Rt

where

T = fundamental period of the structure in the direction under consideration - = seismic importance factor Cv = a numerical coefficient dependent on the soil conditions at the site and the seismicity of the region, as set forth in Table 2.5 (UBC Table 16-R) W = seismic dead load

R = a factor that accounts for the ductility and overstrength of the structural system, as set forth in Table 2.4 (UBC Table 16-N) Z = seismic zone factor, as set forth in Table 2.3a (UBC Table 16-I). Note that Z does not directly appear in the base shear formula. It does, however, affect the seismic coefficients C„ and Cv.

Figure 2.34. Irregularity due to out-of-plane offsets of vertical elements. This irregularity exists when there is discontinuity in the lateral-load-resisting path. In this figure, the columns and other elements such as the transfer girder and the cantilever girder support a discontinuous wall or a frame-column. They must be designed for special seismic load combination given in the 1997 UBC and ASCE 7-02.

Figure 2.34. Irregularity due to out-of-plane offsets of vertical elements. This irregularity exists when there is discontinuity in the lateral-load-resisting path. In this figure, the columns and other elements such as the transfer girder and the cantilever girder support a discontinuous wall or a frame-column. They must be designed for special seismic load combination given in the 1997 UBC and ASCE 7-02.

Since collector elements of diaphragms and columns supporting stiff elements are particularly vulnerable to earthquake damage, these elements must be designed for the estimated maximum axial forces that can realistically develop in these elements. Design of the diaphragm itself is not addressed in seismic codes. Typically it is designed for the above-load combinations or as a special reinforced concrete shear wall.

Figure 2.35. Irregularity due to nonparallel system. A nonparallel system irregularity exists when a lateral system is not parallel to or symmetric about the building's orthogonal axes.

TABLE 2.2 Vertical Irregularities

Irregularity type and definition

Reference fig.

Requirement

1. Stiffness irregularity — Soft story Fig. 2.26 A soft story is one in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.

2. Weight (mass) irregularity Fig. 2.27 Mass irregularity shall be considered to exist where the effective mass of any story is more than 150% of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.

3. Vertical geometric irregularity Fig. 2.28 Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral-force-resisting system in any story is more than 130% of that in an adjacent story. One-story penthouses need not be considered.

4. In-plane discontinuity in vertical lateral- Fig. 2.29 force-resisting element

An in-plane offset of the lateral-load-resisting elements is greater than the length of those elements.

5. Discontinuity in capacity—weak story Fig. 2.30 A weak story is one in which the story strength is less than 80% of that in the story above. The story strength is the total strength of all seismic-resisting elements sharing the story shear for the direction under consideration.

No penalty. Use dynamic analysis to determine lateral-force distribution.

No penalty. Use dynamic analysis to determine lateral-force distribution.

No penalty. Use dynamic analysis to determine lateral-force distribution.

Use special seismic load combinations for members below discontinuity.

Increase seismic loads for members below discontinuity by a factor = Q„.

The base shear as specified by Eq. (2.3) is subject to three limits: 1. The design base shear need not exceed

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