Resilient design is gaining prominence in the building industry. Resiliency is about holistically assessing buildings for potential risks, and then engaging the building owner to make cost-effective risk-management decisions to mitigate those risks after a disruptive event.
Buildings face many potential structural hazards, ranging from earthquakes to hurricanes to snowstorms to tornadoes. To guide engineers, architects, contractors, and regulatory authorities in determining structural design criteria of new or renovated buildings, the American Society of Civil Engineers (ASCE) publishes Minimum Design Loads and Associated Criteria for Buildings and Other Structure (ASCE 7-16). This standard provides required minimum design loads for dead, live, soil, flood, tsunami, snow, rain, atmospheric ice, earthquake, and wind hazards. The standard is adopted by local and regional jurisdictions throughout the United States and globally.
ASCE 7 is updated every six years, and ASCE currently is in the process of developing ASCE 7-22, with subcommittees meeting regularly to assess industry-wide changes that will inform the new standards.
It is important to note that the prescriptive requirements in building codes are meant to provide a “life-safe” structure based on a design lifespan of 50 years and are not intended to allow for building occupancy after a design event. Hospitals, schools, and other essential facilities are the exception and are implicitly designed to be “operational” after a design event. Some building owners may desire a more resilient building with enhanced performance criteria—and this is where Performance-Based Design comes in.
Wind Loads Subcommittee
As a member of the ASCE 7 Main Committee and the ASCE 7 Wind Loads Subcommittee, I am sharing my professional experience to update the standards, as well as looking beyond the baseline. Specifically, as secretary of the Wind Performance-Based Design task group under the Wind Loads Subcommittee, I am collaborating with engineers across the country to develop Performance-Based Wind Design (PBWD) language to be included in the new standards. Similar to the existing Performance-Based Seismic Design (PBSD) methodology in the current ASCE 7-16, the newly created PBWD language will look beyond baseline prescriptive to predictive performance.
Performance-based wind design procedures can be desirable to building owners wishing to quantify wind disruption or loss hazard, improve wind resistance toughness, or evaluate structural system resilience. Building or structure disruption, loss, toughness, and resilience are influenced by response to strength-level wind effects due to rare wind events, as well as service-level wind effects under frequently occurring wind events. Performance-based procedures identify the source of loss or disruption and seek to mitigate undesirable outcomes through value-based design decisions informed by design objectives. Selection of design objectives should consider the ramification of structural and nonstructural response upon the building occupants, the ability of the structure to provide services following a design wind event, and preservation of structural safety.
Performance design objectives should consider that a large proportion of wind loss damage from high winds is the secondary but associated effect of nonstructural component damage, including water penetration through breaches in the building envelope. Additionally, direct loss or interruption of service can occur due to motions causing objectionable occupant comfort or nonstructural damage.
Computational and Modeling Tools
Although ASCE 7-16 (soon ASCE 7-22) provides essential prescriptive requirements, structural engineers and building owners have opportunities to plan beyond the baseline and assess wind load risks with new and evolving computational and modeling tools, including advanced structural analysis techniques and wind-tunnel testing. Engineers can study how building shape, square footage, program, location, climate variations, and extreme conditions can predict potential failure during a building’s design life.
While these advanced modeling tools are exciting, several risks can be mitigated by implementing simpler design strategies, such as increase parapet height to protect adjacent buildings from ballast blown off the roof, specify tighter spacing of roofing system attachments, specify enhanced performance requirements for glazing systems, and including robust language in project specifications regarding rooftop unit attachment to the primary structure.
Successful building design is a collaborative process between engineers, architects, contractors, and building owners. Through engaged planning incorporating resilient principles, building owners can better understand potential risks that may or may not be covered under current building codes and make informed decisions that ultimately will save them money and avoid downtime after a design event. Through our involvement with the ASCE 7 Wind Loads Subcommittee and Performance-Based Wind Design task group, HGA has the unique opportunity to become an industry expert in this new, evolving method and technology.
Kevin Borth, PE, SE, is a structural engineer at HGA and a member of the ASCE 7-22 Main Committee and Wind Loads Subcommittee. He serves as secretary of the Performance-Based Wind Design task group of the ASCE 7-22 Wind Loads Subcommittee.