Understanding Seismic Design Philosophy for Buildings

Seismic design philosophy is a critical consideration for the construction industry, especially in areas where earthquakes are frequent and potentially catastrophic. The fundamental goal is to ensure that buildings can withstand minor but frequent shaking without sustaining significant damage, while allowing for repairable damage in non-load-bearing components.

Seismic Design Philosophy

During an earthquake, the primary members of a building that carry vertical and horizontal forces must remain undamaged under minor but frequent shaking. This is crucial to maintain structural integrity and prevent catastrophic failure. However, parts of the building that do not carry load can sustain repairable damage. The design philosophy embodies a balance between flexibility and stiffness, allowing the building to absorb and dissipate seismic energy without failing.

Seismic Body Waves and Structural Modelling

Earthquakes generate two categories of seismic body waves: primary waves (P-waves) and secondary waves (S-waves). P-waves, or compressional waves, and S-waves, or shear waves, travel through the interior of the Earth from the focus to the surface. Understanding these waves is essential for seismic design, as their velocities depend on the elastic constants and densities of the materials and properties of the surrounding medium.

To effectively design earthquake-resistant structures, a mathematical model of the building structure must be constructed. This model helps to determine member forces and structure displacements resulting from applied loads, imposed displacements, and P-Delta effects. The model should include the stiffness and strength of elements crucial for force distribution and deformation, as well as represent the spatial distribution of mass and stiffness throughout the structure.

Building as an Elastic Structure

The concept of treating buildings like elastic structures is akin to how a motorbike or car travels over potholes. The idea is to 'absorb' the energy from minor shaking while maintaining overall structural integrity. Seismic-resistant structures must be designed to be slightly elastic, allowing them to sustain lateral forces without becoming too rigid. This approach is exemplified by structures such as Taipei 101, where a design that is slightly elastic can better withstand lateral forces from earthquakes.

Key attributes of earthquake-resistant structures include:

Lateral Force Resistance: The main part of the structure that adjoins the ground surface must resist significant lateral forces, often through the use of supplemental systems such as base isolators or energy dissipation devices. Inertia and Stability: The building should have sufficient mass and inertia to remain stable without substantial horizontal movement during an earthquake. This is achieved through careful balance between stiffness and flexibility. Seismic Proofing: Non-structural components, such as partitions and cladding, may be allowed to move to prevent damage that would otherwise compromise the building's overall integrity.

In summary, earthquake-resistant design philosophy focuses on creating buildings that can absorb and dissipate seismic energy while maintaining their structural integrity. By understanding seismic waves and structuring mathematical models accordingly, engineers can enhance the resilience of buildings in earthquake-prone regions.