Earthquakes often occur due to slip between tectonic plates along a geological fault in the earth's crust. Earthquakes result in various types of ground motion as seismic waves propagate through the earth. As these waves pass the location of a structure, the associated ground motion subjects the structure to lateral forces (primarily) and vertical forces (to a lesser degree).

The following is a short animation that illustrates the origin of seismic waves and their effect on a single story structure.

Ground motion at the base of a structure results in dynamic loads (forces) distributed throughout the structure based on the stiffnesses of structural elements (restoring forces) and the distribution of mass (inertial forces). The most accurate methods of design for seismic loads involve comprehensive dynamic analyses of structures. However, simplified analytical techniques (typically referred to as equivalent static force or equivalent lateral force procedures) are provided in model building codes for the design of low-rise buildings subjected to seismic loads. Since most wood structures are classified as low-rise buildings, this tutorial focuses on understanding and utilizing the equivalent static (lateral) force procedures specified in the 2000 International Building Code (IBC) and the 1997 Uniform Building Code (UBC).

Support for development of the Seismic Loads Tutorial was provided by CUREE, PEER, and WSU. The Seismic Loads Tutorial was designed and created by the following Research Assistants and Faculty at Washington State University: Mike Dodson, Cameron Knudson, Zach Davidson, Aaron Henson, Dave Pollock (Professor), Michael Symans (Professor), and Ken Fridley (Professor).