APESS 2017 Smart Structures Technology Summer School - Yokohama, Japan - Technical Visit to Shinjuku Mitsui Building's TMD

For the second update of 2018, I'm going to cover a more interesting aspect of the Asia-Pacific-Euro Summer School on Smart Structures Technology (APESS 2017) that I introduced in the last post, that being one of the technical site visits that we attended during the three weeks. 

So, empezamos...

Given that Japan is prone to both earthquakes & strong typhoons, and has numerous densely populated urban areas, not least the Greater Tokyo Metropolitan Area which is the largest in the World, it obviously served as a more than suitable location to demonstrate some of the most state-of-the-art smart structure and structural control technologies in operation today. And well this bore true for the first of our visits, which entailed a guided tour of the interior of Japan’s largest Tuned Mass Damper (TMD) weighing a total of 1800 tons, which is equal to 6.5% of the entire structure's effective weight. It consists of 6 individual TMDs of 300 tons each and is located on the roof of the 55 story Shinjuku Mitsui Building in Tokyo (Figure 1.), to which it was retrofitted in 2015. The area is generally closed to the public, but access was permitted to the APESS '17 attendees under the supervision of our tour guide, who was also the original design engineer of the TMD system. This visit was particularly special for me, as I've had a keen interest in TMDs for quite a while now and have even wrote about one (TMD in Taipei 101) in a previous post in this blog as being an inspiration for my current path in engineering. As in the case of the TMD in Taipei 101, each of the 6 TMDs in the Mitsui Building is a pendulum-type TMD that uses the simple relationship between the mass and cord length of a pendulum to "tune" its period of oscillation to that of the structure so that when strong winds blow, the TMDs will sway out of phase to the main structure, mitigating the magnitude of vibration. Using this system of tuning means that the TMDs do not require electrical power, and so can be relied upon whatever the situation. 

Figure 1. Shinjuku Mitsui Building in Tokyo

Figure 2. TRUSS ERSs Farhad Huseynov, Matteo Vagnoli, John Moughty, Antonio Barrias on top of Shinjuku Mitsui Building in Tokyo, Japan

Figure 3. presents a photo of one of the 6 TMDs suspended from a purpose-built steel frame by 8 steel cables. The mass is positioned horizontally by four hydraulic pistons, or "oil dampers" that work to stabilize the mass and transfer its restorative energy into the structure, while not interfering with the mass itself. Figure 4. provides a more informative viewpoint of the oil dampers where it can seen how their diagonal connections help to avoid piston buckling, while increasing the TMD's range of motion. 

Figure 3. One of Six 300 ton Tuned Mass Dampers in Shinjuku Mitsui Building

Figure 4. Full TMD Configuration (Hori, et al., 2016)

This system, or similar, is essential for exposed tall structures of regular plan, i.e. low aerodynamic design, as high winds create vortices on either side of the structure that vibrate the building in an out-of-plane manner, as shown in Figure 5. The greatest threat of such a phenomenon is when the vortices "shed" at a period close to one of the structure's first few modes of vibration, at which point resonance may occur and the probability of damage is at a maximum. In Mitsui Building's case, the TMDs were placed at the top of structure as this is the location of the maximum displacement of the first mode of vibration, and as such it is tuned to the building's first mode, which changes slightly throughout the day as people come and go for work providing additional mass in a somewhat uneven manner. 

Figure 5. Vortex Shedding around Cylinder (Van Dyke, 1982)

The estimation of the wind speed required for vortex shedding to occur can be to assessed using the equation below, where the frequency of vortex shedding (f), in Hertz, is obtained using; wind speed (V), structure's diameter (D) and a dimensionless parameter known as the Strouhal Number (S) which can be adjusted to reflect the shape of the structure.

f=VS/D                                                      Eq (1.)

Usually, TMDs are only relied upon to mitigate sway in high winds, as their placement at the top of the structure can do little for violent Earthquake induced vibrations emanating from the ground, which are usually catered for by either base isolation or more strategically positioned oil dampers, as is the case in the 5th to 10th floors of the Mitsui Building (see Figure 6.). However, long period Earthquake waves can have a more global effect on a structure of this size and can cause significant displacements at roof top level and may even cause resonance to occur. For this reason the Mitsui Building's TMD system designer elected to fit multiple TMDs that are orientated off center from each other on either side of the building. This allows each of the 6 TMDs to oscillate slightly out of phase from one another when required and can also provide some torsional damping due to their position in plan (see Figure 6.). 

Finally, Figure 7. presents the results of a simulated test case where the Mitsui Building was subjected to the Great East Japan Earthquake. The Results show a significant reduction in rooftop displacement, which equates to about an additional 5% damping to the entire structure (Hori, et al., 2016).

Figure 6. TMD and Oil Dampers Arrangement in Shinjuku Mitsui Building (Hori, et al., 2016) 

Figure 7. Test of TMD under Simulation of the Great East Japan Earthquake (Hori, et al., 2016) 

References

Hori, Y., Kurino, H. & Kurokawa, Y. (2016) "Development of large tuned mass damper with stroke crontrol system for seismic upgrading of existing high-rise building" International Jrn of high-rise buildings, Vol 5, no.3, pp 167-176

Van Dyke, M. (1982) "An Album of Fluid Motion" 4th Edition. The Parabolic Press, Stanford, California.

APESS 2017 Smart Structures Technology Summer School - Yokohama, Japan - An Overview

Happy 2018 to all,

So with the new year here, it begins the final leg of the PhD and the TRUSS ITN project as whole, so it's about time to rejuvenate this blog by catching up on some technical aspects of my project and on some of the activities and dissemination that has been carried out over the past 12 months or so. Firstly, I'll begin with some training and dissemination by covering my time at the Asian-Pacific-Euro Summer School on Smart Structures Technology (APESS 2017) held in Yokohama National University from July 17th to August 4th. 

The summer school was attended by 60 postgraduate and postdoctoral researchers from 3 Continents and entailed over 2 weeks of lectures from over 30 international speakers from both academia and industry, followed by field tests on real bridge structures and experimental group work using state-of-the-art sensing technology. The 3 week course also included cultural events, technical site visits, technology demonstrations from local engineering companies in the Yokohama area and attendance to a smart structures technology conference (ANCRiSST) held in Tokyo National University.

APESS 2017 was the tenth edition of the summer school since its inception and was only the second time it was open to European applicants. Of the 60 attendees, only 6 were European, 4 of which were TRUSS ITN ESRs: Myself (ESR10), Antonio Barrias (ESR11), Matteo Vagnoli (ESR9) & Farhad Huseynov (ESR7). 

APESS 2017 Group Photo

The 3 weeks of lectures and projects are designed to fill any gaps in standard engineering education which are necessary for the advancement of the fields of; Advanced structural engineering and dynamics; Structural control theory and application; Smart structures technology, Sensing and materials; Structural health monitoring and assessment.

The final week of the summer school provided an opportunity for the students to apply the knowledge attained during the previous two weeks of lectures through a group competition that entailed the modal assessment of a real in-service bridge using wireless accelerometers, in addition to conducting laboratory experiments using Fiber Grated Bragg sensors on a beam. A final group-style presentation format was used to describe selected methodologies employed by each team and to present the results obtained. Each group was judged on the accuracy of results, but also on the level of innovation, originality and understanding of the methodologies employed. When it was all over, somehow, I managed to find myself (white t-shirt) in the winning group and received an award from Prof. Yozo Fujino, Chairman of APESS (center) and APESS coordinators Dr. Dionysius Siringoringo and Dr. Mayuko Nishio on the far right and left, respectively. 

APESS 2017 Winning Group

Damage Detection in Bridges; Why it's becoming a Necessity

Hi all,

In this post I'll be covering some general aspects of why my project (ERS10) is required. In particular I will discuss the growing necessity of damage detection in bridges and some historical development down the years. I hope you'll enjoy!

The identification of structural damage in bridges is a research topic that has generated significant attention in recent years. The primary reason for its surge in popularity is an aging road and rail infrastructure, which is subjected to traffic loading conditions that far surpass their original design criteria. This unprecedented increase in loading is accelerating structural fatigue, which in turn reduces service-life. Some fatigue assessments carried out on the most common reinforced concrete bridge types constructed in Brazil since 1950 found that shorter span bridges, in the range of 7 to 10 meters, may have their fatigue performance in danger if a 100 year design-life is required [1]. As a consequence, it was deemed that a straightforward, non-destructive assessment method of bridge deterioration is urgently required.

Currently, non-destructive assessment methods entail visual inspections, hammer tests and localised damage assessment methods. These methods, although useful and inexpensive, have numerous limitations; they are infrequent, taking up to nine years between inspections [2], dependent on the competence of inspectors and are confined to localised damage and external deterioration, while the true global bridge condition remains relatively unknown. Additionally, as bridge infrastructure continues to age and deteriorate, the frequency of inspection must increase to counteract the reduction in safety of these structures. This task is made more difficult due to its sheer enormity. Recent figures show that Europe's bridge count is circa one million, and of Europe's half a million rail bridges, 35% are over 100 years old [3]. This leads to a need to reduce uncertainty regarding bridge condition through other, more efficient means apart from traditional inspection techniques.

The concept of using measured vibrations to discern damage in structures has been employed for some time. For instance, some early research by German engineers in the 1950's used vibration intensities, attained from measured accelerations, as an empirical indicator of damage in buildings [4]. The use of monitoring a bridge's natural frequency over time to detect damage in structures was originally proposed by Adams et al. [5] in the late 1970s. It was a promising development as frequency is a product of a structure's mass and stiffness, and it was thought that monitoring natural frequencies over time would show how a structure's stiffness declined. However, there are many limitations to this methodology, for instance; changes in frequency would not locate damage accurately, as cracking in different locations can cause frequency changes of equal magnitude. 

Apart from natural frequencies, other modal properties such as mode shapes, damping ratios and modal curvatures have been traditionally used to detect damage. For instance, cracking in a cross-section will increase internal friction and thus raise the value of the section's damping ratio, however, damping ratios are heavily influenced by vibration amplitude and measuring them from vibration data produced large standard deviations, which impair their accuracy and effectiveness as a reliable damage indicator.  

The core problem is that bridges are monitored over long periods of time and are subjected to large temperature fluctuations, harsh storms and numerous traffic scenarios. These varying conditions affect changes to a bridge's stiffness and mass in a non-linear manner, which in turn alters the bridge's modal properties. This is evident in Peeters & De Roeck's [6] assessment of the Z-24 Bridge in Switzerland, where significant variation in the bridge's natural frequency was observed when the ambient temperature dropped below freezing point (see Figure 1). The cause of this bi-linear behaviour was attributable to the newly solidified ice in the bridge deck and supports contributing to its stiffness.

Figure 1.  Z-24 Bridge - Natural Frequency v Temperature - after [6]

So, that's all I will cover for now. I hope the above few paragraphs give you an idea of the need of an efficient condition assessment methodology of bridges across Europe, and that it also portrays some of the difficulties imposed by using vibration data, in particular, modal properties.

See you again soon!

Bibliography

[1]  Rodrigues, F., Casas, J.R. & Almeida, P. (2013). "Fatigue-Safety assessment of RC bridges. Application to the   Brazilian highway network", Structure and Infrastructure Engineering, Vol. 9, N. 6, 2013, pp.601-616.

[2]  Federal Highway Administration. (2008) "Bridge Evaluation Quality Assurance in Europe", Technical Report Document, FHWA-PL-08-016, March.

[3]  MAINLINE. Maintenance, renewal and improvement of rail transport infrastructure to reduce economic and environmental impacts. (2013) Deliverable D1.1: "Benchmark of new technologies to extend the life of elderly rail infrastructure" European Project. 7th Framework programme. European Commission.

[4]     Koch, H.W. (1953). Determining the effects of vibration in buildings, V.D.I.Z., Vol. 25, N. 21, pp. 744-747

[7]  Adams R.D., Cawley P., Pye C.J., Stone B.J. (1978) "A vibration technique for non-destructively assessing the integrity of structures." Journal of Mechanical Engineering Science. 20: 93–100.

[6]  Peeters, B & Roeck, G.D. (2001) " One-year monitoring of the Z24-Bridge: environmental effects versus damage events ". Earthquake Engineering and Structural Dynamics, 30, 149-171.

Hola!

Hello and welcome to my blog...

As you're probably aware, this blog is designed to keep you all up to date with my hectic social life and wild adventures! Instagram in text form, if you will. Of course I'll also try to keep everyone informed of my progress with my TRUSS ITN project too! 

So to begin with a short introduction for those who don't know me; my name is JJ Moughty, short for John James, I'm 26 and from Longford, Ireland. I'm a graduate of Civil Engineering from N.U.I. Galway and, subsequently, from Trinity College Dublin where I completed a MSc. in Structural and Geotechnical Engineering. After graduating I found employment in the offshore oil & gas industry with Wood Group Kenny (WGK) in their Galway office where I specialised in the design and analysis of deep water drilling systems for semi-submersible vessels and drillships. 

The majority of projects I competed while with WGK were fatigue analyses of subsea wellheads. This is probably due to the fact that I've always been very interested in structural dynamics and how structures behave. It's an interest I've had ever since I discovered how the one of the World's tallest skyscrapers (Taipei 101, see picture below) uses an enormous steel pendulum ball, suspended from the 90th floor, to maintain equilibrium during typhoons and earthquakes.  It achieves this by swaying out of phase to the structure at its natural period of oscillation in order to counteract the external environmental forces. This interest in structural behavior is also probably why I now I find myself relocated in Barcelona as one of the 14 fortunate Early Stage Researchers (ESRs) that make up TRUSS ITN.

Taipei 101 

TRUSS, or Training in Reducing Uncertainty in Structural Safety, is an Innovation Training Network (ITN), funded through European Union's Horizon 2020 research and innovation programme. My project's title is "Assessment of bridge condition and safety based on measured vibration level", which is ESR10. It is based in Universitat Politecnica de Catalunya under the supervision of Prof. Joan Ramon Casas. The motivation behind ESR 10 is to determine the structural condition of aging bridges as they decline due to a number of degradation processes over time, such as; creep, corrosion and cyclic loading from traffic and environmental effects. Recent figures show that Europe's bridge count is circa one million, and of Europe's half a million rail bridges, 35% are over 100 years old, which justifies the considerable amount of research being conducted in the area at the moment. 


My project is also in collaboration with the Spanish Engineering company COMSA, where I'll be spending some months on secondment in their Barcelona offices. This will increase my exposure to innovative environments in both academia and industry, while also allowing me to learn first hand how projects of this nature are completed in the privet sector.  

That's all for now. I'll be back soon to fill you all in on my progress with ESR10 and life in Barcelona!

Slán go fóill!