The design of structures resistant to natural events like earthquakes and hurricanes involves distinct approaches due to the nature of the forces involved and associated risks. As professionals are unpredictable and sudden events, underscored by designing and constructing structures in these areas, your role is crucial in implementing these design standards. Below, we explain why standards are stricter for earthquake-resistant design based on seismic performance, while the design for high-speed winds generally remains within elastic philosophy, albeit with notable exceptions.

Structure collapse after Hurricane Harvey hit Houston, TX in 2017.

Predictability and Preparedness

Earthquakes, being unpredictable and sudden events, underscore the gravity of our work. Structures must survive and limit damage to protect lives and enable safe evacuations. The lack of prior warning necessitates a detailed focus on post-damage performance in earthquake-resistant design, a crucial aspect for ensuring occupant safety.

Unlike earthquakes, hurricanes are predictable events, often with days of advance notice. This predictability plays a significant role in the high-speed wind-resistant design, allowing evacuations and preparations that can significantly reduce human life loss. While material damage can be extensive, the predictability of hurricanes permits the implementation of effective mitigation strategies, thereby reducing the emphasis on the post-damage behavior of structures.

Nature of Forces

Earthquakes: These events induce cyclic dynamic forces, causing significant deformations in all directions. This complexity necessitates structures capable of dissipating energy through controlled inelastic deformations. Your expertise in understanding these complexities is crucial for designing structures that can withstand these forces without collapsing, ensuring safety during an earthquake.

Winds: While wind loads may seem to apply more uniform and constant forces in a single direction, especially in tall structures like skyscrapers or long-span bridges, winds can induce cyclic effects due to turbulence and variability in wind direction and speed. This can cause significant vibrations and oscillatory movements, necessitating nonlinear response analysis and design strategies addressing energy dissipation.

Post-Event Impact and Recovery

Earthquakes: Continuous functionality of structures is crucial for immediate community recovery. Seismic-resistant standards seek to minimize damage and ensure post-event stability, enabling swift repair and reutilization of affected structures.

Winds: While recovery is also essential, the ability to predict and prepare allows for a design approach balancing structural strength with economic and practical viability. Structures must withstand wind loads without failing, but the primary focus is preventing collapses and maintaining structural safety.

Structural Behavior

Ductility and Rigidity: In seismic-resistant design, ductility is vital to absorbing and dissipating energy without collapsing. This is achieved through controlled inelastic deformation of materials, enabling the structure to maintain integrity during repetitive loading and unloading cycles.

Rigidity and elastic resistance are more important for wind loads to prevent excessive displacements and ensure overall stability. Maintaining the structure within the elastic range ensures that stresses in materials do not exceed their elastic limits, avoiding catastrophic failures. However, the cyclic and nonlinear behavior due to wind effects must be considered in some structures, like tall buildings and long-span bridges, where the impact of the vortex-induced vibrations is more considerable.

Collapse of structures after the 2023 Turkey earthquake.

Regulatory Development

Seismic Codes: Evolved in response to devastating earthquakes, highlighting the need for performance-based design approaches and ductility. This has led to particular and detailed standards to ensure safety and structural performance during and after earthquakes.

Wind Codes have also evolved but focus more on keeping structures within the elastic range and simplifying design. This is not a wrong practice, but we must know that the design criteria are very uncertain. While damages can be significant, estimable and mitigation strategies have influenced a different approach to wind-resistant design. In the case of some particular structures, a more detailed and nonlinear analysis is necessary.

Design Efficiency

Seismic-resistant design requires additional considerations to ensure ductility and energy dissipation, which can be more expensive and complex. Designing for controlled inelastic deformations and ensuring post-damage performance entails more detail in design and construction.

Wind Design: Maintaining design within the elastic range minimizes structural damage and reduces the need for significant post-event repairs, resulting in a more economical and straightforward process. This approach ensures that structures can withstand wind forces without suffering significant permanent damage, maintaining functionality and safety in the long term. Due to cyclic wind effects, a more complex and detailed approach is necessary for particular structures, but this is not detailed in the standards.

Importance of Stricter Standards for Winds

It is crucial to prioritize wind analysis with more rigorous approaches. At the same time, evacuations can protect people in coastal areas, regions like islands where evacuation is not feasible, and where additional safety challenges are faced. Therefore, standards must be stricter regarding wind-resistant structure design.

An example highlighting the importance of this approach is the collapse of the Tacoma Narrows Bridge in 1940. This collapse was caused by vibrations induced by the wind, a phenomenon known as "flutter." This incident underscores the need to consider cyclic wind effects and vibration periods in structures. Often, these aspects are not adequately reviewed in design, leading to catastrophic failures.

Ensuring structures can withstand extreme winds without collapsing is vital to protect lives and properties in all vulnerable regions. Additionally, it is essential that structures of greater importance, such as bridges, hospitals, schools, shelters, and food distribution centers, be designed for immediate occupancy following an extreme wind event. This will ensure continuity of essential services and facilitate quicker and more efficient recovery.