During an event of earthquake, many different type of building failures were observed in the past. So, it is very much important to understand, building failure patterns and reasons of that to reduce the building failure ratio by improving it against future seismic events.
It is quite important to understand what is pounding effect, reasons of occurrence and types of it, remedial measures to avoid it and some capturing of past failures.
Among the possible structural damages, seismic induced Pounding is one of the main causes of severe building damages and commonly observed in several earthquakes. The non-structural damage involves, pounding or movement across separation joints, between adjacent structures.
What is Seismic Pounding?
The Seismic Pounding is simply
defined as, collision of buildings constructed in close proximity with each
other.
Why Seismic Pounding occur?
The principle reason for the
seismic pounding is;
- Insufficient separation in between the
adjacent buildings.
It may also be the result of a
combination of many other factors such as;
- Different
dynamic characteristics of adjacent structures,
- The unexpected
severity of the ground motion,
- Non-compliance with code
provisions; particularly for lateral and torsional stiffness due to inadequate
building configuration and structural framing system, and
- Cumulative
tilting due to foundation movement.
This phenomenon is mostly
observed in the old buildings that were constructed before the advent and
popularity of earthquake resistant design principles.
Type of damage occur due to
Seismic Pounding?
Two types of pounding damage
can occur;
1. Local damage at the point of
impact
2. Global damage; resulting
from the energy and momentum transfer; caused by collision.
Local damage is caused by the
collision force while global damage depends on the dynamic properties of both
buildings at the time of collision. This type of damage arises when buildings
are built without separation right up to property lines in order to make
maximum use of the space. When floor of these buildings are constructed of the
same height, damage due to pounding usually is not serious. If this is not the
case, there are two problems.
When the floors of adjacent buildings are at
different elevations, the floor of each structure can act like rams, battering
the columns of the other building. When one of the buildings is higher than the
other, the lower building can act as a base for the upper part of the higher
building; the lower building receives an unexpected large lateral load while
the higher building suffers from a major stiffness discontinuity at the level
of the top of the lower building.
What is Separation joint /
Seismic gap ?
A separation joint / Seismic
gap is the distance between two different building structures - often two wings
of the same facility - that allows the structures to move independently of one
another.
Codal provisions?
Although many current codes
specify a minimum seismic gap; but, it is still inadequate as codes necessarily
lag behind the current research and fail to include the effect of other
parameters that affect the structural deformation. Seismic codes and
regulations worldwide specify minimum separation distances to be provided
between adjacent buildings, to prevent pounding, which is obviously equal to
the relative displacement demand of the two potentially colliding structural
systems.
History of Pounding Effect:
Past seismic codes did not give
definite guidelines to prevent pounding, due to economic considerations
including maximum and usage requirements, especially in the high density
populated areas of cities, there are many buildings worldwide which are already
built in contact or extremely close to another, that could suffer pounding
damage in future earthquakes.
Several example of building
damage have been observed due to pounding in past earthquakes (Watch Video).
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