Ground Motions and Structures - Earthquake Engineering for Structural Design - page 372

372 Earthquake Engineering for Structural Design

Figure 8.65 Soft stories: (a) Soft ground story (pilotis story);
(b) Soft story (Gioncu and Mazzolani, 2002)

The lateral response of these buildings is characterized by a large rotational ductility
demand, which is concentrated at the end sections of the columns of the first story,
while the superstructure behaves like a quasi-rigid body (Mezzi and Parducci, 2005).
All the inelastic deformations are concentrated in the soft story. This structural type was
the cause of many collapsed buildings during the 1999 Kocaeli earthquake (Turkey).
The UBC (1997) defines the soft story when its stiffness is less than 70% of the one of
the story above or less than 80% of the mean stiffness of the three storeys above
(Valmundsson and Nau, 1997, Magliulo et al, 2002). A solution to avoid the failure of
pilotis buildings consists in the introduction of some partial rigid ductile walls in the
first story around the staircase, in order to respect the above conditions.
Massirregularity can be detected by comparing the story weights (Fig. 8.66).
The effective mass is given by the dead load of the structure at each level plus the