To view the figures and tables associated with this article, please refer to the flipbook above.
In the aftermath of disasters, people are often left wondering what they could have done to prevent the catastrophic loss of life and property damage. In some cases, the answer is in policy change, such as warning notification systems or mandatory evacuation procedures. In the world of codes and standards, we examine the performance of engineered structures, looking for lessons learned and areas where our codes and standards can be improved. Major storm events can provide the data and information needed to advance our abilities to protect the health, safety, and welfare of the public. ASCE provides the bridge between data collected in the field and implementation in codes and standards.
ASCE is home to more than 75 standards, and SEI supports nearly a third of these standards. ASCE/SEI has deployed several teams of structural engineers to assess post-disaster performance. How did the structures perform during the flood, tornado, or seismic event? Was the hurricane a design-level wind event? Did the flood event exceed the design mean recurrence interval? Did the structures perform as expected given the hazard? The goal of these deployments is to study engineered structures, particularly those designed to codes with design requirements comparable to ASCE/SEI standards, and determine whether any of the assessments could lead to changes in codes and standards. Lessons learned from two such post-disaster deployments directly informed the latest editions of ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures and ASCE/SEI 24 Flood Resistant Design and Construction.
The Atlantic hurricane season in the United States runs from June 1 through November 30, with the greatest activity occurring in August through October. The 2017 hurricane season was particularly active, with 17 named storms, 10 hurricanes, and six major hurricanes. These storms produced high winds, heavy rainfall, and storm surges. Hurricane Irma passed by St. Thomas in the U.S. Virgin Islands on September 6. Hurricane Maria was not far behind, passing to the south of St. Thomas on September 19. Each of these storms carried an estimated maximum wind speed of 160 to 165 mph.
In October of 2017, ASCE/SEI deployed a team of three engineers to study the wind effects from both Hurricanes Irma and Maria on St. Thomas in the U.S. Virgin Islands. The effort included comparing the observed effects of the damage caused by the storms to the wind provisions in ASCE/SEI 7-16. The fieldwork included four areas: a comparison of design wind speeds with estimated event wind speeds, wind speed-up effects due to the topography of the island, performance of solar panel arrays, and windborne debris regions along with the performance of impact-resistant glazing. The complete post-disaster assessment results were published in Hurricanes Irma and Maria in the U.S. Virgin Islands: Building Performance Observations and Recommendations for ASCE 7.
The team found that the estimated wind speed for St. Thomas was roughly 20% higher than the ASCE 7-16 ASD design wind speeds, and close to the 7-16 ultimate design wind speeds. The observed damage from the storm primarily appeared to be to elements that were designed to earlier building codes, though it was clear in several cases that the building construction did not comply with design requirements. For example, masonry cells with reinforcing steel were ungrouted, and rooftop equipment was not mechanically fastened to the roof structure. Figures 1-3 show structural failures at a post office, and Figures 4-5 show a rooftop unit blown off of its curb, damaging the roof membrane and structure and allowing water infiltration. The majority of the failures observed, when construction was not a consideration, were failures of non-structural elements, such as window frame anchorage and building envelope maintenance.
The topographic study found that the wind speed-up effects were greater for 2D ridges than for escarpments or 3D axi-symmetrical hills. It was difficult to classify the terrain features in the field using ASCE 7-16. Fig. 6 shows a topography map of one of the studied sites, and Fig. 7 shows the challenge posed to the practicing engineer in calculating these speedup effects. Given these findings, one of recommendations of the study was to provide wind speeds with topographic effects incorporated. This provision was incorporated in ASCE/SEI 7-22 as a result of the study’s recommendation.
Observed solar array damage was primarily due to panel detachment from the rails and from impact from windborne debris, shown in Fig. 8. The ASCE/SEI 7-28 committee is working to add wind load criteria for ground-mounted solar panels based on the team’s recommendations. The team’s windborne debris observations validated the current ASCE/SEI 7 test missile criteria and confirmed that the omission of windborne debris criteria for exterior walls for most Risk Category II buildings is appropriate. Recommendations for improvement to the standard included the review of roof assembly windborne debris criteria for higher risk category buildings in windborne debris regions, which is currently under review by the ASCE 7 Wind Load Subcommittee.
Hurricanes are major disasters and can be accompanied by significant flooding events. Hurricane (Superstorm) Sandy is a key example. This storm formed in late October 2012 and made landfall in New York City as a post-tropical cyclone on October 29, bringing with it massive hurricane-drive storm surges up to 13.8 feet in height.
The ASCE Urban Flood Study after Hurricane Sandy in 2012 was commissioned to study flood damaged buildings in lower Manhattan. The study focused on lower Manhattan as its dense urban environment and extensive subterranean spaces could complement other post-storm studies of single-family residential neighborhoods. The seven-person study team consisted of structural, forensic, civil-hydrology, and MEP engineers. The team conducted site visits, interviews, and research at five high-rise buildings, the South Street Seaport area, an electrical power plant, and a hospital.
The high-rise buildings in the study were observed to have taken on 20 to 30 feet of water during Superstorm Sandy, with up to 5 feet of water above the ground floor and typically two below-grade levels fully inundated. Observed structural damage to the high-rise buildings was minimal due to the robust column and wall sizing at the base of the structures. However, the inundation of the below-grade mechanical room levels wiped out the building systems. The high rises lost their electrical panels, substations, transformers, hot water heaters, heating equipment, refrigeration equipment, ductwork, conduits, fire water pumps, elevators, and fuel storage. These below-grade levels took 4-7 days to pump out all the water, weeks to clean up, and months to make sufficient repairs to restore power. Repairs to the building systems and relocation of building systems to upper levels of the buildings were designed and constructed in the years that followed.
Superstorm Sandy’s flood elevations and extents exceeded the 100-year flood across large areas of the city. A recommendation from the study was to locate mechanical and electrical equipment above the 500-year flood elevation so that the risk of loss of mechanical systems is less than 10% in 50 years and the buildings can continue to operate after a flood event. This recommendation was realized in ASCE 24-24 when the Design Flood transitioned to the 500-year floodplain extent and 500-year flood elevation for Risk Category II structures.
Buildings that had emergency generators located above the flood waters still struggled to fuel their generators after Superstorm Sandy. The hospital in the study had fuel pumps to its generators in a dry-floodproofed enclosure. The enclosure failed to remain watertight and the pumps subsequently shorted out. Fuel sent to the site needed to be hauled up 13 flights of steps to the emergency generator as elevators were taken out by the flood. Building operators need to allow sufficient fuel storage to operate emergency equipment for an extended period and emergency equipment needs to be placed in areas not susceptible to flooding or out of reach. Requirements were adopted in ASCE 24-24 for Flood Design Class 3 and 4 buildings to provide a means to connect to temporary power above the Design Flood Elevation or fuel supplies for emergency generators to provide at least 72 hours of power to critical functions.
Dry floodproofing measures failing or being overtopped is a common observation among post Hurricane field assessments. For Flood Design Class 3 and 4 buildings, ASCE 24-24 now requires dry floodproofed areas comprised of multiple stories below grade to have at least two shafts to allow for the automatic passage of water to the lowest physical floor to limit the accumulation of flood loads on a floor system in the event that the dry floodproofing measures fail.
In other data collected from Hurricane Sandy on Long Island, houses were observed to be suspended on helical piles where all the surrounding soil was scoured away. Imagine a house supported on toothpicks. This led the ASCE 24-24 committee to incorporate helical piles into the standard and to require designs to account for all soil conditions over the design life of the helical anchors, including scour and erosion.
Observations for flood mitigation outside of disasters have also lent themselves to proposed additions in ASCE standards. It is common in the South for homes to be built with no basement and with the lowest floor constructed as a slab-on-ground. As flood maps change, flood elevations and insurance premiums rise, and homeowners experience the effects of flood, many owners have chosen to elevate their existing homes. In many instances, slab-on-ground houses, which were not designed to span, have been elevated on piers or walls to become suspended slabs. Some of these slabs subsequently failed. New language in ASCE 24-24 prohibits the elevation of a slab-on-ground unless the existing slab is assessed, strengthened where required, and supported to meet the load requirements of ASCE 7 and ASCE 24.
The provisions of standards such as ASCE/SEI 7 and 24 establish the minimum requirements to provide a reasonable level of safety, health, and general welfare through structural strength and stability in the face of natural disasters. These codes and standards are sometimes challenged by catastrophic events, when engineers ask themselves, how can we do better. Each edition of the standard evolves based on not only the latest research and new use cases, but also on recommendations from these post-disaster deployments that assess how provisions stand up to their ultimate real-life test when people and communities are depending on them. ASCE is at the forefront of enhancing codes and standards by providing engineers with this unique knowledge and understanding of structural performance post-natural disaster.■
Joint Summer Series
The Coalition of American Structural Engineers (CASE), the National Council of Structural Engineers Associations (NCSEA), and the Structural Engineering Institute of the American Society of Civil Engineers (SEI) are proud to announce their second Joint Summer Series with four free webinars and three accompanying STRUCTURE magazine articles centered around a topic with the potential to broadly impact all aspects of structural engineering, from education and research to design and construction and business practice. This year’s topic is “Disaster Response and Lessons Learned.”
Register for the webinars at https://program.acec.org/-slug-2026-joint-summer-series-disaster-response.
About the Authors
Cherylyn Henry, PE, F.SEI, F.ASCE, is a senior project manager with ZAPATA, and was part of ASCE’s deployment to the U.S. Virgin Islands following Hurricanes Irma and Maria. Henry is the chair of the ASCE/SEI 7-28 Wind Load Subcommittee.
Jessica Mandrick, PE, SE, is a partner at Gilsanz Murray Steficek and was part of the ASCE 24 Urban Flood Study Team. Mandrick currently serves on the ASCE/SEI 7-28 Flood Load Subcommittee and is the current chair of ASCE/SEI 24: Flood Resistant Design and Construction.