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.

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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.

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This month I had a chance to attend the 11th East-Asia Pacific Conference on Structural Engineering  (EASEC) in Taipei, Taiwan. It was a good conference, with many subjects and some of them are becoming very popular in civil engineering. The main topic of conference in structural engineering. This year special theme is about sustainability in civil engineering. The host of the conference is National University of Taiwan, which have done a tremendous work to put out such a good organization and arrangement.

with former advisor Prof. Iswandi Imran of ITB

This year conference is special because it was chaired by my prof, Prof. Fujino, so that many participants are from our research group. As always, conference is a good opportunity to learn what people have been doing and also to develop a network. I met a lot of old colleagues from previous EASEC conference (Bali 2003), and several alumni as well as my former lectures from ITB. I wrote two papers in this conference, one is a keynote as the second author co-written with Prof. Fujino and the other as the first author.

presenting a paper

Taipei itself is a nice city, a little similar with Jakarta but more humane. Transportation is not as good as it is in Tokyo, but they are cheap and readily available. Taxi fares in particular are very reasonable, while subway network is not as dense as it is in Tokyo, so we need to walk  a lot to reach our destination when using subway.

Me with Taipei 101 on the background

The most obvious place in Taipei is Taipei 101 tower. It is the tallest existing building in the world, soon to be replaced by Burj Dubai. I had a chance to visit the building by taking the world’s fastest elevator.  The building is equipped with TMD to prevent lateral vibration. In fact the elevator itself is equipped with control system.

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.

The May 12 2008 Sinchuan earthquake was felt in northern area of Vietnam. Today we received the records from the instrumented Bai Chay Cable-Stayed in Ha Long Bay, about 1,154 km from the earthquake epicenter. The bridge experienced moderate shaking (acceleration RMS of 3.2 cm/s2) at the top of the 95m tower that caused 5 cm of displacement. The Bai-Chay Bridge is currently the world’s longest center span for the single-plan Cable Stayed Bridge (435 m). It is designed by the Japan Bridge & Structure Institute Inc, Pacific Consultants Int Tokyo, and constructed by Shimizu Corp and Sumitomo Mitsui Const. Co.Ltd. The bridge was just opened to traffic in December 2006

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.

]]> http://dionsiringo.wordpress.com/2008/04/03/okutama-cable-stayed-bridge-tmd-for-vortex-induced-vibration/feed/ 4 dionsiringo okutama cable-stayed bridge TMD main spring and damper http://dionsiringo.wordpress.com/2008/03/19/tes-destruktif-skala-penuh-jembatan-overpass/ http://dionsiringo.wordpress.com/2008/03/19/tes-destruktif-skala-penuh-jembatan-overpass/#comments Wed, 19 Mar 2008 14:44:15 +0000 dionsiringo http://dionsiringo.wordpress.com/?p=42 ]]>

Minggu terakhir February saya habiskan di kota Vienna, mengunjungi kolega perusahaan konsultan engineering yang bergerak di bidang Structural Health Monitoring, namanya VCE. Ini adalah satu dari sedikit perusahaan yang menjadi SHM sebagai core-bussinessnya. Institusi lain biasanya bergerak di bidang riset atau proyek pemerintah.

Satu hal yang menarik dari kunjungan tersebut adalah bagaimana perusahaan tersebut dapat memadukan solusi engineering dan bisnis. Tak pelak, banyak pihak yang menganggap bahwa SHM masih berada pada tataran riset dan belum siap untuk terjun ke bisnis. Pihak VCE menjawab tantangan tersebut dengan cerdas. Satu hal yang mungkin kurang adalah keandalan solusi ilmiahnya. Untuk inilah mereka mengajak kerjasama dengan pihak universitas, salah satunya universitas tempat saya bernaung. Bentuk kerjasama seperti ini menjanjikan win-win solution. Mereka membutuhkan expertis sementara pihak universitas perlu belajar celah bisnis dan diversifikasi wilayah kerja.

Sebagai tahap awal dari kolaborasi riset kami dihadapkan dengan problem full-scale destructive test dapada sebuah highway bridge di Vienna. Jembatannya sederhana, post-tensioned overpass bridge, satu bentang 42 m. Sebelum dihancurkan, karena pelebaran jalan di bawahnya, serangkaian test dinamik dikerjakan pada jembatan tersebut. Kemudian, satu persatu kabel prategangnya dipotong. Pada waktu bersamaan getaran struktur direkam dan dimonitor. Dari hasil analisa getaran pada tiap tahap kerusakan, diharapkan kita dapat belajar mendeteksi moda-moda getaran yang disebabkan oleh kerusakan tersebut.

The recent growing interest in bridge assessment and monitoring has led to the instrumentation of many bridges in Japan, especially the long-span cable-supported ones. Since most part of Japan is located on a seismically active area, these permanent instrumentation installations provide high-quality seismic records every time an earthquake occurs. Such records are essential to gain insights into real behavior of bridges and to evaluate the adequacy of bridge seismic design codes.

The work reported here presents case studies of the application of multiple-input multiple-output (MIMO) SI to three long-span bridges in the Tokyo Bay area. The methodology used in this tudy is based on the System Realization using Information Matrix (SRIM) , which utilizes correlations between input–output data for realization of a state-space model and estimation of modal parameters. To provide a comprehensive discussion, this paper includes: (1) a brief explanation of the SI method, (2) numerical verification using a benchmark cable-stayed bridge, (3) application of the SI to the Yokohama Bay Bridge, Rainbow Bridge, and Tsurumi Fairway Bridge using the records from the Chuetsu-Niigata earthquake, and (4) evaluation of identification results.

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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.

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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.

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