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 sufficient 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
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.
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 pin-connected 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.
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 cross-section,
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.
Tall buildings have generally been made with a rigid steel skeleton,
sheathed in the lightest materials to keep out the weather.
Alternatively, reinforced concrete, where the compression-resisting and
protecting concrete surrounds the tough, tension-resisting steel,
integrated into a single body, has been used. Such structures have never
failed (when properly built on good foundations), and stoutly resist
demolition. When the lower supports of a steel skeleton are destroyed,
the weight of the building seems to crush the lower parts and the upper
parts descend slowly into the pile of debris. Monolithic
reinforced-concrete buildings are diffcult to demolish in any fashion.
The World Trade Center towers used neither a steel skeleton nor
reinforced concrete. They were designed as square tubes made of heavy,
hollow welded sections, braced against buckling by the building floors.
Massive foundations descended to bedrock, since the towers had to be
safe against winds and other lateral forces tending to overturn them.
All this was taken into consideration in the design and construction,
which seems to have been first-rate. An attempt to damage the buildings
by a bomb at the base had negligible effect. The strong base and
foundation would repel any such assault with ease, as it indeed did. The
impact of aircraft on the upper stories had only a local effect, and
did not impair the integrity of the buildings, which remained solid. The
fires caused weakening of the steel, and some of the floors suddenly
received a load for which they were not designed.
What happened next was unexpected and catastrophic. The slumped
floors pushed the steel modules outwards, separating them from the floor
beams. The next floor then collapsed on the one below, pushing out the
steel walls, and this continued, in the same way that a house of cards
collapses. The debris of concrete facing and steel modules fell in
shower while the main structure collapsed at almost the same rate. In 15
seconds or so, 110 stories were reduced to a pile 9 stories high,
mainly of steel wall modules and whatever was around them. The south
tower collapsed 47 minutes after impact, the north tower 1 hour 44
minutes after impact. The elapsed times show that the impacts were not
the proximate cause of collapse; the strong building easily withstood
them. When even one corner of a floor was weakened and fell, the
collapse would soon propagate around the circumference, and the building
would be lost.
It is clear that buildings built in this manner have a catastrophic
mode of failure ("house of cards") that should rule out their future
construction. It is triggered when there is a partial collapse at any
level that breaks the continuity of the tube, which then rolls up
quickly, from top to bottom. The collapse has a means of propagation
that soon involves the whole structure, bypassing its major strengths
and impossible to interrupt. There is no need for an airliner; a simple
explosion would do the job. There were central tubes in the towers, for
elevators and services, but they appeared to play no substantial role in
the collapse, and were not evident in the pictures or wreckage.
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