A3 Seismic Construction Physics

Topic

Modern seismic design is based on a response spectrum: for any given building—depending on its natural oscillation period—there is an expected design acceleration during an earthquake. This value is determined by the zone's characteristic acceleration and the way local soil conditions amplify or attenuate that acceleration. From this acceleration, the equivalent lateral force acting on the building is derived—a force proportional to the building's mass. Meanwhile, the relative displacement of each story with respect to the one below depends on this force and the story's lateral stiffness—a property whose role in structural equilibrium was established in the first module of this catalog.

The core concept of contemporary seismic design is the acceptance that, during a severe earthquake, a building may enter a non-elastic state—deforming beyond its purely elastic range—provided that collapse is prevented. This requires the structural system to possess ductility: the capacity to undergo significant displacement beyond the onset of permanent deformation without losing its load-bearing ability. This capacity to deform without failure allows for the dissipation of seismic energy through loading and unloading cycles within the structural material, preventing the energy from accumulating to the point of structural rupture.

Base isolation systems take this logic a step further: rather than allowing the structure to absorb and dissipate seismic energy, they partially decouple the building from ground motion—a decoupling that can drastically reduce the seismic forces ultimately transmitted to the structure.

Thus, this module—which begins by considering acceleration based on a building's oscillation period—concludes by demonstrating that seismic safety is not achieved by preventing structural deformation, but by designing *how* that deformation occurs: whether by dissipating energy through controlled ductility or by preventing that energy from reaching the structure altogether through base isolation.

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