The equations used to determine Seismic Design Forces throughout the United States as well as the rest of the world are based on historical data that has been collected during past earthquakes. As the level of knowledge and data collected increases, these equations are modified to better represent these forces. The heavily instrumented San Francisco (1989-Loma Prieta) and Las Angeles (1994-Northridge) earthquakes increased this knowledge dramatically. Major shortcomings in the force levels predicted by the codes in effect at the time have led to the development of considerably more complex equations that more accurately address items such as equipment locations within a building, soil factors, etc.
Historically, there have been three independent codes used in various areas of the United States. In the Northeast, the Building Officials Code Administration (BOCA), developed to address concerns local to the area such as high snow loads, was the predominate code. In the Southeast, the Southern Building Code Congress International (SBCCI), developed the Standard Building Code (SBC), which tended again to focus on local concerns like wind, is the most common building code. While seismic requirements were addressed by these codes, the design criteria were not particularly severe. West of the Mississippi and more commonly used internationally is the Uniform Building Code (UBC). Because of the more severe conditions present in the western United States, the UBC was required to address snow, wind, and seismic loads on a more equal footing.
The first code version significantly affected by the seismic data collected in the early 1990's was the 1997 UBC. Several new factors were introduced to the design equations to account for soil, fault proximity and type, specifics of the equipment location within the structure, and typical dynamic response characteristics for particular kinds of equipment.
It is important to recognize that the newer codes predict a significantly higher seismic design load than past codes. This is particularly true for equipment located in upper levels of buildings. In some instances using the newer code criteria it will be found that attaching heavy, unstable equipment located on an upper floor will not be practical with concrete anchorage. A connection directly to steel will be required.
Depending on the geology of the installation site, a vertical force component may also need to be considered when evaluating seismic loads. When incurred in the codes, this force is typically a fixed percentage of the horizontal seismic load.