Page 188 - Dynamic Loading and Design of Structures

Page 163

Figure 4.16 Irregular configurations in plan (a), (b) and in elevation (c), (d).

●The distribution in plan and/or in elevation of the mass of the structure is not reasonably
uniform.
●The aspect ratio (height to length) of a building is high (more than about 4).
●There are abrupt changes in lateral load resistance along the height of the building (see
Figure 4.16c).
●There are abrupt changes in lateral stiffness along the height of the building, due to
termination of stiff elements (such as walls or heavy partitions) and/or due to the presence
of setbacks; see Figure 4.16c, d.

There are several reasons for avoiding problematic configurations; they have to do with:
●our inability to accurately predict the (inelastic) response of irregular structures subjected to
strong earthquakes;
●the tendency of damage to concentrate in the weakest parts of a structure; this is true,
regardless of whether dynamic or other refined analysis has been used in the design;
●the increased cost required for providing to an irregular structure the same seismic
resistance as in a similar regular structure.
Some of the problems mentioned previously, particularly the ones related to irregularities in
plan, can often be tackled effectively by splitting a building into smaller parts separated by
seismic gaps, so that each individual part becomes a regular structure. Seismic gaps should
account for a substantial part of the anticipated