Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 328

Figure 9.1. Poor-quality concrete in a column
9.2.1. Pathologies linked to overall behavior
The seismic loads that cause the most damage are horizontal inertia loads the
structure has not been designed to withstand: permanent loads do less damage
because they are directed downwards. Horizontal loads have to be distributed
between transverse bracings , vertical structures designed to transfer inertial loads
down to the foundations. As far as the overall behavior of any structure is
concerned, two main problems are typically identified:
- the layout of the transverse bracing structures is such that the center of stiffness
is offset from the inertial load resultant, hence the structure is subject to a vertical
axis torsion that tends to overload bracing located on the edges of the building.
Figure 9.2 shows a building block after torsion around a stiff portion, namely the
stairwell. The right hand part, supported by a reinforced concrete column, has
collapsed due to a general rotation imposed on the vertical axis;
- bracing structures do not have any continuous stiffness and geometry along the
height of the building; consequently, at levels where discontinuities occur, seismic
loads have to be transferred to a “diaphragm” horizontal plane. Moreover, since
stiffness can differ significantly between two successive levels, strains are
concentrated in the less rigid storeys. This is evident in Figure 9.3, which shows a
rather stiff villa built on piles that had its lower level destroyed by displacements
imposed by the overall motion.