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Über dieses Buch

Topics in Dynamics of Civil Structures, Volume 4: Proceedings of the 31st IMAC, A Conference and Exposition on Structural Dynamics, 2013, the fourth volume of seven 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 Structural Dynamics, including papers on:

Modal Parameter Identification for Civil Structures

Vibration Control of Civil Structures

Cable Dynamics

Damage Detection Models for Civil Structures

Data-Driven Health Monitoring of Structures & Infrastructure

Experimental Techniques for Civil Structures

Human-induced Vibrations of Civil Structures

Structural Modeling for Civil Structures

Inhaltsverzeichnis

Frontmatter

Chapter 1. Accurate Damping Estimation by Automated OMA Procedures

Systems and techniques for fast damage detection based on vibration analysis are becoming very attractive in different engineering fields. Modal-based damage detection algorithms are well-known techniques for structural health assessment. However, the lack of automated modal identification and tracking procedures has been for long a relevant limit to their extensive use. The development of several automated output-only modal identification procedures in the last few years has led to a renewed interest in modal-based damage detection. However, robustness of automated modal identification algorithms, computational efforts and reliability of modal parameter estimates (in particular, damping) still represent open issues. In this paper, a novel algorithm for automated output-only modal parameter estimation is adopted to obtain reliable and very accurate modal parameter estimates. An extensive validation of the algorithm for continuous monitoring application is carried out based on simulated data. The obtained results point out that the algorithm provides fairly robust, accurate and precise estimates of the modal parameters, including damping ratios. This may potentially lead to a standardized, extensive characterization of modal damping ratios in structures, which is useful to gain knowledge about damping mechanisms in structures and to develop predictive models.

C. Rainieri, G. Fabbrocino

Chapter 2. Real-Time Structural Health Monitoring and Damage Detection

Structural health monitoring (SHM) contains continuous structural vibration monitoring, extraction of damage sensitive features of structure from measurements, and statistical analysis of those features to detect and locate the damage in structures. In other words, SHM involves data collection, continuous monitoring and analyzing them in real-time. Changes in modal properties of structures during their service life are strongly related to damage in structures, which makes accurate estimation of modal properties an essential step in SHM. Therefore, both monitoring and accurate identification of real-time modal properties (modal frequency, damping ratio, and mode shape) are of crucial importance in order to have a good estimate in SHM. In this paper, a real-time modal identification techniques along with a damage detection algorithm based on inter-story drift calculation has been developed for SHM. The modal identification technique is based on the modification of standard spectral analysis tools for real-time data, and utilizes running time windows to keep track of time variations of structures’ modal properties. On the other hand, the damage detection algorithm makes use of inter-story drifts, which is calculated by narrow-band filtering the recorded data around modal frequency and very sensitive to structural damage, by estimating the contribution of each identified mode of structure. A software package called

REC_MIDS

is developed for real-time modal identification. The software includes various user-selectable algorithms to identify modal properties, as well as options to plot their time variations and animations. The software has been tested with the ambient vibration data recorded from the Hagia Sophia Museum, a 1500 year-old historical structure in Istanbul, Turkey. Modal properties of the structure have been identified accurately in real-time. Results of the Hagia Sophia test have been compared with the previous studies conducted by different researchers. Comparison shows that the results of the

REC_MIDS

are in good agreement with that of the previous studies.

Yavuz Kaya, Erdal Safak

Chapter 3. Improved Substructure Identification Through Use of an Active Control Device

Increasingly, researchers turn to substructure identification for civil structural health monitoring for a variety of reasons. First, substructure identification provides inherent damage localization. Second, substructure identification provides greater numerical conditioning than full structure identification because only a small subset of the degrees of freedom are considered in each analysis. Third, substructure identification is perfectly suited for a decentralized implementation within a network of wireless sensors. This implementation can realize cost savings in installation and operation.While the benefits of substructure identification are many-fold, current research shows that certain structure-substructure combinations admit poor performance. Research demonstrates that single story substructures in a shear building are poorly identified (if at all) when interstory acceleration response is low in a specific frequency range. This result shows that portions of the structure are unable to be identified properly with substructure identification.To overcome these results, this paper temporarily re-purposes a structural control device to change the global dynamics of the structure to improve substructure identification at a particular story. A control law for an active mass damper is developed to increase the interstory response at a particular story, which, when implemented, will improve substructure identification of that story. The control law is developed in simulation and will later be tested experimentally on a four story, 12 ft. steel building excited by base motion.

Charles DeVore, Erik A. Johnson, Richard E. Christenson

Chapter 4. Hybrid MPC: An Application to Semiactive Control of Structures

In clipped LQR, a common strategy for semiactive structural control, a primary feedback controller is designed using LQR and a secondary controller clips forces that the semiactive control device cannot realize. However, when the primary controller commands highly non-dissipative forces, the frequent clipping may render a controller far from being optimal. A hybrid system model is better suited for semiactive control as it accurately models the passivity constraints by introducing auxiliary variables into the system model. In this paper, a hybrid model predictive control (MPC) scheme, which uses a system model with both continuous and discrete variables, is used for semiactive control of structures. Optimizing this control results in a mixed integer quadratic programming problem, which can be solved numerically to find the optimal control input. It is shown that hybrid MPC produces nonlinear state feedback control laws that achieve significantly better performance for some control objectives (e.g., the reduction of absolute acceleration). Responses of a typical structure to historical earthquakes, and response statistics from a Monte Carlo simulation with stochastic excitation, are computed. Compared to clipped LQR, hybrid MPC is found to be more consistent in the reduction of the objective functions, although it is more computationally expensive.

Wael M. Elhaddad, Erik A. Johnson

Chapter 5. Findings with AVC Design for Mitigation of Human Induced Vibrations in Office Floors

In recent years, there have been extensive active vibration control (AVC) studies for the mitigation of human induced vibrations in a series of office floors, in which such vibrations are deemed to be ‘problematic’ and have been found to affect only certain sections of the floors. These floors are predominantly open-plan in layout and comprise of different structural configurations for their respective bays and this influences their dynamic characteristics. Most of the AVC studies have comprised extensive analytical predictions and experimental implementations of different controller schemes. The primary measures of vibration mitigation performance have been by frequency response function (FRF) measurements, responses to controlled walking tests, and in-service monitoring, all tests with and without AVC.

This paper looks at AVC studies in three different office floor case studies in past field trials. Some of the estimated modal properties for each of these floors from experimental modal analysis (EMA) tests are shown as well as some selected mode shapes of fundamental modes of vibration. These reflect the variability in their dynamic characteristics by virtue of their different designs and thus the potential for their ‘liveliness’ under human induced excitation. An overview of some of the controller schemes pursued in the various field trials are mentioned as well as a brief insight being provided into some challenges encountered in their designs and the physical siting of the collocated sensor and actuator pairs used in the field trials. The measure for the vibration mitigation performances in this work is in the form of uncontrolled and controlled point accelerance FRFs which show attenuations in the target modes of vibration between 13 and 18 dB. These tests also show the variability in vibration mitigation performances between the various controllers.

Donald Nyawako, Paul Reynolds, Malcolm Hudson

Chapter 6. Power Requirements for Active Control of Floor Vibrations

Recent research has made significant developments towards improved Active Vibration Control (AVC) technology for the mitigation of annoying vibrations in floor structures. However, there are very few examples of permanent AVC installations in floor structures; this is in part due to the requirement of a continuous power supply and the ensuing electricity costs. This paper investigates potential improvements to AVC from the perspective of the choice of control algorithm. Firstly, the use of model-based controllers as opposed to the direct output feedback controllers that have been used in most prior research effort is considered. For a model-based (MB) controller, because the controller can be designed to target modes within a specific frequency band of interest, it is possible that control effort is used more effectively for a given reduction in response. Secondly, the potential benefits of using a switching-off rule to reduce the actuator effort during periods of low structural response is investigated. Future actuators and amplifiers could incorporate a switching-off rule similar to this in order to minimise the overhead costs associated with running the amplifier. The changes in potential electricity consumption for the previously declared control laws are experimentally determined through direct measurement of the power drawn by the actuator. The results from these analyses are compared and conclusions drawn.

M. J. Hudson, P. Reynolds, D. S. Nyawako

Chapter 7. Tuning TMDs to “Fix” Floors in MDOF Shear Buildings

Many researchers have examined the optimal design of tuned mass dampers (TMDs) for vibration reduction in single-degree-of-freedom (SDOF) and multiple-degree-of-freedom (MDOF) systems. This work focuses on the design of damped TMDs to “fix” selected floors of a harmonically base-excited building in shear such that the vibrational amplitudes of the selected floors are zero. This method does not require the fixed floors, referred to as “fixed nodes,” to coincide with the floors to which the TMDs are attached. This paper presents the proposed tuning method and addresses the feasible arrangements of the fixed and TMD floors. The proposed tuning method is demonstrated with a simulation of a 3-DOF shear building, along with a discussion on the effects of mistuning on the TMD performance.

Jennifer Rinker

Chapter 8. Precise Stiffness Control with MR Dampers

Mag can be used not only as controllable damping devices but also to emulate a controllable positive or negative stiffness in combination with the dissipative force. However, the dissipative nature of MR dampers constrains the stiffness control. This work formulates the problem of combined stiffness and damping control with MR dampers if the damper is subjected to pure harmonic motion. A new method is presented that ends up in precise stiffness emulation with MR dampers, also when the sum of the stiffness and dissipative forces is constrained by the semi-active nature and residual force of MR dampers. The new control concept is applied to a semi-active tuned mass damper with an MR damper (MR-STMD). The numerical and experimental results demonstrate that the MR-STMD outperforms the passive TMD significantly.

Marcin Maślanka, Felix Weber

Chapter 9. Employing Hybrid Tuned Mass Damper to Solve Off-Tuning Problems for Controlling Human Induced Vibration in Stadia

A key objective in the design of any sports stadium is to include maximum number of spectators with minimum obstruction in the visual cone. This functional requirement often results in employing one or more cantilevered tiers, which in turn culminates in more slender grandstands often with relatively low natural frequencies and modal damping ratios. These natural frequencies may sometimes fall in the range of frequencies of human movement which can possibly excite the structure in resonance resulting in vibration serviceability issues. One of the available techniques to reduce excessive responses is to use passive vibration control techniques such as Tuned Mass Dampers (TMD). However, the off-tuning problem is a substantial drawback of this technique, whereby changes in natural frequencies caused by crowd-structure interaction may detune the TMDs. This paper presents a study into the possibility of using hybrid (combination of active and passive control) technology to augment the vibration serviceability of sports stadia. It shows a comparative analysis of vibration mitigation performances that are likely to be attained by utilising a passive TMD and the proposed HTMD. An appropriate control scheme is utilised with the proposed HTMD to deal with the off-tuning issues in TMDs caused by crowd loading, and is shown to be effective.

Nima Noormohammadi, Paul Reynolds

Chapter 10. Semi-Active TMD Concept for Volgograd Bridge

The Volgograd Bridge in Russia is known not only for its record length but also for the large amplitude vibrations induced by wind in May 2010. This paper describes the development of a new semi-active TMD with a magnetorheological damper (MR-STMD) that was installed on the Volgograd Bridge in fall 2011. The main feature of the MR-STMD concept is that the real-time controlled MR damper emulates a controllable stiffness force and a controllable friction force. The controllable stiffness force augments or diminishes the stiffness of the passive springs and thereby tunes the MR-STMD frequency to the actual frequency of the bridge. The controllable friction force generates frequency dependent energy dissipation. The small-scale prototype was experimentally tested on the 19.2 m long Empa bridge for various modal masses and disturbing frequencies. After that, the full-scale MR dampers were tested at Empa by hybrid testing for the expected frequencies and amplitudes of the bridge. Finally, the frequency controllability of one full-scale MR-STMD was verified at the University of the German Armed Forces, Munich. All tests confirm that the new technology can compensate for the frequency sensitivity of passive TMDs and works at high efficiency.

Felix Weber, Johann Distl, Marcin Maślanka

Chapter 11. A New Shape Memory Alloy-Based Damping Device Dedicated to Civil Engineering Cables

Most of civil engineering cable structures are subjected to potential damages mainly due to dynamic oscillations induced by wind, rain or traffic. If vibration amplitudes of bridge cables for example are too high, it may cause a fatigue phenomenon. Recently, researches had been conducted dealing with the use of damping devices in order to reduce vibration amplitudes of cables. Thin shape memory alloy (SMA) NiTi (Nickel-Titanium) wires were used as a simplified damping device on a realistic full scale 50 m long cable specimen in Ifsttar (Nantes - France) laboratory facility, and its efficiency was shown. It has been done using finite element simulations, as well as experimental test methods. The aim of this work is to link the wire material behavior with the local damping induced along the cable qualitatively. Indeed, thermomechanical energy dissipation of the NiTi-based wires enables their damping power. The hysteretic behavior in NiTi-based alloys demonstrates a consequent dissipation because of an exothermic martensitic transformation and then an endothermic reverse transformation.

G. Helbert, L. Dieng, T. Lecompte, S. Arbab-Chirani, S. Calloch, P. Pilvin

Chapter 12. Using Pall Friction Dampers for Seismic Retrofit of a 4-Story Steel Building in Iran

Past earthquakes in Iran have caused severe damage to existing steel buildings without adequate resistance and ductility against earthquakes. Competent methods for seismic retrofitting are required in order to prevent damage and casualty. Among effective seismic retrofit methods, passive control reduces seismic vulnerability by mitigating seismic demand and increasing ductility. One of the most suitable methods in passive control system is to use pall friction damper in the braced steel structures. Main advantage of this friction damper is its almost rectangular force-deformation hysteretic loops with high-energy dissipations, without any need to specific technology. In this paper, while introducing the performance of pall friction dampers and their design, seismic retrofit of an existing 4-story steel simple frame in Iran is investigated by using such dampers.

Seyed Mehdi Zahraei, Alireza Moradi, Mohammadreza Moradi

Chapter 13. Modal Identification of a 5-Story RC Building Tested on the NEES-UCSD Shake Table

A full scale five-story reinforced concrete building was built and tested on the NEES-UCSD shake table. The purpose of this experimental program was to study the response of the structure and nonstructural systems and components (NCSs) and their dynamic interaction during seismic excitation of different intensities. The building specimen was tested under base-isolated and fixed-based conditions. In the fixed-based configuration the building was subjected to a sequence of earthquake motion tests designed to progressively damage the structure. Before and after each seismic test, ambient vibration data were recorded and additionally, low amplitude white noise base excitation tests were conducted at key stages during the test protocol. A quasi-linear response of the building can be assumed due to the low intensity of the excitation and consequently modal parameters might change due to the structural and nonstructural damage. Using the vibration data recorded by 72 accelerometers, three system identification methods, including two output-only (SSI-DATA and NExT-ERA) and one input-output (DSI), are used to estimate the modal properties of the fixed-base structure at different levels of structural and nonstructural damage. Results allow comparison of the identified modal parameters obtained by different methods as well as the performance of these methods and studying the effect of the structural and nonstructural damage on the dynamic parameters. The results show that the modal properties obtained by different methods are in good agreement and that the effect of structural/nonstructural damage is clearly evidenced via the changes induced on the estimated modal parameters of the building.

Rodrigo Astroza, Hamed Ebrahimian, Joel P. Conte, Jose I. Restrepo, Tara C. Hutchinson

Chapter 14. Modal Testing of a Repaired Building After 2010 Chile Earthquake

A 24 storey reinforced concrete residential building in the city of Concepcion, Chile, was severely damaged during the 2010 M

W

8.8 Maule earthquake. After the earthquake structural elements at the base of the building were repaired in an attempt to restore the structure to its original state. A modal test using ambient vibrations was conducted on this repaired building to determine its dynamic properties. Additional studies using ambient vibrations at near free field locations confirm that ground conditions may have contributed to seismic amplification of the ground shaking at frequencies that were dominant in the seismic response of this high-rise building. This amplification of ground shaking can be considered an important contributing factor to the damage suffered by this building.

Manuel Archila, Ruben Boroschek, Carlos E. Ventura, Sheri Molnar

Chapter 15. System Identification and Displacement Profiles of Multi-Span Skewed Bridges with Seat Type Abutments

Skewed bridges are classified as irregular structures due to the geometry of the deck and bents. The evaluation of their dynamic response is challenging as it requires a combination of several modes of vibration. In this study, the results of ambient vibration tests performed on four bridges in British Columbia, Canada are used to identify the dynamic properties and the displacement profiles of multi-span skewed bridges with seat type abutments. The frequencies of vibration, the modes of vibrations and the modal dampings are identified using frequency and time domain techniques. In addition, the directionality in the transverse and longitudinal response for skewed bridges with different levels of lateral restraint and deck flexibility is discussed. This paper improves the understanding of the dynamic response of skewed bridges, in particular their lateral response to seismic loads. This understanding contributes to having a better assessment of the seismic demands that skewed structures will undergo and to the development of displacement based design methods for these structures.

Seku Catacoli, Carlos E. Ventura, Steve McDonald

Chapter 16. Robustness of Modal Parameter Estimation Methods Applied to Lightweight Structures

On-going research is concerned with the losses that occur at junctions in lightweight building structures. Recently the authors have investigated the underlying uncertainties related to both measurement, material and craftsmanship of timber junctions by means of repeated modal testing on a number of nominally identical test subjects. However, the literature on modal testing of timber structures is rather limited and the applicability and robustness of different curve fitting methods for modal analysis of such structures is not described in detail. The aim of this paper is to investigate the robustness of two parameter estimation methods built into the commercial modal testing software B&K Pulse Reflex Advanced Modal Analysis. The investigations are done by means of frequency response functions generated from a finite-element model and subjected to artificial noise before being analyzed with Pulse Reflex. The ability to handle closely spaced modes and broad frequency ranges is investigated for a numerical model of a lightweight junction under different signal-to-noise ratios. The selection of both excitation points and response points are discussed. It is found that both the Rational Fraction Polynomial-Z method and the Polyreference Time method are fairly robust and well suited for the structure being analyzed.

Kristoffer A. Dickow, Poul Henning Kirkegaard, Lars V. Andersen

Chapter 17. A Comparative Study of System Identification Techniques Under Ambient Vibration

This paper presents the dynamic properties of the Confederation Bridge extracted by four output-only system identification algorithms applied to vibration monitoring data. The purpose of this study is to evaluate the efficiency and accuracy of different modal identification methods, particularly in the presence of high level of uncertainty and noise related to field measurement data. The modal estimates obtained using these alternative approaches are compared and verified against the modal properties from the finite element model.

Muhammad S. Rahman, David T. Lau

Chapter 18. Some Implications of Human-Structure Interaction

On structures, humans may be active which may cause structural vibrations as human activity can excite structural vibration modes. However, humans may also be passive (sitting or standing on the structure). The paper addresses this subject and explores the implications of having passive humans present on the structure. It is not conventional to model the presence of passive humans when predicting structural response, but nevertheless it is instructive to investigate which effect they do in fact have on structural behavior and modal characteristics of structures. Such investigations are made in the present paper.

Lars Pedersen

Chapter 19. Evolution of Dynamic Properties of a 5-Story RC Building During Construction

A full scale five-story reinforced concrete building was built and tested on the NEES-UCSD shake table. The purpose of this experimental program was to study the response of the structure and nonstructural systems and components (NCSs) and their dynamic interaction during seismic excitation of different intensities. The building specimen was tested under base-isolated and fixed-based conditions. Furthermore, as the structure was being built, an accelerometer array was deployed in the specimen to study the evolution of its modal parameters during the construction process and due to placement of major NCSs. A sequence of dynamic tests, including daily ambient vibration tests, impact/free vibration and forced vibration (white noise base excitation) tests, were performed on the structure at different stages of construction. Several state-of-the-art system identification methods, including two output-only (SSI-DATA and NExT-ERA) and one input-output (OKID-ERA), were used to estimate the modal properties of the structure (natural frequencies, damping ratios and mode shapes). The results obtained allow to compare the modal parameters obtained from different methods as well as the performance of these methods and to investigate the effects of the construction process and NCSs on the dynamic properties of the building specimen.

Rodrigo Astroza, Hamed Ebrahimian, Joel P. Conte, Tara C. Hutchinson, Jose I. Restrepo

Chapter 20. Structural Dynamic Parameter Identification and the Effect of Test Techniques

Ambient and forced excitation test techniques are both widely used to dynamically identify civil engineering structures. Two floor levels of a newly constructed building, the Charles Institute at University College Dublin, were tested using both techniques. Both floor designs were identical although the layout of partitions above and below each were different. The objective of the tests was to determine the most appropriate test procedure and also to identify whether the layout of partitions contributes in a significant manner to dynamic response. It was found that at low levels of excitation, ambient test levels, the dynamic response of both floors was identical. In contrast, at higher vibration excitation levels, during forced vibration testing, the floor responses were substantially different. The differing modal parameters identified are attributed to an amplitude dependent response resulting from engagement, or not, of the partitions in the dynamic response of the system. The practical significance of this finding is that it is imperative to consider, and test at, the in-service vibration amplitude expected for a floor system.

A. Devin, P. J. Fanning, C. J. Middleton, A. Pavic

Chapter 21. Comparison of Damping Models for Space Flight Cables

A model to predict the dynamic response of space flight cables is developed. Despite the influence of cable harnesses on space structures’ dynamics, a predictive model for quantifying the damping effects is not available. To further this research, hysteretic and proportional viscous damping were incorporated in Euler-Bernoulli and Timoshenko beam models to predict the dynamic response of a typical space flight cable, using hysteretic dissipation functions to characterize the damping mechanism. The Euler-Bernoulli beam model was used to investigate the hysteresis functions specifically, and it was determined that including hysteretic dissipation functions in the equations of motion was not sufficient to model the additional modes arising in damped cables; additional damping coordinates in the method of Golla, Hughes and McTavish will be necessary to predict damping behavior when using dissipation functions for this case. A Timoshenko model that included viscous and time hysteresis damping was developed as well, and will ultimately be more appropriate for cable modeling due to the inclusion of shear and rotary inertia terms and damping coefficients.

Kaitlin Spak, Gregory Agnes, Daniel Inman

Chapter 22. Forced 3D Nonlinear Dynamics of a Hanging Cable Under Multiple Resonance Conditions

The forced, nonlinear, 3D dynamics of an elastic cable is analyzed by means of a reduced 4 d.o.f. model, already obtained several years ago by some of the Authors of this paper. The system is analyzed in the case of multiple internal resonance conditions and a 1:1 external primary resonance condition. The reduced model, because of a strong intrinsic symmetry due to the fact that anti-symmetric in-plane and out-of-plane modes have the same natural frequency (Irvine’s theory), is in principle not able to catch some interesting classes of motion, such as ballooning, which on the other hand have been observed in experimental tests. In the present paper, an imperfection between the equations ruling the in-plane and out-of-plane components is introduced through an internal detuning, which simulates the slight difference between the frequencies of the two involved modes, which is plausible as a consequence of the initial curvature of the cable as well as obtainable through more refined analytical models. A discussion on the similarity and differences with the solutions previously obtained is presented. Regions of non-regular response in the excitation control parameter plane are located and ballooning trajectories are analyzed.

R. Alaggio, F. Benedettini, G. Rega, D. Zulli

Chapter 23. Computationally Efficient Design of Semiactive Structural Control in the Presence of Measurement Noise

Designing control strategies for smart structures, such as those with semiactive devices, is complicated by the nonlinear nature of the feedback control, secondary clipping control, and other additional requirements such as device saturation. The authors have previously developed an approach for semiactive control system design, based on a nonlinear Volterra integral equation (NVIE) that provides a low-order computationally efficient simulation of such systems, for state feedback semiactive clipped-optimal control. This paper expands the applicability of the approach by demonstrating that it can also be adapted to accommodate more realistic cases when, instead of full-state feedback, only a limited set of noisy response measurements is available to the controller. This extension requires incorporating a Kalman filter estimator, which is linear, into the nominal model of the uncontrolled system. The efficacy of the approach is demonstrated by a numerical study of a 100-DOF frame model, excited by a filtered Gaussian random excitation, with noisy acceleration sensor measurements to determine the semiactive control commands. The results show that the proposed method can achieve more than two orders of magnitude improvement in computational efficiency while retaining a comparable level of accuracy.

Mahmoud Kamalzare, Erik A. Johnson, Steven F. Wojtkiewicz

Chapter 24. Optimal Nonlinear Control Using a Non-quadratic Cost Function for Scalar Systems

Various types of controllers have been studied and implemented to mitigate the effects of excitations from natural hazards. Linear control laws are most often applied, for active devices as well as part of the controller for semiactive devices, primarily due to their simple design and use. However, researchers have investigated nonlinear control of structures, showing performance improvements relative to that with linear controllers. The optimal linear control law that minimizes a quadratic cost function can be obtained using typical solvers like

Matlab

’s

lqr

command. In contrast, the optimal nonlinear controller generally requires minimizing a non-quadratic cost function, which is difficult and analytical solutions may only exist when the cost function is in particular forms. This paper presents a comparison of some of the analytical and numerical methods for finding optimal nonlinear controllers, particularly for cost functions that are even order powers of the states and quadratic in the control. A scalar model (

i.e.

, a scalar state-space equation) is used for the comparisons since analytical solutions exist for some of the methods. Even though the methods would all result in the same linear control law, it is demonstrated that the methods’ differing assumptions give rise to different optimal nonlinear control laws, with different performance in different excitations.

Elham Hemmat-Abiri, Erik A. Johnson

Chapter 25. Optimal Sensor Placement with a Statistical Criterion for Subspace-Based Damage Detection

Subspace-based fault detection algorithms have proven to be efficient for the detection of changes in the modal parameters for damage detection of vibrating structures. With these algorithms, a state-space model from the reference condition of a structure is confronted to output-only vibration data from a possibly damaged condition in a chi

2

test on a damage detection residual. The outcome of this test is compared to a threshold to decide if there is damage or not. In this paper, the problem of optimal sensor placement for this damage detection algorithm is considered based on the statistical properties of the chi

2

test. Using a model of the structure, sensor positions are chosen such that the non-centrality parameter of the chi

2

distribution is maximized for a certain set of damages. It is anticipated that this approach would, indirectly, lead to a maximization of the power of the test. The efficiency of the approach is shown in numerical simulations.

Michael Döhler, Kenny Kwan, Dionisio Bernal

Chapter 26. Application of Efficient Model Correction for Damage Assessment Using Limited Measurements

The purpose of this research is to employ the model updating technique to conduct the structural damage detection with insufficient measurements. First, the stochastic subspace identification technique is used to identify the system mode shapes from the limited measurement, then the mode shape expansion (MSE) technique was introduced to reconstruct the mode shapes in all degree of freedoms. Second, in cooperated with the expanded mode shapes the damage detection technique, called Efficient Model Correction Method (EMCM), is used to identify the damage location as well the damage severity. To investigate the effectiveness of the proposed MSE technique and model updating approaches, numerical studies with three types of sensor distributions and five damage scenarios were investigated. The results indicated that when the weighting coefficient of the proposed MSE technique is properly selected, the ability of reconstructing the mode shape is appreciated. For damage detection, data collected from the shaking table test of a six-story steel frame structure was used. With limited measurements the MSE technique together with the applicability of the EMCM, damage detection of the frame structure is conducted. The ability EMCM for damage detection is also discussed. The study concluded that damage detection through mode shape expansion is possible if the expanded mode shapes are consistent with the exact mode shapes. The Efficient Model Correction Method can also provide good results of structural damage detection.

Yi-Cheng Wu, Chin-Hsiung Loh

Chapter 27. Theory Based Sensitivity Analysis and Damage Detection of Steel Roof Sheeting for Hailstone Impact

In this paper, a first-order design sensitivity analysis is presented for dynamics responses of roof sheeting under hailstone impacts. The presented design sensitivity analysis computes the rate of various response changes with respect to the relevant design variable for roof sheeting. For this purpose, exploiting the finite element method, the dependence of response measures, such as deflection, stress and strain on the thickness and material properties of the steel sheeting, is implicitly defined. Then the minimum required thickness for each steel grade under different loading conditions for sheeting, are suggested by considering two conditions according to the hailstones sizes: no damage (yielding) for hailstones of small sizes (20 mm or less) and no penetration (failure) for hailstones of large sizes (up to 100 mm).

P. Sharafi, Lip H. Teh, Muhammad N. S. Hadi

Chapter 28. Long-Term Dynamic Monitoring of an Offshore Wind Turbine

Future Offshore Wind Turbines will be hardly accessible; therefore, in order to minimize O&M costs and to extend their lifetime, it will be of high interest to continuously monitor the vibration levels and the evolution of the frequencies and damping ratios of the first modes of the foundation and tower structures. Wind turbines are complex structures and their dynamics vary significantly in operation in comparison to stand still parked conditions due to changes in operating conditions or changing ambient conditions. State-of-the-art operational modal analysis techniques can provide accurate estimates of natural frequencies, damping ratios and mode shapes. To allow a proper continuous monitoring during operation, the methods have been automated and their reliability improved, so that no human-interaction is required and the system can track changes in the dynamic behaviour of the offshore wind turbine. This paper will present and discuss the approach and the first results of a long-term monitoring campaign on an offshore wind turbine in the Belgian North Sea.

Christof Devriendt, Filipe Magalhães, Mahmoud El Kafafy, Gert De Sitter, Álvaro Cunha, Patrick Guillaume

Chapter 29. Nondestructive Evaluation of Surface Crack Depth in Concrete

Subsurface damages in concrete may cause considerable durability loss in the structure since they are not visible from the surface and let water and other chemicals to penetrate into the structure and cause reinforcement corrosion. Ultrasonic methods have been widely applied for defect detection in concrete, however, there are not many applications of the method found in literature for subsurface damage identification. Specifically, when the damage is deeper considering the distance from the top surface. Therefore, in this study concrete blocks including subsurface cracks with different depths as well as a sound concrete block were tested by applying ultrasound. Wave parameters such as velocity and attenuation for each case were evaluated. Rayleigh wave and longitudinal wave velocities do not change for different subsurface crack depths. Even though, wave energy attenuates more in case of a shallower crack, it is difficult to correlate the attenuation rate with crack depth. Still, attenuation rate is an indicative parameter for subsurface damage identification. Numerical results obtained by Boundary Element Method (BEM) analysis are in good agreement with experimental ones. Stack imaging procedure applied to ultrasonic echo data is used as a complementary technique for subsurface crack depth identification. Reflection at the crack depth can be clearly identified.

Ninel Alver, Masayasu Ohtsu

Chapter 30. An Improved Methodology for Anomaly Detection Based on Time Series Modeling

In this paper, results from on-going studies for extending a damage detection approach previously developed by the authors are presented. The methodology in the previous form was able to detect and locate the damage/anomaly successfully by using free response acceleration data employing a sensor clustering based time series modeling approach. The basic idea behind the methodology is that an Auto-Regressive model with eXogenous input (ARX model) between the outputs of a structure can be related to the structural properties of the system without using the input (excitation to the structure) or any other information. The improved version can detect, locate and quantify mass, stiffness and damping changes separately in a numerical model by using acceleration, velocity and displacement data. Based on the results, the potential and advantages of the methodology under investigation are discussed. Its limitations and shortcomings in the current version are also addressed along with proposed solutions and future work plans.

Qipei Mei, Mustafa Gul

Chapter 31. Response Surface Model Updating for Nonlinear Structures

This paper presents a procedure to update nonlinear finite element models in time. In the proposed method, accurate response surface models are constructed and evaluated to replace the finite element model at every time step of the analysis. Then, the optimization problem of model updating is formulated and solved iteratively leading to histograms of the updated model parameters. This methodology is beneficial in extracting more information from measured signals and compensate for the error present in the regressed response surface models. The proposed method was verified through a numerical case study of a steel frame with global nonlinearity. Appropriate design and model orders were successfully established and the optimization in time performed well in the simulated scenarios under the assumption of noise free and noisy measurement data.

Golnaz Shahidi, Shamim N. Pakzad

Chapter 32. Application of Multivariate Statistically Based Algorithms for Civil Structures Anomaly Detection

Two multivariate statistics based damage detection algorithms are explored in conjunction with optical fiber sensors for long-term application of Structural Health Monitoring. Two newly developed data driven methods are investigated, for bridge health monitoring, here based on strain data captured by Fiber Bragg Grating (FBG) sensors from 4-span bridge model. The most common and critical damage scenarios were simulated on the representative bridge model equipped with FBG sensors. Acquired strain data were processed by both Moving Principal Component Analysis (MPCA) and Moving Cross Correlation Analysis (MCCA). The efficiency of FBG sensors, MPCA and MCCA for detecting and localizing damage is explored. Based on the findings presented in this paper, the MPCA and MCCA coupled with FBG sensors can be deemed to deliver promising results to observe and detect both local and global damage implemented on the bridge structure.

Masoud Malekzadeh, Mustafa Gul, F. Necati Catbas

Chapter 33. System Identification of a Three-Story Precast Concrete Parking Structure

A half-scale, three-story, precast concrete building resembling a parking garage was tested on the UCSD-NEES shake-table in May–July of 2008. This was the capstone test of a multi-university/industry research project aimed at the development of seismic design guidelines for precast concrete diaphragms. The shake-table tests were designed to induce damage on the building progressively through earthquake records of increasing intensity. Low-amplitude white-noise base excitations as well as scaled earthquake tests were performed between large-amplitude earthquake records. In this study, modal parameters of the test structures are identified using a deterministic-stochastic subspace identification method based on low-amplitude as well as high-amplitude seismic test data. The changes in the identified modal parameters are compared to the progressive damage of the building. Reduction of the identified natural frequencies and increase of the damping ratios indicate loss of stiffness and development/propagation of cracks, while the changes in the mode shapes point to the location of damage.

Andrea Belleri, Babak Moaveni, Jose I. Restrepo

Chapter 34. Structural Damage Localization Using Sensor Cluster Based Regression Schemes

Automatic damage identification from sensor measurements has long been a topic of interest in the civil engineering research community. A number of methods, including classical system identification and time series analysis techniques, have been proposed to detect the existence of damage in structures. Not many of them, though, are reported efficient for higher-level damage detection which concerns damage localization and severity assessment. In this paper, regression-based damage localization schemes are proposed and applied to signals generated from a simulated two-bay steel frame. These regression algorithms operates on substructural beam models, and uses the acceleration/strain responses at beam ends as input and the acceleration from an intermediate node as output. From the regression coefficients and residuals three damage identification features are extracted, and two change point analysis techniques are adopted to evaluate if a change of statistical significance occurred in the extracted feature sequences. For the four damage scenarios simulated, the algorithms identified the damage existence and partially succeeded in locating the damage. More accurate inferences on damage location are drawn by combining the results from different algorithms using a weighted voting scheme.

Ruigen Yao, Shamim N. Pakzad

Chapter 35. Operational Modal Analysis Based on Multivariable Transmissibility Functions: Revisited

Transmissibility functions are output-only functions that have one major advantage. When well-defined these functions become independent from the spectral content of the acting forces. A method for Operational Modal Analysis (OMA) based upon these type of functions would therefore not be affected by any colored or harmonic forces exciting the structure. This is a major advantage compared to existing techniques in the field of OMA. Transmissibility based Operational Modal Analysis (TOMA) was first introduced a few years ago and this paper will summarize the basics behind this new field of study. Secondly the recently developed p-TOMA will be presented and illustrated by means of a numerical experiment.

Wout Weijtjens, Gert de Sitter, Christof Devriendt, Patrick Guillaume

Chapter 36. Diagnosis of Building Vibration Sources via Time-Frequency Analysis

Diagnosing excessive building vibration can be problematic because of long transmission paths. The excitation may be far displaced from the occupant thus increasing the number of potential sources. Source identification is further exacerbated by the intermittent behavior of the problematic excitation. Long time vibration records, on the order of many minutes, hours, or even days are sometimes required to capture a problematic event or the pattern of the problematic events. Analysis of the long time records with traditional spectral processing methods is usually not effective as the ensemble averaging clouds the intermittent features that are found objectionable by the occupants. Methods that rely on identifying peaks are also ineffective because a single transient peak is not typically the source of problem vibration levels. The Short Time Fourier Transform (STFT) is well suited to analyzing long records of building vibration because they provide the insight necessary to separate and identify intermittent excitation sources creating problematic building vibrations. This paper will describe the STFT in relation to analyzing long time record building vibration. The processing technique is applied to diagnose the source in a residence where the occupant complained about floor vibration even though no easily identifiable source was found.

Linda M. Hanagan, Martin W. Trethewey

Chapter 37. Medial-Lateral Gait Patterns in Healthy Adult Walkers

With over a decade of research into the causes and effects of footbridge lateral excitation, models have emerged to describe pedestrian motion and force patterns. Authors have suggested, however, that insufficient data are available for model comparison. Thus, the medial-lateral (M-L) ground force, centre of pressure (CoP) location, and centre of mass (CoM) location were collected for over 300 healthy adult male and female footsteps. The data were collected using two AMTI force plates (1000 Hz) and a Vicon motion capture system (100 Hz). Using MATLAB, the data were analysed: the subsequent qualitative and quantitative observations are the topic of this paper. Inter- and intra-subject trends were observed among force-time correlations, CoP and CoM paths, and force-CoP plots. Additionally, the M-L force data were also compared to the inverted pendulum model to assess the model’s accuracy in predicting individual step behaviour. Conclusions are drawn that while walkers exhibit consistency in M-L force strategy over repeated footsteps, their steps do not tend to match the population mean. Furthermore, the inverted pendulum model proves poor in predicting either the population or the individual participants. This supports the theory that lateral pedestrian motion and forces should be modelled as stochastic processes.

Daniel Claff, M. S. Williams, A. Blakeborough, J. Stebbins

Chapter 38. Quantifying Differences Between Walking Locomotion on Rigid and Flexible Pavements

This work explores the differences between walking locomotion on rigid pavement and a particular type of flexible pavements made of EVA75 foam having thickness of either 10 or 20 mm. The aim is to analyse how human movements, in terms of trajectories of different body parts, are influenced by rigidity of such pavements. This research line is ultimately directed towards evaluation of the influence of flexible pavement on the vibration perception by human test subjects walking over vibrating decks of lively footbridges and floors.

The experiments were carried out in the Structures Laboratory at the University of Warwick, UK. A part of a 20 m long walkway was monitored using six infrared cameras. Three test subjects, instrumented with reflective body markers were asked to walk over the walkway made of concrete. Nominally the same experiments were then conducted on the walkway covered with one 10 mm thick layer of foam (ethyl vinyl acetate EVA75) first, and then with two layers of foam having thickness of 20 mm. Mechanical properties of the foam and its influence on the vertical ground reaction forces (GRFz) depending on the number of layers are studied. The trajectories of the reflective markers attached to the human anatomical landmarks are traced using the Vicon motion capture system. Averaged amplitudes of movement for chosen markers are compared for walking over the three supporting surfaces. While the first peak in the GRFz decreases when increasing thickness of the foam the magnitudes of displacements and accelerations of all analysed markers increase. The findings reveal that human walking on surfaces of different flexibility can be characterised by acceleration of body segments as well as by the way the forces propagate through the body via skeletal system.

M. V. Istrate, S. Zivanovic, A. Lorenzana, N. Ibán, H. V. Dang

Chapter 39. Using MSD Model to Simulate Human-Structure Interaction During Walking

Increasing vibration serviceability problems of modern pedestrian structures have drawn researchers’ attention to detailed modelling and assessment of walking-induced vibration on floors and footbridges. Stochastic nature of human walking and unknown mechanisms of their interaction with the structure and surrounding environment, make it difficult to simulate. Ignoring these complexities has rendered the current design methods to a rough approximation of reality which often leads to considerable over or under-estimation of the structural response yielding unreliable assessment of vibration performance.

Some aspects of human-structure interaction (HSI), such as synchronization, have been studied extensively, mostly in the lateral direction. But, despite of its much bigger significance, effects of walking pedestrians on dynamic properties of structures in the vertical direction are mostly ignored. This is mainly due to the lack of credible HSI experimental data in the vertical direction as well as models capable of simulating the interactions between the two dynamic systems.

To address this gap, this paper tries to adapt a classic single degree of freedom mass-spring-damper (MSD) model of human body to illustrate the effects of walking pedestrians on dynamic properties of structures. Parametric studies were carried out to analyse effects of the human model dynamic properties on coupled system response. This MSD model can be seen as the basic building block of realistic human body models which are currently being developed to address both biomechanical specifics and HSI effects on structures occupied and excited by walking human.

E. Shahabpoor, A. Pavic, V. Racic

Chapter 40. Effect of Sensory Stimuli on Dynamic Loading Induced by People Bouncing

Prediction of dynamic loads induced by groups and crowds of people bouncing is a hot topic among designers of grandstands and floors in entertaining venues. Using motion capture technology transferred and adapted from biomedical research, this study aims to investigate effect of visual, auditory and tactile cues on the ability of people to coordinate or synchronise their bouncing movements in groups of two. The numerical results showed a great significance of such stimuli on people’s mutual interaction during bouncing, signifying that their effect should be considered in developing much-needed models of crowd dynamic loading of structures due to coordinated rhythmic activities.

Vitomir Racic, James M. W. Brownjohn, Shu Wang, Mark T. Elliot, Alan Wing

Chapter 41. Design and Construction of a Very Lively Bridge

In recent years, an increasing number of light structures has been reported to exhibit substantial vertical vibrations when exposed to pedestrian-induced dynamic loading. It is believed that pedestrians interact with lively structures by altering their walking style and changing the dynamic properties of the vibrating system. As the existing vibration serviceability guidelines do not address these pedestrian-structure interaction effects, they cannot predict the structural dynamic response accurately. Fundamental understanding of the pedestrian-structure interaction is currently limited since most reported observations are of qualitative nature. To improve understanding and develop models of human interaction with lively structures, a purpose-built experimental facility that can be excited by human walking is required.

This paper describes design and construction of a 19.9 m long, low-frequency and lightly damped experimental bridge for studying pedestrian-structure interaction. The challenge to design a relatively heavy and low-frequency footbridge in the limited space of the Structures Laboratory at the University of Warwick, UK, was met by adopting a traditional steel-concrete composite structural system. The experimental data collected on the "Warwick Bridge" during first six-months of structural life are presented to characterise both its static and dynamic behaviour. Dynamic testing of the bridge revealed that, with an achieved fundamental natural frequency of 2.4 Hz, the corresponding damping ratio of 0.5%, and an opportunity to tune the dynamic properties as required, the key design criteria were successfully met.

S. Živanović, R. P. Johnson, H. V. Dang, J. Dobrić

Chapter 42. Experimental Results from a Laboratory Test Program to Examine Human-Structure Interaction

Assembly-type structures, subjected to crowd-induced rhythmic excitation, are typically designed to avoid excessive vibration. Most current design guidance recommends using the dynamic properties of the empty structure to assess the susceptibility of a given design to vibration. This recommendation does not incorporate the effects of human-structure interaction where the dynamic properties of the combined system are different from that of the empty structure. To assess the consequences of this recommendation, an experimental study was completed to identify the effects of human-structure interaction on the dynamic properties of the empty structure. The study investigated the influence of posture, mass ratio, and natural frequency of the structure when considering human-structure interaction of a passive crowd. The results of this study are presented and discussed with respect to the prediction of the dynamic response of a structure when the human-structure interaction effects are incorporated or neglected. Because it is likely that the dynamic response of the structure will be overestimated when utilizing the properties of the empty structure, a method for appropriately incorporating the effects of human-structure interaction is needed. An overview of the methods utilized in the study is presented and the results are examined to improve the understanding of the influential factors involved in human-structure interaction.

Kelly A. Salyards, Nicholas C. Noss

Chapter 43. Alleviation of Wind-induced Vibrations of Railings in a Building

It was observed that wind-induced vibrations had created destructive effects on corner balcony railings in an oceanfront condominium building. The vibrations of the railings caused disconnections of welds at the ends of the pickets, and noise was generated, which created discomfort to the residents of the building. This paper presents a method to mitigate the wind-induced vibrations and noise of the balcony railings. The noise generation mechanisms were investigated; buffeting and vortex-induced vibration were identified as the two main mechanisms. A finite element (FE) model of a railing was developed using commercial FE software ABAQUS. The FE model was validated by modal testing results and used as a basis for later modifications. Design modifications were made for the railing and simulated using ABAQUS; increasing the stiffness of the railing was proposed as a solution based on the FE simulation results and the constraints imposed. A modified railing was installed on one of the corner balconies according to the solution and tested. The experimental results and on-site monitoring on the modified railing indicate that the noise issue has been resolved, and all of the affected railings were demolished and replaced with the modified railings.

C. M. Hou, W. D. Zhu

Chapter 44. Operational Modal Analysis of a Slender Footbridge to Serviceability Purposes

This work proposes the dynamic analysis of a pedestrian walkway. This walkway is placed within the ‘Ospedale di Circolo’ Hospital complex in Varese. In order to prevent any vibration serviceability issue a dynamic analysis has been committed. Ambient vibration data and vibration data in operating conditions, that is, while people walked along the footbridge, were collected. In order to assess the serviceability of this structure, a complete modal analysis was first performed. This allowed identifying the main resonances of the structure in the range of frequencies potentially at risk because of a possible synchronization with the walking people loading. Vibration levels were then verified according to ISO 10137, and other international standards. A comparison between vibration levels determined according to the normative regulations and those measured in operating conditions is finally proposed.

Anna Cappellini, Stefano Manzoni, Marcello Vanali, Elena Mola

Chapter 45. Optimal Sensor Placement for Structural Health Monitoring of Power Transmission Tower-Line Systems

This paper presents a numerical pre-test finite element modeling and optimal sensor placement study for power transmission structures. The number, geometry, repetition and importance of such structures require easier, quicker and cheaper monitoring methods. Vibration-based health monitoring methods determine the modal characteristics of the structure via a limited number of sensors. These characteristics are intrinsic properties, so that a variation in them may be induced by structural damage. Only a limited number of degrees-of-freedom can be measured for the system identification process. By developing a finite element model for the tower-line structure, these degrees-of-freedom can be identified. Prior to any modal analysis, a geometrically non-linear static analysis of the structure is required. Based on these results, two methods are employed to determine the optimal sensor number and locations. Both are formulated with the use of the modal properties of the structure model. The first scheme maximizes the independence of the target modal shape matrix in an iterative process, where those degrees-of-freedom that do not contribute to the independence of the target modes are eliminated. The second scheme is based on a mass-weighting of the previous one. Correlation results are developed between the tower and the tower-line structures in order to verify the influence of the lines in the modal characteristics. In order to simulate experimental measuring, modal properties are altered by adding Gaussian noise which determines the effect on the number and location of the sensors. It is concluded that employing the tower-line system is more accurate than considering only the tower structure; and the result of sensor placement is improved for structural health monitoring purposes.

José Antonio Vergara, Rafael Castro-Triguero, David Bullejos, Rafael Gallego, Diego Zamora

Chapter 46. Evaluation of Economical Dynamic Exciters for Vibration Testing of Bridges

Quantitative data describing condition and performance is essential for evaluating the structural health of bridges. Dynamic testing is a common approach for globally characterizing bridges in a quantitative sense. Dynamic testing is most commonly accomplished for full-scale bridge structures through either forced vibration testing or ambient vibration testing methods. Forced vibration testing offers many advantages, but is generally not a practical or economical approach for many bridges due to the high cost of providing controlled excitation, limits to the excitation that can be supplied, and interference with the normal operation of the bridge. The writers have been investigating the feasibility of using low-cost, small-scale dynamic exciters for forced vibration testing of short to medium span bridges. The exciters being evaluated have a unit cost that is comparable to a typical accelerometer, and could be deployed in numbers using a spatially distributed setup for forced vibration testing. This paper presents and describes the results of a laboratory evaluation program conducted for these devices. Their capabilities and operating characteristics are compared with a more conventional linear mass shaker. The preliminary results of a vibration test using these devices on an in-service highway bridge are also discussed.

E. V. Fernstrom, J. L. Carreiro, K. A. Grimmelsman

Chapter 47. Assessment of Floor Vibrations for Building Re-use: A Case Study

Developments involving building re-use can present numerous challenges with respect to evaluation of structural floor capacity and establishing controls for serviceability. Retrofit strategies can be particularly challenging when engineering drawings of the structure are scant, or even non-existent. This paper presents a case study involving renovation of a light-weight mezzanine warehouse floor into executive office space. During early construction the floor was observed to be particularly lively, attributable to its lightweight, long-span construction and lack of non-structural elements. The author was engaged by the client to assess the floor and evaluate expected performance following fit-out. This included field measurements of frequency response and footfall vibrations, development of a computer model and correlation of the model with the field data. Simulations were conducted to validate random force models presented in the literature. The force and response models were then employed to assess expected performance of the floor. All modeling was conducted in the absence of engineering drawings, providing valued lessons on FEM techniques for floor systems. Measurements were also conducted following partial fit-out to track the change in the floor’s dynamics and footfall responses.

Brad Pridham

Chapter 48. Experimental Validation on a Continuous Modulated Wave-Form Command Shaping Applied on Damped Systems

A new point-to-point command-shaping control strategy for oscillation reduction of damped simple harmonic oscillators is implemented experimentally on a scaled crane model. The effect of damping on the shaper frequency and duration is investigated. The performance of the proposed shaper is simulated numerically and compared to the results obtained experimentally. It is shown that, the proposed wave-form command profiles are capable of eliminating the travel and residual oscillations for systems with different damping ratios. It is shown that ignoring the system damping may result in unwanted oscillations. Unlike traditional impulse and step command shapers, the proposed command profiles have smoother intermediate acceleration, velocity, and displacement profile. In some ranges, the proposed technique produces faster maneuvers with smoother intermediate acceleration, velocity, and displacement profiles.

Khaled A. Alhazza

Chapter 49. Quantification of Damping Effect of Humans on Lightly Damped Staircases

There is experimental evidence that the people interacting with structures are not only an active load but also affect the structural properties. Particularly, considerable damping ratio value changes are often experienced. This fact assumes a relevant importance in assessing the structure serviceability against vibrations due to pedestrian induced loads. There is therefore ground to find methods capable of estimating the effect induced by people interacting with a structure, in terms of both changes of modal parameters and of loading effect. This work aims at presenting a model, partly based on modal approach, able to describe people effect on a structure. Each person is modelled as a two degrees of freedom spring-mass-damper system and is introduced locally on the structure. Several tests on a lightly damped staircase were then carried out to validate the model.

Anna Cappellini, Stefano Manzoni, Marcello Vanali

Chapter 50. Robust Design Optimization of Steel Moment Resisting Frame Under Ground Motion Uncertainty

In the design of a steel moment resisting frame, uncertainties may arise from a variety of sources, such as ground motion, mass, and damping ratio, etc., that may cause variation in seismic demand and capacity. These uncertainties need to be taken into account to ensure the desired margin of safety for required performance objectives. FEMA 350, a reliability based design guideline, can be employed to mitigate safety concerns by satisfying minimum confidence level requirements for performance objectives. However, in these existing design codes, variation in seismic demand is not explicitly considered in the design process. According to the FEMA 350 procedure, seismic demand is calculated with a suite of seismic records, considering ground motion variability, while only median demand is used in the subsequent calculation of demand to the capacity ratio and confidence level. In this paper, variation of seismic demand due to ground motion variability is considered explicitly as a

robustness measure

, and the mean value of seismic demand is treated as a

safety measure

. A Robust Design Optimization of steel moment resisting frame methodology is proposed, which is featured as a multi-objective optimization problem with the variation of seismic demand, mean value of seismic demand and cost as three objectives. In the optimization problem, optimal steel section sizes are sought to minimize these three conflicting objectives. The proposed methodology is then demonstrated through a multi-story multi-bay steel moment resisting frame design and solved with a Non-dominated Sorting Genetic Algorithm-II. With three competing objectives, the proposed methodology provides a set of designs in the form of a Pareto Front, which is robust, safe and economical. Furthermore a uniformity drift ratio requirement is proposed to ensure efficient designs.

Zhifeng Liu, Sez Atamturktur, Hsein Juang

Chapter 51. Structural Assessment of Fort Sumter Masonry Coastal Fortification Subject to Foundation Settlements

In historic unreinforced masonry structures, one of the most critical structural issues is the differential settlement of foundations. Due to unreinforced masonry’s brittle nature with a fairly low tensile strength, brittle cracking can occur due to tensile stresses introduced by foundation settlements. This study demonstrates the development and calibration of a finite element model and the use of this model for structural analysis under differential settlements of a casemate of Fort Sumter, a masonry coastal fortification best known as the site where the first shots of The American Civil War were fired in 1861. Development of accurate finite element models for historic masonry structures presents numerous challenges in the acquisition of non-linear material properties, and irregular geometry. Furthermore, these challenges are exacerbated because of the configuration of coastal fortifications, as these structures have characteristic designs unique to the distinct functionality of defense, such as cold-joints between disjointed structural components. The non-linear material behavior of the brick and mortar assembly is obtained from laboratory tests on samples obtained at the site. High definition laser scanning is used on irregular geometry to obtain the details of accumulated structural damage and degradation, including differential foundation settlements. The uncertain interface behavior at the cold joint between the scarp wall and the casemate of the fort is assessed using in-situ vibration tests. The finite element model developed is utilized to study the settlement magnitudes critical to the stability of the casemates of Fort Sumter for a variety of possible soil settlement configurations.

Sez Atamturktur, Saurabh Prabhu, Rick Dorrance

Chapter 52. Condition Assessment of a Coal Mine Shiploader

This paper describes the vibration tests and analyses conducted for the structural condition assessment of a shiploader structure in Colombia. This structure has been in continuous operation since the early 1980s and the owner of the facility is interested in assessing the current status of the structure. The purpose of the work conducted was to establish, through advanced structural engineering analyses, the capacity of the structure under different loading conditions. The analytical structural assessment was complemented with a visual assessment of the structure and field vibration tests. As the structure is a highly complex system and likely to behave nonlinearly, the need to calibrate the FE model for a linear condition of the structure was deemed necessary. The results of a series of OMA tests were used to update a detailed FE model of this complex structure. Once the model was updated, a number of analyses were conducted to determine the performance of the structure under different loading configurations and to identify critical areas of the structure where maintenance should be conducted regularly.

Carlos E. Ventura, Freddy Pina, Steve Yee, Christopher Prychon

Chapter 53. Experimental Assessment of Structure Borne Noise Generated by a Braking Resistor

This paper presents an application of modal analysis to the assessment of the structure borne noise level for a braking resistor to be mounted on the floor of a train carbody. This topic is becoming more and more important since the dynamic behavior of auxiliary components attached under the carbody strongly affects the passengers’ comfort. In particular, a braking resistor contains one or more fans used to cool the electrical resistances that dissipate the braking energy by Joule effect. The rotation of these fans can represent a relevant source of vibrations even in presence of low eccentricity.

In order to assess the transmitted vibrations before the real carbody has been built and to optimize the supporting elements of the braking resistor, an auxiliary fixture has been designed to reproduce the carbody mechanical impedance at the mounting point of the braking resistor. Thus, the first step is to design the fixture to have the same mechanical impedance of the carbody under construction (through finite element analysis); then, to measure its mechanical impedance along all three directions through impulsive tests and finally to assess the braking resistor’s behavior when mounted on the fixture and operated as during real life conditions.

F. Braghin, F. Cheli, G. Galli

Chapter 54. Pre-Test Nonlinear FE Modeling and Simulation of a Full-Scale Five-Story Reinforced Concrete Building

A full-scale reinforced concrete (R/C) building specimen, furnished with a variety of nonstructural components and systems, was built and tested on the UCSD-NEES outdoor shake table. The building specimen was subjected to a sequence of dynamic tests including scaled and unscaled historical earthquake ground motions. In order to simulate and predict the nonlinear dynamic response of the building specimen, a detailed three-dimensional nonlinear FE model of the structure was developed using the FE analysis software DIANA. By comparing the pre-test simulated results with the experimental results, the effects of the nonstructural components on the dynamic response of the building can be inferred. This paper describes the building test specimen and the nonlinear FE modeling and response simulation. Key numerical results are compared with their experimental counterparts and potential sources of discrepancies are discussed.

Hamed Ebrahimian, Rodrigo Astroza, Joel P. Conte, Jose I. Restrepo, Tara C. Hutchinson

Chapter 55. The Dynamic Stiffening Effects of Non-Structural Partitions in Building Floors

It is commonly known that full-height non-structural partitions of a fitted out floor structure affect its dynamic properties, with increase in floor mass and modal damping being commonly quoted in floor design guidelines. As a consequence, it is generally accepted that the non-structural elements usually reduce the response of floors to walking excitation. There is very little understanding of the effects of full-height partitions on the stiffness of building floors and this effect is generally not taken into account in floor design guidelines.

This paper is therefore focused on establishing experimentally the effects of full-height non-structural partitions on dynamic stiffness of a full-scale real-life composite building floor. Modal testing data are presented for three construction phases of the floor: from a completely bare floor via partially to fully-fitted floor. The effects of the partitions are shown by comparing the measured frequency response functions (FRFs) at the same location for different construction phases and the estimated key modal properties of the floor corresponding to these phases. This kind of multi-phase measurements on a real-life floor structure during construction is very rare due to its logistical complexity and long-time required to gather data through all of the phases.

It is shown that the partitions significantly affect measured FRFs by increasing damping, and in particular, floor stiffness. It is also shown that the mode shapes are changed by the partitions. The magnitude of the changes is quantified experimentally which is one of the first attempts to do this on a real-life floor structure using high-quality FRF measurements.

C. J. Middleton, A. Pavic

Chapter 56. Experimental Characterization and Predictive Modeling of a Residential-Scale Wind Turbine

As the demand for wind energy increases, industry and policymakers have been pushing to place larger wind turbines in denser wind farms. Furthermore, there are higher expectations for reliability of turbines, which require a better understanding of the complex interaction between wind turbines and the fluid flow that drives them. As a test platform, we used the Whisper 500 residential scale wind turbine to support structural and atmospheric modeling efforts undertaken to improve understanding of these interactions. The wind turbine’s flexible components (blades, tower, etc.) were modeled using finite elements, and modal tests of these components were conducted to provide data for experimental validation of the computational models. Finally, experimental data were collected from the wind turbine under real-world operating conditions. The FAST (Fatigue, Aerodynamics, Structures, and Turbulence) software developed at the National Renewable Energy Laboratory was used to predict total system performance in terms of wind input to power output along with other experimentally measurable parameters such as blade tip and tower top accelerations. This paper summarizes the laboratory and field test experiments and concludes with a discussion of the models’ predictive capability.

LA-UR-12-24832

.

Jordan B. Chipka, Andrew R. Lisicki, Chuong T. Nguyen, Stuart G. Taylor, Gyuhae Park, Curtt N. Ammerman, Charles R. Farrar

Chapter 57. Sizing Optimization of Trapezoidal Corrugated Roof Sheeting, Supporting Solar Panels, Under Wind Loading

The dimensions of trapezoidal roof sheeting supporting solar panels are optimized so that a minimum amount of steel is required for a specific range of wind loads. Sheets of different grades and different thicknesses along with different ranges of wind speeds are considered. In order to simulate the behavior of the corrugated sheets analysis is carried out for two limit states: strength and serviceability. For both limit states, the objective is to minimize the weight per unit area of the panels. First, optimum cross-section is obtained to meet the strength conditions. Then the deflection is controlled for serviceability. The optimum dimensions for each steel grade and loading condition are determined. The optimization problem is formulated as a multi-objective problem that aims to minimize the section’s weight and maximize the section elastic modulus under the wind loading condition. A graph theory based approach is presented for the sizing optimization, employing an applied graph theory approach for the multi-objective all pairs shortest path problem. The proposed methodology addresses the sizing optimization problem to determine the geometry of the thin-walled cold-formed steel cross-sections that satisfy the design topological constraints.

P. Sharafi, Lip H. Teh, Muhammad N. S. Hadi

Chapter 58. Establishment of Optimized Digging Trajectory for Hydraulic Excavator

Efficiency of digging operation of hydraulic excavator is affected by the geometry of digging trajectory. To improve the operation efficiency, appropriate trajectory design is important. In this report, we developed a simulation model to establish optimized trajectories. This model involves a soil and a front linkage model of an excavator. A Distinct Element Method (DEM) is adopted for the soil model. The DEM is able to describe soil behavior during digging operation and to estimate digging soil volume and load quantitatively. Generative force and energy consumption of hydraulic cylinders are calculated by the kinematic model which solves inverse dynamics of front linkage of hydraulic excavators. Geometric shapes of the trajectory are determined with some trajectory parameters in this study. Using these generated trajectories and the simulation model, consumed energy and digging soil volume of every trajectory is calculated. Analyzing these results, the optimal digging trajectory is established. Effectiveness of the optimized trajectory is validated by comparing with a skilled operator’s digging trajectory which is reproduced on the simulation. In addition, efficient trajectory for horizontal ground is established by using this developed simulation model.

Tatsuya Yoshida, Takayuki Koizumi, Nobutaka Tsujiuchi, Kan Chen, Yozo Nakamoto

Chapter 59. Modeling, Simulation, and Optimization of California High-Speed Rail Bridge Under Earthquakes

To meet ever-growing demands on California transportation infrastructure, the California High-Speed Rail (CHSR) Project is underway inspired by the successful high-speed train systems worldwide. Seismic risk tends to be a critical concern to the CHSR bridges in California on the structural engineers’ side as the proposed CHSR bridges will be in close proximity to several major seismic faults such as the San Andreas and Calaveras faults. Considering the social-economic functions of high-speed rail bridges, the optimal design of such bridges based on advanced structural modeling, seismic response simulation and performance evaluation are of significant importance to guarantee the dedicated high-speed train services after earthquakes. These issues are addressed in this paper for a CHSR prototype bridge testbed. To mitigate the seismic risk, seismic isolation strategies and segmental displacement control techniques (slotted hinge joints) are integrated into the design of this bridge. Numerical modeling and seismic response simulation are carried out to predict the performance of this bridge. An optimization problem in the context of the probabilistic Performance-based Earthquake Engineering (PBEE) methodology is proposed and illustrated conceptually to optimize the seismic isolator parameters to obtain a satisfactory trade-off between the deformation of the bridge piers and the deformations of the seismic isolators (inducing rail deformations).

Yong Li, Joel P. Conte
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