Major type of Failures
Lifeline structures
are those which are directly related to the facilities for existence of
society. Nowadays many cities are facing problem to maintain the lifeline
structures after the earthquake event. These structures are very important and
have to be safe.
There
are many lifeline structures among them bridge is also a very important
lifeline structure that provide commuting and also transportation of foods, oils,
water etc. History of bridge failure starts before 1907 when Qbec bridge fail.
There are many failure modes of bridge, main two types of them are:
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| Photo: collapse of cypress street viaduct (http://pubs.usgs.gov/dds/dds-29/web_pages/oakland.html) |
Shear Failure:
When
earthquake event happens at that time due to the shaking of the ground
structure feels the large lateral load depends up on the structure’s self
weight, amplitude of ground acceleration, soil condition etc. Total load it has
to resist as horizontal force so shear force on the column will increase and
across the limit of capacity of the column section. When there is the diagonal
crack on the structure then we can say there is shear failure. Due to the lack
of transverse reinforcement it occurs generally within the ¼ of height from the
ground level.
Bending failure (buckling):
During
earthquake horizontal force acting on the structure, in bridge structure most
of the structural weight is concentrate on the top of column so due to the
shaking and displacement more than usual bending moment will apply to the
section of column. When excessive bending will apply then section cannot bear
and firstly more loads on outer longitudinal reinforcement if load is more than
the capacity of reinforcement then it starts to buckle. Due to lack of the
longitudinal reinforcement and also the transverse tie reinforcement at that
reason this failure occurs. It is also depends on the section and length ratio
or slenderness of the column, column with more slenderness ratio or long column
will fail faster under this phenomenon. Buckling
failure is the main failure for the truss bridge also where when the
compression force on any compression member exceeds the capacity of that member
then it starts to buckle. Most of the truss bridges fail under the buckling
failure mode because steel have good enough tensile strength than compressive
strength.
One mitigation method used to prevent such failures.
There are
many mitigation method used to increase the seismic performance of structures.
For mitigation of concerned failures we have to consider the weak points and
then strengthen on those areas. During design and construction we have to
concentrate on key areas of failure.
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| Strengthening of column |
Shear failure can reduce by providing the sufficient transverse
bars in the area of more shear zone. We can provide the additional transverse
bars on the column by jacketing or by chipping and inserting as like the old
one. We can also reduce the shear failure by increasing the shear area i.e.
cross section. Addition of longitudinal bars also contributes to the shear
strength of column. During the construction we have to care about the
curtailment of the lap bars from the foundation.
Bending failure occurs due to the
insufficient steel reinforcement. Mitigation of such failure is by providing
excessive longitudinal reinforcement either during the construction or by steel
jacketing for the existing structures. For the slender column casing will
provide both longitudinal reinforcement and larger cross section which reduce
the slenderness of the column. Also the link of the columns provides more
strength on bending y sharing of forces. For truss bridges we can provide more
compressive strength by other material on composite section or addition of the
member. We can also reduce the compressive force by linking of the members that
will reduce the member length also reduce the slenderness.
