New paper : Noncontact Operational Modal Analysis of Structural Members by LDV February 2, 2009
Posted by dionsiringo in Vibration, mechanical vibration.add a comment
A system that uses ambient vibration and two laser Doppler vibrometer (LDV) is developed for noncontact operational modal analysis of structural members. The system employs natural excitation technique (NExT) to generate the cross-correlation functions from laser signals, and the eigensystem realization algorithm (ERA) to identify modal parameters of structural members. To facilitate simultaneous modal identification, time-synchronization technique and construction of cross-correlation functions from ambient response of laser signals are proposed. Performance of the proposed system is verified experimentally by evaluating the consistency and accuracy of identification results in different measurement conditions. The work presented here is an extension of the previous study, where a modal-based damage detection method using LDV was formulated.
In the present study, application of LDV for structural parameters identification of a combined dynamical system is proposed. A model that represents the connection properties in terms of additional stiffness and damping is developed, and its importance for structural damage detection is discussed. The study shows that the presence of simulated damage in a steel connection can be detected by tracking the modal phase difference and by quantifying the additional stiffness and damping.
Observed Dynamic Characteristics during Progressive Damage Test of a Flyover Bridge December 4, 2008
Posted by dionsiringo in Bridge Engineering, Destructive Test of Bridge, Structural Monitoring, Vibration, mechanical vibration, paper work.add a comment
Progressive destructive test of a structure is an important and rare opportunity to observe the change in structural characteristics. During the step-by-step demolition process, we can measure structural response and learn how the dynamic characteristics evolve. This insight is valuable in evaluating structural performance and in setting the benchmark for typical deterioration of structure. In this paper, we discuss the evolution of dynamic characteristics of a reinforced concrete bridge during a destructive test.
An instrumented box girder flyover bridge is sequentially damaged by cutting the tendons of its main span. Ambient vibration measurement was performed during destructive test and dynamic characteristics of the bridge were evaluated in each damage stage. The results show the change of natural frequencies is clearly visible as an indicator of global damage presence, while the change in damping distribution may be used as the local damage indicator.
IABSE Chicago and UIUC Trip October 1, 2008
Posted by dionsiringo in Structural Monitoring, Vibration, proceeding, visit.add a comment
Last week I attended the 17th IABSE Congress in Chicago. My colleague Dr. Nagayama presented a paper with me as the a co writer. The conference was great. Most of participants are engineers from big firms and famous consultants companies. Few are from academics. This sort of mixed audience is good for a change. It’s always a good thing to get feedbacks from engineers.
This is the paper presented at IABSE congress. It’s a modification of our previous works on bridge monitoring using ambient and seismic response. There are lots of paper that are very interesting to me, especially about long span bridge and tall building. I’ll post them next time.
Couple of days before IABSE Conference we visited the UIUC at Urbana-Champaign, where I visited Prof. Spencer’s Lab and participated on measurement of an old truss bridge using IMOTE2. The measurement was lead by Nagayama, couple of students from UIUC assisted us. It was a great experience. The collaboration was featured in UIUC research buletin.
Okutama Cable-Stayed Bridge : TMD for supression of Vortex-Induced Vibration April 3, 2008
Posted by dionsiringo in Bridge Engineering, Jembatan Cable-stayed, Teknik Sipil, Vibration, mechanical vibration, visit.4 comments
The Okutama Cable-stayed Bridge
Recently I had a very rare opportunity to go inside a cable-stayed bridge girder. The bridge is the Okutama Bridge, a single pylon cable-stayed bridge located at the Okutama area west part of Tokyo. The span length is 160 + 105 meter and the girder width is 12 m. The A-shape tower is made of steel reinforced concrete . The main girder is composed of double-span continuous steel double-box girder. Bridge construction was completed in 1996 and has become a part of the Okutama-Ohme Line in west part of Tokyo ever since.
We went inside the bridge following a group of Korean engineers, who are interested in the Tuned Mass Damper (TMD) system installed in this bridge. Unlike any other bridges that have TMD installed on the towers, in this bridge the TMD was installed on the girder.
The main reason for TMD installation is to suppress the Vortex-Induced Vibration (VIV) in vertical direction of the grider. As mentioned in the paper published by the designer and owner of the bridge, wind tunnel test of the current design girder revealed that vortex-induced vibration in vertical and torsional direction were evident under the wind velocity of 10 and 45 m/s, respectively. Both types of vibration had caused vertical amplitudes that were higher than the permissible one.
Tuned Mass Damper (0.8 ton of mass), main spring (white coil),and adjustable cantilever arm (in the middle)
In order to suppress the vibration, two measures were considered, one is aerodynamic measures by increasing aerodynamic damping using flaps, and the other is by mechanical measures such as installing a tuned-mass damper. The aerodynamic measure was abandoned for aesthetic and pedestrian safety precautions reasons, the TMD measure was selected instead.
Eight units of TMD were installed inside the girder, where each consists of mass (0.8 ton), springs (two of them: main and supporting springs) and a cylinder type of oil damper. The main spring, which has a form of coil, is connected to the mass by steel cantilever. Due to limited space inside the girder, the natural frequency of TMD is controlled by changing the position of the mass through an internal adjusting mechanism.
The bridge and its vibration control system are very impressive. In fact, it is the first girder VIV controlled bridge in Japan. However, when we visited the site, I was quite surprise to see that not so many passerby or vehicles crossing the bridge.
Structures with Human-Induced Vibration – How Serviceability Requirement Improves Vibration Design Concept July 25, 2007
Posted by dionsiringo in Vibration, mechanical vibration, opinion, paper work.1 comment so far
In the past few years, human body motions have quite often caused serious structural vibration problems. We have seen several excessive vibration problems caused by human motion during the service of structures. Human-induced vibrations were sometimes not considered in vibration suppression design due to the fact that the problem itself is primarily serviceability problem. Main considerations for structural dynamic design are safety against the occurrence of major vibration impact to the structures such as: earthquake, wind-induced vibration, traffic-induced vibration or/and impact-induced that might lead to structural failure at a catastrophic level. Human-induced vibrations, which were perceived only as serviceability problem in term of annoyance and disturbance to the users, accordingly have not been addressed properly in design code.
Several latest reports on cases, where human-induced vibrations were found excessive and annoying, however, have changed the common perspective on this problem. The closing of Millenium Bridge in London right after its completion due to excessive human-induced lateral vibration is one major case that took public attention. Thus following this report and some other previous cases, researcher, structural engineers and building authority have work together to accommodate users convenience requirements (serviceability) to provide better structures that suppress the anticipated human-induced vibration.
Monitoring of bridges and transportation infrastructures using vibration techniques July 15, 2007
Posted by dionsiringo in Bridge Engineering, Jembatan Cable-stayed, Jembatan Suspensi, Structural Monitoring, Vibration, proceeding.1 comment so far
The rapid growth of Japanese economy during the 1960’s intensified construction of bridges and transportation infrastructures system to meet the expansion of industrial activities. Since then, the total infrastructure stocks have accumulated considerably. These include development of national railway line and the highway networks that are still continuing until now. In twenty years from now, the bridges constructed around seventies will be more than fifty years old. Without proper maintenance they will be deteriorating and degradating in function. It is anticipated that by the year 2020 the number of aging road bridges will constitute half of the total road bridge networks. Consequently, even though Japan has been long considered as a country that is active in new construction of infrastructures, maintenance has becoming an increasingly important issue nowadays.
Maintenance, however, is not only the issue for aging bridges and transportation infrastructures but also for the newly constructed ones. In order to maintain infrastructure condition continuously, comprehensive monitoring systems have been introduced at the beginning at its service life. Many newly constructed bridges especially the long-span ones are now being closely monitored. They are instrumented with embedded sensors that allow continuous –and in some cases, online monitoring. For smaller scale bridges, the monitoring process constitutes routine inspection using portable sensors. A routine inspection or an overall monitoring of bridge after certain severe natural environmental condition such as earthquake provides a tool to evaluate structures condition and to detect possible structural defects. For this purpose, the vibration based SHM is considered superior to other non-destructive assessment methods due to its characteristics that allow for global system monitoring as well as detection of local structural defects determined from changes in the vibration characteristics.
System identification of suspension bridge using ambient response May 28, 2007
Posted by dionsiringo in Bridge Engineering, Jembatan Suspensi, Structural Monitoring, Vibration.add a comment
Performance of a suspension bridge under wind, seismic and other live loads depends upon its structural properties such as mass, stiffness and damping and their distribution. Although these properties can be modeled using sophisticated analytical models, the real behaviors of the bridge remain to be verified from a full-scale vibration test. The fullscale vibration test would facilitate identification of dynamic characteristics (e.g. natural frequency, damping ratio and mode shape), whose quantities serve as the basis for validating and/or updating analytical models of the structure, as well as providing the actual structural properties and boundary conditions. Furthermore, frequent measurements and analysis of these characteristics will facilitate the evaluation of structural safety and health monitoring.
There are two most common techniques for vibration test of a bridge, namely, the measured-input test and the ambient vibration test. In the measured-input tests, the structure is excited by artificial means using large inertial shakers or drop weights. Measured input excitation is usually applied at a single location where the force input to the structure can be monitored. The tests with measured inputs are usually conducted on small- or moderate-span bridges. The results are generally sufficient for modal identification since the inputs can be well defined and the excitations can be optimized to the response of vibration modes of interest. However, the test features that require extensive instrumentations and disruption of traffic have made frequent tests less favorable. Furthermore, in the case of large and flexible bridges (such as cable-stayed and suspension bridges), where the natural frequencies of the predominant modes are closely spaced within the frequency range 0–1 Hz, the controlled use of specific exciters to obtain significant levels of response is often difficult and costly. In such cases, ambient vibration becomes the only practical means of exciting the structure. This type of test makes use of ambient environment effects such as wind, traffic load, and environmental load as excitation force.
Dynamic characteristics of a curved cable-stayed bridge April 29, 2007
Posted by dionsiringo in Bridge Engineering, Earthquake, Jembatan Cable-stayed, Structural Monitoring, Vibration.add a comment
An assessment of dynamic characteristics of the 455 m Katsushika–Harp curved cable-stayed bridge is presented. Dynamics characteristics such as natural frequencies, mode shapes and modal damping ratios are obtained from seismic response of the bridge by employing a time-domain multi-input multi-output (MIMO) system identification (SI) technique. The technique makes use of base motions and superstructure accelerations as pairs of inputs–outputs to realize the coefficients of state-space system matrices. The SI results indicate the occurrence of many closely spaced modal frequencies with spatially complicated mode shapes. Fourteen global modes in the ranges of 0.45–2.5 Hz were identified, in which the girder motion dominated most of the modes. The tower modes were associated with girder modes and were characterized by the lowly-damped motion. Using identification results from six earthquakes, the effects of earthquake amplitude on modal damping ratios were observed.
Laser Doppler Vibrometer for Damage Detection April 29, 2007
Posted by dionsiringo in Structural Monitoring, Vibration, mechanical vibration.add a comment
The paper presents a study on the application of a laser Doppler vibrometer (LDV) for structural health monitoring using ambient vibration. The work covers three important issues namely, data acquisition, system identification, and structural damage detection. A two-laser system is employed for data acquisition using ambient response of the structure. Modal parameters are estimated by the eigensystem realization algorithm, after first deriving the impulse response functions from both laser responses using the Natural Excitation Technique.
In damage detection, a new matrix-updating-based method is proposed. The essential feature of this method is the non-iterative solving technique of inverse problem, which allows damage to be located and quantified by employing the modal parameters obtained before and after damage. Numerical simulation and laboratory-scaled experiments using bolted lap joint plate demonstrate that the proposed technique can detect locations and magnitude of damage with incomplete modal information.
