Geoscience Reference
In-Depth Information
Figure 35. Onagawa — Tank and building overturned; significant subsidence
experienced throughout the area during the earthquake
3.4
Movement
of
Structure
from
Its
Foundation
due
to
Sliding,
Rotation,andOverturning
A structure requires a strong foundation and an adequate anchoring system. In the
absence of either, a tsunami's hydrostatic lateral and buoyant loading as well as
hydrodynamic loading and foundation scouring can result in the structure's
movement off its foundation. When forces exceed the capacity of the anchoring to the
foundation, a structure can fail through sliding, rotation, or overturning. Figures 36 -
39 show examples of structures moved from their foundations.
3.5
Impact
Loads
In addition to hydrostatic and uplift forces, tsunamis subject coastal structures to
impact loads during inundation. Forces include unsteady hydrodynamic drag forces,
impulsive forces, debris impact forces, and debris damming forces. Water flowing
around a structure applies hydrodynamic drag forces to the structure. The drag forces
are a combination of the lateral forces caused by the pressure forces from the moving
mass of water and the friction forces generated as the water flows around the
structure (see figure, next section). Impulsive forces are very short duration loads
caused by the leading edge of a surge of water impinging on a structure. As the surge
passes through a structure, all structural components receive the brunt of impulsive
forces — applied sequentially but not at the same time. Debris impact forces are short
duration loads due to the impact of large floating objects that act as projectiles and
strike individual structural components. Debris damming increases the exposed
surface area and thus increases the hydrodynamic loading on the structure. Figures 40
- 43 show structures subjected to impact loads.
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