Building Irregularities

The impact of irregularities in estimating seismic force levels, first introduced into the Uniform Building Code in 1973, long remained a matter of engineering judgment. Beginning in 1988, however, some configuration parameters have been quantified to establish the condition of irregularity, and specific analytical treatments have been mandated to address these conditions.

Typical building configuration deficiencies include an irregular geometry, a weakness in a story, a concentration of mass, or a discontinuity in the lateral-force-resisting system. Although these are evaluated separately, they are related and may occur simultaneously. For example, a tall first story can be a soft story, a weak story, or both, depending on its stiffness and strength relative to those above.

The 1997 UBC quantifies the idea of irregularity by defining geometrically or by use of dimensional ratios the points at which the specific irregularity becomes an issue requiring remedial measures (see Figs. 2.26 through 2.35). It should be noted that not all irregularities

Figure 2.26. Stiffness irregularity; soft story. Soft story exists if at any given story;

1. The story stiffness is <70% of that of the story above

2. The story stiffness < 80% of the average stiffness of the three stories above

(Use story drifts for stiffness comparison.)

Figure 2.27. Mass irregularity. Mass irregularity exists if mass of any story is more than 150% of the mass of the story below or above. M2> 1.5M1 or 1.5M3.

require remedial measures. Some, such as stiffness, mass, and geometric irregularities, may be accounted for by performing dynamic analysis. See Tables 2.2 and 2.3.

The irregularities are divided into two broad categories: 1) vertical; and 2) plan irregularities. Vertical irregularities include soft or weak stories, large changes in mass from floor to floor, and large discontinuities in the dimensions or in-plane locations of lateral-load-resisting elements. Buildings with plan irregularities include those that undergo substantial torsion when subjected to seismic loads or have reentrant corners, discontinuities in floor diaphragms, discontinuity in the lateral force path, or lateral-load-resisting elements that are not parallel to each other or to the axes of the building. The definitions of these irregularities are found in Tables 2.2 and 2.3.

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