Ground Motions and Structures - Earthquake Engineering for Structural Design - page 371

Ground Motions and Structures 371

Weak story (irregularity in strength) (Fig 8.64) The story strength is the total strength
of all lateral force-resisting elements in a given story for the direction under
consideration. For the shear walls, the strength is the shear capacity of walls. In the
framed structures, the strength is given by the shear capacity corresponding to the
flexural strength of columns. Weak stories are usually found where vertical
discontinuities exist, or where member size or reinforcement has been reduced. UBC
(!997) (Valmundsson and Nau, 1997, Magliulo et al, 2002) defines a weak story when
its strength is less than 80% of the storey above. The weak story effect consists in the
concentration of inelastic activity which can produce the total or partial collapse of the
story itself (WHO, 2006). In order to prevent this situation, an over-strength is
recommended, in such a way that the differences in strength do not exceed 20%.
Soft story (irregularity in stiffness) (Fig. 8.65). The increasing of story stiffness is
given by the non-structural infill panels (masonry, concrete or metal). The soft story is
the one where these infill panels are missing due to functional demands (Fig. 8.64). A
very widespread structural configuration in existing buildings is characterized by the
absence of these infill panels at the ground floor, while they are present at the elevation
stories. This configuration is called pilotis configuration, giving rise to a soft first story,
which allows a good use and distribution of the space at the ground floor, but it is very
dangerous from the seismic point of view.

Figure 8.64 Weak story (after Gioncu and Mazzolani, 2002)