Cs 12Cv RT23

where Ca = seismic coefficient for velocity dependent (intermediate and long period) structures.

FIGURE 6.2 Contour map of the United States showing effective peak acceleration, Aa. (Source: From Minimum Design Loads for Buildings and Other Structures, ASCE 7-95, American Society of Civil Engineers, Reston, Va., with permission.)

Coefficients CV and Ca are based on the soil profile and are determined as follows. From the descriptions in Table 6.13, determine the soil profile type for the site under consideration. From Fig. 6.2, determine the effective peak acceleration, Aa. Enter Tables 6.14 and 6.15 with Aa and the soil type to find coefficients CV and Ca. For the cases noted in Table 6.14, CV depends upon the effective peak velocity-related acceleration, AV, Fig. 6.3.

The response modification factor, R, depends upon the structural bracing system used as detailed in Table 6.15. The higher the factor, the more energy the system can absorb and hence the lower the design force. For example, ordinary moment frames are assigned a factor of 3 and special moment frames a factor of 8 (see Art. 9.7.1). Note that the forces resulting from the application of these R factors are intended to be used in LRFD design, not at an allowable stress level (see Art. 6.12).

A rigorous evaluation of the fundamental elastic period, T, requires consideration of the intensity of loading and the response of the structure to the loading. To expedite design computations, T may be determined by the following:

TABLE 6.13 Soil Profile Descriptions for Seismic Analysis


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