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Über dieses Buch

Dynamics of Civil Structures, Volume 2: Proceedings of the 36th IMAC, A Conference and Exposition on Structural Dynamics, 2018, the second volume of nine from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of the Dynamics of Civil Structures, including papers on:

Modal Parameter Identification

Dynamic Testing of Civil Structures

Control of Human Induced Vibrations of Civil Structures

Model Updating

Damage Identification in Civil Infrastructure

Bridge Dynamics

Experimental Techniques for Civil Structures

Hybrid Simulation of Civil Structures

Vibration Control of Civil Structures

System Identification of Civil Structures

Inhaltsverzeichnis

Frontmatter

Chapter 1. Experimental Modal Analysis Study of a Standing Soldier and Rifle System

The response of the human body to shock and vibration has been a subject of interest to many researchers in the aerospace and automotive industry. In a new study, an experimental modal analysis of a rifle-armed, standing soldier in a standard firing position was performed. The purpose of the study was to determine the modes of vibration of the soldier-weapon system in order to gain an understanding of its response during the firing event. The weapon firing accuracy, especially during closely-repeated semi-automatic or fully-automatic fire, as well as the energy transmitted to the body of the soldier depend not only on the weapon itself but also on the soldier body’s dynamic characteristics. Rapid weapon fire does not allow a soldier to consciously control muscles needed to bring back the weapon to its original position, so the weapon location after each fire is influenced more by the dynamic characteristics of the human-rifle system. The experimental modal analyses were performed using multi-averaged, impact-force and electrodynamic shaker force excitation (mainly sine sweeps) and roving triaxial acceleration response at various locations on the body. It was observed that testing of human subjects poses significant difficulties since an increasing number of measurement averages could lead to muscle fatigue and ensuing tremors that could negatively influence coherence. The soldier stance during the tests could also change due to the unconscious need to adjust to a more comfortable body position. The positioning of the accelerometers was difficult since attachment could be made to skin only. While there was in general large variability in soldier size, mass and body strength, the study allowed the identification of some lower modes of interest which appeared to have the same mode shape, albeit at different frequencies, for all the various individuals tested. The signals were acquired with a National Instruments hardware and processed using ModalView software. For an average size soldier the first mode occurred at approximately 2 Hz. The first mode shape exhibited combined bending (backward) and twisting characteristics which are generally seen in a “up and to the right” motion of a right-shoulder held weapon during firing. More modes in the 0–20 Hz range were identified.

Razvan Rusovici, Joshua Drew, Brian Fischer, George Kontis, Terrence F. Rice, Francis J. Battersby, Michael Pavlisak

Chapter 2. Performance Characterization of Modal Identification Algorithms, the Case of Automated Modal Analysis of Palazzo Lombardia

In this paper the study of the analysis of the data coming from the permanent monitoring system of Palazzo Regione Lombardia, one of the tallest buildings in Milano is presented. The system works acquiring both the vibrations of the building and its static behaviour. The vibration data are used to carry out continuous modal analysis to the aim of detecting possible changes of the structure. Two algorithms for modal parameter analysis are presented and compared through Monte Carlo simulations. The results of this comparison allowed to choose the best data analysis procedure to be used on the real data measured under operating conditions. The first results coming from the modal parameter extraction are presented.

Marcello Vanali, Marta Berardengo, Stefano Manzoni, Giorgio Busca, Elena Mola

Chapter 3. Dynamic Characterization of the Little Belt Suspension Bridge by Operational Modal Analysis

The (new) Little Belt Bridge, opened in 1970, is a Danish suspension bridge with largest span being 600 m, and a total length of 1700 m. During the design and construction phase, detailed analysis of the dynamic properties of the bridge were carried out both by hand calculations and by measurements on a scale model. Recently, the bridge was measured using a setup of 45 simultaneous responses, 30 vertical and 15 lateral, distributed over the main span. Operational modal analysis was carried out on the data set, and the first two vertical bending and torsional modes were compared to those of the model. It was found that the first vertical bending mode was 0.156 Hz, which is very near the frequency predicted by the original scale model. In total nine modes are reported, with frequencies from 0.156 to 0.808 Hz, and with damping values between 0.38% and 9.74%. This paper also demonstrates and discusses the use of inexpensive geophones and general measurement equipment for use in OMA applications based on the experience from this bridge measurement.

Silas S. Christensen, Michael S. Andersen, Anders Brandt

Chapter 4. The Prediction of Vibrations for Light Structures in Presence of Moving People

Recently, a model to describe the vibration of light structures (e.g. footbridges, staircases) was proposed by the authors of this paper. Such a model was developed with the aim of being accurate with a high number of people occupying the structure for long times. The present paper analyses the behaviour of the same model in the case of transient excitation of the structure. This allows to assess the accuracy of the model also in this further situation.

M. Berardengo, L. Drago, S. Manzoni, M. Vanali

Chapter 5. On Stationarity and the Interpretation of the ADF Statistic

The paper considers the nature of stationarity of a time series or signal, and how it may be quantified. It is argued that a subjective assessment is as effective as one based on mathematical definitions, if one actually has finite samples of data, and that such an assessment is fundamentally based on the number of cycles of the dominant periodic component visible in the sample. It is shown by dimensional analysis that one of the most often-used measures of stationarity – the Augmented Dickey-Fuller (ADF) statistic – supports this hypothesis. The paper should be of interest not just to engineers, but also to econometricians, or anyone concerned with time series analysis and the impact of nonstationarity.

K. Worden, I. Iakovidis, E. J. Cross

Chapter 6. Simulation of People’s Movements on Floors Using Social Force Model

Vibration serviceability assessment of floors has been traditionally based on a scenario of a single person walking along a path which will generate maximum vibration level. This is due to the difficulty of predicting the real positions and paths of the walking people. With such a design scenario, it is possible to obtain calculated responses, which could be both over- or under-estimated, depending on the specifics. This could be due to considering only one person walking along one walking path in the simulations. This aspect in the design guidelines could be improved if realistic modelling of people’s movements is utilised. Hence, this paper examines the performance of the social force model to simulate the behaviour of people’s movements on floors. This method has been widely used to model a crowd of people in evacuation and panic situations. However, it has been reported in the literature that this approach could be used to model people’s movements in normal situations as well. The simulation carried out in this paper focuses on the interaction between walking people themselves and between walking people and the surrounding boundaries in typical office floors. The results show that reasonable and realistic behaviour of the floor occupants could be obtained using the social force model. Furthermore, utilising the ‘heatmap’ can help the designers to visualise and obtain information about the proportion of time spent by walking individuals at various points on the floor. This approach can be adopted in a more realistic procedure for the vibration serviceability assessment of floors.

Ahmed S. Mohammed, Aleksandar Pavic

Chapter 7. Using Correlation Functions as Free Decays

It is a general assumption in OMA that correlation functions are free decays. In multiple input OMA this assumption also implies that any column in the correlation function matrix is to be considered as multiple output free decays. This assumption is discussed in this paper together with issues concerning estimation and application of correlations functions in OMA.

Rune Brincker, Sandro Amador, Martin Juul

Chapter 8. Footbridge Vibrations Predicted by Stochastic Load Model

Actions of humans on footbridges may result in structural vibrations that may be annoying to bridge users potentially rendering footbridges unfit for their intended use. Hence, it is useful to make predictions of footbridge vibrational performance already at the design stage involving estimation of levels of vibrations in the footbridge. Nowadays both deterministic and stochastic approaches are available for such evaluations. The have primary focus on probability-based approaches for predicting levels of floor vibrations. The predictions involve employing Monte Carlo simulations and the initial setting up of a stochastic framework describing the action of a walking person. The paper investigates the influence of selected decisions made by the engineer when setting up the basis for the prediction of levels of vibration in the footbridge.

Lars Pedersen, Christian Frier

Chapter 9. Non-structural Masses and Their Influence on Floor Natural Frequencies

Excessive floor vibrations are problematic and may potentially render a floor unfit for its intended use. A design-stage check of vibrational performance of a floor design would encompass design-stage estimates of floor dynamic characteristics such as floor natural frequencies. Non-structural masses such as furniture might be present on the in-service floor. For a prediction of floor dynamic characteristics it is not common to account for the fact that non-structural masses elevated above the floor plane may contribute with inertial energy as a result of their horizontal motion occurring during vertical floor vibration. The paper addresses this subject by setting up a finite element model for the floor, which also accounts for an elevation of the non-structural masses. It is shown how different configurations of non-structural masses influence floor natural frequencies. For the investigations, the elevations and weights of the masses are modelled as random variables and Monte Carlo simulations are used for setting up the random configurations of non-structural masses across the floor area.

Christian Frier, Lars Pedersen, Lars Vabbersgaard Andersen

Chapter 10. Probabilistic Analysis of Modal Properties for Floor Systems with Uncertain Support Conditions

Traffic and construction work as well as internal sources may cause vibration of floors in buildings. Potentially, this leads to annoyance for people living or working in the buildings—especially when resonance occurs as a result of excitation frequencies coinciding with eigenfrequencies of the floors. Hence, proper design of floors requires insight into the dynamic properties of the system in order to avoid resonance. In this context, the boundary conditions for the floor—or the connections to the main structure—play an important role. A floor clamped along the entire edge reacts differently than a floor which is simply supported. However, whereas the floor system may well be described in terms of material and geometry, an assessment of the supports can be difficult. Often, calculated eigenmodes and eigenfrequencies do not match those identified for a real floor system and this is, to a great extent, due to uncertain and poorly described supports. Hence, the paper suggests a probabilistic approach focussing on the dynamic properties of the floor given uncertain support conditions. Especially, a rectangular concrete floor, representative of a floor in an office or residential building, is assessed regarding its eigenfrequencies. A stochastic model is introduced for the rotational stiffness of the supports, and a numerical analysis is performed in order to quantify how uncertainty related to the supports for the floor system transfers into uncertainty of its eigenfrequencies.

Lars Vabbersgaard Andersen, Christian Frier, Lars Pedersen

Chapter 11. Usage of MEMS Capacitive Acceleration Sensors for Structural Monitoring

Bridge Structural Monitoring has been a recurring topic over the last few years. A focus has been placed on avoiding any future catastrophic bridge collapses by having a better understanding of the mechanical reactions caused by traffic and environmental conditions. This desire for a better understanding has caused a rise in the usage of sensor technologies which measure the vibrations of a structure.The IEPE (Integrated Electronic Piezoelectric) or MEMS Capacitive technologies are widely used accelerometer families for structural monitoring. The IEPE technology is ideal for smaller structures where higher frequencies are being focused on. While the MEMS capacitive technology is best used for larger structures which experience DC or low frequency signals.In the first part of this paper, we will detail the technical specificity of MEMS Capacitive technologies. Some application oriented technical considerations will be made such as driving long cables, EMC protection and thermal stability. In the second part of this paper, three applications will be described where Kistler MEMS Capacitive Technologies have been used for Modal Investigation and Structural Monitoring. Details on sensor selection, implementation and signal computing relevant to those applications will be provided.

Marine Dumont, Dan Wolf

Chapter 12. Using the Random Decrement Technique on Short Records with Varying Signal-to-Noise Ratios

An earlier paper by the authors discusses potential issues in the practical implementation of the Random Decrement Technique (RDT), particularly when the data has a low signal-to-noise ratio (SNR), and when filtering, which distorts the data and affects the results, is necessary. This is the case for practically every measurement of full-scale buildings, typically done at very low ambient response amplitudes. The RDT procedure used also enables a form of error quantification which allows the engineer or data analyst to judge how reliable the result is. In one of the examples in the earlier paper, it was shown that for a certain SNR, an improvement in the damping estimate is evident when the recording length is significantly longer. For the studied structure, this meant that for better results the recording had to be 72 h (or about 46,000*T where T is the building period of interest) instead of the 4 h initially considered. Even then it required a much longer record duration, estimated to be equivalent to a month of recording, to arrive at an acceptably accurate result. However, such a very long record length is impractical. The current study thus firstly aims to determine what is the minimum SNR (maximum noise level) given say just a 1-h recording, to achieve a certain level of accuracy. This is done by varying the noise level and therefore the SNRs, and applying the mentioned RDT procedure and comparing SNR against the calculated error. The SNRs of ambient response measurements of a full-scale building with and without a supplementary damping system are then viewed considering the finding on minimum SNR.

Ronwaldo Emmanuel R. Aquino, Yukio Tamura

Chapter 13. Why a Curb Shouldn’t Be Kicked to the Curb: The Importance of Non-Structural Elements in Dynamic Modelling

Occupant footfalls are often the most critical sources of floor vibration on the elevated floors of buildings, especially if rhythmic activity is expected (e.g. dancing or aerobics). Achieving reasonable vibration levels on these floors requires sufficiently stiff and massive floor structures to effectively resist the forces exerted by larger groups of people. In many cases, further vibration control is provided by Tuned Mass Dampers (TMDs). A difficulty for engineers in modelling buildings for these scenarios can be exacerbated due to the uncertainty provided by non-structural elements (e.g. non-load bearing partitions, floor toppings, curbs or railings).In this paper, three case studies are presented of modelling structures in order to predict vibrations due to rhythmic activity. The first structure is a sports arena which features a large cantilevered balcony upon which dancing by 600 people was expected to occur. The structural design included TMDs to control these expected vibrations. Validation testing conducted once construction was complete indicated that the balcony was significantly stiffer than expected, and a complete redesign of the TMDs was required. The second structure is a long-span floor office tower that was designed with light steel trusses. Modelling predicted excessive vibration from aerobic activity on the amenity floor, which was proposed to be mitigated with TMDs. Validation testing indicated that the measured frequencies were almost 250% higher than those in the model, completely removing the need to implement TMDs. The third case study is a project consisting of two hospital towers that were nearing completion. Peer-review modelling indicated expected marginal exceedance of the required criteria, so the decision was made to measure the as-built floors. Measurements showed that frequencies were considerably higher than predicted, and extraordinarily high damping.In all three case studies, it was concluded that non-structural elements were the cause of the large discrepancies between modelled and measured dynamic properties.

Michael J. Wesolowsky, Melissa Wong, Allan Raun, John C. Swallow

Chapter 14. Dynamic Behavior of a 130 Years Old Building Under Excessive Sound Pressure

The Casineum in the City of Luzern, Switzerland, has been built in 1883. In 1911 a major reconstruction added a floor on top of the building and significantly changed the roof load bearing structure. The Casineum serves as a Disco since about 2004. The admissible sound pressure was increased from 98 dBA to 100 dBA recently. As a consequence of respective complaints the structural response to such excitation was measured. In contrary to the façade the roof vibrations were found as being not acceptable. The critical frequency region is f = 30..0.80 Hz where the bass speakers are powering. Acting on either vibration source or transfer path found not being feasible the receiver (the roof) was given a closer look at. The result: The roof structural details are so complex that a straightforward structural solution could not be estimated without experimental validation. Therefore, an Experimental Modal Analysis was performed using ambient as well as white noise excitations.

Reto Cantieni

Chapter 15. Scenario Based Approach for Load Identification

In output only analysis the load identification has been a puzzle for several years. Different techniques have been purposed to cope with the inversion problem that lies within this field. However it has been shown, that most methods struggle to obtain robust and consistent results in cases of modal truncation and noise contaminated signals. In the light of these challenges, a scenario based method is proposed. This approach utilizes model updating along with mode shape expansion to obtain a reliable numerical model of the given structure. Then, by evaluating a series of rational load scenarios, it is possible to obtain a reasonable input identification – both the spatial distribution and the temporal variation of the load. The method is demonstrated numerically and experimentally.

Michael Vigsø, Marius Tarpø, Jannick B. Hansen, Rune Brincker, Christos T. Georgakis

Chapter 16. The Realisation of an Inerter-Based System Using Fluid Inerter

Many lightly damped flexible structures suffer from unwanted vibrations. Typically a tuned-mass-damper (TMD) can be used to reduce unwanted vibrations of a specific mode of vibration. The inerter is a novel passive vibration control device offering a wide range of potential applications in engineering practice. It has been analytically proven to be an effective device for controlling unwanted vibrations in structural systems. One of the most effective control strategies employing an inerter is the tuned inerter damper (TID) whose inerter element is connected in series with parallel connected spring-damper. When the inerter element is in parallel with the damper element, it is then called Parallel Viscous Damper Inerter (PVID). In this paper, we will introduce a new passive modal vibration control strategy for the PVID based on a fluid inerter combined with a linear spring connected in parallel. The fluid inerter produces inertance by the acceleration of the fluid inside a helical pipe coiled around the outside of the main fluid chamber. The fluid inerter has both inertance and damping in one device and these properties are coupled to each other. Hence, it is a particular challenge to tune both parameters to fit with optimized values resulting from a design analysis. In this paper, a new analysis will be presented for this device that demonstrates how the PVID with a fluid inerter can be modelled to achieve the targeted parameters.

Predaricka Deastra, David J. Wagg, Neil D. Sims

Chapter 17. Experiences from the Five-Year Monitoring of a Long-Span Pontoon Bridge: What Went Right, What Went Wrong and What’s Next?

The Bergsøysund Bridge is a 930-m-long end-supported pontoon bridge located in Norway, and has been the target of a 5-year-long, extensive monitoring program. Herein, we will describe the unique structural characteristics of the bridge. The monitoring system has been under continuous expansion and revision, and consists of sensors monitoring both the excitation and the response of the bridge. Quantification of the uncertainties of the modelling methodology for structures of this nature has been the main goal, for which purpose modal analysis has been an indispensable tool. Modal analysis has also been used to study the effects the environment has on the structure’s dynamic behaviour. We discuss the limitations of the results from modal analyses. Furthermore, we rise the question of how long monitoring campaigns may continue to provide useful information of this bridge and similar civil structures.

Knut Andreas Kvåle, Ole Øiseth, Anders Rønnquist

Chapter 18. Development of a 3-DOF Structural Displacement Sensor Based on a Two-Stage Kalman Filter

Structural displacement is one of the important indicator for monitoring and assessing the safety of civil infrastructures. GPS-RTK has been one of the widely used sensor for displacement measurement, but the GPS-RTK has low sampling rate and its precision and accuracy are easily affected by stability of satellite and environmental conditions. To overcome the limitations of GPS-RTK, a novel 3-DOF structural displacement sensor is developed in this study. The developed sensor measures 3-DOF displacement, velocity and acceleration of large-scale civil structures based on data fusion of acceleration measured from a force feedback accelerometer, and velocity and displacement obtained from a low cost GPS-RTK using two-stage Kalman filtering. The developed 3-DOF structural displacement sensor offers the following advantages over the existing GPS-RTK sensors: (1) The proposed sensor can measure 3-DOF displacement, velocity and acceleration simultaneously, (2) A better accuracy (around 2 mm) and a better sampling rate (up to 100 Hz) can be achieved, compared to the conventional GPS-RTK sensors, and (3) The performance is less affected by weather conditions and multi path problems, which deteriorate the performance of conventional GPS-RTK sensors. The performance of the proposed sensor was validated through a series of lab scale tests and a field test conducted on Yeongjong Grand Bridge.

Jun Yeon Chung, Kiyoung Kim, Jaemook Choi, Hoon Sohn

Chapter 19. Effects of Pedestrian Excitation on Two Short-Span FRP Footbridges in Delft

Reported in this paper is an evaluation of the vibration behaviour of two footbridges in The Netherlands having main spans of about 15 m. Short-span footbridges over canals and rivers are embedded in the landscape of Delft and elsewhere. Increasingly, these bridges are made of Fibre-Reinforced Polymer (FRP) composites, utilising the high-strength and light-weight nature of the material, and taking advantage of fast installation and low maintenance costs. Due to low mass, these FRP bridges might be sensitive to dynamic excitation by human actions. The two footbridges investigated in this paper have the main bearing structure which consists of either two or four longitudinal beams made of vacuum infused FRPs with foam cores connected by an FRP deck. Modal testing revealed that fundamental vertical natural frequency of the two structures at 4.8 and 6.1 Hz is in the range typical of similar structures made of concrete and steel, whilst the corresponding damping ratio for the wider, slightly cambered, bridge was exceptionally high at 7.9%. The vibration response to dynamic force by people walking was typically up to 1 m/s2. While both of these light-weight structures performed satisfactorily under the regular pedestrian loading, the higher frequency – higher damping structure represents an example of successful control of pedestrian-induced vibrations by means of longitudinal restraints at the end supports and slightly curved shape of the main structure.

Stana Živanović, Justin Russell, Marko Pavlović, Xiaojun Wei, J. Toby Mottram

Chapter 20. Vibrational Response of Structures Exposed to Human-Induced Loads

Structures like pedestrian bridges, staircases and floors become lighter and more slender. As a consequence human-induced vibrations in resonance with the structure become increasingly important when considering the serviceability of a structure. The present paper describes the vibrational response using models for pedestrian loads and the accompanying assessment of the serviceability of a structure exposed to human-induced vibrations. The presented approach uses natural frequencies, modal masses and structural damping to determine the structural vibrations. This allows for more flexible and elegant structures when considering human comfort, which often is dimensioning for light structures with large spans. A criteria based on frequency limits is not sufficient to ensure satisfactory vibration serviceability, especially for light structures. The approach is compared with vibration measurements on a structure before and after the installation of tuned mass dampers.

Jonas Syders Knudsen, Nikolaj Grathwol, Svend Ole Hansen

Chapter 21. Protection of Critical Assets from the Effects of Ground Vibrations

The approaches to control of vibrations in critical science, technology, and healthcare facilities focus on three key elements: (1) the vibration source; (2) the vibration path; and, (3) the vibration receiver. Techniques for modelling, assessment, and control vary depending on the nature of the vibration disturbance. Among the most challenging sources to control are ground motions generated by construction activities and/or heavy industrial processes that contaminate the site environment near vibration-sensitive facilities.In this paper we discuss recommended approaches to control and management of the effects of ground vibration effects on ultra-sensitive research facilities. Approaches to design assessments and concepts for control are presented, with focus placed on management of the source, characterization of the path, and treatment of the vibration receiver. Illustrative examples and key findings from field testing, modelling, and monitoring are presented from a project involving a research facility located next to a construction site. A discussion on current challenges is provided together with suggestions for future research.

Brad Pridham, Nick Walters

Chapter 22. Experimental Characterisation of Dynamic Properties of an All-FRP Truss Bridge

Fibre Reinforced Polymers (FRPs) have increasingly been utilised for construction of pedestrian bridges due to high strength- and stiffness-to-weight ratios, low maintenance costs and quick installation. Their relatively low mass and stiffness make these bridges potentially susceptible to vibration serviceability problems, which increasingly govern the design. Currently, the wider application of FRPs in civil engineering is hindered by the lack of experimental insight in dynamic performance of as-built structures. This paper presents an experimental investigation on a 25 m long glass-FRP truss footbridge in Italy. Ambient vibration tests were conducted to identify the dynamic properties. The peak-picking method and stochastic subspace identification approach were employed for modal parameter identification. The two methods produced very consistent results. Eight vibration modes were identified in the frequency range up to 10 Hz. Two lateral flexural vibration modes having natural frequencies of 5.8 and 9.6 Hz were identified, as well as two vertical flexural modes (at 7.5 and 8.1 Hz) and four torsional modes (at 2.1, 2.7, 4.8 and 9.3 Hz). Damping ratios for all modes up to 10 Hz except the eighth mode were above 1.2%.

Xiaojun Wei, Giosue Boscato, Justin Russell, Alessandro Adilardi, Salvatore Russo, Stana Živanović

Chapter 23. Modal Parameter Uncertainty Estimates as a Tool for Automated Operational Modal Analysis: Applications to a Smart Building

The knowledge of modal parameter uncertainties derived from operational modal analysis (OMA) can greatly improve automated decisions by providing information about the quality of the modal identification. Yet so far, this information has been largely ignored in continuous monitoring studies on civil infrastructure, especially with respect to buildings. In this paper, we implement an automated version of Covariance Based Stochastic Subspace Identification on a highly instrumented smart building. An expansion of the technique estimates uncertainty bounds for all modal parameters. Through a series of full scale experiments, we demonstrate how uncertainties are valuable tools in various contexts of automation. These include the identification and removal of badly-fitted modes, the identification of periods of high signal-to-noise ratio, and the validation of reference sensors selection.

Rodrigo Sarlo, Pablo A. Tarazaga

Chapter 24. Modeling Human-Structure Interaction Using Control Models: External Excitation

Modeling the human-structure interaction (HSI) phenomenon on flexible structure when a person is performing an activity such as dancing remains a challenge. Human activity on a flexible structure can generate excessive vibrations that can cause serviceability problems. Traditional models for human-structure interaction considered the human as a mass-spring-damper system that do not allow the use of external excitation to the human (e.g. music). Recently, researchers have proposed the use of control systems to model the human that will allow the use of external excitation. However, the use of control laws for modeling human-structure interaction has only been performed when the human is standing. This paper focuses on expanding these models to model people in motion. The paper presents an experimental study describing testing on a cantilever structure. The uncertainty of the models is estimated using Bayesian inference.

Ahmed T. Alzubaidi, Juan M. Caicedo

Chapter 25. Measurement of Human Loads Using Computer Vision

The applications of computer vision techniques in civil engineering are becoming increasingly popular with their promising capabilities such as easy and low-cost deployment, contactless measurement solutions and accurate reconstruction of structural finite element models. This paper aims to explore the possibilities of using several computer vision techniques on the reconstruction of load time histories. The motivation is to find a suitable method that could be applied both in the laboratory environment and in the field for the prediction of excessive loads that are exerted upon assembly type structures and specifically on stadiums. Virtual feature extraction and tracking methods are applied on the video recordings of test subjects while jumping and bobbing on structures instrumented with load cells and accelerometers. The results are compared with the sensor based measurements to assess the accuracy levels and feasibility of the methods.

Ozan Celik, Chuan-Zhi Dong, F. Necati Catbas

Chapter 26. Automatic Detection of Structural Deficiencies Using 4D Hue-Assisted Analysis of Color Point Clouds

Recent developments in the fields of robotics and remote sensing technologies such as 3D laser scanners and photogrammetric approaches have provided an unprecedented opportunity to collect a massive amount of data from infrastructure systems in a contactless and nondestructive manner, which can potentially improve the structural health monitoring process. However, the complex nature of these geometrically accurate and high-resolution 3D models makes it inefficient and time-consuming to manually analyze and manipulate them and automating this process continue to pose a challenge. Thus, procedures that automate the data processing in order to detect a variety of damages are desired to make full use of these modern inspection technologies as a tool for infrastructure integrity assessment and asset management. The aim of this paper is to present a new algorithm to automatically identify and evaluate structural deficiencies in massive 3D point clouds of complex infrastructure systems. This approach takes advantage of both local geometry and color data properties associated with each point to improve the damage detection capabilities in a variety of scenarios. Linear and non-linear transformations from the RGB color space to non-RGB spaces were performed to increase separability between the damage and the structure and to achieve robustness to changes in illumination. Recently, a complex and large-scale gravity dam in Maryland, USA has served as a test bed for the developed methodology. In this experiment, a multi-scale photogrammetric computer vision approach was utilized to generate accurate and highly detailed 3D models of the targeted dam. In order to maximize the accessibility and to overcome geometric constraints, different multi-rotor Unmanned Aerial Vehicle (UAV) platforms with varied payload and maneuverability capabilities, each equipped with different optical sensors were used in this study. Experimental results demonstrate that the presented 4D point cloud analysis method can accurately detect and quantify a variety of anomalies from spalling to moisture infiltration in exposed concrete structures.

Ali Khaloo, David Lattanzi

Chapter 27. Human Activity Benchmark Classification Using Multilayer Artificial Neural Network

Human induced floor vibrations have recently been proposed to track human activity for a number of applications such as health care and surveillance. For example, floor vibrations can be used to identify human falls, or the existence of an intruder in a room. In these applications, the acceleration signals should be classified accurately to eliminate false positives. In this paper, a multi-layer artificial neural network is used to classify floor vibrations. Data from a previously published benchmark problem, which consists of seven types of human activities, is used to train and test the algorithm. Results show the capabilities of a multilayer artificial neural network in human activity classification.

Ramin Madarshahian, Juan M. Caicedo, Nicholas Haerens

Chapter 28. Cracking Influence on Dynamic Parameters of Reinforced Concrete Floors

In view of the need to satisfactorily provide good performance of structures in their service situations, it is desired to understand the influence of some reinforced concrete particularities on the dynamic parameters of the structure. These particularities are fundamental in static analysis of pavements design. Starting from spans, geometry, and structural typologies common in real buildings, this paper studies the influence of cracking and non-constant reinforcements on the vibration frequency of concrete floors. It is shown that the reinforced concrete properties significantly impact the floor’s dynamic parameters; this impact is stronger when the floor is slender and deformable. Some of the common structural typologies, as the ribbed/waffle slabs, are even more susceptible to those influence. Knowing the main variables that may be responsible for excessive vibration phenomenon, it was possible to prevent issues during the design process.

William Ferreira Miranda, Suzana Moreira Avila, Graciela Nora Doz

Chapter 29. Paradigm Shift in Structural Vibration Serviceability: New Assessment Framework Based on Human’s Experience of Vibration

Reliable assessment of structural vibration serviceability during the design process is still a great challenge for the designers of pedestrian structures, such as footbridges and floors. Witness to this is the report of the UK Institution of Structural Engineers that approximately half of its 27,000 members, worldwide, have dealt with vibration serviceability complains related to the code-compliant designs. Although structures are meant to be designed to provide function/comfort for human users, evaluation of the ‘experience’ of the human users is conspicuously absent from structural design guidelines. This paper highlights the distinctive features of the Interaction-based Vibration Serviceability Assessment (I-VSA) method, proposed by the authors, and compares the results of the I-VSA with those of the current guidelines for two full-scale structures. It further proposes that: (1) the level of vibrations received by human users is a significantly more informative design parameter than maximum response levels at a certain locating on the structure, which may or may not be experienced; and (2) a deep understanding of the ‘perception’ of vibration by humans is needed to link the level of vibrations received by the occupants with their ‘experience’ from this vibration.

Erfan Shahabpoor, Aleksandar Pavic, Vitomir Racic, Hootan Rezaei

Chapter 30. State-of-the-Art and Future Directions for Predictive Modelling of Offshore Structure Dynamics Using Machine Learning

Ramboll Oil and Gas are leading the field in the development of Structural Health Monitoring Systems (SHMS) for offshore structures. This paper outlines the State-of-the-Art process for predictive maintenance that Ramboll have developed and implemented for offshore structures. This system is one of the first, if not the only one, that creates a maintenance schedule based on knowledge of the structure’s current state.The State-of-the-Art methods of today, as adopted by Ramboll, encompass advanced analysis methods ranging from linear and non-linear system identification, expansion processes, Bayesian FEM updating, wave load calibration, quantification of uncertainties from measured data, damage detection and structural re-assessment analysis to Risk- and Reliability-Based Inspection Planning (RBI) analysis.The paper will be the first in a series of papers that will outline various promising methods contributing to an even better understanding of the issues at stake in the offshore structures context.

U. T. Tygesen, K. Worden, T. Rogers, G. Manson, E. J. Cross

Chapter 31. Structural Identification of a Five-Story Reinforced Concrete Office Building in Nepal

This study is focused on system identification, finite element (FE) modeling, and FE model updating of the five-story masonry-infilled reinforced concrete building of the National Society of Earthquake Technology (NSET), located in Kathmandu, Nepal. The structure, shown in Fig. 31.1, was subjected to a series of ground motions during the 2015 Gorkha Earthquake and its aftershocks.

Mehdi M. Akhlaghi, Supratik Bose, Babak Moaveni, Andreas Stavridis

Chapter 32. Structural Identification for Dynamic Strain Estimation in Wind Turbine Towers

Fatigue is a common issue in steel structures such as wind turbine towers, which is caused by cyclic wind and wave excitations. Therefore, estimation of the remaining fatigue life of the structural and foundation system is of concern. For this purpose, continuous monitoring of the structure is necessary to obtain strain data at fatigue critical points. Since installing and maintaining strain sensors in critical underwater location is difficult, strain data is often available only from a few sensors at accessible locations. Using these sparse sensors, the strain time histories at fatigue critical points can be estimated using estimation techniques. These techniques can identify the structural system using limited measured response data and a system model. In this paper, we implement a model updating approach followed by modal expansion to estimate the strain time history at critical points in a numerical case study representing an offshore wind turbine tower. The acceleration response of the structure is simulated using a finite element model and polluted with Gaussian white noise to represent measurements. The measurements are then used for model updating and strain estimation. The accuracy of the methods and their robustness to the measurement noise and model uncertainty are investigated. The estimated strain response time histories can later be used as input to an appropriate fatigue damage model to estimate the current state of fatigue damage in the system.

Mansure Nabiyan, Hamed Ebrahimian, Babak Moaveni, Faramarz Khoshnoudian

Chapter 33. Modal Parameter Identification from Measurements of Vehicle-Bridge Interaction

The rise of output-only modal analysis has offered an economical and efficient way to identify modal parameters of civil engineering structures, namely natural frequencies, damping ratios and mode shapes. However, since the forcing is unknown, it is not possible to directly estimate modal masses, and estimates of damping ratios may be inaccurate. With the advancement of wireless sensor networks both vehicle and bridge responses can be simultaneously measured. This offers the possibility of estimating true Frequency Response Functions (FRFs), since the vehicle acceleration gives an estimate of the force input to the bridge. Hence in principle it is possible to estimate modal masses and more accurate damping ratios. However, the spatial and temporal variation of the moving load from a passing vehicle gives challenges to this idea and precludes the direct use of existing single-input-multiple-output (SIMO) system identification methods. Even if the system is treated as a multiple-input-multiple-output (MIMO) one, the inputs are highly correlated so existing methods for these systems are not applicable either. For this reason, a two-stage strategy is proposed to modify an existing method to solve this moving load problem.

Yi Liu, John MacDonald, Dario Di Maio

Chapter 34. Bridge Structural Identification Using Moving Vehicle Acceleration Measurements

Identification of dynamic characteristics of structures is a desired objective for existing infrastructure and has been accounted as a serious challenge for civil engineers. In this research, a structural identification method is proposed, which is capable of identifying dynamics of structures using sensor data inside vehicles passing over a bridge. The methodology utilizes a special type of identification algorithm facilitated by Expectation Maximization (STRIDEX) that is capable of identifying systems using mobile data networks. In this study, it is assumed that the mobile sensor measurements are the accelerations inside rigid vehicles and are primarily a mixtures of accelerations caused by the road roughness and bridge dynamic acceleration. With this regard, a stochastic State-Space model represents the equation of motion for a linear dynamic vehicle-bridge system consisting of an impure input. The observation vector is treated as a linear mixture of two sources that are not known. Therefore, the problem turns to a Blind Source Separation (BSS) procedure that is aiming to draw out the bridge vibrations from the mixture. An algorithm called Second Order Blind Identification (SOBI) has been utilized for source separation and validated using simulation. The entire algorithm, including both SOBI and STRIDEX acting together, could successfully identify natural frequencies and mode shapes of a numerical bridge model.

Soheil Sadeghi Eshkevari, Shamim Pakzad

Chapter 35. NDE of Additively Manufactured Parts via Directly Bonded and Mechanically Attached Electromechanical Impedance Sensors

Additive Manufacturing (AM) allows increased complexity which poses challenges to quality-control (QC) and non-destructive evaluation (NDE) of manufactured parts. The lack of simple, reliable, and inexpensive methods for NDE of AM parts is a significant obstacle to wider adoption of AM parts.Electromechanical impedance measurements have been investigated as a means to detect manufacturing defects in AM parts. Impedance-based NDE utilizes piezoelectric wafers as collocated sensors and actuators. Taking advantage of the coupled electromechanical characteristics of piezoelectric materials, the mechanical characteristics of the part under test can be inferred from the electrical impedance of the piezoelectric wafer. Previous efforts have used piezoelectric wafers bonded directly to the part under test, which imposes several challenges regarding the applicability and robustness of the technique. This paper investigates the use of an instrumented clamp as a solution for measuring the electromechanical impedance of the part under test. The effectiveness of this approach in detecting manufacturing defects is compared to directly bonded wafers.

C. Tenney, M. Albakri, C. B. Williams, P. Tarazaga

Chapter 36. Classification of Human Walking Patterns through Singular Value Decomposition Projection

Sensing of structural vibration provides a rich source of information that can be used in structural health monitoring and impact/fault localization among other applications. In this paper, acceleration measurements from vibration sensors (accelerometers), installed in an operational smart building (Virginia Tech’s Goodwin Hall), are used to classify footsteps of different kinds from building occupants. Goodwin Hall is a 160,000 square foot five story building instrumented with over 200 accelerometers mounted to the building’s structure. Singular value decomposition (SVD) projection is used to classify measured data into categories seeded with training data. Contrary to the black box machine learning approach, the SVD framework allows classification parameters to be easily modified and their effects visualized to be understood. Better understanding of the classification problem and its dominant parameters will allow the development of more accurate and robust algorithms for classification of a wide variety of signals.

Ellis Kessler, Pablo A. Tarazaga

Chapter 37. Dynamic Characterization of a Prestressed Concrete Bridge by Strain and Acceleration Measurements

Structural health monitoring systems for bridges typically employ a mixture of different types of sensors to measure and track long-term structural performance under various environmental and service live loads. A significant challenge in designing such monitoring systems is to optimize the quantities and locations of the different sensor types to achieve the monitoring objectives while minimizing instrumentation costs. The final instrumentation design is usually a compromise between an ideal instrumentation plan and one that incorporates economic and practical considerations and constraints. It follows that there is merit in exploring the use of different types of sensors to accomplish more than one characterization objective. This paper presents a study to explore the limits of using both strain gage and accelerometer measurements from a prestressed concrete highway bridge for the dynamic characterization of the structure. The bridge specimen used for this study is primarily instrumented with strain gages to measure the stresses induced by heavy trucks. A smaller array of six accelerometers is also included specifically to measure characterize the vibration response of the bridge due to different truck loads, configurations and crossing speeds. The accelerometers are installed along the transverse and longitudinal centerlines of the bridge span. Strain gages are installed on each beam in the bridge cross section at 28%, 32% and 50% of the longitudinal span length locations. The monitoring system operates in a triggered acquisition mode in which the strain gages and accelerometers are both sampled at 200 Hz during truck crossing events. Strain and vibration measurements from truck crossing events are individually processed and evaluated to identify the dynamic characteristics, and the combination of the two types of measurements for dynamic characterization are evaluated and discussed.

Kirk A. Grimmelsman

Chapter 38. Evaluation of a New Energy-Based Human Tracking Method in a Smart Building Using Floor Vibration Measurements

Tracking occupants in an indoor environment has applications in intruder detection, emergency response and evacuation (e.g., locating an occupant in a burning building), and energy saving (through activity-based control of building lighting and HVAC system). In this document, we show that tracking occupants in an indoor environment can be done using the floor vibration caused by occupant footstep impacts. In order to track an occupant, each footstep impact location must first be estimated. For that purpose, we evaluate the performance of a newly developed energy-based localization (multilateration) method for the case of localizing occupant footsteps in a real-life operational smart building. The new method is based on the fact that the energy of the impact-generated wave will be attenuated as the wave travels away from the impact location. Localization is achieved using a network of vibration sensors (accelerometers) placed underneath the walking floor, which provides a non-intrusive and tamper-proof localization system. The new method has small computational time and requires a relatively small sensor data sampling rate. It is anticipated that the new method will have a smaller footstep localization error compared to conventional time of flight/arrival methods. Occupant tracking experiments show that the new method has a promisingly small localization error.

Sa’ed Alajlouni, Pablo A. Tarazaga

Chapter 39. Innovative Sensing by Using Deep Learning Framework

Structures experience large vibrations and stress variations during their life cycles. This causes reduction in their load-carrying capacity which is the main design criteria for many structures. Therefore, it is important to accurately establish the performance of structures after construction that often needs full-field strain or stress measurements. Many traditional inspection methods collect strain measurements by using wired strain gauges. These strain gauges carry a high installation cost and have high power demand. In contrast, this paper introduces a new methodology to replace this high cost with utilizing inexpensive data coming from wireless sensor networks. The study proposes to collect acceleration responses coming from a structure and give them as an input to deep learning framework to estimate the stress or strain responses. The obtained stress or strain time series then can be used in many applications to better understand the conditions of the structures. In this paper, designed deep learning architecture consists of multi-layer neural networks and Long Short-Term Memory (LSTM). The network achieves to learn the relationship between input and output by exploiting the temporal dependencies of them. In the evaluation of the method, a three-story steel building is simulated by using various dynamic wind and earthquake loading scenarios. The acceleration time histories under these loading cases are utilized to predict the stress time series. The learned architecture is tested on acceleration time series that the structure has never experienced.

Nur Sila Gulgec, Martin Takáč, Shamim N. Pakzad

Chapter 40. The Role of Control-Structure Interaction in Deployable Autonomous Control Systems

Modern, innovative structures often rely on auxiliary control devices to suppress excessive vibrations. Currently, most control devices are permanent installations designed specifically for the intended structure and further tuned to a particular structural property. The concept of a deployable, autonomous control system (DACS) is proposed herein where the notion of deployability implies the system can be rapidly implemented on a range of structures while the autonomy aspect facilitates real-time positioning of the system and generation of active control forces. A small-scale prototype targeting short-term control applications of lightweight structures is presented. The prototype consists of an electromagnetic mass damper (EMD), unmanned ground vehicle (UGV) equipped with vision sensors, and computational hardware. The UGV transfers inertial control forces generated by the EMD through the tires which eliminates the need for a rigid connection to the structure and enables rapid deployment for short-term applications. A simultaneous localization and mapping (SLAM) solution utilizing the mobility of the UGV and on-board vision sensors facilitates autonomous positioning of the device at desired locations on the structure. This study investigates the role of control-structure interaction (CSI) by examining the effect of base motion on the position controlled EMD and interaction between the structural response and UGV dynamics. Different controller formulations are presented and comparatively assessed to illustrate the inherent interaction effects. Experimental results confirm CSI has a negligible impact on EMD position tracking while demonstrating the importance of considering UGV dynamics in the controller formulation.

K. Goorts, S. Narasimhan

Chapter 41. Support Vector Machine-Based Face Direction Detection Using an Infrared Array Sensor

Facing direction detection plays a critical role in human computer interaction, such as face recognition and head pose estimation in biometric identification, spatial microphone/loudspeaker devices, virtual reality games and etc. Currently detection methods are mainly focused on extracting specific patterns of various facial features from the user’s optical images, which raises concerns on privacy invasion and these detection techniques do not usually work in the dark environment. To address these concerns, this paper proposes a pervasive solution for a coarse facing direction detection using a low pixel infrared thermopile array sensor. Support vector machine (SVM) method is selected as the classifier. Two methods for feature extraction are compared. One is based on pre-defined features and the other is based on pre-trained convolutional neural network (CNN) model. The detection accuracy resulted from using pre-defined features reaches 87% for identifying five different facing directions up to 1.2 m. However, this accuracy is largely descended when the detection range is increased to 1.8 m. Interestingly, the accuracy resulted from using pre-trained CNN features, however, demonstrates a reliable and steady performance up to 1.8 m. The accuracy keeps above 95% at both detection ranges (1.2 and 1.8 m). This proposed solution leads to many advantages, such as low resolution leading to no intention on privacy invasion, and the low-cost intriguing a potentially large market for human computer interaction in smart home appliances control and computer games.

Zhangjie Chen, Hanwei Liu, Ya Wang

Chapter 42. Estimation of Remaining Useful Life of a Fatigue Damaged Wind Turbine Blade with Particle Filters

Structural maintenance operations in wind energy sector are steering towards condition based maintenance (CBM) which requires prognostic estimates of existing condition of the wind turbine (WT) structural systems that is damage propagation and remaining useful life (RUL). WT blades are highly vulnerable structural components that are subjected to continuous cyclic loads of wind and self weight variation. A method for estimation of RUL of wind turbine blades considering the fatigue mode of failure is proposed in this paper. Stochastic life expectancy methods that use Bayesian updating with measurements of evolving damage for damage propagation estimation have proven to be reliable in RUL estimation. In this study probability density functions for the RUL of WT blades are estimated for diffident initial crack sizes and particle filtering method is used for forecasting the evolution of fatigue damage addressing the non-linearity and uncertainty in crack propagation. The stresses on a numerically modeled life size onshore WT blade subjected to turbulence are used in computing the crack propagation observation data for particle filters.

Bhavana Valeti, Shamim N. Pakzad

Chapter 43. A Numerical Investigation of a Gravity-Compensated Nonlinear Energy Sink for the Passive Control of Flooring Systems

Flooring systems are subjected to a variety of human-induced and mechanically-induced loads which can vary in amplitude, frequency, and location. Furthermore, the properties of flooring systems and the acceptable levels of vibration can change during the life of a building as it transitions between multiple different uses. Tuned mass dampers (TMDs) can be effective at controlling floor vibration; however, their effectiveness is limited because TMDs must be tuned and can only effectively control vibrations across a narrow band of frequencies. Recently, a passive mass damper, known as a gravity-compensated nonlinear energy sink (GCNES), was proposed to mitigate vertical vibrations. The unique geometric nonlinearity used to produce this device’s stiffness element compensates for the vertical offset resulting from the weight of the device and allows it to dynamically achieve a cubic nonlinearity. This strong nonlinearity allows the GCNES to interact with the flooring system across a broad range of frequencies. In this paper, a numerical model of a flooring system with a GCNES attached is developed. This model is then used to investigate the effectiveness of the GCNES, in comparison to the TMD, at controlling floor vibrations. The results of this study show that, while the TMD is more effective when mitigating excitations at the particular frequency it is tuned to, the GCNES can provide effective vibration control across a wide range of frequencies near the system’s resonance point.

J. R. Ramsey, N. E. Wierschem

Chapter 44. Characterizing Structural Changes to Estimate Walking Gait Balance

We present a method for improving left-right walking gait balance using structural floor vibration sensing by characterizing changes in structural properties in the sensing area. Understanding and measuring human gait balance can be used to assess overall health status, mobility, and rehabilitation progress. The key research challenge is that structural properties in the sensing area may differ from one footstep location to the next, resulting in inaccurate footstep force and balance estimations. To address this challenge, our method performs sensor selection using the insight that some sensors in the sensor network are in a similar structural region as the footstep location and, therefore, are not as effected by the observed variations in structural properties as the other sensors. We evaluate the performance of our method by conducting uncontrolled real-world walking experiments in a residential structure. This evaluation shows that our method achieves a 1.6X reduction in force estimation error and a 2.4X reduction in balance estimation as compared to the baseline approach.

Jonathon Fagert, Mostafa Mirshekari, Shijia Pan, Pei Zhang, Hae Young Noh

Chapter 45. Load Rating of a Reinforced Concrete T-Beam Bridge Through Ambient Vibration Testing and Finite Element Model Updating

As the load demands on highway bridges increases, it is essential that the load rating procedures reliably assess the condition of existing structures. In addition, conventional design office load rating techniques cannot be used for bridges without structural plans, which indicates the need for a more advanced load rating procedure. This paper presents a methodology to compute the live load-carrying capacity of reinforced concrete T-beam bridges, which can be applied for bridges with structural plans or with missing or limited design information. The method involves modal identification of bridge using ambient vibrations and finite element model updating using vibration characteristics for capacity estimation. A simply supported T-beam bridge located in Virginia is selected for field-testing to verify the proposed method. The bridge is composed of five spans of the same length, 12.95 m for each, with a total length of 65.4 m and a width of 8.864 m. A total of nine accelerometers are installed to bridge to collect acceleration data for 15 min at a sampling rate of 500 Hz. The modal properties of the bridge are determined using enhanced frequency domain decomposition technique. The initial finite element model of the bridge is updated such that the modal properties of the bridge match the field measured parameters. The load effects and capacity of the bridge are determined and used to calculate the load rating factor. The rating factors obtained from the proposed method and traditional design office load rating procedures are compared. The results indicate that the proposed method can reveal the reserve capacity of bridges.

Abdou K. Ndong, Mehrdad S. Dizaji, Mohamad Alipour, Osman E. Ozbulut, Devin K. Harris

Chapter 46. Identifying Modal Characteristics of Reinforced Concrete Bridges Using Smartphones

This paper explores the use of smartphones as vibration measurement device to identify modal properties of reinforced concrete (RC) bridges. Two in-service RC bridges are instrumented with both conventional accelerometers and smartphone accelerometers. One of the tested bridges is a simply supported RC T-beam bridge structure and the other one is a simply supported RC slab bridge with a skew angle of 15 degrees. The vibration testing includes both traffic-induced ambient excitations and impact hammer excitation. The natural frequencies of the bridges are identified from operational modal analysis using the data obtained from both conventional and smartphone accelerometers at a single point and a peak-picking technique. In addition, the modal properties of two bridges are extracted using data obtained from a dense network of sensors and by employing enhanced frequency domain decomposition method. To assess the correlation between the modal properties identified from smartphone data and data obtained from traditional sensor, statistical analyses are conducted. Results show that there is a good agreement between the modal characteristics extracted from smartphone and reference sensor data as well as those obtained from a dense instrumentation.

Abdou K. Ndong, Osman E. Ozbulut, Devin K. Harris

Chapter 47. Model Updating and Damage Assessment of a RC Structure Using an Iterative Eigenvalue Problem

This study discusses the model updating performed on a two-story reinforced-concrete masonry-infilled building using the dynamic response obtained during ambient vibration measurements. Five infills were demolished at four stages creating four damage states. The numerical model of the building is developed based on the obtained material properties and geometry. The model updating process minimizes a target function defined as the least-square optimization of an inverse eigenvalue problem. The reference model is then updated using the experimental results in each stage of damage to assess the damage. The dynamic properties obtained from the updated model match well their experimentally-estimated counterparts. Also, the model can reliably locate the damage introduced to the structure.

Michele Tondi, Seyedsina Yousefianmoghadam, Andreas Stavridis, Babak Moaveni, Marco Bovo

Chapter 48. Modal Properties of a Model of a Chinese Pagoda

China has been building wood frame pagodas since 200 ad during the Han Dynasty. A pagoda is essentially a multi-story tower, and each story is a separate building consisting of all structural components. This paper describes a series of shake table tests conducted on a one-fifth scale model of a seven-story pavilion-style wooden pagoda recently conducted at Tongji University in Shanghai, China. The main purpose of these tests was to investigate the seismic performance of this model pagoda. The model of the pagoda was subjected to different levels of excitation simulating ground shaking generated by earthquakes. White noise excitation was also used to determine the dynamic characteristics of the structure. The results of the modal analyses conducted using the various datasets are presented and discussed in this paper. The parameters studied include modal frequencies, damping and modes shapes of the structure. The variation of these parameters with the increasing levels of ground excitation is investigated, and these are correlated to the nonlinear behaviour and damage observed during each test. A detailed analysis of the mode shapes of the structure indicates that a soft-story behaviour of the first floor of the structure is noticeable with increasing level of shaking. Considering that the pagoda’s connections using the Dou-gong style are made of wood only, the behaviour of the structure during extreme levels of excitation was excellent and very minor damage was observed. The mortise-tenon joints of the timber frame dissipated most of the seismic energy.

Yajie Wu, Xiaobin Song, Carlos E. Ventura

Chapter 49. Advanced Fourier-Based Model of Bouncing Loads

Contemporary design guideline pertinent to vibration serviceability of entertaining venues describes bouncing forces as a deterministic and periodic process presentable via Fourier series. However, fitting the Fourier harmonics to a comprehensive database of individual bouncing force records established in this study showed that such a simplification is far too radical, thus leading to a significant loss of information. Building on the conventional Fourier force model, this study makes the harmonics specific to each individual and takes into account imperfections in the bouncing process. The result is a numerical generator of stochastic bouncing force time histories which represent reliably the experimentally recorded bouncing force signals.

Vitomir Racic, Jun Chen, Aleksandar Pavic

Chapter 50. Defining Groupings and Classification of Human Gait Using Correlation of Ground Reaction Force Measurements

Classification of a person’s gait through quantitative methods has wide reaching applications in security, marketing, and healthcare. This study uses ground reaction force (GRF) measurements of healthy subjects and patients with osteoarthritis (OA) to define groupings and classify subjects. In addition to grouping classes into known qualities (gender, diagnosis, etc.), this work introduces classes based on groups coming directly from correlating GRFs from different subjects. Using correlation allows new groupings to be established and more accurate classification results because continuous force time histories are compared as opposed to conventional discrete force data (i.e. peaks). Two new classes are introduced from the data which can be classified with a 92% accuracy, and the physical meaning of these classes is investigated. Comparison of a single healthy person’s walking pattern to multiple classes builds a signature that could be used to identify specific individuals. Additionally, patients suffering from OA do not correlate well with healthy groupings and can be distinguished from healthy subjects. This allows for the possibility of using GRFs to track patients’ rehabilitation. It is expected that as a patient progresses through a rehabilitation program and begins to recover, their walking patterns will become more consistent and be more highly correlated with healthy groupings.

Ellis Kessler, Pablo A. Tarazaga, Robin Queen
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