Rafael Aguilar

Operational modal analysis of historical constructions using commercial wireless platforms

Operational modal analysis is currently applied in structural dynamic monitoring studies using conventional wired-based sensors and data acquisition platforms. However, this approach becomes inadequate in cases in which aesthetic concerns arise (e.g., in cultural heritage buildings) or in which the use of wires greatly impacts the monitoring system cost and creates difficulties in the maintenance and deployment of the data acquisition platforms. In these cases, the use of a WSN and MEMS would clearly benefit the applications. This work illustrates an attempt to apply the wireless technology for the structural health monitoring of historical masonry constructions in the context of operational modal analysis. The article presents the state of the art of the wireless monitoring systems making a review of the applications in the civil engineering field. Subsequently, commercial wireless-based platforms and conventional wired-based systems are applied to study one laboratory specimen and one structural element from a 15th Century building located in Portugal. The results achieved in this study showed that, in comparison to the conventional wired sensors, the commercial wireless platforms have poor performance with respect to the acceleration time series recorded and the detection of modal shapes. However, reliable results were obtained for the measured frequencies.

A wireless sensor network platform for structural health monitoring : enabling accurate and synchronized measurements through COTS+custom-based design

Abstract Structural health monitoring has long been identified as a prominent application of Wireless Sensor Networks (WSNs), as traditional wired-based solutions present some inherent limitations such as installation/maintenance cost, scalability and visual impact. Nevertheless, there is a lack of ready-to-use and off-the-shelf WSN technologies that are able to fulfill some most demanding requirements of these applications, which can span from critical physical infrastructures (e.g. bridges, tunnels, mines, energy grid) to historical buildings or even industrial machinery and vehicles. Low-power and low-cost yet extremely sensitive and accurate accelerometer and signal acquisition hardware and stringent time synchronization of all sensors data are just examples of the requirements imposed by most of these applications. This paper presents a prototype system for health monitoring of civil engineering structures that has been jointly conceived by a team of civil, and electrical and computer engineers. It merges the benefits of standard and off-the-shelf (COTS) hardware and communication technologies with a minimum set of custom-designed signal acquisition hardware that is mandatory to fulfill all application requirements.

Continuous Stiffness Monitoring of Cemented Sand through Resonant Frequency

Mixture formulation and in-situ quality control of the stabilized soils often represent difficult and challenging tasks. The present paper addresses the possibility of using a variant to a recently developed non-destructive technique for continuous monitoring of stiffness of hardening materials as a supporting means to the above-mentioned tasks. The material to be tested is placed inside a polycarbonate mold placed in simply supported conditions. The technique is based on the continuous monitoring of the first resonant frequency of this composite beam, which evolves as a consequence of the hardening of the material, and can be correlated with its E-modulus. The usefulness and potential of this experimental methodology for mixture formulation and quality control of stabilized soils is shown through a series of tests conducted on laboratory since the instant of mixing until 7 days. The conducted tests include complementary methodologies of characterization such as E-modulus measured on specimens with strain instrumentation, as well as monitoring with recourse to bender-extender elements.

Dynamic Identification and FE Updating of S. Torcato Church, Portugal

San Torcato Church is located near the city of Guimaraes, Portugal. The construction of the church started in 1871 and completed in recent years. The church combines several architectonic styles, like Classic, Gothic, Renaissance and Romantic. This “hybrid” style is also called in Portugal as “Neo-Manuelino”. At the moment, the church has significant structural problems due to soil settlements of the main facade. Cracks can be observed in the main and lateral facades. The cracks are visible from the outside and inside of the temple. The bell-towers are leaning, and the arches in the main nave present vertical deformations. Non-stabilized phenomena are present in the structure. To stabilize the damage, a structural intervention is planned to occur soon and the church is already monitored to assist the intervention. The paper clearly presents the problem with emphasis to the dynamic analysis carried out before the structural strengthening, namely: the experimental tests with output-only techniques for frequencies, damping and mode shapes estimation, and FE model updating analysis.

Operational Modal Monitoring of Ancient Structures using Wireless Technology

Operational Modal Analysis is currently applied in structural dynamic monitoring studies using conventional wired based sensors and data acquisition platforms. This approach, however, becomes inadequate in cases where the tests are performed in ancient structures with esthetic concerns or in others, where the use of wires greatly impacts the monitoring system cost and creates difficulties in the maintenance and deployment of data acquisition platforms. In these cases, the use of sensor platforms based on wireless and MEMS would clearly benefit these applications. This work presents a first attempt to apply this wireless technology to the structural monitoring of historical masonry constructions in the context of operational modal analysis. Commercial WSN platforms were used to study one laboratory specimen and one of the structural elements of a XV century building in Portugal. Results showed that in comparison to the conventional wired sensors, wireless platforms have poor performance in respect to the acceleration time series recorded and the detection of modal shapes. However, for frequency detection issues, reliable results were obtained, especially when random excitation was used as noise source.

Prototype WSN Platform for Performing Dynamic Monitoring of Civil Engineering Structures

Structural Health Monitoring represents the present and future of the civil engineering since, until few years ago, structural diagnosis works had been performed with few resources regarding to experimental techniques. Precisely in the field of monitoring sensors, the progress of new technologies based on wireless communications and Micro-Electro-Mechanical- Systems (MEMS) are of high interest for replacing the handle difficult wired sensors. However, three major limitations of the commercial off-the-shelf technology on WSN (combination of MEMS and wireless technology) for performing dynamic monitoring were identified by means of: (1) not enough sensitivity of the accelerometers; (2) low resolution of the ADC embedded; and (3) lack of synchronization algorithms implemented. This paper presents a new prototype system conceived for performing dynamic monitoring civil engineering structures. This system was jointly conceived by a team of civil, electrical and communication engineers and is a combination of the last technology on high resolution MEMS accelerometers and the state of the art of communication technologies. Despite the fact that the prototype system needs more improvements; the results of several rounds of validation experiences confirm the feasibility for its consideration as an alternative to the conventional wired based sensors.

SEISMIC VULNERABILITY ASSESSMENT OF A 17th CENTURY ADOBE CHURCH IN THE PERUVIAN ANDES

ABSTRACT A large part of Peruvian heritage corresponds to Colonial Andean churches, which are economically and culturally essential for the development of several villages. Particularly, adobe churches are very vulnerable to earthquakes due to the brittle material behavior, the weak connections, and the absence of a rigid diaphragm. This article aims at performing the seismic vulnerability assessment of an adobe heritage church: “Virgen de la Asuncion” of Sacsamarca located in the province of Ayacucho in the Andean region of Peru; using a rapid screening method and a more complex numerical analysis. First, the damage survey was carried out through visual inspection and infrared thermography, while the material characterization was performed by applying flat-jack tests. Later, the seismic vulnerability assessment was carried out in two stages. The first was based on simplified criteria for masonry structures, and the other on a pushover analysis of global performance. From this evaluation, it was possible to conclude that the church is indeed vulnerable to Peruvian seismic demands.

Identification of the Tensile Force in Tie-rods of Historical Constructions

This paper addresses the problem of the bending curvature due to self-weight of tie-rods when using dynamical approach to identify the tensile force in tie-rods of historical constructions. Although several dynamic testing methods have been proposed in the literature, the effect of bending curvature due to self-weight of the rods on their frequency values has not been studied. In this work, the bending curvature due to self-weight of tie-rods with small cross-section-to-length ratios is proven to have significant effect on their frequency values of the first vibration mode at low tensile stresses. As a result, the accuracy of the identified tensile force in tie-rods will be affected if the effect is not accurately considered. Four tie-rod specimens of different characteristics were tested in laboratory by dynamic tests. A numerical model was developed for axially loaded tie-rod using a FE program, assuming Euler beam with uniform cross-section and rotational springs at both supports. By calibrating the experimental and numerical results, the most suitable dynamical analysis for tie-rod models to take into account the effect of bending curvature due to their self-weights is concluded. In particular, the analysis should be performed in two steps: (i) first, the static geometric non-linear analysis to obtain the deflected shape of the tie-rod due to its self-weight and an applied tensile force; (ii) then, the modal analysis is run on the deflected tie-rod to achieve the frequencies and mode shapes via free vibration at that applied tensile force. When the effect of bending curvature due to self-weight of tie-rods is neglected, the frequency of the first mode should be excluded. Based on these conclusions, two techniques to identify in-situ the tensile stress in tie-rods are discussed. They are frequency-based identification techniques that minimize the measurement errors. In addition, a methodology to estimate a range of tensile stress using a formula or two selfconstructed standard charts is proposed.

Dynamic identification and monitoring of St. Torcato Church

The paper is related to the San Torcato Church, in Guimarães, Portugal. At the moment, the church has significant structural problems due to soil settlements. Cracks can be observed on the main and the lateral façades, the bell-towers are leaning, and the arches in the nave exhibit a failure mechanism with cracks and vertical deformations. Non-stabilized phenomena are present in the structure. To stabilize the damage, a structural intervention is planned to occur soon and the church is already monitored to follow the intervention. The paper clearly presents the problem with emphasis to the dynamic analysis carried out before the structural strengthening, namely: the experimental tests with output-only techniques for frequencies, damping and mode shapes estimation, FE model updating analysis and dynamic monitoring

Mechanical Characterization of Adobe Constructions Using Flat Jack Tests: Case Study of the Virgen de la Asunción de Sacsamarca Church

This article presents the results of an experimental campaign using the flat jack technique in an adobe church called “Virgen de la Asuncion” located in Sacsamarca, Peru. The main mechanical properties of this construction were determined using the single and the double flat jack tests. The flat jack test is considered as minor destructive test (MDT), which is appropriate for historical buildings due to the temporary and easily repairable damage generated. Single and double flat jack tests were carried out on the north and east facades of the church. The single flat jack test allowed the estimation of the local compressive stress levels through the continuous monitoring of the state of strain. The double flat jack test allowed determining the local stress-strain behavior in each facade of the church. The mechanical properties obtained in this research are similar to values obtained by other authors for historical adobe buildings using different methods.