Sinchuan Earthquake felt in Vietnam
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
Okutama Cable-Stayed Bridge : TMD for supression of Vortex-Induced Vibration
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.
Tes Destruktif Skala-Penuh Jembatan Overpass
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.
System identification applied to long-span cable-supported bridges using seismic records
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.
Structures with Human-Induced Vibration – How Serviceability Requirement Improves Vibration Design Concept
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
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.
Entrepreneurial universities
In the spirit of the slogan “2006, The Indonesian Year for Science”, it worth asking the question how much our scientific communities have contributed to the economy. With the current 0.3 percent of national GDP invested in research and technology, some might argue that it is too soon to ask for such a contribution. But while increasing national investment in research is vital, it is also essential to measure how much economic growth has — and will — emanate from these knowledge investments. New strategies should be developed to avoid the pitfall of the so-called “European paradox”; the conjecture that while European countries have consistently been leading players with top levels of scientific output, they lag behind in the ability to convert this knowledge into wealth-generating power.
During the New Order government of the 1980s and 1990s Indonesia experienced a similar kind of European paradox, only on a smaller scale. Back then, the promotion of high technology was initiated from the top, on the assumption this technology would automatically function as the accelerator of economic growth..
System identification of suspension bridge using ambient response
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.
Seismic Monitoring of Cable-Stayed Bridge
Strong motion data acquired from instrumented bridges during seismic events provides an excellent opportunity to gain insight into the behaviour of bridges and performance of their components. Using system identification, modal parameters of bridges can be estimated and the performance during various level of earthquake can be studied. In this study dynamic behaviour of Yokohama-Bay Cable-Stayed Bridge is investigated using seismic response recorded from six earthquakes. Modal parameters of the structure are estimated using system realization of state–space model.
The realization method used here is based on the system realization using information matrix (SRIM), which makes use of the correlations between earthquake input and output data to identify the coefficient matrices of state–space model. Identification results from six earthquakes show that the system identification can be used to capture global behaviour of the bridge by estimating modal parameters and also to explain local behaviour of its component such as performance of link-bearing connections during earthquake.
Dynamic characteristics of a curved cable-stayed bridge
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.
