Seismic Design Factors
The following factors affect and are affected by the design of the building. It is important that the design team understands these factors and deal with them prudently in the design phase.
Torsion: Objects and buildings have a center of mass, a point by which the object (building) can be balanced without rotation occurring. If the mass is uniformly distributed then the geometric center of the floor and the center of mass may coincide. Uneven mass distribution will position the center of mass outside of the geometric center causing "torsion" generating stress concentrations. A certain amount of torsion is unavoidable in every building design. Symmetrical arrangement of masses, however, will result in balanced stiffness against either direction and keep torsion within a manageable range.
Damping: Buildings in general are poor resonators to dynamic shock and dissipate vibration by absorbing it. Damping is a rate at which natural vibration is absorbed.
Ductility: Ductility is the characteristic of a material (such as steel) to bend, flex, or move, but fails only after considerable deformation has occurred. Non-ductile materials (such as poorly reinforced concrete) fail abruptly by crumbling. Good ductility can be achieved with carefully detailed joints.
Strength and Stiffness: Strength is a property of a material to resist and bear applied forces within a safe limit. Stiffness of a material is a degree of resistance to deflection or drift (drift being a horizontal story-to-story relative displacement).
Building Configuration: This term defines a building's size and shape, and structural and nonstructural elements. Building configuration determines the way seismic forces are distributed within the structure, their relative magnitude, and problematic design concerns.
Regular Configuration buildings have Shear Walls or Moment-Resistant Frames or Braced Frames and generally have:
Low Height to Base Ratios
Equal Floor Heights
Symmetrical Plans
Uniform Sections and Elevations
Maximum Torsional Resistance
Short Spans and Redundancy
Direct Load Paths
Irregular Configuration buildings are those that differ from the "Regular" definition and have problematic stress concentrations and torsion.