The entire building is meshed using four-node structural shell elements (SHELL181). Shell thickness is uniform (1 m) for the wall and slab, and the basemat thickness is 2 m.
Mesh size is based on the shortest wavelength to be resolved. In this case, the flexural wavelengths can be predicted using the classical formula:[3]
where λb is the flexural wavelength, B is the plate bending inertia per unit-length, ω is the circular frequency, and m” is the plate mass per unit-length.
A cutoff frequency of 50 Hz is specified, which gives a bending wavelength of about 12 m. Using six elements per wavelength is sufficient and results in a mesh size of 2 m.
To simplify the problem in order to better illustrate the method, the following assumptions apply:
Soil structure-interaction (SSI) effects are ignored
Ignored effects include the effects of waves reflected back to the soil/structure interface, and waves radiated following the structural motion. This assumption is conservative for very stiff, rock-like soil situations. The loading can be modeled as an enforced displacement (or acceleration) at the boundaries.
Damping is homogeneous within the structure.
A rigorous treatment would require a distinction between prestressed (external shell and dome) and reinforced concrete parts damping. Because most of the structure is made of reinforced concrete, uniform damping is applied. This assumption is valid because the high-frequency seismic events are not expected to damage the building significantly. High-frequency events are less harmful to large, ductile concrete structures, but more detrimental to smaller, stiffer objects such as secondary components.