In which regions of the United States are houses required to be designed for earthquake loads?
Special seismic provisions of the International Residential Code (IRC) are required for detached one- and two-family houses located in Seismic Design Categories (SDC) D1 and D2 and townhouses located in SDC C, D1, and D2. All houses located in SDC E and portions of the houses that have irregular configuration in accordance with Section R301.2.2.2.2 of 2003 IRC are required to be engineered. Examples of irregularities include vertical wall offsets, floors with openings exceeding 12 feet, offsets in floor levels in the same story, etc. Seismic Design Categories are defined in Figure R301.2(2) of the 2003 IRC.