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Behavior of Columns During Earthquakes |
The behavior of columns during earthquakes is very important since column
failures may lead to additional structural failures and result in total building collapses.
For example, in the Mexico City earthquake, the most frequent observed cause of structural
failure was linked to inadequate beam-to-column and slab-to-column connections. Building
configurations may cause columns to be over stressed. Columns of unequal length result
in unequal load distribution and hence failure. Column construction and underlying
soils also are factors in column failures. The slides in this set illustrate these
and a variety of other column failure types.A column is a supporting pillar consisting
of a base, a cylindrical shaft and a capital. These columns may be made of welded
steel or reinforced concrete in which steel rods (rebar) are imbedded in the concrete.
These steel rods may be tied together with metal strips at intervals (ties), or they
may be wrapped spirally around the column with metal strips (spirally wrapped). All
the metal reinforcing is embedded in the concrete. The Uniform Building Code requires
that columns be confined by such spirals or ties. A spirally-confined column is basically
more ductile than a tied column, but in lateral motion this ductility can only be
achieved if ductility also exists at the column-to-beam connection at the top of the
column.Shear is a condition caused by forces that tend to produce an opposite but
parallel sliding motion of the body's planes. A shear wall is a wall in a building
designed to absorb these forces. A soft story is a story in a building (often the
first story) with few partitions and/or open exterior walls (such as garage doors,
large windows, etc.). This openness makes it vulnerable to earthquake damage. Sometimes
the configuration of the building causes the columns to be over stressed, and when
they fail, the building fails. This was the case with the Olive View Hospital, which
was severely damaged in the 1971 San Fernando California earthquake. The first two
floors were soft stories in which the principal support for the floors above were
columns. When earthquake shaking caused the upper floors to move as a unit as much
as two and a half feet, the first and second floor columns could not accommodate such
a large displacement, and they failed. The result was that the entire building had
to be demolished. Another building configuration that resulted in column failure is
shown in photos 7 and 8, the Imperial County Services Building. During the 1979 El
Centro, California, earthquake, the building was destroyed due to column shortening
by compression at the east end of the building.Short columns are often damaged in
earthquakes. A column twice as long is eight times more flexible. If the structure
contains both short and long columns, the load will be concentrated in the shorter
columns. The short columns are less subject to buckling and hence are capable of receiving
high vertical loads. But under lateral loading (forces from the side), the short,
stiff columns receive more than their share of the load, and fail. The condition may
be avoided by equalizing the length and hence the stiffness of the columns. Columns
and piers may be more resistant to failure in earthquakes if they are designed to
be more shear-resistant. Extra column ties or spiral wraps should be provided in the
end sections of all columns and in the beam-to-column connections. The Uniform Building
Code (UBC), published by the International Conference of the Building Officials, gives
requirements used in the design of buildings within the United States. The UBC contains
design guidelines for earthquake- resistant features in buildings. The Code is revised
every three years, incorporating new information gleaned from building failures such
as those shown in this slide set. About the Earthquakes - Several earthquake events
are represented by these slides of column damage. The statistics for these earthquakes
follow: San Fernando, California. At 14:00:41.8 UTC (6:01 A.M. local time) on February
9, 1971, a Ms 6.5 earthquake occurred twenty miles north of downtown Los Angeles (34
deg 25' N, 118 deg 24' W) at a depth of 8.4 km (5 miles). The San Fernando earthquake
was felt over an area of 80,000 square miles. The statistics include 58 dead and $497.8
million in damage. Esmeraldos, Ecuador. A Ms 6.7 earthquake occurred in Esmeraldos,
Ecuador, on April 9, 1976, 2:08 A.M., causing 10 deaths and moderate damage.El Centro,
California. A Ms 6.8 earthquake occurred on October 15, 1979, 3:16 P.M., in El Centro,
California. The earthquake caused $30 million in property damage and injured 91 people.Mexico
City, Mexico. On September 19, 1985, 7:17 A.M. local time, a Ms 8.1 earthquake occurred
on the Pacific coast of Mexico. However, the damage was concentrated in a 25 square
kilometer area of Mexico City, 350 km northeast of the epicenter. Within a population
of 18 million effected, an estimated 10,000 people were killed, and 50,000 were injured.
In addition, 250,000 people lost their homes, over 800 buildings crumbled, and property
damage amounted to more than $5 billion.San Salvador, El Salvador. A Ms 5.4 earthquake
occurred in San Salvador, El Salvador, on October 10, 1986, 11:49 A.M., killing 1,000
and causing $1.5 billion in damage.Whittier, California. A Ms 6.0 earthquake occurred
on October 1, 1987 at 6:42 A.M. in Whittier, California. The earthquake caused $358
million in property damage and eight deaths.Loma Prieta, California. On October 17,
1989, 5:04 P.M., a Ms 7.1 earthquake occurred in the Santa Cruz mountains of California.
The Loma Prieta earthquake resulted in more than $7 billion in property damage ($2.5
billion in San Francisco alone). The earthquake destroyed 414 single-family homes,
104 mobile homes, 97 businesses and three public buildings. It damaged 2,575 businesses
and 18,306 houses, displacing 12,000 people from their homes. It caused 67 deaths
and 3,757 injuries. Northridge, California. On Monday January 17, 1994 at 4:31 A.M.,
a Ms 6.8 earthquake occurred twenty miles west- northwest of downtown Los Angeles
(34 deg 13' N, 118 deg 32'W) at a depth of 20 km (12 miles). Northridge statistics
include 56 dead, 25,000 dwellings uninhabitable, $15-$20 billion in damage.
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