Transcript The Collapse of Buildings

Matakuliah
Tahun
Versi
: S0182/Studi Kasus Dalam Teknik Sipil
: Juli 2005
: 01/01
Pertemuan 04
Bangunan Tinggi
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Learning Outcomes
Mahasiswa dapat membandingkan kasuskasus yang terjadi dengan berbagai
alternatif yang dipilih  C4
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Outline Materi
• Analisa pemecahan masalah
• Beberapa alternatif pemecahan masalah
• Kasus kegagalan konstruksi yang mungkin
terjadi
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The Collapse of Buildings
• Buildings, like all structures, are designed to support certain loads
without deforming excessively. The loads are the weights of people
and objects, the weight of rain and snow and the pressure of wind-called live loads--and the dead load of the building itself. With
buildings of a few floors, strength generally accompanies sufficent
rigidity, and the design is mainly that of a roof that will keep the
weather out while spanning large open spaces. With tall buildings of
many floors, the roof is a minor matter, and the support of the weight
of the building itself is the main consideration. Like long bridges, tall
buildings are subject to catastrophic collapse.
• The causes of building collapse can be classified under general
headings to facilitate analysis. These headings are:
• Bad Design
• Faulty Construction
• Foundation Failure
• Extraordinary Loads
• Unexpected Failure Modes
• Combination of Causes
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The Collapse of Buildings
• Buildings, like all structures, are designed to support certain loads
without deforming excessively. The loads are the weights of people
and objects, the weight of rain and snow and the pressure of wind-called live loads--and the dead load of the building itself. With
buildings of a few floors, strength generally accompanies sufficent
rigidity, and the design is mainly that of a roof that will keep the
weather out while spanning large open spaces. With tall buildings of
many floors, the roof is a minor matter, and the support of the weight
of the building itself is the main consideration. Like long bridges, tall
buildings are subject to catastrophic collapse.
• The causes of building collapse can be classified under general
headings to facilitate analysis. These headings are:
• Bad Design
• Faulty Construction
• Foundation Failure
• Extraordinary Loads
• Unexpected Failure Modes
• Combination of Causes
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The Collapse of Buildings
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Bad design does not mean only errors of computation, but a failure to take
into account the loads the structure will be called upon to carry, erroneous
theories, reliance on inaccurate data, ignorance of the effects of repeated or
impulsive stresses, and improper choice of materials or misunderstanding of
their properties. The engineer is responsible for these failures, which are
created at the drawing board.
Faulty construction has been the most important cause of structural failure.
The engineer is also at fault here, if inspection has been lax. This includes
the use of salty sand to make concrete, the substitution of inferior steel for
that specified, bad riveting or even improper tightening torque of nuts,
excessive use of the drift pin to make holes line up, bad welds, and other
practices well known to the construction worker.
Even an excellently designed and constructed structure will not stand on a
bad foundation. Although the structure will carry its loads, the earth beneath
it may not. The Leaning Tower of Pisa is a famous example of bad
foundations, but there are many others. The old armory in St. Paul,
Minnesota, sank 20 feet or more into soft clay, but did not collapse. The
displacements due to bad foundations may alter the stress distribution
significantly. This was such a problem with railway bridges in America that
statically-determinate trusses were greatly preferred, since they were not
subject to this danger.
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The Collapse of Buildings
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•
•
Extraordinary loads are often natural, such as repeated heavy snowfalls, or the
shaking of an earthquake, or the winds of a hurricane. A building that is intended to
stand for some years should be able to meet these challenges. A flimsy flexible
structure may avoid destruction in an earthquake, while a solid masonry building
would be destroyed. Earthquakes may cause foundation problems when moist filled
land liquefies.
Unexpected failure modes are the most complex of the reasons for collapse, and we
have recently had a good example. Any new type of structure is subject to
unexpected failure, until its properties are well understood. Suspension bridges
seemed the answer to bridging large gaps. Everything was supported by a strong
cable in tension, a reliable and understood member. However, sad experience
showed that the bridge deck was capable of galloping and twisting without restraint
from the supporting cables. Ellet's bridge at Wheeling collapsed in the 1840's, and the
Tacoma Narrows bridge in the 1940's, from this cause.
The conservative, strong statically-determinate trusses were designed with pinconnected eyebars to be as strong and safe as possible. Sad experience brought the
realization of stress concentration at the holes pierced in the eyebars. From earliest
times, it has been recognized that tension members have no surprises. They fail by
pulling apart when the tension in them becomes too high. If you know the tension,
then proportioning a member is easy. A compression member, a column, is different.
If it is short and squat, it bears its load until it crushes. But if you try to support a load
with a 12-foot column that will just support the load with a 1-foot column, you are in
for a surprise. The column bends outward, or buckles, and the load crashes to earth.
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The Collapse of Buildings
• Suppose you have a beam supported at the ends, with a load in the
center. You know the beam will bend, and if the load is too great, it
may break apart at the bottom, or crush at the top, under the load.
This you expect. However, the beam may fail by splitting into two
beams longitudinally, or shearing, or by the top of the beam
deflecting to one side or the other, also called buckling. In fact, a
beam will usually fail by shearing or buckling before breaking.
• A hollow tube makes a very efficient column or beam. If you think
about it, it is the material on the surface that most resists buckling
and bending. A column that is modified from a compact crosssection, like a cylinder, to an extended cross-section, like a pipe, can
still support the same load per unit area, but with much greater
resistance to buckling. As a beam, one side is in compression and
the other in tension, while the pipe cannot buckle to one side or the
other. When you do bend a pipe, notice that it crushes inward
reducing the cross-section to a line, which bends easily. Tubes need
to be supported against buckling. Such a tube has a very high ratio
of strength to weight, and hence strength to cost.
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The Collapse of Buildings
• Suppose you have a beam supported at the ends, with a load in the
center. You know the beam will bend, and if the load is too great, it
may break apart at the bottom, or crush at the top, under the load.
This you expect. However, the beam may fail by splitting into two
beams longitudinally, or shearing, or by the top of the beam
deflecting to one side or the other, also called buckling. In fact, a
beam will usually fail by shearing or buckling before breaking.
• A hollow tube makes a very efficient column or beam. If you think
about it, it is the material on the surface that most resists buckling
and bending. A column that is modified from a compact crosssection, like a cylinder, to an extended cross-section, like a pipe, can
still support the same load per unit area, but with much greater
resistance to buckling. As a beam, one side is in compression and
the other in tension, while the pipe cannot buckle to one side or the
other. When you do bend a pipe, notice that it crushes inward
reducing the cross-section to a line, which bends easily. Tubes need
to be supported against buckling. Such a tube has a very high ratio
of strength to weight, and hence strength to cost.
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The Collapse of Buildings
• Suppose you have a beam supported at the ends, with a load in the
center. You know the beam will bend, and if the load is too great, it
may break apart at the bottom, or crush at the top, under the load.
This you expect. However, the beam may fail by splitting into two
beams longitudinally, or shearing, or by the top of the beam
deflecting to one side or the other, also called buckling. In fact, a
beam will usually fail by shearing or buckling before breaking.
• A hollow tube makes a very efficient column or beam. If you think
about it, it is the material on the surface that most resists buckling
and bending. A column that is modified from a compact crosssection, like a cylinder, to an extended cross-section, like a pipe, can
still support the same load per unit area, but with much greater
resistance to buckling. As a beam, one side is in compression and
the other in tension, while the pipe cannot buckle to one side or the
other. When you do bend a pipe, notice that it crushes inward
reducing the cross-section to a line, which bends easily. Tubes need
to be supported against buckling. Such a tube has a very high ratio
of strength to weight, and hence strength to cost.
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Why the World Trade Center
Buildings Collapsed
A Fire Chief ’s Assessment
• World Trade Center tower construction
• In terms of structural system the twin towers departed completely
from other high-rise buildings. Conventional skyscrapers since the
19th century have been built with a skeleton of interior supporting
columns that supports the structure. Exterior walls of glass steel or
synthetic material do not carry any load. The Twin towers are
radically different in structural design as the exterior wall is used as
the load-bearing wall. (A load bearing wall supports the weight of the
floors.) The only interior columns are located in the core area, which
contains the elevators. The outer wall carries the building vertical
loads and provides the entire resistance to wind. The wall consists
of closely spaced vertical columns (21 columns 10 feet apart) tied
together by horizontal spandrel beams that girdle the tower at every
floor. On the inside of the structure the floor sections consist of
trusses spanning from the core to the outer wall.
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Why the World Trade Center
Buildings Collapsed
A Fire Chief ’s Assessment
• Bearing walls and Open floor design
• When the jet liners crashed into the towers based upon knowledge
of the tower construction and high-rise firefighting experience the
following happened: First the plane broke through the tubular steelbearing wall. This started the building failure. Next the exploding,
disintegrating, 185-ton jet plane slid across an open office floor area
and severed many of the steel interior columns in the center core
area. Plane parts also crashed through the plasterboard-enclosed
stairways, cutting off the exits from the upper floors. The jet
collapsed the ceilings and scraped most of the spray-on fire
retarding asbestos from the steel trusses. The steel truss floor
supports probably started to fail quickly from the flames and the
center steel supporting columns severed by plane parts heated by
the flames began to buckle, sag, warp and fail. Then the top part of
the tower crashed down on the lower portion of the structure. This
pancake collapse triggered the entire cascading collapse of the 110story structure
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Why the World Trade Center
Buildings Collapsed
A Fire Chief ’s Assessment
•
Steel Framing
The most noticeable change in the modern high-rise construction is a
trend to using more steel and shaping lightweight steel into tubes, curves,
and angles to increase its load bearing capability. The WTC has tubular
steel bearing walls, fluted corrugated steel flooring and bent bar steel truss
floor supports. To a modern high rise building designer steel framing is
economical and concrete is a costly material. For a high-rise structural
frame: columns, girders, floors and walls, steel provides greater strength per
pound than concrete. Concrete is heavy. Concrete creates excessive
weight in the structure of a building. Architects, designers , and builders all
know if you remove concrete from a structure you have a building that
weights less. So if you create a lighter building you can use columns,
girders and beams of smaller dimensions, or better yet you can use the
same size steel framing and build a taller structure. In News York City where
space is limited you must build high. The trend over the past half-century is
to create lightweight high buildings.
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Why the World Trade Center
Buildings Collapsed
A Fire Chief ’s Assessment
• Steel Framing
To do this you use thin steel bent bar truss construction
instead of solid steel beams. To do this you use hollow
tube steel bearing walls, and curved sheet steel
(corrugated) under floors. To do this you eliminate as
much concrete from the structure as you can and
replace it with steel. Lightweight construction means
economy. It means building more with less. If you reduce
the structure’s mass you can build cheaper and builder
higher. Unfortunately unprotected steel warps, melts,
sags and collapses when heated to normal fire
temperatures about 1100 to 1200 degrees F.
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Why the World Trade Center
Buildings Collapsed
A Fire Chief ’s Assessment
• Steel Framing
The fire service believes there is a direct relation of fire resistance to
mass of structure. The more mass the more fire resistance. The
best fire resistive building in America is a concrete structure. The
structures that limit and confine fires best, and suffer fewer collapses
are reinforced concrete pre WWII buildings such as housing projects
and older high rise buildings like the empire state building, The more
concrete, the more fire resistance; and the more concrete the less
probability of total collapse. The evolution of high- rise construction
can be seen, by comparing the empire state building to the WTC.
My estimate is the ratio of concrete to steel in the empire state
building is 60/40. The ratio of concrete to steel in the WTC is 40/60.
The tallest building in the world, the Petronas Towers, in Kula
Lumpur, Malaysia, is more like the concrete to steel ratio of the
empire state building than concrete to steel ratio of the WTC.
Donald Trump in New York City has constructed the tallest
reinforced concrete high-rise residence building.
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Recommendations for constructing the
new high rise buildings on ground zero
• The steel columns, girders and floor beams should be
encased in masonry or other more effective fire
retarding material. Spray-on fire retarding is ineffective.
Post fire investigations reveals the spray on fire retardant
has scaled off and steel beams and concrete and steel
floor slabs crack and allow flame spread.
•
• Lightweight bar joists should not be used to support
floors in high-rise buildings. The National Fire Protection
Association has shown unprotected steel bar joist fail
after five or ten minutes of fire exposure.
•
• For life safety in high-rise buildings bring back the
smoke proof tower. This allows people to escape fire
using smoke free stairways.
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Recommendations for constructing the new high
rise buildings on ground zero
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Stairs and elevator shaft ways should be enclosed in masonry to prevent
smoke spread.
Heating ventilation and air condition HVAC systems should be provided by
unit system serving only one or two floors. Central air system serving 10 or
20 floors creates shaft ways and duct systems that penetrate fire rated
floors walls partitions and ceilings. Smoke spreads throughout ducts of
central HVAC systems.
The high rise building framework should be skeleton steel framing not
center core steel column framing. There should be no bearing wall high rise
construction. Reduce the size of open floor design.
Increase the thickness of concrete in floor construction. The two or three
inches of concrete over corrugated steel fails during most serious high rise
fires and must be replaced.
Automatic sprinklers should protect all high rise buildings. Firefighters can
extinguish approximately 2,500 square foot of fire with one hose line. Two
hose steams may quench 5,000 square feet of fire. The World Trade Center
floor areas were 40,000 square feet in area.
Federal, State and Port Authority buildings should comply with New York
City building codes and actually in some cases should exceed them.
Remember building codes are only minimum standards.
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