Hurricane-proofed downtown skyscrapers unexpectedly vulnerable to ‘bouncing’ winds

(Press-News.org) Houston, we have a problem. The ‘Space City’ boasts 50 buildings over 150 meters tall. These were designed to withstand hurricanes, to which Texas is prone. But on May 16th, 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 disruptions, businesses temporarily closing, and the need for repairs.

Why was the structural damage so much larger than expected? A new study in Frontiers in Built Environment has now provided the answer to this conundrum. Its findings carry lessons for the future design of tall buildings and the planning of city centers, not only in Houston.

"Here we show that a type of highly localized 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 Dr Amal Elawady, an associate professor at Florida International University, and one of the study’s authors.

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 utilizes 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.

An ill wind

Here, the authors analyzed 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 to 226 meters tall. All conformed to the construction standard which dictates that tall buildings be designed to withstand winds up to 67 meters per second, corresponding to a category 4 hurricane.

Wind speeds measured in downtown Houston during the derecho didn’t come close to this construction standard, as they peaked at 40 meters per second. Nonetheless, as illustrated by the study, facade panels on these tall buildings were dislodged while 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 July 8th, 2024. The maximum wind speed measured in downtown Houston during Beryl was 36 meters per second, comparable to the derecho.

Seeing which way the wind blows

The researchers proceeded to simulate downbursts and hurricanes at the Wall of Wind experimental facility, whose 12 jet fans can generate wind speeds up to 70 meters 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 meters from the first, to mimic interference from neighboring buildings.

The authors compared two conditions which 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 Omar Metwally, a doctoral student who was the study’s first author.

"On top of this, downbursts create intense, localized forces which can exceed typical design values for hurricanes, especially on the lower floors of a tall buildings.”

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.

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