Building Rigidity and Interstory Drift
Buildings need to control movements that occur in the structure to prevent damage to the main elements as well as non-structural elements such as curtain walls and interior partitions. An important parameter used by the Chilean National Code NCh433.Of96  (hereafter referenced as NCh433) to meet this objective is the interstory drift where restrictions are imposed for torsional effects and to overall building lateral displacements and interstory drifts. Section 5.9.2 limits displacement between centers of gravity to two per thousand of the interstory height and Section 5.9.3 limits the incremental displacement of any point on the floor of the building with respect to the centers of mass, to one per thousand of that height for a total allowed displacement of three per thousand.
Regarding these drift restrictions, Guendelman et al. presented the study “Perfil Bio-Sismico de Edificios/Bio-seismic Profile of Buildings" . This study performed a seismic review of reinforced concrete buildings which included 2262 actual buildings constructed in Chile. The study established a set of seismic indicators, whose values are compared with those considered
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Height (H) and Period (T)
FIG. 10—Distribution of rigidity ratio H/T verified in 2262 buildings in Chile .
satisfactory according to general experience. Among the several indicators noted in the study, two are specifically interesting to understand the rigidity and deformation characteristics of buildings in Chile:
(1) Total height/translational first mode period (H/T) index. This index, with dimensions of speed, has been considered a good estimate of the rigidity of a building. In general terms, it may be noted that values of H/T between 20 and 40 m/s identify flexible buildings, between 40 and 70 those of normal stiffness, and 70, to a maximum of 150, the rigid ones. Figure 10 shows all 2262 build rigidities superimposed.
(2) Maximum interstory drift: Shown in Fig. 11 is the frequency distribution of the 2622 buildings reviewed. From Figs. 10 and 11, Guendelman et al.  found that a large majority of concrete buildings in Chile are more rigid when compared to other similar buildings in other countries. Furthermore, the Chilean code imposes demanding interstory drift restrictions. Both of these items likely contributed to the successful performance of those facades reviewed after this event.