Skyscrapers built to withstand major hurricanes fare much more poorly in less powerful windstorms known as derechos, researchers have found, raising questions for cities worldwide over the resilience of tall buildings as the climate emergency worsens.
A team from Florida International University’s (FIU) civil and environmental engineering department studied the unexpectedly severe damage caused to buildings in Houston, a city with 50 skyscrapers of 492ft (150 metres) or more, during the 16 May 2024 derecho.
They found that the storm’s long line of fast-moving thunderstorms spawned “downburst” winds peaking at 90mph that bounced off the buildings and inflicted considerable damage, especially to the facades of structures designed to withstand stronger, category 4 hurricane-force wind speeds of up to 156mph.
The same buildings, by contrast, were virtually unscathed during category 1 Hurricane Beryl in July, when sustained wind speeds were similar to those of the earlier derecho, but without their more erratic, up and down nature, or explosive bursts at or near ground level.
The results were published on Friday by the peer-reviewed science website Frontiers in Built Environment. The FIU study focused on five of Houston’s tallest and most iconic buildings but, the researchers say, it could have profound implications for cities elsewhere as the climate crisis and soaring ocean temperatures fuel stronger and more frequent severe weather events, including hurricanes, fires and floods.
They stress that the wind speeds in a derecho, which can vary from far below major hurricane strength to match or exceed it, is not as consequential as how that wind is dispersed. A “unique characteristic” of a downburst, they say, is how the wind blows outwards in all directions when it reaches the ground.
“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, an FIU doctoral student and the report’s co-author.
“On top of this, downbursts create intense, localized forces which can exceed typical design values for hurricanes, especially on the lower floors of tall buildings.”
Metwally called it a “one-two punch effect” that the FIU team predicts will become an even worse problem for states around the Gulf of Mexico, where a 0.34F rise per decade over the last half-century is twice the rate of oceans globally.
Amal Elawady, professor of structural and wind engineering at FIU, and the team’s leader, said the research would also have relevance in other countries, where regulations for building design and wind loads are often calculated primarily with hurricane categories in mind.
“It’s not only a US issue. Downbursts are also very common and very frequent in Europe and worldwide,” she said.
“How a building responds to a thunderstorm is different from the way they respond to hurricanes, so it’s something that needs to be considered, not just for the buildings, but also for the components, like the cladding, the envelope of the building.”
Metwally said he hoped the research would lead to a re-evaluation of regulations and design of future tall buildings, as well as urban planning, as officials became more aware of the complexity and potential negative outcomes of downburst events.
The FIU analysis focused on Houston’s Chevron Building Auditorium, CenterPoint Energy Plaza, El Paso Energy Building, RRI Energy Plaza, and Wedge International Tower, all built between 1962 and 2003 and between 518ft and 742ft tall. Construction standards require them to withstand winds up to 67 metres per second, or category 4 hurricane strength.
During last year’s derecho, facade panels were dislodged and cladding damaged, especially on corners and lower floors. Broken glass and other hazardous debris fell on to downtown streets and the aftermath brought significant socio-economic impacts including traffic disruptions, businesses temporarily closing, and a huge bill for clean-up and repairs.
The FIU team ran simulations of the downbursts and hurricanes on modeled replicas at the university’s Wall of Wind experimental facility in Miami, funded by the National Science Foundation. Suction on the sides of buildings was substantially more evident during downburst events, explaining the ripping away of cladding and broken windows that did not occur during the hurricane.
“It’s not likely that a tall building will fail under wind, either hurricane or downburst,” Elawady said.
“But it causes damage, debris and water intrusion, and once you have a broken window you have a change in the internal pressure in the building and then the total force on the building is different,” she said.
Ongoing and future FIU research will look at the effects of downbursts on transmission lines, lighting poles, telecommunication towers and low-rise buildings as well as more studies on skyscrapers.
“It’s a very complex problem that needs to be thoroughly studied, and we’re trying our best to better understand it,” Elawady said.
