on the movement of structure is normally very prominent and may alter or even mask deformations caused by other possible effects, such as those specifically linked to long-term damage. An adequate post-processing of the results is then needed to separate the reversible components, normally related to environmental effects, from the irreversible ones, which may be connected to active processes causing further damage. Among these, irreversible damage and deformation can be related with gradual phenomena (such as long-term damage linked with creep) or sudden effects caused by earthquake or strong wind episodes. An accurate numerical model must be available to interpret the results and correlate the causes identified (measurements related to actions) with their effects (deformations or displacements measured at different critical points of the building) in light of hypotheses on the configuration and condition of the structure. Characterizing the action in the time domain will later allow its numerical simulation and comparison between the numerical prediction and the actual response measured. Monitoring must be carried out over a period long enough to cover the entire duration of the cyclic actions at work. Although the minimum acceptable period is a complete year, additional annual cycles may be also necessary to confirm the trends observed and characterize the long-term evolution. In some cases (especially for very important structures) a permanent monitoring system may be considered. Moreover, the monitoring system must be designed to allow redundant measurements, allowing results to be interpreted in a more consistently and certain way. For instance, displacements or Knowledge based assessment D9.4 10

13 rotations of a façade experiencing a gradual out-of-plumb can be measured in combination with related crack opening experienced at the junction of the façade with perpendicular walls. The adequacy of the monitoring systems for cultural heritage structures has given place to an additional set of requirements. These requirements stem from general criteria on conservation and restoration of architectural heritage. These requirements, already mentioned in deliverable D9.2, are discussed here again due to their large importance in the selection of the technologies and systems applied to the different case studies. Acceptable monitoring systems should cause null, or almost null, impact on the original structure in terms of alteration or damage. This condition leads to the preference for efficient systems requiring superficial, light and small devices. Systems and devices not requiring any physical/mechanical connection or insertion on the material surfaces (for instance, thermovision or radar interferometry) are advantageous from this point of view. All the equipment and auxiliary devices and particularly the methods utilized to fix the sensors to the surfaces should not cause any meaningful and lasting damage to the original material. Chemical or physical lasting effects resulting from the use of adhesives need to be evaluated. The devices for fixing the systems and devices should avoid or minimize the need for inserted anchors for surface chemical gluing to the original material. If insertions are needed, they should preferably be located in joints rather than in stone surface. In the case of walls or vaults with valuable fixed artistic contents (frescoes, mosaics), the use of adhesives or insertions will normally not be possible. Even if the system is intended to work during a limited period, the devices and auxiliary components should be sufficiently durable and free of possible deterioration problems (for instance, metal corrosion) which may cause deterioration to the original material and surfaces. However, monitoring systems will normally be implemented for a limited period of time, while also needing maintenance and substitution operations. It will be important to ascertain the possibility of dismantling or substituting partially or totally the system without causing any meaningful impact or deterioration on the surfaces and material. The original aspect and aesthetics of the construction and surfaces need to be preserved when the structure is in use and visitors are allowed during the monitoring period. However, it may be adequate to present the systems and devices in a distinguishable way (without compromising the aesthetics of the structure) in order to permit the recognition of on-going works and studies carried out on the building. Knowledge based assessment D9.4 11

14 4 GENERAL APPROCHES Given the nature of the problem analysed, focussing on seismic assessment, most of the monitoring approached applied to the case studies have considered a combination of static and dynamic measurements. Static monitoring is aimed at the continuous measurement of gradually, slow-varying parameters over a long period, while dynamic monitoring involves the intensive measurement of sudden variations caused by isolated and short-lived actions (such as microtremors) over a brief interval of time. Static monitoring requires the regular measurement of small variations over lengthy periods of time. Normally, a few measurements per hour, or even per day, may be enough to characterize the variations caused by daily climatic cycles or other periodical or gradual effects. In turn, dynamic monitoring is intended to characterize the dynamic response of the building and requires a highly dense measurement of the response of the structure during a short period of time. Thousands of readings per minute (for instance, 200 readings per second) may be needed to adequately characterize the oscillation of the structure caused by an external source of vibration, and to later carry out the signal processing leading to the determination of significant dynamic properties such as the shapes of the vibration modes, frequencies and damping. The study of the different cases, reported in more detail in the following section (and in even more detail in deliverable D9.5) has been carried out following one or more of the following basic approaches: 1) Periodical or repeated monitoring. Monitoring is attained by applying, in a repeated or periodical way, inspection non-destructive technologies (NDT). Monitoring by repeated or periodical application NDTs may be regarded as an alternative to continuous monitoring in cases with technical or economic constraints. The repeated or periodical application of NDTs may also be considered as a complementary activity to the continuous monitoring. In the latter case, periodical NDT application is aimed at improving or complementing the information yielded by that continuous monitoring. The adequate application of this approach requires always a parallel measurement of environmental conditions and, if possible, structural parameters such as crack openings. Four different NDTs have been used for the purpose of periodical inspection, namely sonic pulse velocity, radar technique, thermovision and crack measurement are considered and discussed below. 2) Repeated dynamic identification. As a particular case of periodical or repeated inspection, dynamic identification tests are of special importance for the characterization of the dynamic response of the building and the updating and validation of structural models. 3) Continuous static monitoring. It consists of the measurement of the gradual variation of wide variety of variables, either environmental (temperature, humidity, wind parameters) or structural (crack openings, displacements, deformations, work stresses...) by means of a number of specific sensors applied at critical locations on the structural surfaces. 4) Dynamic monitoring. It consists of fixed systems, with sensors permanently active, allowing highly dense measurements adequate for the characterization of the dynamic response of the building. Dynamic monitoring aims to capture meaningful vibrational episodes caused by earthquakes, microtermors or wind. Dynamic monitoring can be carried out using, in turn, three alternative (or complementary) methodologies. Firstly, the information may only be recorded when the amplitudes measured (or the frequencies) surpass a predefined threshold, allowing for the measurement of only meaningful vibrational episodes. Secondly, the system may be programmed to carry out periodical Knowledge based assessment D9.4 12

15 measurements on a daily ore seasonal basis. Thirdly, a continuous recording and storage of the response of the building may be undertaken (requiring a far more sophisticate and expensive system) allowing the latter extraction of meaningful episodes. The purpose and applicability of these possible systems is discussed into more detail in the guidelines as part of deliverable D10.4. Knowledge based assessment D9.4 13

16 5 APPLICATION TO DIFFERENT PROBLEMS 5.1 DIFFERENT PROBLEM TYPES As already mentioned in deliverable D9.3, the case studies considered have involved a different problem types concerning the specific objectives and applied methodology. The different problems identified are a consequence of the meaningful differences shown by the case studies with regards to the seismicity of the location, the quality of the construction materials and techniques, the damage condition, the effect of past or recent earthquakes and the efficiency of possible preexistent interventions. The role and the use of monitoring are discussed in the subsequent sections for the different problem types together with the specific application to a selection of case studies. The following problem types are discussed: increase the knowledge on the structural behavior using Structural Heath Monitoring (SHM) as alternative to the execution of strengthening interventions; precautionary strengthening in order to increase of the seismic capacity; study of the vulnerability of structures and evaluation of the need for seismic improvement; study of the vulnerability of buildings damaged by earthquake and design of provisional strengthening measures to prevent for further collapses and stop on-going damage process activated by the earthquake; repair and seismic strengthening (improvement of seismic behavior) of structures severely damaged by earthquake; validation of effectiveness of adopted strengthening solutions by using monitoring. The description of the application to the case studies focusses mainly on a critical analysis of the monitoring methodologies utilized and the results obtained. No specific reference is made to the technical characteristics of the equipment utilized, as these were already presented in deliverable D9.2. In turn, the description of the buildings referred to is provided in deliverable D9.1 and a full description of the studies carried out on them, including the inspection, structural analyses and intervention designs, is offered in deliverable D APPLICATION Increase of knowledge as alternative to strengthening In the first problem type discussed, monitoring is regarded as a possible strategy to avoid, when possible, a seismic strengthening intervention. The possibility of avoiding the strengthening intervention may emerge from the increase of knowledge on the structural behavior, allowed by Structural Health Monitoring (SHM). This strategy is mainly applicable to buildings for which there is partial evidence (because of the structural typology, limited damage and the result of previous structural analyses) of a sufficient or almost sufficient seismic capacity. Instead of implementing a new strengthening to forcefully grant the full capacity of the building, it is preferred to rely upon a detailed continuous monitoring as a way to assess the condition of the building and possibly gather additional evidence on the seismic positive response of the building. When the knowledge level on a specific structure is sufficiently high, the damage state, the structural response and the vulnerable elements may be adequately characterized. This characterization allows for the definition of safety thresholds allowing to keep the building under control by means of a SHM system, and to postpone the execution of possible interventions unless a worsening of the structural conditions is recorded. Within the project, this strategy has been applied to the Arena of Verona. Knowledge based assessment D9.4 14

17 Arena of Verona In the Arena of Verona, and following the concept of monitoring as alternative to intervention, the SHM system has been designed and installed in order to monitor its mechanical behaviour in a detailed way. The evaluation of the measured quantities, and in particular the study of their changes over time, has allowed useful indications in the definition of the structural behaviour and in the determination of the presence or occurrence of damage phenomena. The monitoring implemented has included the acquisition of the vibrational characteristics of the monument by means of acceleration transducers and the control of the surveyed crack pattern through the implementation of displacement transducers installed on the main cracks. The acquired data have constantly related to the environmental parameters (temperature and relative humidity). For this problem type, and given the critical character of an accurate identification of the response of the building, it is important to evaluate the accuracy of the adopted numerical models on the basis of the actual behaviour. Possible seismic events, adequately monitored, offer a relevant opportunity for the validation of the numerical models. More clearly, the models must be evaluated taking into account the information recorded on both the static displacements (deformation of the controlled cracks) and the dynamic response of the monument. As in most of the case studies considered, the monitoring system includes both static and dynamic subsystems (Figure 5.1). The static system monitors permanently the behaviour of the surveyed crack pattern to control the evolution of cracks related to specific failure mechanisms and, especially, to constantly control the most dangerous damage. It was observed that in case of minor/moderate seismic events occurring during the monitored period, the static sensors proved to be very sensitive and were able to record even very small displacements/rotations. Cross correlating static records with the variation of environmental parameters was then possible to filter the influence of temperature and relative humidity on the crack opening and evaluate if a crack, and hence the controlled failure mechanism or macroelement, was stable or not. In turn, the dynamic system has been implemented to measure and record the vibration characteristics of the structure and extract the modal parameters in order to understand the dynamic behaviour of the monument. It has been observed that natural frequencies are largely influenced by the variation of environmental conditions (especially temperature), showing a clear cyclic behaviour on a yearly basis (Figure 5.2). During the period analysed, no significant variations of the identified natural eigenfrequencies due to damage progress were recorded. The dynamic trigger-based system gives also the possibility to capture some minor and moderate seismic events during the monitoring period. The record of the base acceleration and the response of the structure under dynamic loadings are of great importance from a structural point of view. More specifically, it is demonstrated that natural frequencies are strongly influenced by the level of vibrations. Thus during seismic events (even if of minor magnitude) it is noticed a clear decrement of frequencies induced, probably, by both the larger amount of energy transmitted to the structure (compared to ambient vibrations) and the slight non-linear behaviour that masonry structures show even under moderate seismic loadings. The system utilized for the Arena of Verona has been actually very intensive and is composed of sixteen single-axial accelerometers (acceleration transducers), twenty linear potentiometers (displacement transducer) and four integrated sensors of temperature and relative humidity. The installed monitoring system allows the analysis of a huge amount of data taking into account three main aspects: (i) control of variations of the static measurements, (ii) daily extraction of the fundamental modal parameters; (iii) registration and analysis of possible seismic events. Given the huge dimensions of the structure it was decided to develop and implement a hybrid system in which static sensors are connected through six wireless slaves placed around the perimeter of the Arena to the principal acquisition unit and data are transmitted via a radio antenna, whereas the accelerometers are connected with three wired slaves that transmit the signals to the master via ethernet cables. Knowledge based assessment D9.4 15

18 Figure Location of static and dynamic sensors in the Arena. Figure Development of the first eight natural frequencies of the wing in function of time Precautionary strengthening The second type of monitoring is applied to structures for which a precautionary strengthening to increase of seismic capacity is considered. In this case, a building for which there is possible evidence of insufficient seismic capacity is investigated with the aim of defining a seismic upgrading. In order to define respectful strengthening operations, the envisaged strengthening solutions may be based on step-by-step approaches relying upon an extensive use of monitoring. The response of the structure for minor earthquakes, at subsequent intervention increments, may be monitored as a way to determine the optimal strengthening level. Within the case studies considered, those of the Former Casa da Bragança Foundation Headquarter, Jerónimos Knowledge based assessment D9.4 16

19 Monastery, and the minarets in Bosnia fall within this category. The two last cases are discussed in more detail herein Jerónimos Monastery The health monitoring plan envisaged for Jerónimos Monastery resorts to a limited number of sensors (e.g. a pair of reference accelerometers, strain gauges at critical sections, temperature and humidity sensors, etc.). Data is intended to be stored periodically and the monitoring system should be able to send an alarm. Environmental and loading effects should be studied and the presence of damage should be detected by the global modal parameters. Damage identification methods should be applied to the structure after filtering the environmental effects. The aim of the dynamic methods is to confirm and locate the (possible) damage in a global way; During the control of the intervention, upgrading measures will be carefully studied and implemented if this proves to be profitable. The monitoring has included both a static and a dynamic system (Figure 5.3 and Figure 5.4). The static monitoring system aims at measuring the deformation of two columns in the main nave, ambient and surface temperature, relative air humidity and wind velocity and direction. The measurement system focusses on the columns due to their critical resisting behaviour as part of the structure. The static monitoring system actually implemented is composed of 6 temperature sensors, of which 4 are installed in the North and South walls and the other two on the top of the columns and in the nave extrados; 2 uniaxial tilt meters installed on top of the columns with larger vertical out-of-plumb and in the extrados of the nave, with measurement orientation in the transverse direction of the nave; a combined sensor to measure temperature and relative air humidity; and an ultrasonic 2D anemometer to measure the wind velocity and direction. The system includes a datalogger for the data acquisition and data record, with a Global System for Mobile (GSM) communication device, which allows data remote downloading by phone line. The sampling rate of the static system is one sample per hour. This rate is considered sufficient to observe the temperature variation during one day cycles. The temperature sensors are distributed in the structure to evaluate the effect of the temperature gradient in the response of the structure. TS1 TS6 TL1 TH TS5 W W TS2 TL1 TS5 TS6 TL2 TS4 TS3 TS2 TS1 TH D TS3 TL2 TS m Figure 5.3 Static monitoring system: plan and section of the main nave. Knowledge based assessment D9.4 17

20 Frequency [Hz] NEW INTEGRATED KNOWLEDGE BASED A2 A1, R m Figure 5.4 Dynamic monitoring system. Location of the sensors Static Regression for the First Frequency Temperature [ºC] Figure Variation of first frequency with temperature. The dynamic system is currently implemented is composed by two strong motions recorders with 16 bits ADC analysers provided with batteries. One triaxial force balance accelerometer is connected to each recorder by cable. Two points were selected to install the sensors. One sensor was installed at the base of the structure near the chancel, and the other sensor at the top of the nave and in the extrados. The sampling frequency is equal to 100 Hz. It must be noted that the actual number of sensors installed in the church is not enough to monitor the mode shape changes. During a first phase and until further sensor upgrade, the dynamic monitoring is carried out in terms of resonant frequencies. The sampling is undertaken with the following schedule: (1) the recorders are activated for low acceleration levels and are therefore activated when a micro tremor (or strong wind) occurs at the site; (2) every month, a record of 10 minutes is performed to detect frequency shifts (Figure 6); this allows to separate the influence of environmental conditions and to compare the consecutive dynamic responses before and after the occurrence of significant events; (3) seasonally, 10 minutes records are taken every hour and during one complete day to observe, again, the influence of environmental conditions in the dynamic response of the church. In particular, these measurements allowed the detection of a clear influence of the temperature on the static behaviour of the structure. For the study of the environmental and loading effects all data acquired in the strong motion recorders are used. During the monitored period, the majority of the triggered events occurred during working hours due to the road traffic, special events inside the church (like mass or concerts), and minor Knowledge based assessment D9.4 18

21 earthquakes. With respect to the programmed events, it is noted that it was more difficult to estimate the modal parameters during the night period due to the low ambient excitation level Minarets in Bosnia-Herzegovina The study, oriented to a set of similar structures (several minarets) rather than to an individual one, the study has been carried out using dynamic identification instead of true monitoring. It must be noted that, in real cases, true static or dynamic monitoring (meaning the installation of permanent sensors) may not be possible due to economic or technical constraints. In these cases, an alternative sort of monitoring can made by carrying out a periodical or repeated inspection, as already mentioned. According to this approach, it is envisaged to use dynamic identification also in the future, on one or more minarets, as a means to detect possible variations in the dynamic parameters. More in detail, this repeated inspection would be carried out during and after the implementation of possible seismic strengthening. The dynamic investigation campaign was performed at the beginning of January 2012 by means of the application of output-only identification techniques. A compact unit provided with 24-bit digital acquisition modules, connected to piezoelectric mono-axial acceleration transducers, composed the acquisition system. Once fixed the transducers to the structure in the selected positions, tests consisted in acquiring data over a predetermined period, at a specific sampling rate. Eight accelerometers were used, considering six test setups and 36 acquisition points for the minaret of the Hadži-Alija mosque in Počitelj, whereas eight sensors, five setups and 30 acquisition points for the minaret of the Koski Mehmed-pasha mosque in Mostar. In both cases two reference sensors were kept in the same position at the top of the minarets during the whole test; for each setup a typical acquisition consisted in a record length of samples, resulting in an acquisition time of approximately 22 min at a sample rate of 100 SPS (Samples per Second). In both cases the extraction of the modal parameters lead to very good and accurate results and it was possible to identify 18 structural modes (with associated natural frequencies, modes shapes and damping estimations) of the minaret in Počitelj and 12 structural modes of the minaret in Mostar Study of vulnerability and evaluation for seismic improvement The study oriented to the characterization of the vulnerability of structures and evaluation of the need for seismic improvement is applied to general cases for which there is insufficient previous knowledge on the seismic capacity. It is oriented to evaluate the seismic capacity of the building based on a detailed application of inspection, monitoring and structural analyses. The problem involves, in fact, many different situations and the technologies to be used must be adequately selected and dimensioned in proportion to the seismicity of the location, the importance of the building and the economic and technical means available. This scheme is applicable especially to buildings located in moderate or low seismic places, which, in spite of not having experienced severe earthquakes in the past, may show potential vulnerability under future earthquakes due to the absence of specific seismic resistant structural features. This general scheme has been applied, with certain variations, to the majority of the case studies chosen for the project, including Mallorca cathedral, Ras Cherratine Medersa, the Ambel preceptory and Hagia Sophia Museum in Trabzon. Most of these cases are located in low seismic regions. One of the main aims of dynamic monitoring, for this problem type, is allow for the calibration and validation of the numerical models later utilized for seismic assessment. Obviously, dynamic monitoring is also useful to identify essential information on the dynamic response of the building. Knowledge based assessment D9.4 19

22 Mallorca Cathedral Given the significance of Mallorca cathedral as one of the largest and more daring structures built during the Gothic period in Europe, a complex monitoring system has been implemented including both static and dynamic subsystems. The dynamic monitoring system has been programmed to continuously measure, record, and wirelessly transfer the records of the accelerations without having to set up an activating threshold. It was decided to implement this type of monitoring because the amplitude of the seismic motion expected in the island of Mallorca is low and may be similar in magnitude to frequent wind or other vibrational ambient effects. Under these conditions, the low thresholds necessary to record the effect of seismic motion would allow for the frequent and spurious activation of the recorders by other frequent environmental effects. However, the system is connected to a GPS antenna in order to discipline it to global time. Hence, it is possible to extract from the recorded information meaningful episodes corresponding to seismic events detected by nearby seismic stations, or corresponding to meaningful wind episodes also detected by the static monitoring. The system is composed of digitizer, data acquisition system, GPS antenna, an internet router and the three tri-axial accelerometers. It was decided to install two of the accelerometers on arches of the central nave. In order to choose the appropriate locations, a modal analysis was performed using a finite element model of the cathedral. The normalized modal displacements (for the first ten mode shapes) were compared and those showing the largest displacements were considered for the placement of the sensors (Figure 5.6). Finally, a third accelerometer (named Soil Station) was placed at the ground level of the cathedral close to the third buttress from the façade. The system worked properly from 17 th December 2010 to 13 th September Due to some technical problems it stopped working from 14 th September 2011 up till to 17 th May A new monitoring period started from 18 th May 2012 and is still working. During the monitored period, the website of European-Mediterranean Seismological Centre ( has been checked periodically for the occurrence of any earthquake with significant magnitude that might produce a recordable effect to the cathedral. Following some seismic events, it has been noticed that regional earthquakes with magnitude higher than 4, and teleseismic earthquakes with magnitude higher than 8 are of interest. For this purpose, the timefrequency distribution of each channel has been computed to determine whether the earthquake was arriving with enough energy. In the post-processing of the information for any earthquake, the time-frequency distributions are calculated by 100 seconds Hanning windows, which is considered enough to obtain appropriate frequency resolution and to avoid side lobes. Power spectral densities, coherence and two transfer function estimators (H1 and H2) for different combinations of pairs of channels of S1-Soil Station and 145 Station-Soil Station are computed before, during, and after the considered seismic events. Frequencies with coherence higher than 0.8 are considered for transfer function calculations. Knowledge based assessment D9.4 20

23 Figure Investigated points for accommodating sensors (in blue) and the final chosen locations (in red). Figure Time-frequency distribution of the EW component at 145 Station for Menorca earthquake of During the monitored period it has been possible to capture one local seismic event, corresponding to the Menorca earthquake (31/7/2011, Figure 5.7), and six regional earthquakes with epicentre in Lorca (11/5/2011), Alagüeña (10/7/2011), Gulf of Lion (2/7/2011 and 7/7/2011) and Northern Italy (17/7/2011 and 25/7/2011). Also, It has been possible to register the teleseismic earthquake of Honshu occurred in Japan (11/3/2011). Considering the range of interest of natural frequencies of the cathedral, it was found that the effective duration of regional earthquakes is less than 250 seconds and for the local ones is less than 100 seconds. Also, It was noticed that captured regional and local earthquakes had frequency contents able to excite the range of interest of natural frequencies of the structure. Some effects observed in the dynamic response, even for very low amplitude motion, are attributed to the nonlinear response of the building. Hence, the appearance of multiple close peaks for the same mode in the spectral diagrams is believed to be due to a breathing behaviour caused by existing cracks. Knowledge based assessment D9.4 21

24 The use of a long term dynamic monitoring in Mallorca Cathedral (for nine months so far) has provided a better understanding of its dynamic behaviour compared to that obtained by means of the punctual dynamic identification (dynamic test). The continuous dynamic monitoring has permitted the capture of larger amplitude vibration (several orders of magnitude larger than in the dynamic test) during the occurrence of seismic tremors or strong wind episodes. This information has provided a more reliable measurement of natural frequencies. However, both techniques should be regarded as complementary ones. Due to the sensible effects of climatic environmental actions over the variation of natural frequencies, it was decided to carry out a detailed analysis of the influence of temperature on the natural frequencies. The correlation with temperature of some individual structural elements was studied along with the correlation with exterior temperature. An Infrared (IR) camera of type Thermo GEAR G120 produced by NEC Company was installed in the Cathedral to characterize the temperature on a large region of the inner surface of the structure. The camera was adjusted to take an IR photo every 30 min. The temperatures of different structural elements, including columns, arches, vaults and clerestory walls, were calculated from the IR photos. Good qualitative agreement was found between the temperature changes and variation of natural frequencies, thus providing further evidence on the dependency between temperature and frequencies. For the monitored period the modes number 2, 3, 4 and 8 were continuously identified, The effect of temperature variation resulted in a measurable change in the natural frequencies of those modes. It has been observed that the change in the value of the natural frequencies is more satisfactorily correlated with the outdoor temperature than that measured at the interior surface of the structural members, Although Mallorca Island is a low to moderate seismic zone, the usage of high sensitive accelerometers has allowed for the capture of very low motion earthquakes, For those seismic events the only non-linear effect so far observed is the doubling of some frequency peaks. This peak doubling is probably due to a breathing crack effect. It is expected that the capture of higher motions in the near future may allow an improvement of the characterization of the dynamic response including such non-linear effects Ras Cherratine Medersa In this case the dynamic characterization has only relied upon dynamic identification having been carried out in a punctual way. Again (as in the case of the Minarets), future tests may be carried out, in particular after possible meaningful seismic events, as a way to identify the influence of damage in the response of the structure. The dynamic identification was carried out under the effect of ambient vibration only. Four uniaxial accelerometers of type Bruel&Kjær 8313C were used in the tests. For planning tests, a simplified FE model of the structure was prepared and used to decide on the locations of the points at which meaningful measurements could be taken and on the duration of the tests. Sixteen different setups were planned to be carried out in the available two days. The first eight setups lasted for approximately 20 minutes whereas the other eight setups lasted for about ten minutes. A sampling rate of 4096 Hz was used. Each setup had two test points, one for reference and a moving one. The identification of the natural frequencies and mode shapes was then utilized to update and validate a more detailed FE model encompassing the entire structure. However, some difficulties were found in the upgrading of the model due to the complex dynamic response of the building. This complex response is due, among other aspects, to the fact that the building is connected to other surrounding ones within an urban texture. In spite of the attempts to numerically simulate these connections, the amount of uncertainties existing on the nature and real distribution of the connections limited the possibility of an efficient and fully satisfactory updating of the model Ambel Preceptory The Ambel Preceptory, in Spain, is the only case study involving an earthen structure considered within the project. In spite of being located in a low seismic place, the possible response may be Knowledge based assessment D9.4 22

25 insufficient even for the low intensity earthquakes expected in the location due to the very large dimensions of the building (for an earthen construction) and its severely damaged condition. The monitoring laid-out has been oriented to evaluate the influence of the existing damage on the strength of the building and also to upgrade and validate the numerical model later used for seismic assessment. Instead of both dynamic and static monitoring, the study of the building has been performed by combining true static monitoring (with permanent sensors) with periodic dynamic inspection. The latter was preferred to continuous dynamic monitoring due to its significantly minor cost. The static monitoring has been implemented for the simultaneous measurement of crack openings in a large portion of the building surfaces during an arbitrarily long period of time, in addition to the measurement of the climatic environmental parameters (temperature and humidity). Continuous crack monitoring was also used to assess the influence of temperature and humidity conditions on the structural behaviour change. The system was implemented during a limited period of time (of about 1 month) with LVDT s at opposite moments of the annual thermal cycle with digital crack meters (in summer and also in winter) for which the occurrence of the maximum and minimum temperatures are expected (earthen buildings are particularly sensitive to these factors). Static risk assessment by means of crack monitoring was carried out to know the influence of temperature and humidity conditions on the structural behaviour changes. Data on crack width increase and reduction over time were collected and processed. The proposed arrangement was used for an overall minimum time span of 13 months. Dynamic identification tests (dynamic test) in which velocity values were recorded in different steps and after were used to extract dynamic parameters (natural frequencies, mode shapes and damping) for both the global structure and individual structural members. The dynamic monitoring system was used to measure the velocity parameters of some selected points using geophones. Two dynamic identification campaigns were performed. The goal was to determine the dynamic properties of the building by means of environmental vibrations measurements. Dynamic monitoring was carried out in different seasons, thus allowing for the influence of temperature on natural frequencies variations (earthen buildings are particularly sensitive to these effects). One of the main results of the studies carried out, which have also included the testing of specimens in the laboratory, is found in a significant influence of the climatic environmental parameters on the strength of the material. However, and due to this significant influence, there is significant uncertainty in the characterization of the material for the purpose of preparing a numerical model. The current case study highlights the need for further studies on the direct correlation between salt and moisture contents and compressive strength; on freeze and thaw cycles on compressive strength; on thermal coefficients for these materials; on the effect of moisture on swelling and shrinking of the given rammed earth material; on reasonable values for Young s modulus in relation to moisture content and also on how such Young s moduli are to be calculated. In spite of this uncertainty, dynamic parameters were useful in calibrating and adjusting the structural numerical model (integrated modelling technology). In particular, modal updating can be performed in an iterative way comparing experimental dynamic parameters with numerical dynamic parameters. Finite element analysis software was used to establish a preliminary model, compare it with the whole structural dynamic behaviour measured experimentally, then modify relevant model parameters, and fit the theoretic data with the test result to get a reasonable structural model. This method can be used to control the response of the building before, during and after possible interventions Hagia Sophia Museum in Trabzon In addition to its intrinsic architectural value, Hagia Sophia Museum in Trabzon includes very valuable byzantine frescoes. Therefore, the studies and possible intervention solutions must consider the need to preserve the integrity of this valuable artistic contents. However, and due to Knowledge based assessment D9.4 23

26 economic constraints, no true dynamic monitoring has been applied. The dynamic characterization of the building has relied upon an accurate dynamic identification carried in a punctual dynamic test. As mentioned for other buildings, the repetition of the dynamic test may provide a sort of monitoring with the possibility of identifying possible alterations on the dynamic properties or response after possible seismic motions, and also during and after the implementation of possible upgrading solutions. Static monitoring is performed in combination with the dynamic identification tests. The dynamic identification of the structure was carried out under the ambient vibration taking advantage of heavy traffic nearby, wind from the sea and the effect of sea waves in the close vicinity. For the tests uniaxial seismic accelerometers with a frequency range of 5Hz to 1500Hz were used. The sensors were placed in different parts of the structure according to different setups. As for static monitoring, the aim has been to track any possible change in some cracks existing in the structure, as in the wall that supports the nartex part as well as the barrel vaults of the naves. The variation of the crack activity with temperature inside the structure has been analyzed in detail. Within the crackmeter there is also a thermometer to measure the variation of temperature inside the museum. The use of this thermometer is aimed to observe the influence of the temperature on the propagation of the crack as well as on the behavior of the crack in general. Regarding the dynamic identification, the measurements were taken according to 3 different setups with sensors located (1) at all corners of the external walls in two directions, (2) in two directions on top of main columns carrying the dome and (3) in two directions on top of the main columns and corner walls. Up to 9 accelerometers were simultaneously used. The values were measured in a relatively warm temperature for a duration of 2 hours. It is expected to have some variation in the frequency values if the same measurement is taken in different temperatures and a longer time. The calibration of the numerical model aimed to decrease the difference between experimental results and the analytical results. For this purpose, spring coefficients simulating the bearing conditions were adjusted to improve the agreement between numerical and experimental frequencies and modal shapes. The spring rigidity- soil bearing coefficient was decreased around 25% with respect to their initial values to reach the acceptable levels of agreement. Monitoring is aimed to facilitate the identification of the state and progression of the main structural crack. It will also serve as an early warning system that will provide a base for any further inspection. In addition to permanent monitoring, it is envisaged to execute deep grouting in the main pillars and in some structural walls with voids. The possibility and efficiency of a possible lime mortar injection in such members has been investigated by means of radar technique and boroscopy. Knowledge based assessment D9.4 24

27 Figure The placement of the accelerometers in Hagia Sophia Museum in Trabzon. Figure Spectrum obtained by EFDD based on the data taken from the Hagia Sophia Museum Study and stabilization of buildings damaged by earthquake Another type of problem has consisted of the study of the vulnerability of buildings severely damaged by earthquake. The problem also includes the design of provisional strengthening measures to prevent for further collapses and stop the on-going damage process activated by the earthquake. This strategy is oriented to analyze the response of the damaged building and to design provisional stabilization to control or stop further damage, including possible partial collapses activated by the earthquake. Monitoring is applied before and after the execution of the stabilization technique in order to obtain information on the response of the damaged building and also to control and validate the efficiency of the applied solution. Within the project, this approach has been applied to the Civic Tower and the church of San Silvestro in L Aquila Civic Tower in l Aquila The monitoring system has included both static and dynamic devices (Figure 5.10). In addition to the monitoring, a dynamic identification was carried out using a large number of sensors. The dynamic identification campaign took place in July The dynamic behaviour was evaluated in the damaged conditions of the Tower after the earthquake. The main aim was to identify the global dynamic response of the structure after the provisional strengthening interventions executed by the firemen during the emergency phase. Another important objective of Knowledge based assessment D9.4 25

28 the dynamic tests was the definition of the optimal layout of the SHM system sensors and the characterization of the dynamic properties for FE modelling calibration purposes. Dynamic identification tests were carried out on the Tower and on the two wings of the palace (Eastern and Southern facades) directly connected with the Tower in order to evaluate the dynamic behaviour of the whole structural complex and appraise if the Tower-palace system had still an unitary dynamic response even after the high level of damage and disconnection induced by the earthquake. As it can be noticed from the damage survey, huge cracks appeared along the connection between the Tower and the palace. It was decided to use 32 single-axial acceleration transducers. (a) Figure Layout of the static system composed by 5 displacement transducers, 1 inclinometer, 5 strain gauges and 6 thermal sensors (left). Layout of the dynamic system with 8 acceleration transducers (right). The tests consisted of acquiring data in 3 different registrations over a predetermined period, at a specific sampling rate. Output only identification techniques were used (Operational Modal Analysis). The recorded ambient vibrations were related to the wind excitation and to the urban traffic. In this specific case SHM was applied to control on site the effectiveness of specific provisional strengthening solutions applied to the monument after the damage inducted by the earthquake. When a monitoring system is kept active during and after the execution of strengthening interventions it is possible, in fact, to validate their applicability and evaluate their performance both at a local and global level. A dynamic and static monitoring system installed on the Tower in July 2010 is continuously assessing its structural conditions. It acquires vibration characteristics, evaluates the opening or reclosing of the main cracks, controls the inclination and monitors the deformation of steel ties. The system gives interesting information on the progression or stability of the assessed damage pattern, with reference to the already carried out stabilisation actions and the foreseen strengthening interventions. These readings are constantly related to environmental parameters (temperature and relative humidity). The evaluation of the measured quantities, and in particular their changes over time, gives a strong indication in assessing the structural behaviour of the building. The static system includes devices to measure a set of displacements and strains at critical points of the building. It is composed by 18 static channels: 3 displacements transducers installed on representative cracks in the lower part of the Tower to control the crack width; 2 displacements transducers installed on the gap between Tower and palace to control the relative displacements of the two structures; 1 inclinometer to control the displacement of the Tower s top in two in-plane orthogonal directions; 5 strain gauges on the existing metal ties of the Tower to control the strain variation; 6 thermal sensors to control both air and walls temperatures in different points of the structure. Data from the static system are registered every 30 minutes. (b) Knowledge based assessment D9.4 26

29 The static system is continuously controlling the behaviour of the main surveyed damage. It is possible to note a quasi-linear dependency of crack opening from temperature. From a general point of view, it is possible to state that the damage pattern induced by the earthquake is stable. However, combining data recorded by the inclinometer and the displacement transducer installed between the Tower and the palace one can identify a very slight movement of the Tower along the East-West direction toward the main square. This phenomenon is constantly monitored in order to intervene quickly in case of a worsening of the structural stability of the Tower. The dynamic system has been implemented to measure and record the vibration characteristics of the structure and extract the modal parameters in order to understand the dynamic behaviour of the monument. The continuous dynamic monitoring has several connected purposes: to characterize the dynamic response from ambient vibrations along with its dependence with environmental parameters m (Figure 5.11); to capture the dynamic response in the occasion of possible seismic events; to calibrate reference Finite Element models based on the daily extraction of modal parameters from recorded vibrations. The dynamic trigger-based system, used in combination with the continuous monitoring, gives also the possibility to capture some minor and moderate seismic events during the monitoring period. The dynamic monitoring system is composed of 8 high sensitivity piezoelectric accelerometers connected to an acquisition unit. Three reference sensors are fixed at the base of the structure to record the ground acceleration both in operational conditions and during seismic events. The positioning of the other acceleration sensors on the tower prospects was decided following the results of the dynamic identification. High-density (80 samples per second) dynamic information is continuously recorded. Given its significance, special attention is devoted to the monitoring of the variation of the temperature in the building. Temperature influences in fact largely both the static and dynamic monitoring output, and a good characterization of its variation and distribution within the structure is necessary for a correct interpretation and post-processing of both results. As in other cases, it has been observed that natural frequencies are largely influenced by the variation of environmental conditions (especially temperature), showing a clear cyclic behaviour on a yearly basis. The installed monitoring system stores a large amount of static and dynamic data every day and thus significant effort is devoted to the analysis of the recorded information. The research activities are currently focused on the implementation of automatic procedures for both static and dynamic data processing. The automatic procedure applied to dynamic data elaborates the acquisition files and automatically estimates modal parameters from measured vibrations. Figure Evolution of the estimates of the first and fifth natural frequencies. During the first two years of monitoring no worrying displacements, increments of strain and ongoing damage phenomena have been recorded. It is possible to state that the damage pattern induced by the earthquake is stable. Knowledge based assessment D9.4 27

30 Church of San Silvestro As in the previous case, the design of permanent interventions is postponed and only provisory stabilization solutions have been implemented. In this context, the role of monitoring becomes fundamental as a way to survey the stability of the damaged structure and the adequate performance of the provisional interventions. The monitoring system has been very useful to evaluate possible changes in the structural condition from both static and dynamic points of view. Monitoring is also important for the continuous assessment of the provisory stabilization solutions. The main problems experienced by the building due to the recent Abruzzo earthquake were the partial overturning of the bell tower, the façade and the side chapel and of the apses. Also, shear cracks were observed in the apses and a partial collapse of the roof. The study has included dynamic identification for the bell tower and the façade, and both static and dynamic permanent monitoring. The seismic performance has been evaluated by means of a FE numerical model utilized to simulate the collapse mechanisms that were activated during the earthquake, to study the model under damaged conditions and finally to assess the effectiveness of the temporary interventions already applied to the structure. The church is under constant and continuous monitoring (both static and dynamic). The dynamic identification was carried out during July Other sensors were applied in addition to the already applied sensors used for monitoring, other sensors were applied. In order to find the natural frequencies and associated modal shapes, Output-Only Identification Techniques, based on random ambient vibration with enough capacity to excite all the structure, were used. The measurement of the dynamic response of the structure was carried out under the effect of wind and traffic. The static monitoring includes 2 tiltmeters applied to the bell tower and the façade in two different directions, 7 displacement transducers on the most severe cracks and 2 sensors for the temperatures of air and walls. The system acquires data and records the measurements every 30 minutes. The data coming from static monitoring collected between the 23 rd of July 2010 and the 5 th of February 2012 have been analysed. Crack openings and tilting data have been related to temperature and show a reversible and cyclic character. The dynamic monitoring system of St. Silvestro is composed of 8 high-sensitivity piezoelectric accelerometers placed in the bell tower. These accelerometers are connected to the acquisition unit which sends the data through internet connection directly to the University of Padova. Three reference sensors are fixed to the base of the bell tower to record the ground acceleration. Two accelerometers are placed at a height of m and three at m. The accelerometers location was decided after the dynamic identification carried out before the installation of the system. The dynamic data are sampled at high frequency (80 samples per second) and the environmental vibrations are continuously recorded. In particular, dynamic data collected between the 23rd of July 2010 and the 6th of February 2012 have been analysed in detail. It was observed that the changes of the natural frequencies were only due to temperature. Also the damping showed a variation with the temperature. Both static and dynamic systems will be active during and after the execution of the seismic strengthening, during a period after the intervention for at least two years. Knowledge based assessment D9.4 28

31 Figure Sensors used for the static monitoring (left) and dynamic identification of the structure (right). Green: displacement, Ciano: temperature, Orange: tiltmeter). Figure Damping vs. time for the first mode Repair and seismic strengthening of structures damaged by earthquake A different problem type has involved the repair and seismic strengthening (improvement of seismic behaviour) of structures severely damaged by earthquake. After the application of provisory stabilization measures in structures severely damaged by earthquake, there is the need to design a more definitive solution oriented to repair and sufficiently improve the future seismic response. In this type of problems, the application of step-by-step procedures based on monitoring is of large interest to limit the amount of strengthening provided to the structure. This approach has been applied to the complex of S. Biagio and S. Giuseppe, and the church of S. Marco in l Aquila Church of S. Biagio and Oratory of S. Giuseppe Before the installation of the Structural Health Monitoring (SHM) system, a dynamic identification campaign took place in May The dynamic behaviour was evaluated in the resulting conditions of the two buildings after the earthquake. The main aim was in fact to identify the global dynamic behaviour of the structures in the damaged state, i.e. after the provisional strengthening interventions carried out by firemen during the emergency phase to improve their safety conditions after the earthquake. Other important objectives of the dynamic tests were the definition of the optimal layout of the SHM system sensors and the characterization of the dynamic properties for FE modelling calibration purposes. The dynamic identification was carried out according to 5 test Knowledge based assessment D9.4 29