Ground Motions and Structures - Earthquake Engineering for Structural Design - page 369

Ground Motions and Structures 369

(a) (b) (c)

Figure 8.61 Regular and irregular vertical configurations; (a) Compact shape;
(b) Set-back configuration; (c) Off-set configuration

From the horizontal structural configuration point of view , it has been already
pointed out the conceptual importance of the ability of a building to vibrate separately
in two directions, without torsional coupling. The symmetric shapes have the ability to
fulfill this demand. The ideal shape is undoubtedly the double symmetrical one (Fig. 8.
62a), because the center of gravity coincides with the center of rigidity, so the
translational and torsional motions are uncoupled. Contrary, it is well known that
asymmetric plan-wise buildings undergo translational as well as torsional coupled
motions during seismic actions and, therefore, they are subjected to non-uniform plan
distribution of damage. Figure 8.62b presents some usual unsymmetrical plans. When
these shapes are proved to be unavoidable, it is desirable to use seismic separations to
divide the building into independent parts, which provide a regular seismic behavior
(Fig. 8.63).

(a)
Figure 8.62 (continues)