Issue link: http://alpine.uberflip.com/i/493471
1 2 3 4 5 6 7 8 3.11 SEISMIC LOADING Seismic Events Over sixty percent of the land area of the USA is considered seismically active. Certain regions of the country are more prone to heavy seismic activity than other areas, examples being California, Alaska and Hawaii. Structures in these regions are required to be designed for specific lateral loads imposed through seismic activity. In a seismic event, slippage in the earth's crust releases energy that is transmitted along the surface of the earth as a series of waves, similar to the way that waves travel across water when the surface is disturbed. These waves can produce an up-and-down motion, a sideways motion, or both. The type and severity of the motion depends on the amount of initial energy released, the distance from the epicenter, type of ground fault and soil characteristics. The back-and-forth movement can cause brief accelerations of 1g or higher in strong earthquakes. This ground vibration changes its magnitude throughout the duration of a seismic event. The vibrations usually taper off, or dampen, in a few seconds, although the waves can continue for several minutes. Aftershocks are earthquakes of lesser magnitude than the main earthquake. They may occur for hours or days after the main earthquake and originate near the initial epicenter. Seismic Design Categories The International Building Code assigns a Seismic Design Category to each location in the USA based on earthquake probability, occupancy, and soil characteristics. Categories A and B are assigned to locations that do not require any seismic design. Structures built in Category C locations require some special detailing, but one and two family dwellings are exempt from the seismic provisions. Categories D1, D2, and E require successively more load resistance and attention to prescriptive details. Map shown for illustration purposes only. See the IBC or ASCE7 for actual seismic loading maps and data. ENGINEERED BY ALPINE S P E C I F Y I N G / D E S I G N I N G