Types of Systems

The changing needs of society and advancing capabilities of engineers have caused the development of several types of systems for withstanding seismic loads. The greater concern for performance (cost of damage, interruption of operation, less tolerance for performance, etc.) and the recent increases in the estimates of expected earthquake intensity have caused designers to have to make structures:

  • insensitive to uncertainty in ground motion
  • insensitive to damage
  • stronger and stiffer (or maybe weaker or less stiff - this will be discussed at length later)

However, increased abiltity to do advanced computer analysis and new technologies for earthquake protection have given designers the capability to use several different types of systems. These systems can be separated into those that remain elastic and those that are allowed to respond nonlinearly. Systems that have nonlinear response are desirable for several reasons and the class will be focused on these systems.

Elastic Systems

The elastic approach consists of making the structure strong enough and stiff enough to remain elastic during the design earthquake. This is difficult and expensive to do when the design earthquake is large. It is difficult to determine the maximum load the structure will face, due to the uncertainty involved in determining the design earthquake and its associated ground motion parameters. This approach is not usually used except for very important structures such as nuclear power plants.

Inelastic Systems

Inelastic systems rely on different variations of the nonlinear fuse concept to control the level of forces the structure will see. One or more elements are designed to become damaged, while the rest of the structural system suffers little or no damage. Several different types of nonlinear system are shown below.

A large part of the course will be spent on the major design question for nonlinear systems: determining the design displacements.