David O. Prevatt
T+T Icons In Science & Technology Volume 3
We have all seen dramatic footage of tornadoes powering across the landscape, destroying everything in their path within minutes. These gigantic, rotating columns of air can produce wind speeds in excess of 200 mph and can be as much as a mile (1.6 km) wide. While most tornadoes travel for only one or two miles, some can continue for more than 100 miles. The most deadly occurences of tornadoes are the so-called tornado outbreaks in which many hundreds of tornadoes may be spawned in an area within a 24 to 48 hour period.
On average 1,200 tornadoes occur in the United States every year. Tornadoes have been observed in many parts of the world, including Europe, Japan and Bangladesh, but three out of four tornadoes occur in the United States. Tornadoes are formed during atmospheric instability, caused when two air masses – one warm (moist), one cool (dry) impact each other. Along this unstable boundary, changing in wind direction, and an increase in wind speed and height, creates a horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical, which extends 2-6 miles wide. During the spring of each year, conditions in the US are ideal for tornado-formation in the mid-western states and in Texas. It is hard to imagine how the devastation by such a force of nature can be reduced by humans through technological interventions. But structural engineers like Dr. David O. Prevatt are doing just that. Associate Professor (Structures) at the University of Florida, Gainesville, in the U.S. since 2007, Prevatt has studied hurricanes for over twenty years throughout the CARICOM Caribbean and in the U.S. More recently, he has brought that expertise to bear on tornadoes. His research group has investigated post-tornado damage in Tuscaloosa, Alabama and Joplin, Missouri, and well as damage from tornadoes in Florida – the state having the most number of tornadoes after Oklahoma. Prevatt’s current research is focused on developing engineering solutions to mitigate the impact of these winds on new and existing structures. According to him, a major proportion of tornado damage to houses is preventable but most residential buildings are not engineered structures designed to withstand specific (quantified) tornadic loads. Until very recently, no reliable estimates of tornado loads existed, which had led to the commonly held misconception that tornado forces are so large that no structure can be economically designed to resist them.
Wind forces act counter-intuitively to most peoples’ expectation, in that the wind tends to pull buildings up and away from the ground in opposition to gravitational forces. So, for houses to resist wind loads, they need to have fully-engineered “vertical load paths” consisting of components and connections designed to stay together even under extreme upward suction forces.
The challenging dilemma in mitigating tornado damage is usually the issue of the cost of wind-resistant features. While the knowledge is available to design and construct buildings better able to resist extreme forces of nature, the homebuyer often cannot appreciate the justification for the additional 1% to 2% cost increase that would ensure their safety and reduce damage. Furthermore, building developers are unable to advise homebuyers that solutions do exist, partly because the focus of building codes is more on construction that minimizes loss of life. The low frequency of occurrence of tornadoes reduces the likelihood that a building strengthened to be tornado resilient will be hit. But at the same time, the socio-economic losses from such natural disasters are substantial.
While the loss of life has historically been low, due largely to decades of improvements in forecasting and tornado warnings, 2011 saw a spike in the number of tornado-related fatalities. The cumulative damage from tornadoes from 2000 through 2011 is about US $19 billion, or 15% of the economic loss from hurricanes over the same period. For example, the economic losses from two tornadoes in 2011 amounted to nearly 3% of the gross domestic prodcut (GDP) for the states of Alabama and Missouri because the paths of those tornadoes went through densely populated areas in Tuscaloosa and Joplin respectively.
With an ever-increasing population living in “Tornado Alley” (an area in the United States where tornadoes are most frequent) and the effect of changes in climate, it is prudent to improve the structural systems used in housing. Far more is known about wind forces but much less about how those forces are transferred within light-framed wood buildings. Prevatt believes more investment in research would bring the reward of reduced annual tornado losses.
To ensure a building can resist tornado-induced loads, all connections between the main structural components must be strengthened. Dr. Prevatt explains the concept by comparing structures to chains, which are only as strong as their weakest links. For typical houses, the roof covering must be fastened to the roof rafters, which, in turn, must be securely anchored using metal straps to the structural member along the top of the walls. The walls must, in turn, be firmly connected to the foundation using steel bolts and, in the case of wood-framed construction, large, stiff metal washers.
Dr. Prevatt is a recognised expert on wind hazards, forensic engineering and the structural performance of low-rise buildings and building envelope systems i.e. To determine the tornado-resistance of buildings, Prevatt estimates the strengths of past tornadoes from damage surveys to understand how buildings fail i.e. what are the weakest connections or components. His research group at the University of Florida reproduces tornado loads within the laboratory to test critical structural components and connections, and structural modifications. The goal is to establish appropriate size and strength ranges for the nailed connectors, metal ties and anchor bolts needed in tornado-resilient construction.
David Otway Prevatt was born to Trinidadian parents, Otway and Zilma Prevatt in Nassau, Bahamas in 1964, but spent most of his life in Trinidad, growing up in Port-of-Spain, Cocorite and Petit Valley. He became interested in science at a very young age, always curious about building structures and fascinated by the way nature’s forces (winds, earthquakes and water) continually changed the physical world. He windsurfed and enjoyed competitive sailing races at the Trinidad and Tobago Yachting Association. This was where he first appreciated that the power of the wind could be harnessed using technology but, at the same time, it could be very destructive if its power was ignored.
Prevatt was also influenced by his father, a land and quantity surveyor who showed great concern for people and their need for safe and affordable housing. He grew up in a close knit, extended family, with parents who instilled in their children the importance of post-secondary education and having a successful career.
He attended Philips Street Primary School, then Trinity Junior School in Port-of-Spain, from which he won a scholarship in 1975 to attend St. Mary’s College. He says, “I was a good but not a great student at St. Mary’s. I did what I had to do most of the time, but I was also easily distracted from studying. The big change for me came in Form 4, when I took after-school lessons in mathematics and additional math from Mr. Martineau in Belmont. He was a great teacher and he helped me appreciate how much fun these subjects were, and in fact, how easy they were. I was able to gain the confidence I lacked to tackle the work, and since then, and I’ve never looked back.”
School was not all work though. He was a member of the Sixth Trinidad Sea Scouts, which fostered in him a sense of duty and service to others and the benefits of hard work and cooperation. He also represented St. Mary’s College in basketball and athletics. Academically, he particularly enjoyed physics, due in large measure to the enthusiasm brought to the classroom by his teacher, Denise Padmanaban, whose approach in the physics labs really opened his eyes to the world of possibilities that scientific enquiry could bring, and the fun of experimentation.
He went on to study civil engineering at The University of the West Indies (UWI), St Augustine. After graduating in 1985 with his bachelor’s degree, he took up a post with the Ministry of Works in the Design and Engineering Department. There, he was responsible for designing structures with careful consideration to building loads, and he worked on the construction and dismantling of temporary Carnival facilities such as the North Stand, Grand Stand and main stages at Victoria Square and Adam Smith Square.
In 1993, Prevatt went to Clemson University, South Carolina, where he completed his M.Sc. and Ph.D. degrees in civil engineering. For his doctoral degree, he conducted extensive research on the wind uplift performance and testing of commercial roofing systems. His work on mechanically-attached single ply membrane roofing systems addressed existing knowledge gaps between how roofs are tested and how they actually behave when installed on a building. Prevatt’s research showed that it is necessary to monitor the loads throughout the structure rather than relying only on the surface pressure distribution to predict failure. He established minimum specimen sizes needed to predict in situ behaviour of roofs subjected to extreme wind loads. The work was important given the need for modernized testing protocols within the industry.
After completing his Ph.D., Dr. Prevatt worked with the Boston-based ENR500 consulting engineering firm, Simpson Gumpertz and Heger Inc. His work there focused on the repair, design and renovations of building envelope systems e.g. roofing, plaza waterproofing, wall systems and fenestration (window design) in historic and contemporary structures. In 2004, he took up an appointment as Assistant Professor of Civil Engineering at Clemson University. His work as a consultant in the years following included: wind tunnel studies to determine components, cladding and MWFRS loads and pedestrian-level wind speeds for high-rise buildings; hurricane risk assessment; and forensic investigations.
Prevatt has been the recipient of many research grants, totalling over US $4 million. He has published 18 peer-reviewed journal articles and presented over 35 papers at national and international engineering conferences. In January, 2012, he was awarded the prestigious Faculty Early Career Development (CAREER) award by the National Science Foundation, to study and develop tornado-resilient residential communities. His damage surveys following the tornadoes that devastated Tuscaloosa and Joplin provided useful information for assessing whether or not wind-resistant building practices adopted in Florida against tornadoes could be applied to mitigate tornado damage in other parts of the US.
As a teacher, Prevatt has taught structural analysis, experimentation and instrumentation, steel design and wind engineering. He has also taught forensic engineering which enables engineers to determine the causes of failure of structures. He is a member of the American Society of Civil Engineers (ASCE); the ASCE 7 Wind Load subcommittee and the Wood Structures committee, and he serves as Director of the American Association of Wind Engineering (AAWE). He has testified before the United States congressional subcommittees on three occasions, on behalf of ASCE and AAWE.
In a world that is increasingly in need of engineers to help solve technological and socio-economic problems, Dr. David Prevatt naturally encourages young people to enter a field that has been exciting and rewarding for him professionally, enabling him to tackle the challenges of the built environment and particularly in the fight against natural disasters. For him, “Civil engineering is a unique profession in which one is able to take an idea for a building, engineer it through calcuulations, draw it to produce a blueprint and then eventually have this design become a reality. It is a most satisfying feeling to know that a building once represented by mathematical calculations or sketches on paper, will now serve a community of people well into the future.” His simple advice to those just starting out is that, “Life is short, so work hard and follow your passion.”