In Commemoration of the Awarding of the Japan Academy Prize to Distinguished YNU Professor Fujino:
“Bridge for Disaster Prevention to Cross into the Future” (Part 1)
Professor Yozo Fujino of the Institute of Advanced Sciences (IAS), Yokohama National University (YNU), was awarded the Japan Academy Prize this year (2019). In commemoration of this award, his series of research will be introduced in two parts, comprising (Part 1) “Starting point: an Analysis of mysterious vibrations on a small pedestrian bridge” and (Part 2) “Into the present: New developments in social infrastructure and urban disaster prevention”, to give a glimpse into how his research came to be and how it became popular.
Starting point: an Analysis of mysterious vibrations on a small pedestrian bridge
First, let us take a look at the website of Japan Academy for an explanation about the Japan Academy Prize (1). ‘The Japan Academy Prize has been awarded since 1910 to persons who have achieved notable research landmarks or who have authored particularly outstanding academic papers or books. The awards ceremony has been held every year since 1911, marking its 109th year in 2019. The ceremony has been graced by the presence of His Majesty the Emperor of Japan since 1949, with both the Emperor and Empress attending since 1990.
Past awardees include Drs. Hisashi Kimura, Jokichi Takamine, Hideyo Noguchi, Nobutsuna Sasaki and Kyosuke Kindaichi, and Drs. Hideki Yukawa, Shinichiro Tomonaga, Kenichi Fukui, Reona Esaki, Masatoshi Koshiba, Ryoji Noyori, Akira Suzuki, Toshihide Masukawa, Makoto Kobayashi, Shinya Yamanaka, Isamu Akasaki, Satoshi Omura, Takaaki Kajita, Yoshinori Osumi and Tasuku Honjo, who have all later received the Nobel Prize. (The rest of the list has been omitted)
No further explanation is probably necessary. Two YNU graduates, Akira Fujishima (graduated from the Faculty of Engineering in 1966) and Takuzo Aida (graduated from the Faculty of Engineering in 1979), have been awarded the prize.
The prize was awarded for the research titled “Vibration and its Control in Large Civil Structures”. According to the “award review summary”, Prof. Fujino was awarded for the discovery and exploration of causes of vibrations in large civil structures based on measurements, and for further establishing and being an international leader in the study of structure control. As examples, the summary lists his discovery and elucidation of the phenomenon of “synchronization of human walking” for predicting vibrations on the Millennium Bridge in London more than ten years before the vibration problems on the bridge surfaced in 2000, and his achievements in the study of structure control such as “tuned liquid dampers”, and “structural health monitoring” that led to technology for monitoring the state of civil structures.
Now let us look back on Prof. Fujino’s achievements, especially on the “synchronization of human walking” that served as the starting point of his series of research.
One day in 1988, when he was an associate professor at the Faculty of Engineering, University of Tokyo, he received a telephone call from an old acquaintance who was an engineer.
“Professor, there is a footbridge that sways from side to side whenever it is crowded with pedestrians. Could you help us out?”
His old acquaintance sounded frustrated. It was the period of the bubble economy, when long-span bridges like the Great Seto Bridge and Akashi-Kaikyo Bridge were popular. At that time, unless you had been a bit eccentric, no one would have been interested in small footbridges.
However, Prof. Fujino felt and sensed something when he heard the words, “a bridge that sways from side to side”.
Bridges are known to sway when people walk on it. It happens in such instances as when a line of people synchronize their steps and move forward. But that would only produce vibrations that move up and down. He had never heard of vibrations from side to side that were caused by walking.
- This might be something unknown. He got very interested.
“So, it’s not up and down?”
“Seeing is believing. Please come and have a look.”
This is how Prof. Fujino got involved in dampening vibrations on footbridges.
He headed to a speedboat racecourse in Saitama Prefecture. The name of the bridge was the Great Toda Park Bridge. “Great” was in name only, since it was just a footbridge that was about 180 meters long and a little more than 5 meters wide. The newly painted yellow path stood out (Figure 2). The footbridge was at the entrance of the racecourse and connected the stands with the bus terminal over the water of the racecourse.
When Prof. Fujino arrived, it was in the middle of a race. There were 20,000 spectators filling the stands. The reverberating sounds of both joy and disappointment drowned out the roars of the engines.
The bridge swayed after the last race, when the multitude of spectators crossed the bridge to head towards the bus terminal. The engineer who took him there said, “The bridge always sways when it is crowded”. Prof. Fujino waited on the footbridge until the races ended. After the races ended and the surface of the water got calm, a wave of people came rushing in.
Prof. Fujino focused his senses on his feet. A little while after it started to become crowded, the bridge started to sway. It was just a small vibration at first, definitely side to side. The vibration gradually increased. Here and there, he heard voices yelling out, “Hey, it’s swaying!”. It could have been due to the amplitude being a few centimeters, but it was difficult to walk. Some people were holding on to the handrails and walking cautiously. The vibration continued for about 10 minutes and eventually stopped.
Prof. Fujino was a researcher in the field of civil engineering. His expertise was bridge engineering, so he was engaged in research on bridges. He mainly conducted wind tunnel experiments. A wind tunnel experiment is a method of observation and measurement by blowing artificial wind on a model of a bridge. Since a small model is used, it is good for getting a bird’s eye view of the movement of a long-span bridge. However, the footbridge that Prof. Fujino was standing on did not sway because of the wind, it swayed because of people.
Prof. Fujino asked the engineer. “Is there any place where I can see the entire bridge?”
The engineer pointed towards the top of the stands. “There is a VIP room. You can see the entire area from there.”
The entire racecourse could be seen from the VIP room. The footbridge was on the right side of the view. The setting sun was casting a long shadow of the bridge on the surface of the water.
On the next race day, Prof. Fujino observed the bridge from the VIP room (Figure 3). As the crowd grew, the bridge swayed. When the bridge swayed, the crowd seemed to sway together as if they were a single mass of people.
-I see, this is what it means to be able to have “a bird’s eye view on things”. Prof. Fujino was impressed about peculiar things.
In nature, there is a phenomenon called “synchronization (sync)”. When a group of fireflies repeatedly flash on and off together or when pacemaker cells in the heart pulsate, they are spontaneously vibrating without being commanded (2). People crossing the bridge were not walking on command, too. They were just walking as they pleased and then naturally fell in sync. Why did this happen?
The next time Prof. Fujino visited the speedboat racecourse, he brought a video camera and accelerometers. Up in the VIP room, Prof. Fujino set the video camera’s focus on the bridge.
The camera clearly tracked the movement of people and the accelerometers accurately recorded the movement of the bridge. The next thing to do was analyzing the video. It is called image analysis nowadays, but you can only imagine how much work it entailed back in those days. Among the pedestrians that were filmed, Prof. Fujino selected several people that had distinctive clothing or appearance and recorded each person’s position every 0.2 seconds of the video. This was continued for the ten or so minutes it took for people to cross the bridge.
To begin with, people did not walk perfectly straight forward. When they put out their left foot, their body moved to the left, and when they put out their right foot, their body moved slightly to the right. The direction of their feet, length of their stride and their speed were all different. However, as it became crowded, the swaying to the left and right became uniform and the vibration of the bridge intensified. As it became less crowded, their pace of walking returned to what it was and the vibration would cease.
Why does the synchronization occur? If there are many people among the pedestrians who happen to thrust their feet at the same time and direction, a little bit of force is exerted in that direction. The bridge moves a little bit and it becomes harder to walk. People who find it harder to walk will then try to find balance by taking their next step on the other side. The force exerted on the bridge becomes even stronger and people who thrust their feet towards the same direction increase. This way, the movement of the crowd becomes more uniform and eventually, the entire crowd becomes a single mass and the bridge vibrates. This is the “elucidation of synchronization of human walking” that is listed first among the reasons for Prof. Fujino’s award.
Based on the analysis and a proposal by Prof. Fujino, dampers to suppress vibration were installed. On the first race day after the completion of construction work, the staff gathered in the VIP room and held their breath while watching the crowd reach the bridge. Would the bridge sway? While multiple binoculars were focused on the bridge, it started to become crowded, but the bridge did not sway. The bridge became full of people, but even then, it did not sway. The bridge did not sway at all until the last person had crossed the bridge. Applause filled the VIP room.
Prof. Fujino wrote a paper on vibration and synchronization of human walking on the footbridge and published it in an overseas academic journal in English (3). However, as mentioned earlier, the interest of the public was focused on long-span bridges. No one paid heed to the paper about vibrations on a small footbridge and so it continued to lay dormant.
(Continued in the next article)
Note: This article was written based on an interview with Prof. Fujino in April 2019 and by referring to supporting documents and materials. Prof. Fujino’s words have been edited and reconstructed.
1. The Japan Academy https://www.japan-acad.go.jp/en/activities/
2. Strogatz, S. (2004). Sync: The emerging science of spontaneous order. Penguin UK.
3. Fujino, Y. et al. “Synchronization of human walking observed during lateral vibration of a congested pedestrian bridge” Earthquake Engineering and Structural Dynamics. (22) 741-758 (1993)
（Masahiro Nakagawa, IAS Strategic Planning Manager）