A linear time history analysis (LTHA) shall consist of an analysis of a linear
mathematical model of the structure to determine its response, through direct
numerical integration of the differential equations of motion, to a number of
ground motion acceleration time histories compatible with the design response
spectrum for the site. The analysis shall be performed in accordance with the
provisions of this Section. For the purposes of analysis, the structure shall be
permitted to be considered to be fixed at the base or, alternatively, it shall be
permitted to use realistic assumptions with regard to the stiffness of
foundations. The acceleration time history (ground motion) is applied at the
base of the structure. The advantage of this procedure is that the time
dependent behavior of the structural response is obtained.
Three-dimensional analysis: Where three-dimensional analysis is performed,
ground motions shall consist of pairs of appropriate horizontal ground motion
acceleration time histories (in two orthogonal horizontal directions) that shall
be selected and scaled from individual recorded events. Appropriate ground
motions shall be selected from events having magnitudes, fault distance, and
source mechanisms that are consistent with those that control the maximum
considered earthquake. Where the required number of recorded ground motion
pairs are not available, appropriate simulated ground motion pairs shall be used
to make up the total number required. For each pair of horizontal ground
motion components, an SRSS spectrum shall be constructed by taking the
square root of the sum of the squares of the five-percent-damped response
spectra for the components (where an identical scale factor is applied to both
components of a pair). Each pair of motions shall be scaled such that for each
period between 0.2T and 1.5T (where T is the natural period of the fundamental
mode of the structure) the average of the SRSS spectra from all horizontal
component pairs is not less than 1.3 times the corresponding ordinate of the
design response spectrum, determined in accordance with Sec 2.5.4.3
