a. General. Earthquake ground motions for the design and evaluation of Corps CHS are the Operating Basis Earthquake (OBE) and the Maximum Design Earthquake (MDE) ground motions. Seismic forces associated with the OBE are considered unusual loads. Those associated with the MDE are considered extreme loads. Earthquake loads are to be combined with other loads that are expected to be present during routine operations. b. Operating Basis Earthquake. The OBE is a level of ground motion that is reasonably expected to occur within the service life of the project, that is, with a 50-percent probability of exceedance during the service life. (This corresponds to a return period of 144 years for a project with a service life of 100 years).
c. Maximum Design Earthquake. The MDE is the maximum level of ground motion for which a structure is designed or evaluated. As a minimum, for other than critical structures, the MDE ground motion has a 10 percent chance of being exceeded in a 100-year period, (or a 1000- year return period). For critical structures, the MDE ground motion is the same as the maximum credible earthquake (MCE) ground motion. Critical structures, by ER 1110-2-1806 definition, are structures that are part of a high hazard project and whose failure will result in loss of life. The MCE is defined as the largest earthquake that can reasonably be expected to occur on a specific source, based on seismological and geological evidence. 2-2. Performance Levels
a. General. Various performance levels are considered when evaluating the response of CHS to earthquake ground motions. The performance levels commonly used are serviceability performance, damage control performance, and collapse prevention performance. b. Serviceability performance. The structure is expected to be serviceable and operable immediately following earthquakes producing ground motions up to the OBE level. c. Damage control performance.
Certain elements of the structure can deform beyond their elastic limits (non-linear behavior) if non-linear displacement demands are low and load resistance is not diminished when the structure is subjected to extreme earthquake events. Damage may be significant, but it is generally concentrated in discrete locations where yielding and/or cracking occur. The designer should identify all potential damage regions, and be satisfied that the structure is capable of resisting static loads and if necessary can be repaired to stop further damage by non-earthquake loads. Except for unlikely MCE events, it is desirable to prevent damage from occurring in substructure elements, such as piling and drilled piers, and other inaccessible structural elements.
d. Collapse prevention performance. Collapse prevention performance requires that the structure not collapse regardless of the level of damage. Damage may be unrepairable. Ductility demands can be greater than those associated with the damage control performance. If the structure does not collapse when subjected to extreme earthquake events, resistance can be expected to decrease with increasing displacements. Collapse prevention performance should only be permitted for unlikely MCE events. Collapse prevention analysis requires a Nonlinear Static Procedure (NSP) or Nonlinear Dynamic Procedure (NDP) in accordance with the guidance in Chapter 6.