Structural Health Monitoring and Vulnerability Assessment of Buildings in Earthquake Prone Areas

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1 8th European Workshop On Structural Health Monitoring (EWSHM 2016), 5-8 July 2016, Spain, Bilbao Structural Health Monitoring and Vulnerability Assessment of Buildings in Earthquake Prone Areas More info about this article: Sergey TYAGUNOV, Yuri PETRYNA Chair of Structural Mechanics, Institute of Civil Engineering, Technische Universität Berlin, Berlin, GERMANY Abstract This contribution is dealing with the rapid vulnerability assessment and monitoring of seismic safety of buildings. At that, the SHM technology can play a particular role both at the stage of disaster preparedness (control of the current state of existing structures) and at the stage of post-disaster response and recovery (evaluation of damaged structures after an earthquake). The paper presents preliminary results obtained in the EU project SIBYL - Seismic monitoring and vulnerability framework for civil protection. One of the tasks is to develop a framework for rapid and low-cost in-situ building vulnerability assessment. The proposed approach includes in-situ data collection using visual structural survey, nondestructive testing and operational modal analysis. A simplified integral structural model of the structure for the seismic vulnerability assessment should be developed based on the information collected on site as well as usually quite limited design documentation at disposal. Since vulnerability assessment is subject of significant uncertainties, it is recommended to install SHM systems in representative buildings from various vulnerability classes. The paper describes the developed approach, including the gained experience of insitu data collection and modeling. The concept of SHM systems is discussed in the context of vulnerability assessment and early warning purposes. Keywords: seismic vulnerability assessment, in-situ inspection, operational modal analysis, modelling and simulation 1. INTRODUCTION Structural health monitoring (SHM) has a special relevance in earthquake prone areas, where the existing built environment (in addition to the dead and live/operational loads) is subjected to seismic ground motions. Even though the most of modern urban structures are designed and constructed to resist earthquake loads and the seismic vibrations are mostly lower than the design earthquake loads, one should take into account that the smaller earthquakes are more frequent phenomena and the physical damage (even if not significant from a single earthquake) can be accumulated in the affected structures under repeated seismic events. Besides, there exist still many older structures, which do not comply with the current seismic codes and can be especially seismically vulnerable. Therefore, the SHM technologies can play an appreciable role in assessing the seismic vulnerability of existing building stock and identifying the vulnerable structures. It is true both at the stage of disaster preparedness (meaning the control of the current state of existing structures) and at the stage of post-disaster response and recovery (aiming at the evaluation of damaged structures after an earthquake).

2 The paper presents preliminary results of the EU project SIBYL (Seismic monitoring and vulnerability framework for civil protection) obtained by a multidisciplinary consortium of scientists from Germany, Greece and Italy. One of the main tasks is dedicated to developing an operational framework for rapid and low cost in-situ vulnerability assessment of buildings in earthquake-prone areas. The test sites of SIBYL are located in Thessaloniki (Greece), Cologne (Germany) and L Aquila (Italy). The present work mainly focuses on reinforcedconcrete frame buildings with in-fill walls. The proposed approach includes in-situ data collection using visual structural survey, non-destructive testing (NDT) and ambient vibration measurements in the context of operational modal analysis (OMA). The structural information collected on site in combination with the results of operational model analysis is used for developing a simplified integral structural model (SISM) for evaluating seismic performance of the building. The present paper is focused on the procedure of in-situ measurements (including both structural survey and operational modal analysis) as well as on developing a simplified integral structural model suitable for the seismic vulnerability assessment of structures. However, the vulnerability/fragility analysis itself, e.g. [1, 2], is beyond the scope of the paper. The application of the approach is demonstrated on the building of the Faculty of Philosophy of the Aristotle University in Thessaloniki (AUTH). 2. IN-SITU STRUCTURAL SURVEY AND MEASUREMENTS 2.1 Data collection The following main sources of information can be generally used for the structural modelling: construction documentation (drawings and specifications); in-situ collected data (structural survey and measurements) and simulated design (following to the contemporary codes and standards). The amount of available information differs for each building depending on its age, function, ownership, codes of practice, etc. The Table 1 summarizes a typical list of data required for structural modelling. Along with the listed characteristics, we also indicate their ranking, reflecting their informative relevance in the context of the seismic vulnerability assessment from our point of view. In the first line, for this purpose one needs information about the lateral-load-resisting system and the material of bearing structures. This information can be considered as minimum input data, which allows one to make a first judgement about the seismic vulnerability of the building (based, e.g., on the vulnerability classification of EMS-98 [3]), though, apparently, such a rough estimation involves essential epistemic uncertainty. Accounting for additional building-specific information (see Table 1) would allow reducing the uncertainty and increasing the accuracy of the vulnerability assessment with the use of more sophisticated approaches. If some of the needed data are not available from the construction documentation, they can be collected on site. Otherwise, these parameters can be estimated using simulated design. The simulated design should be based on the contemporary national/international codes and standards; as well one should take into consideration the regional peculiarities of design and workmanship. For this purpose, the information about the occupancy of the building and the year of construction (modification) can be helpful and, therefore, should also be obtained. Keeping in mind the objectives of the project, we engage easily-accessible methods and tools, combining analysis of available original design documentation and the simulated design with a limited/extended in-situ inspection including the visual survey and measurements of the existing structures and the methods of non-destructive testing. 2

3 No. Data type Ranking 1 Lateral load-resisting system and material of bearing structures 1 2 Overall dimensions and shape of the building 1 3 Presence and location of separation joints 1 4 Presence of irregularities (physical or geometrical / in plan or in elevation 1 5 Dimensions and location of structural elements (columns, walls, slabs) 1 6 Cross-sections of the structural members and their material properties 1 (strength, elastic moduli, specific density) 7 Year of construction (modification) 2 8 Occupancy of the building 2 9 Non-structural elements and other building components, which can contribute 2 to the stiffness and/or mass distribution 10 State of the preservation of the building (structural system) 2 11 Depth and type of foundation 2 12 Local soil conditions 2 13 Position of the building with respect to the neighboring buildings 2 Table 1: Data required for structural modelling and vulnerability assessment 2.2 Operational modal analysis (OMA) Since a full structural data cannot be obtained in a short time on site, a significant uncertainty has to be expected from the corresponding structural model. In-situ vibration measurements on the building can help reducing the uncertainty and are, therefore, an important part of the entire data collection procedure. The operational modal analysis usually delivers the natural frequencies and the corresponding vibration modes which shall be used to validate the simplified structural model. If some previous vibration data are available from the past, the current measurements can be used to estimate structural changes, damage or degradation. This may help identifying vulnerable points of the structure. Operational modal analysis in the framework of rapid vulnerability assessment differs from the usual one in one respect: the measurement time and effort should be minimized in view of tolerance to a limited accuracy of the model due to manifold uncertainties. Therefore, it allows for measurements with a minimum set of sensors, for example, two ones placed on the building roof and one on the ground. Ambient vibrations are recorded using seismic stations; each of them composed by a 24 bit DSS-CUBE3 digitizer connected to a 4.5Hz three-component geophone. The sampling rate is set to 400 Hz and the timing is provided by a build-in GPS. The measurement time for ambient vibrations is taken usually between 30 and 60 minutes. The data processing including the format transformation, correction for the instrumental response and filtering in the present study has been provided by our partners from GFZ Potsdam. The identification of the eigenfrequencies and mode shapes is performed by use of the MACEC software [4] on the base of time intervals of 180 s duration. Since only the first bending and torsion modes are of interest, a limited number of sensors is feasible. 3 SIMPLIFIED INTEGRAL STRUCTURAL MODEL A simplified integral structural model (SISM) has been developed in order to avoid a sophisticated finite element analysis of the buildings and provide a simple tool for technical staff of civil protection, which allows focusing on essential structural members during data collection and decision making on site. 3

4 3.1 Dynamic model Real reinforced concrete structures with in-fill walls can be rationally modeled by one integral beam element and one mass for each floor, as depicted in Fig. 1. In the framework of seismic analysis, we consider only one degree-of-freedom (horizontal displacement) and one force (e.g. inertia force) per floor. Figure 1. Real building and its simplified integral structural model (SISM) The corresponding stiffness and mass matrices of such a dynamic system are given below exemplarily for 3 degrees-of-freedom: + = [ + ], = [ ]. (1) (2) The natural frequencies and mode shapes Φ can be obtained numerically from the generalized eigenvalue problem for the stiffness and mass matrices: = (3) with = { }; = {Φ }, =,,. (4) The integral storey stiffnesses k i are calculated by use of a simplified model for frames with in-fill-walls with coupled bending and shear degrees-of-freedom, as described below. 3.2 Coupled shear-bending model Typical buildings with about 2 to 20 storeys cannot be exactly modeled as a beam, irrespective of the fact whether the Bernoulli or the Timoshenko theory is applied. The dimensions of real buildings do not match typical relations suitable for the beam theory. Usually, such buildings exhibit both bending and shear deformations, which are combined depending on the individual stiffness properties. 4

5 Simplified structural beam models for such buildings with coupled shear and bending deformations can be developed in the framework of the displacement method by use of the so-called group degrees-of-freedom (DOF). The procedure developed in the present work is explained below. It shall be applied for each storey individually. We consider a frame reinforced concrete structure with in-fill walls as depicted in Fig. 2,a. The single integral DOF is the horizontal displacement u which corresponds to a horizontal force F. This equivalent beam model shall include both shear and bending deformations, as shown in Fig. 2,b. a) b) Figure 2: Frame structure and its equivalent beam model with a single DOF a) shear in a frame b) shear in a wall c) pure bending in a frame d) bending with longitudinal forces Figure 3: Three characteristic DOF with corresponding forces and displacements The 3 grouped degrees-of-freedom under consideration are the horizontal displacement u, the nodal rotation and the slab rotation or its pendant. The reaction forces to the 5

6 mentioned displacements, which describe the corresponding stiffnesses of interest, are shown in Fig. 3. Such a formulation results in the following coupled stiffness relation: = [ ] [ ] = [ ]. (5) By solving it according to (6) = [ ] =, (6) we get an equivalent beam stiffness with respect to the horizontal displacement u: = with. (7) All reaction forces for the beam members in Fig. 3 are derived according to the classical Bernoulli theory, since typical columns and girders in buildings match the corresponding requirements quite well. The shear stiffness of the walls is also considered according to the classical theory of the thin-walled structures. The discussion of the individual stiffness contributions is, unfortunately, beyond the scope of the present paper. However, it is important to note that the equivalent storey stiffness significantly differs from the pure shear stiffness K uu due to the coupled degrees-of-freedom. 4 CASE STUDY: UNIVERSITY BUILDING AT AUTH (GREECE) 4.1 In-situ measurements and modal analysis According to the original design documents provided by the administration of AUTH, the building of the Philosophical Faculty (FB) was designed and constructed in the sixties of the last century. The bearing structural system of the building is represented by reinforced concrete frames consisting of columns and beams. The building consists of three units separated by two separation joints. The main façade of the building is shown in Fig.4. separation joints Figure 4. The building of the Faculty of Philosophy of AUTH 6

7 During the in-situ inspection in the building, considerable discrepancies with the original design documentation were detected. First of all, the existing building has one additional floor in comparison with the original drawings. Furthermore, sample measurements of the cross-sectional dimensions of structural elements (columns and beams) made in the course of in-situ inspection, showed considerable changes in comparison with the data from the available drawings. It has been revealed lately, that one floor was added and the bearing structural elements were strengthened about 20 years after the original construction. In this situation, an extended/comprehensive in-situ inspection would be necessary to collect the information required for adequate structural modeling. Obviously, such kind of structural modification would not only considerably change the vibration parameters of the building, but also influence its seismic performance. Any computational analyses solely based on the original design drawings, neglecting the structural modifications made after the construction, would produce misleading results and, therefore, inadequate decisions. This fact emphasizes the crucial importance of in-situ inspections and in-situ measurements for assessment of actual structural vulnerability of existing buildings. The vibration measurements in the building were concentrated on the central unit (as this part is used for structural modelling), where sensors were placed at the four corners of the 1st, 2nd, 3rd, and 4th floors. Two sensors were installed on the roof, and two in the underground basement. The two side units were monitored less densely (the sensors were installed at the four corners of the 1st and 4th floors, and one sensor in the semi-basement). The spatial arrangement of the sensors is presented schematically in Fig. 5. For the purpose of investigation, the total duration of measurements was about 20 hours, including, therefore, both day and night time. Figure 5. Schematic representation of the instrumentation layout in Philosophical Faculty building The modal analysis of the recorded data was performed with the help of MACEC 3.3 software [4] using stochastic subspace identification approach [5]. The results of the operational modal analysis are shown in Fig. 6. Two bending modes - in the longitudinal (x) direction and lateral (y) direction as well as a torsional mode could be well identified. One of the obtained results shows that the separation joints are only in part effective, since there is a certain coupling of vibration observed between the central unit of the building and its side ones in all three modes. The torsional mode is, however, less relevant for the seismic vulnerability assessment due to the symmetry of this building. So, we focus on the central unit and consider first of all the bending modes in the structural model described below. 7

8 View from the top Main façade, x-direction Side façade, y-direction 1 st mode (f = 1.60 Hz) 2 nd mode (f = 1.72 Hz) 3 rd mode (f = 1.76 Hz) Figure 6. Results of operational modal analysis for the Philosophical Faculty building 4.2 Finite element model The finite element (FE) model serves in the present work as an intermediate step of structural modeling that can visualize the essential deformation modes of the building. The latter shall be an essential part of the simplified integral structural model (SISM). The FE model has been developed by use of classical beam and shell elements within the LIRA software [6]. Only essential structural elements like columns, elevator shaft, shear walls and slab have been considered. All secondary walls, windows and non-structural elements have been taken into account only as additional masses in an integral manner. It is important to know that the material properties of concrete and masonry members could not been directly measured on site as well as the stiffness of all joints. These parameters are taken according to the usual design practice for such buildings. Of course, this fact introduces an essential uncertainty into the model, which can be generally reduced by a comparison of the measured and calculated modal parameters. Fig. 7 shows the FE model and its deformation modes under the test horizontal loading on the top of the building, put in x- and y-direction separately. One can clearly identify the horizontal displacement, the slab rotation and the individual nodal rotation in the deformation states both in x- and y-direction depicted in Fig. 7,b-c. Fig. 8 shows the first three calculated vibration modes. Except for the torsional mode, there is a good agreement between the numerical and operational modal analysis shown in Fig. 6, keeping in mind essential uncertainties involved. The weak ground floor with no shear walls generally dictates the global behavior of the building. 8

9 a) FE model b) x-direction c) y-direction Figure 7. Finite element model and its characteristic static deformations torsion f 1 = 1.37 Hz bending, y-direction f 2 = 1.83 Hz Figure 8. First calculated vibration modes bending x-direction f 3 = 1.94 Hz 4.3 Simplified structural model and its validation Both the SISM and the finite element model for the building have been developed on the basis of the same geometry, material and structural data. The torsional stiffness and the corresponding torsional mode of vibration have not been considered, as mentioned above. Figure 9. First bending modes of the SISM Two bending modes, calculated from the eigenvalue problem for K and M, are shown in Fig. 9. They match qualitatively well the measured bending modes of the building (Fig. 6) as 9

10 well as those calculated by the finite element method (Fig. 8). The corresponding natural frequencies are generally lower than the measured ones and those calculated from the FE model. The relative differences with respect to the measured frequencies show a good result in view of large uncertainties mentioned above and a quite simplified modeling procedure without FE software. 5 CONCLUSIONS AND FUTURE WORK The data collection procedure is supported by a Microsoft Excel sheet developed by the authors for this purpose. Such a tool should be available at each computer and easy to use by any technician or engineer working in the network of civil protection. This tool delivers automatically the stiffness and mass matrices for each floor, as exemplarily shown in (1, 2). The modal analysis, i.e. the solution of the eigenvalue problem (3) is currently executed within the MATLAB software Based on the gained experience, we can emphasize the importance of both in-situ inspections and in-situ measurements for assessment of actual state of existing structures. Even a limited inspection and short-term monitoring of the state of existing buildings can considerably contribute to the quality of structural models and improve the results of vulnerability analyses. Needless to say, long-term SHM would allow permanent observing of the current state of the structures under study, taking into consideration possible changes due to eventual internal or external interferences. The future steps of our work in this direction are aimed at installing SHM systems in representative buildings of different vulnerability classes. This will serve to the purposes of both vulnerability assessment and early warning. ACKNOWLEDGEMENTS The authors would like to thank the European Commission for the financial support of the present work within the SIBYL Project, Grant ECHO/SUB/2014/ We also gratefully appreciate the help of our colleagues from the GFZ Potsdam and AUTH during the measurement campaign in Thessaloniki. REFERENCES [1] P. Gueguen (Eds). Seismic Vulnerability of Structures. Wiley ISBN: (2013). [2] K. Pitilakis, H. Crowley, A. Kaynia (Eds). SYNER-G: Typology Definition and Fragility Functions for Physical Elements at Seismic Risk, Buildings, Lifelines, Transportation Networks and Critical Facilities. Springer ISBN: (2014). [3] G. Grünthal (Eds). European Macroseismic Scale 1998, Cahiers du Centre Europeen de Geodynamique et de Seismologie, Vol. 15, Conseil de l Europe, Luxembourg (1998). [4] E. Reynders, M. Schevenels, G. De Roeck, MACEC 3.3: A MATLAB toolbox for experimental and operational modal analysis. User's Manual, Report BWM , Department of Civil Engineering, KU Leuven (2014). [5] E. Reynders, System identification methods for (operational) modal analysis: review and comparison. Archives of Computational Methods in Engineering, 19(1), , (2012). [6] LIRA 9.2, Lira Soft Ltd., Kiev, Ukraine. 10