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Architectural miracles triggered by earthquakes: Why do some buildings remain intact even at the epicenter?
In many cases, the impact of an earthquake on a building can result in total destruction, but some buildings can survive the epicenter unscathed. This phenomenon has caused deep thought among earthquake engineering scholars and architectural designers. One of the keys is the "response spectrum," an important tool for evaluating how a structure responds to earthquakes. Response spectroscopy helps engineers understand how to predict and design structures to resist earthquakes by plotting the peak responses of oscillators of many different natural frequencies when subjected to the same base vibrations.
The use of response spectroscopy allows engineers to estimate a building's ability to resist earthquakes during the design process, which involves not only the fundamental frequency of the structure but also the perfect shock-absorbing technology.
The science of strong earthquake motion also often uses ground response spectra generated from data collected by seismographs to assess earthquake damage relevance. This data helps provide risk predictions to ensure buildings remain stable during earthquakes. Based on these assessments, the designer can determine the maximum design force the structure will need to withstand, which is usually derived from the values of the ground response spectrum.
For example, in the 1985 Mexico City earthquake, many mid-rise concrete buildings suffered serious damage because their natural frequencies coincided with the vibration frequencies of the deeper soil lakebed. Conversely, shorter and sturdier buildings showed lower damage rates, suggesting that the vibration characteristics of buildings play a key role in actual earthquakes.
“Buildings must be designed to take into account the vibration characteristics of the ground. As demonstrated in the Mexico City earthquake, the toughness and natural frequency of a structure are critical to its earthquake resistance.”
Response spectroscopy is not only applicable to simple single-degree-of-freedom systems, but also to complex multi-degree-of-freedom systems. Although it can be used in theory, modal analysis is still required for high-level damping settings to more accurately predict the response. . It is worth noting that traditional response spectroscopy is mainly aimed at linear systems, which also limits its scope of application, because the response of nonlinear systems often cannot be simply obtained from the response spectrum.
In 1941, George W. Hausner of Caltech published the first response spectrum calculated from an accelerometer. As journal articles were published, response spectroscopy gradually became the basis for today's earthquake engineering design. This concept has been refined and is now a guide for structural design, especially in earthquake-prone areas.
“Our goal is to use the response spectrum to help design earthquake-resistant buildings, ensuring that disasters and casualties can be reduced in future earthquakes.”
When designing low-rise buildings, the functional mode is mostly the basic mode. This "swinging back and forth" mode is important for designing structures to withstand earthquakes. For high-rise or irregularly structured buildings, complex multi-mode analysis is required to obtain a more realistic response estimate, which involves more complex methods such as nonlinear static or dynamic analysis.
Ultimately, whether a building can survive an earthquake depends not only on design considerations, but also on building materials and construction methods, which will affect its earthquake resistance. For example, in an earthquake last year, engineers discovered that some high-rise buildings built with new materials showed good earthquake resistance, which attracted widespread attention in the industry.
"Ensuring that building structures can withstand earthquakes requires integrating current technology, material science and developments in response spectroscopy."
Finally, as we think about how to improve building design to withstand future earthquakes, should we re-examine building design standards and implementation strategies to better adapt to changing environments and challenges?
