BUILDING RESEARCH INSTITUTE

by providing the public with warning in- The Tornado Damage formation on tornadoes (since 2008) and up-to-the-minute tornado “nowcasts” (since 2010). The Cabinet Office Central Mitigation Project Disaster Management Council mean- ’s Building Research Institute (BRI) conducts activities ranging from while has included tornadoes as an es- tablished weather phenomenon in its research and development on housing, building and urban planning Basic Disaster Management Plan, call- technology, to international training on seismology and earthquake ing for measures to alleviate physical engineering. Against the background of significant tornado damage in damage and improvements in technical Japan earlier this year, this article introduces some of the BRI’s initiatives expertise to help companies implement damage mitigation measures. in tornado research and the launch of a frontline tornado research project to help reduce damage to buildings. How Tornadoes Damage Buildings

n May 6, 2012, a mass of the Fujita Scale (estimated wind speed: In order to get a picture of the damage cold air entered the skies 70–92m/s) near in Ibaraki caused to buildings by the tornado that above Japan. At the same Prefecture, F1–F2 (33–69m/s) in Moka hit Tsukuba, , on May Otime, warm, moist air was in Prefecture and Hitachiomiya 6, 2012, the Building Research Institute moving towards a low-pressure area over in Ibaraki prefecture, and F1 (33–49m/s) joined forces with the National Institute the Sea of Japan. Compounded by elevat- near in Ibaraki Prefecture. for Land and Infrastructure Management ed ground temperatures due to solar radi- These transient winds and the accompa- and began conducting a field survey from ation, this caused major atmospheric in- nying weather conditions left one person that same day. In addition to typical dam- stability from the central Tokai area all dead and fifty-eight injured. In addition age recorded in the aftermath of other the way up to Tohoku in the north, result- to the human toll, 89 homes were also tornadoes (scattering of exterior materi- ing in cumulonimbus clouds accompa- destroyed, with a further 197 partially als, collapse of wooden roof trusses, im- nied by lightning, transient winds and destroyed and 978 damaged (as of June pact from flying debris, etc., see Photos hailstorms. These weather conditions led 13). The trail of destruction had a seri- 2 and 3), the survey found new types of to multiple tornado outbreaks in Ibaraki, ous social impact too. damage that had not been observed previ- Tochigi and Fukushima Prefectures, This is not an isolated case. As ously. Photo 4 shows a two-story wood- causing casualties and substantial damage Table 1 shows, serious damage caused en house whose foundations became de- to countless homes and other buildings by tornadoes has become increasingly tached from the ground, causing the (see Photo 1). common in recent years. The Japan entire structure to overturn. Photo 5 Tornadoes were confirmed at F3 on Meteorological Agency has responded meanwhile shows a five-story reinforced

Photo 2: Destruction caused to a wooden Photo 3: Impact from flying debris house (roof)

Photo 1: A tornado in Tsukuba, Ibaraki Photo 4: A wooden house overturned Photo 5: An apartment building with exten- Prefecture, on May 6, 2012 with its foundations still attached sive damage to non-structural elements

30 The Japan Journal DECEMBER 2012 The Japan Journal DECEMBER 2012 31 Table 1: Summary of major damage caused by tornadoes in Japan since 2002 similar to an Human toll Physical damage Scale (no. of people) (no. of homes) actual tornado, Date Location Fujita Width of affected Length of affected Completely Partially destroyed/ whilst moving Dead Injured scale area (m) area (km) destroyed damaged sideways at the 2002.07.10 Fukaya City, Saitama Pref. F2 100 〜 150 4.5 damaged 11 7 87 same time. 2004.06.27 Saga City, Saga Pref. F2 200 〜 400 8.0 0 15 15 330 Photo 6 2005.12.25 Sakata City, Yamagata Pref. F1 100 9.0 5 33 0 4 shows the ex- 2006.09.17 Nobeoka City, Miyazaki Pref. F2 150 〜 300 7.5 3 143 79 1,101 terior of the 2006.11.07 Saroma Town, Pref. F3 100 〜 300 1.4 9 31 7 32 2009.07.19 Mimasaka City, Pref. F2 200 6.0 0 2 2 76 simulator, with 2009.07.27 Tatebayashi City, Gunma Pref. F1 〜 F2 50 6.5 0 21 14 310 Figure 3 pro- City, Ibaraki Pref. F1 200 〜 300 2.8 0 2 1 105 viding cross- 2009.10.08 Tone Town, Ibaraki Pref. F1 100 〜 200 6.0 0 4 0 121 section views. Kujukuri Town, Pref. F1 20 〜 30 1.6 〜 1.7 0 0 1 36 The simulator Tokunoshima Town, 2011.11.18 F2 100 0.6 3 0 1 2 Kagoshima Pref. consists of a Tsukuba City and other area, 158 partially F3 500 17 1 37 76 main body Ibaraki Pref. destroyed equipped with Chikusei City and other area, F1 600 21 0 3 0 1 partially destroyed Ibaraki Pref. a fan, a self- 2012.05.06 Moka City and other area, 35 partially propelled cradle F1 〜 F2 650 32 0 12 13 Tochigi Pref. destroyed that moves Aizumisato Town, Fukushima F0 300 2 0 0 0 0 partially destroyed Pref. from side to Source: The Japan Meteorological Agency side, a stage that moves up concrete apartment building, which sus- in tests using a conventional turbulent and down, and a control panel. Standing tained extensive damage to non-structural boundary layer wind tunnel. In order to at a full height of around 2.3 meters, the elements such as windows, window identify the effects and damage-causing body has an external diameter of 1.5 frames and aluminum balcony handrails. mechanisms of a tornado, we have to meters. The cradle meanwhile has a range The buildings in these two photographs leave behind existing testing and assess- of 1.4 meters from the center, and a maxi- are thought to have been in or around the ment methods and adopt a new approach. mum translation speed of 0.4 meters per path of the tornado. Nonetheless, the rea- second. When the fan inside the body sons for such extensive damage and the Outline of a Tornado Simulator and spins round, it creates an upward airflow, magnitude of pressure caused by transient Experimental Research which then blows downwards from the winds remain unclear. From the stand- outside. There are eighteen guide vanes point of wind engineering, throwing light In 2010, the Building Research Institute fitted to the upper part of the body, which on phenomena such as these is proving designed and built a tornado simulator to are then angled (0–55 degrees) to force the extremely challenging. experimentally recreate the airflow of a downward flow into a swirling motion. To analyze these atypical examples tornado, in order to ascertain the wind It is possible to vary the maximum of damage in greater detail, it is neces- force characteristics of transient winds and speed and diameter of the swirling flow by sary to run tests that recreate the gust the risk of impact from flying debris (see adjusting the speed of the fan, the angle of load of a tornado and the effects it has on Figure 2). With additional funding cour- the guide vanes and the height of the stage. a building, so as to identify damage- tesy of the Grants-in-Aid for Scientific The airflow conditions created by the sim- causing mechanisms under the relevant Research program, experiments were ulator have been verified against a tornado load. Figure 1 is a graphical illustration then carried out in of what happens when a tornado passes conjunction with re- Figure 1: The effects of forces caused by a tornado close to a building. When the tornado searchers from the passes, its horizontal swirling flow and National Institute for Flying debris vertical upward flow create lots of flying Land and Infrastructure debris. This means that the effects on the Management, the building can be broadly divided into (1) University of and wind pressures caused by the direct ac- the Disaster Prevention Vertical upward flow Movement tion of the air flow, (2) pressures associ- Research Institute at of tornado ated with the variation of the atmospheric Kyoto University. The Horizontal pressure field and (3) impact forces airflow mechanism used swirling flow caused by tornado-borne flying debris. in the simulator is based The characteristics of (1) and (2) differ on a machine developed Upward force Impact from considerably from the wind force gener- at Iowa State University flying debris ated by strong winds during a tropical in the United States and Weight and cyclone. As a result, it is impossible to is able to recreate an un- load-bearing Lateral force realistically recreate the same conditions steady vortex structure force of building

30 The Japan Journal DECEMBER 2012 The Japan Journal DECEMBER 2012 31 BUILDING RESEARCH INSTITUTE

force coefficients,” calculated by divid- Photo 7: The model ing measured wind force data by refer- building in place ence velocity pressure. Figure 4 shows wind force character- istics acting vertically on the roof of the building. The horizontal axis shows the moving position of the simulator in rela- tion to the center of the model (xs) divided by the core radius (Rm). The findings in- Photo 6: The tornado simulator at the dicate that, irrespective of dominant open- Building Research Institute ing, wind force is at its maximum when xs/Rm is -1 or 1. This means that the cen- engineering model (the Rankine vortex ter of the building would be subjected to model) based on tangential wind speed the maximum tangential wind speed when and distribution of low air pressure. the tornado is approaching the building and immediately after the tornado has Wind Pressure Tests Using the passed. In terms of dominant opening, the Tornado Simulator wind force is approximately twice as turned with its foundations still attached, strong when a dominant opening is pres- with most of the upper structure of the Using the tornado simulator, researchers ent. This suggests that preventing large house scattered diagonally forward to- conducted wind pressure tests based on openings from forming in the wall as the wards the right, based on the direction of the premise of a tornado passing directly tornado approaches would effectively re- the tornado. The direction of scattering over a building. The building was based duce the force of the wind on the roof. more or less matches the direction found on a 12-meter high low-rise structure Figure 5 shows results for horizontal in the tests (Figure 6 (b)). With that in measuring 24 meters across by 38 meters wind force characteristics, divided into mind, one possible explanation for this in length. The walls of the model building Direction X, facing the direction of the damage is that the wooden house was were made complete with realistic, uni- tornado head-on, and Direction Y, at overturned and scattered immediately form distributed leakage and a rectangular right angles to the tornado. As with the after the tornado passed close by. dominant opening on the right-hand wall, vertical results, the wind force is at its As these results were obtained under if viewed from the direction of the tornado strongest when xs/Rm is -1 or 1. In terms restricted experimental conditions, there (see Photo 7). The dominant opening was of the direction of the tornado mean- are plans to conduct more systematic designed to simulate a large hole caused while, the results interestingly show a wind pressure tests in the future, taking by the impact of tornado-borne flying greater force diagonally backward to- into account other parameters that need debris. With wind speeds reduced in scale wards the left as the tornado approaches, to be examined. It is hoped that the to 1/10 and distances to 1/350, tests rec- and diagonally forward towards the right planned series of experiments will help reated a tornado with a maximum tangen- immediately after the tornado has passed assess the transient wind load of ap- tial wind speed of 98 meters per second by. Figure 6 compares these findings proaching tornadoes under different cir- and a core radius of 42 meters. against actual damage caused by the tor- cumstances. The results should also pro- The following results for wind force nado in Tsukuba. Figure 6 (a) is an aerial vide evidence to support the damage- characteristics are expressed as “wind photo of a wooden house that was over- causing mechanisms of tornadoes.

Figure 2: Assessment of risks from transient wind due to Figure 3: Cross sections of the tornado a passing tornado simulator Diameter: 1.5m

Tornado airflow simulator 55 degrees (maximum) Recreates unsteady vortex Vane angle 0 degrees Movement structure similar to an actual tornado

Guide vanes Fan Flying debris Transient wind Building Body

Risk of impact from Wind force characteristics Downward Convergence layer height Upward flying debris of transient wind flow flow Movable up and down Stage

32 The Japan Journal DECEMBER 2012 The Japan Journal DECEMBER 2012 33 Figure 4: Vertical force from a passing tornado ulation techniques to explain the mecha- Without With dominant dominant opening opening nisms through which tornadoes cause damage. The Institute is also planning to set out a new approach to tornado- resistant design for the aforementioned

Wind force coefficient critical facilities, based on the following underlying policy. First and foremost is the belief that,

Direction of movement as an extension of conventional wind- resistant design, we should make build- ings safer and improve their functionality by using building materials with superior － wind and impact resistant performances Figure 5: Horizontal force from a passing tornado for parts and components that are most

Direction X Direction Y vulnerable to tornadoes (particularly roofs and openings). Disaster-resistant roof tiles are a prime example that has already been developed. Tests have shown that such tiles are roughly three Wind force coefficient times more resistant to tensile force. In terms of openings meanwhile, it has been reported that laminated glass offers great- er impact resistance than regular glass. Direction of movement As test results from the tornado simulator show, taking steps to make openings more resistant to impact from flying de- bris, to prevent larger openings from － forming wherever possible, is crucial in terms of disaster prevention during a tor- Tornado Damage Mitigation Project for reasons such as these that load and nado, because it helps to reduce the force at the Building Research Institute external force due to tornadoes are not of the wind against roofs. covered under the Building Standard In the United States, researchers Generally speaking, the likelihood of in- Law of Japan. Given that serious torna- have come up with structural calculation dividual buildings being caught in the do damage has become increasingly methods for critical facilities and evacu- path of transient winds from a tornado is common in recent years however, we are ation shelters at risk from tornadoes. extremely low. Incorporating measures going to have to improve the design of These could also provide a valuable into regular structural design would critical facilities that would be crucial to source of reference. There is already a therefore be economically unfeasible. saving lives, protecting property and framework related to design criteria in Tornadoes are also isolated events that maintaining social functions in the place for different scales of tornado, are rarely picked up by normal meteoro- wake of a disaster (emergency facilities, plotted against the Enhanced Fujita logical observation networks. As it large-scale commercial facilities, multi- Scale. As we put that framework into stands, we have insufficient engineering functional facilities, hospitals, schools, practice in the future, the key point will expertise for tornadoes to be reflected in etc.), particularly parts and components be how to reflect findings from damage design and verification procedures. It is that would be vulnerable to a tornado. surveys in Japan and experiments using In light of re- the tornado simulator on the framework. Figure 6: The tornado simulator at the Building cent damage caused Design guidelines for critical facili- Research Institute by tornadoes, the ties in Japan still need to be improved to Estimated path of the tornado Building Research protect against tornadoes. That’s what Institute has started makes the Building Research Institute’s work on a tornado new project such a forward-thinking research project to frontline research project. If we can ac- help reduce dam- tively share research results with the age to buildings. structural engineers who design our The project will homes and critical facilities, and with the use unique research public, in the near future, it will undoubt- Overturned mat foundations Direction of scattering tools such as the edly help to raise awareness of tornado tornado simulator damage and promote measures to protect (a) Actual damage (b) Test results Source: Geospatial Information Authority of Japan and numerical sim- against transient winds and tornadoes.

32 The Japan Journal DECEMBER 2012 The Japan Journal DECEMBER 2012 33