Avoid A Total Blowout

by Stacy Rinella | May 21, 2024 7:00 am

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Approximately 1,200 tornadoes strike the U.S. every year. Many occur in “Tornado Alley,” an area of the Great Plains where dry cool area moving eastward from the Rockies meets humid air pushing up from the Gulf of Mexico, spawning violent storms. Recently, this area of tornado intensity has been expanding eastward to encompass Kentucky, Tennessee, and the northern portions of Mississippi and Alabama, areas not prepared for these extreme hazards. There has also been an increase in tornado swarms, large storm systems generating multiple powerful tornadoes. Since 1980, the duration, frequency, and severity of Atlantic hurricanes have also increased,

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including more of the most powerful storms (Categories 4 and 5). Also, a recent study published in the journal Nature found a fivefold increase in incidents of severe straight-line wind events (e.g. derechos). These worsening phenomena are all connected to global climate change.

In addition to these more extreme weather events, all buildings are periodically subject to strong and sustained winds. Buildings need to be resilient in the face of both acute and chronic wind impacts, especially with the trends toward greater wind-pressure event intensity. Unlike snow, earthquake, or flood loads, wind loads on building enclosure components occur daily. Though often well below a design-level event, the persistent nature of wind loads must be addressed to create resilient building enclosures. Architects should understand how wind impacts buildings, and how to design projects for greater wind resilience, including reasonable opportunities to exceed minimum safety factors and add redundancy to cladding systems.

Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7) is the standard for design wind loads on buildings. Building envelope components are addressed as “Components and Cladding” in the standard. Understanding wind loads on components and cladding are a function of many more parameters than wind speed, such as the building risk category (how essential is the building function?), the position of components on buildings, and local topography and wind obstructions, will help architects appreciate that each project will have unique design requirements to achieve wind resilience. For example, a building on a beachfront property or near the crest of a hill may be subject to higher overall wind loads than a building tucked among others in a dense urban environment, though wind tunnels created by other buildings may subject urban buildings to concentrated wind loads on portions of the structure. It is critical to use the most current design wind data, such as that published in ASCE/SEI 7-22, based on broader data sets and up-to-date science regarding climatology and risk assessment. Experimental data, analytical studies and design experience regarding soffits, overhangs, and ventilated rainscreens led to the development of more detailed wind pressure determinations for these components in recent versions of ASCE/SEI 7.

In the U. S., tornadoes account for more annual loss of life and damage to property than hurricanes and tropical storms. Nevertheless, excepting storm shelters and safe rooms, the full force of tornado winds are rarely considered in the design of structures. However, low-grade tornadoes and other wind events (microbursts or derechos) are more probable than catastrophic tornadoes (~97 percent of tornadoes are EF0 or EF1) and can be considered in the design of common structures. The newest version of ASCE 7-22 has now incorporated tornado wind load design considerations to provide more resilience for the most probable tornado intensities.

Architects need to know where and how failures due to wind occur and design accordingly. Wind pressure events can be long duration, and they vary significantly over different regions of the structure (pressures are often higher in corner and edge zones). Failure of building envelope components are typically due to progressive failure. When a portion of building envelope component (e.g. ridge cap of edge metal) fails or partially fails, it can alter the wind pressure magnitudes and load distributions on adjacent components causing complete failure of that component or other related building envelope components. Understanding the pressure zones on buildings helps in developing product requirements and installation details critical for the integrity of building elements.

With these failure points in mind, architects can develop details and specifications to create wind-resilient building enclosures. Performance testing of building enclosure components is conducted with new materials and installations in ideal conditions. The maximum design loads established by laboratory testing for components may not translate to resilience in actual installations over the life of the building on which they are installed. Rather than using different products to accommodate the various wind pressure zones of a building, installation methods can be adjusted so the same product may be used for the higher wind pressure zones. For example, specifying anchor clip spacing closer than the maximum allowed distance at the ridge, corner, and edge zones of a standing seam metal roof reduces the potential for failures. It is critical these nuances of installation details for high-wind pressure zones are effectively communicated to the construction team and verified by inspection during installation. It is also important to choose materials that are resistant to rot, thermal aging, and UV degradation to improve a building component’s wind pressure resilience.

In completed buildings, regular inspections are helpful to maintain the wind resilience of roofing and cladding. Water leaks or other building enclosure performance issues occurring during below-design-level wind events may indicate that the system lacks resilience to resist a design level event. Spotting and correcting these in advance may prevent catastrophic failures later.

Creating wind-resilient buildings significantly reduces the risks of harm to humans (e.g. flying debris), extensive physical damage to structures, and financial losses due to costly repairs and disruption of use.

Resilience to wind damage is particularly important for large-scale wind events, as there are typically shortages of materials and labor necessary for repairs after these events. Building components or construction techniques which have been verifiable to improved resilience may also lead to lower insurance premiums. For example, the Insurance Institute for Business and Home Safety (IBHS) Fortified program is recognized by several home insurance companies.

The American Institute of Architects now recognizes resilience as part of the architect’s standard of care. In the face of increasingly frequent and intense wind events, designing to minimum standards is not enough. Architects should understand the potential wind hazards for the buildings they design and the applicable wind pressure design standards to address them. Making wind-resilient buildings is critical to life safety and to preserving the long-term value of their clients’ buildings.

Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect and consultant with more than 35 years of experience in sustainable building design. He is director of sustainability with Intertek Building Science Solutions.

 Joseph A. Reed, PE, is a building material performance testing expert with more than 30 years of experience. He is a senior project engineer with Intertek Building and Construction.

 To learn more, follow Alan on LinkedIn at www.linkedin.com/in/alanscottfaia/.

Source URL: https://www.metalarchitecture.com/articles/columns/avoid-a-total-blowout/

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