Ground Motions and Structures - Earthquake Engineering for Structural Design - page 307

Ground Motions and Structures 307

Figure 8.10 Collapse mechanisms of foundations (after Bhattacharya, 2003)

during the earthquake, having a very complex chaotical movement, characterized by
peaks of acceleration, velocity and displacement, produced at different times. After the
end of the seismic action, the structure continues to move under form of free
oscillations , which depend on its level of damping. For strong damping, the
movements stop very quickly, while for weak damping structure continues to move a
long time after the end of the seismic action. Generally, the maximum values of
movements occur during the forced oscillations, but for short seismic actions (such as
pulse loadings), the maximum values can be reached during free oscillations
The structure movements are characterized by vibration modes (Fig. 8.12), being a
superposing of these modes in function of the participation factor. The vibration modes
are horizontal, vertical and torsional. For horizontal modes, generally the most
important for seismic design, the number of vibration modes depends on the number of
masses. But, in the majority of cases, the first three modes are the most important for
the structural analysis. Which mode is determinant for the structural response depends
on the ground motion and the structure characteristics.