Researchers at Florida International University have demonstrated that tall buildings and facade systems designed to withstand hurricanes are vulnerable to ‘downbursts’, winds that are expected to become more common as the climate warms.
Houston boasts 50 buildings more than 150 metres tall. These were designed to withstand hurricanes, to which Texas is prone. However, on 16 May 2024, a derecho – a wide, long-lived windstorm associated with rapidly moving showers or thunderstorms – managed to cause unexpected damage to many of the tall buildings downtown. The socio-economic impact was significant, due to traffic disruption, businesses temporarily closing and the need for repairs.
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The new study has explained why the structural damage was so much larger than expected. According to its authors, its findings carry lessons for the future design of tall buildings and the planning of city centres, not only in Houston.
‘Here we show that a type of highly localised strong winds called downbursts, which were generated during the May derecho, can significantly impact tall buildings and facades due to their unique characteristics in comparison to hurricanes,’ said associate professor Amal Elawady.
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Downbursts are strong downward winds that blow outward in all directions once they hit the ground – and the reason why winds are often much more intense around the ground floor of tall buildings. Elawady leads a research project that utilises the Natural Hazards Engineering Research Infrastructure’s Wall of Wind experimental facility, funded by the US National Science Foundation, to study the impact of downbursts on tall and low-rise buildings in comparison to hurricanes.
The researchers analysed the impact of the May derecho on five iconic buildings in Houston: the Chevron Building Auditorium, the CenterPoint Energy Plaza, the El Paso Energy Building, the RRI Energy Plaza and the Wedge International Tower. Built between 1962 and 2003, these high-rises are 158–226 metres tall. All conformed to the construction standard that dictates that tall buildings be designed to withstand winds up to 67 metres per second, corresponding to a category 4 hurricane.
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Wind speeds measured in downtown Houston during the derecho didn’t come close to this construction standard, peaking at 40 metres per second. Nonetheless, as illustrated by the study, facade panels on these tall buildings were dislodged and cladding was damaged, especially on corners and lower floors. Numerous windows cracked or shattered, raining dangerous debris down into the streets.
In contrast, these tall buildings sustained minimal damage during hurricane Beryl, which hit Houston on 8 July, 2024. The maximum wind speed measured in downtown Houston during Beryl was 36 metres per second, comparable to the derecho.
The researchers proceeded to simulate downbursts and hurricanes at the Wall of Wind facility, whose 12 jet fans can generate wind speeds up to 70 metres per second. These were blasted against a revolving miniature representing a tall building on a 1:350 scale. An identical miniature stood at increasing distances ranging from 0.14 to 0.70 metres from the first, to mimic interference from neighbouring buildings.
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The authors compared two conditions that differed in the variation of the mean wind speed over time: a constant average speed typical of hurricanes and a speed that at first rapidly ramped up, reached a plateau and then ramped down, characteristic of downbursts. The results showed that there was far more suction on the sides of buildings during downburst events than during hurricanes.
‘When strong winds move through a city, they can bounce due to interference between tall buildings. This increases pressure on walls and windows, making damage more severe than if the buildings were isolated,’ said doctoral student Omar Metwally. ‘On top of this, downbursts create intense, localised forces that can exceed typical design values for hurricanes, especially on the lower floors of a tall buildings.’
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This one–two punch effect of interference and downbursts on tall buildings is likely to become an even worse problem soon, as human-induced climate change is already hitting Houston especially hard. The Gulf of Mexico is warming at 0.19°C per decade, twice the rate of the global ocean. Higher temperatures are predicted to bring more frequent and more severe extreme weather.
‘Accounting for the unique effects of downbursts and thunderstorm winds in derechos is essential in urban planning and building design, to protect tall buildings against damage. Current construction guidelines for facades should be re-revaluated to reflect this,’ concluded Metwally.
The research has been published in Frontiers in Built Environment.