HENRY PETROSKI, [email protected]
ALEKSANDAR S. VESIC
Duke University, P.O. Box 90287, Durham, NC 27708 U.S.A.
STRUCTURAL FAILURE: A HISTORICAL PERSPECTIVE
AWARIE BUDOWLANE – PERSPEKTYWA HISTORYCZNA
Abstract For as long as there has been construction, there have been structural failures. Why this is so
will be explored in this talk. Common root causes for failures that have occurred centuries apart will be
discussed. Case studies of failures from ancient times through the present will be employed as illustrative
examples. It will be demonstrated that in spite of great advances in building technology, modern failures
attributable to ancient causes continue to occur. Putting failure in a broad historical perspective will be
proposed as a means of reducing the occurrence of failure in the future.
Streszczenie Awarie budowlane istnieją od zarania działalności budowlanej. Źródła tego stanu rzeczy
są przedstawione w tym wystąpieniu. Wspólne przyczyny awarii oddzielonych od siebie wiekami
są poddane dyskusji. Historie indywidualnych katastrof są uŜyte jako ilustracje omawianych zagadnień.
Odczyt ten postuluje, Ŝe mimo wielkiego postępu technologii budowlanej, wciąŜ pojawia się łączność
między awariami współczesnymi i tymi z zamierzchłej historii. Analiza awarii w szerokim kontekście
perspektywy historycznej jest proponowana tutaj jako sposób na zmniejszenie prawdopodobieństwa
awarii w przyszłości.
1. Introduction
The history of structural engineering is often presented as a succession of outstanding
achievements, with record-setting structures marking ever-forward progress. But the reality
is that the history is punctuated by landmark failures, which have interrupted the monotonic
flow of improvement that is generally seen as being embodied in longer, taller, and more
efficient structures. While the failures may present temporary setbacks in the advancement of
structural knowledge and accomplishment, ultimately it is precisely those failures that enable
the field to rediscover its roots and get back on track with renewed clarity of purpose and an
improved understanding of its limitations.
The earliest structures may have been suggested to the primitive engineer by the chance
discovery of a tree fallen across a stream or a cave carved out of a hillside. Such a found
bridge or shelter would naturally have its shortcomings, and it seems to be in the nature of
engineers to improve the world as we find it. A tree trunk used as a bridge presents numerous
shortcomings: it may not be in a convenient position; its rounded surface allows it to roll out
of position and requires the user to cross over it carefully; it will eventually deteriorate and
become weakened. Such present and anticipated failures are what must have driven the
earliest of engineers to seek to make a better bridge. This could have been done simply by
repositioning the fallen tree so that it provided a more convenient path across the stream.
To make for a more secure bridge, rocks and stones could have been wedged against it to
keep it from rolling sideways. To have a longer lasting bridge, a less rotted tree trunk could
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have been moved into place. Such deliberate actions are what we know as design, and the
products of design are new structures.
The nature of design is timeless, in that the mental process whereby we conceive and
develop a structure is fundamentally the same today as it was millennia ago. The truth of this
is evident when we read the oldest surviving works on engineering. By inference, the nature
of design in ancient times was not fundamentally different from what it was in prehistoric
times, when primitive engineers moved tree trunks to build better bridges. Our store
of experience and knowledge have grown since prehistoric and ancient times, of course, and
we have developed analytical and computational tools that enable us to conceive of and build
structures of which our distant ancestors could hardly have dreamed. Nevertheless, our
advances are rooted in the same timeless mental processes that characterize design. Among
the characteristics of those processes are that they are imperfect and can be misleading, and
thus they sometimes lead to ill-conceived structures that end in failure. This has always been
the case, and there is no reason to think that it will not continue to be the case in the future.
2. Ancient Engineering
Egyptian engineering achievement is epitomized by the Pyramids. It remains debatable
how these massive piles of stones with sophisticated internal structures were built, but it
cannot be doubted that they were designed. Indeed, it is often said that the earliest engineer
who is known by name is the Pyramid builder Imhotep, who flourished in the 27th century
BC. When Imhotep was charged with constructing a tomb for Pharaoh Zoser, he looked to
improve upon the traditional mastaba, which was a relatively low, rectangular burial
structure whose brick walls sloped inward as they rose. Among the improvements advanced
by Imhotep were to build higher and to use stone facing to make what would be called
a stepped pyramid.
Imhotep’s achievement served as a successful structure upon which subsequent engineers
could elaborate and improve. Among the first improvements was to fill in the steps and thus
produce a pyramid with straight rather than serrated edges. Further improvement was
achieved by increasing the angle at which the sides were inclined, thereby producing a more
sharply pointed structure. This aesthetic was achieved in the Meidum pyramid, but after
a few centuries its outer facing failed, leaving exposed its underlying stepped structure.
In the meantime, a pyramid under construction at Dahshur was rising with sides inclined at
an unprecedented 54° angle to the horizontal. Whether driven by actual experience with the
pyramid or just the fear that its steep sides were too daring, at approximately half its final
height the angle of the Dahshur pyramid’s sides was reduced to 45°, thereby causing it to be
known as the Bent Pyramid. In ancient Egypt, as in the modern world, success prompts
structural experimentation to the point of failure.
The ancient Romans also built in stone, of course, but their legacy is characterized more
by rounded than by pointed structures. The rules of thumb governing the proportions of arch
span to pier width for the classic semicircular Roman arch bridge are believed to have been
arrived at through trial and error. Then, as now, it was design that drove trial and failure that
exposed error. The Romans also designed and constructed domes, the most well-known one
being the open concrete dome of the Pantheon. Like the Egyptian pyramids, exactly how this
structure was built remains a topic of some debate, and the function of the stepped rings that
encircle its base continue to be speculated upon. The most convincing explanation for the
rings is that they add mass around the dome’s periphery and thereby buttress its base against
spreading out and cracking, a problem that has plagued masonry domes throughout history.
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The ancient Greeks also built in stone, and they tested the limits of its strength in tension.
Greek architecture relied heavily upon post-and-lintel construction, as epitomized in the
serial columns and architraves of its temples. Vitruvius discusses the design of such temples
in considerable detail, cataloging five classes, each of which he distinguished by the spacing
of the columns relative to their thickness. In discussing the aesthetic and practical advantages
and disadvantages of each of the styles, Vitruvius pointed out that stone or marble
architraves could be used only when the spacing between the columns upon which they rest
was no more than three column widths. When the columns were this far apart, he warned, the
architraves might break. To design a temple with a wider column spacing, Vitruvius asserted,
wooden beams rather than stone or marble architraves were to be used to span the distance
between the columns. In other words, there was a clear recognition that it was the possibility
of failure that defined the limits of structure and architecture and produced the classic look of
Greek temples.
3. Medieval Cathedrals
In the Middle Ages, geometric considerations continued to govern design, just as they
had in ancient times. Vitruvius remained the standard work on architecture and engineering,
and his anthropomorphic rules for proportioning columns were made explicit in the
caryatids. The architectural design of medieval cathedrals was based on geometric principles
rather than on structural ones. The aesthetic of taller columns, higher naves, and increasing
expanses of glass drove the evolution of structure. Considerations of strength and stiffness
generally arose only when a problem surfaced, as it did when the choir and vaulting of
Beauvais Cathedral collapsed in 1284 and defined structural limits.
As Gothic cathedrals had been designed and built higher and lighter, the effects of wind
pressure on their great expanses of wall, window, and roof began to reveal themselves.
Spaces opened up between stones and there naturally arose concerns about failure. Flying
buttress were introduced to counter the effects of the wind, bracing the cathedral against the
wind in the way a man might lean against a heavy piece of timber to raise it to the vertical.
What began as structural retrofits against failure became architectural icons that now
characterize a genre.
4. Renaissance Engineering
The primacy of geometry in design, which Leonardo symbolized in his famous drawing
of Vitruvian man, continued into the Renaissance. What also continued into the Renaissance
was the occurrence of structural failure. When Galileo was forbidden to write further about
the heavens, he turned his attention to more mundane things, including the strength of
materials. He opens his Dialogues Concerning Two New Sciences by pointing out the
limitations of Renaissance engineering. He did so by focusing on failures.
As had been the case since ancient times, moving an obelisk was fraught with danger,
Galileo reminded his readers. If the monolith were not handled carefully, there was the
danger that it could break before it could be erected onto its base. The elaborate precautions
that accompanied the moving of the Vatican obelisk in 1586 provide a prime example of the
difficulty and delicacy of the task. Galileo further reminded his readers that large ships were
also subject to breaking spontaneously upon being launched. That obelisks and ships could
break apart was clear to everyone, but why such things happened remained a mystery.
If smaller obelisks and ships were structurally robust, why were larger versions so subject
to failure? The design of these structures followed strict geometric rules, but when they
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reached a certain size they obviously could not hold themselves together. Galileo pointed out
that designs in nature did not follow geometric rules as closely as Renaissance engineers and
architects did. Corresponding bones of two animals, one three times as long as the other, did
not maintain that same proportion in their cross dimension. Natural designs, Galileo
concluded, took account of something in addition to geometry. That additional factor, he
believed, was the strength of the material involved.
Fig. 1. Corresponding bones in animals whose sizes are in the ratio 1:3
Galileo incorporated the strength of the material in his brilliant analysis of the cantilever
beam. Using a lever principle, he equated the moments of the weight loading the beam at its
end and of the internal force keeping the beam from breaking away from the wall. If the
breaking strength were determined by some independent tensile test, to use modern
terminology, then Galileo’s resulting “formula” would predict the weight that could be
supported at the end of a beam of specified dimensions. Conversely, as a design tool, it
would provide the relationship among the weight supported, the dimensions of the beam, and
the strength of the material of which it was made. Furthermore, Galileo realized, solving the
problem of the cantilever enabled him to explain the behavior of other kinds of beams, such
as simply supported ones. Galileo’s analytical approach was brilliant, befitting the genius
that he was.
Fig. 2. Galileo’s cantilever beam
However, even geniuses can make a mistake, which can lead to failure. Galileo’s analysis
of the cantilever beam effectively yielded what we would today term a design formula of
great utility. His result was applicable to the design of beams of all sorts, including water
piping. The tendency then as now, however, was to apply a factor of safety to a new and
untested result and to progressively reduce that factor of safety as successful results stemming from its use were experienced. In the case of Galileo’s result for the cantilever beam,
when the factor of safety was reduced to about 3, pipes designed according to it began to fail.
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What had happened was that Galileo’s analysis was flawed by an oversimplifying assumption. He had assumed, again in modern terminology, that the stress at the wall was uniformly distributed throughout the entire cross section of the beam. This caused his result to be
off by a factor of 3, which explains why pipes designed with that safety factor began to fail.
It is ironic that Galileo made the error that he did, for in his introductory discussion
leading up to his analysis of the cantilever beam he describes a failure that resulted from
making a wrong assumption. The situation that Galileo described involved a long piece of
marble that was resting on two supports, one near each end, in a storage yard. According to
Galileo, an astute observer saw the marble beam as analogous to an obelisk being moved or a
ship being launched, and so worried that it could break spontaneously. This concern was
dealt with by adding a third support under the middle of the beam, thereby obviating its
collapse. Everyone involved had thought this to be an excellent idea, but after some time the
piece of marble was found to be broken in two. The end supports had settled, leaving the
beam balanced upon the fresh central support, cantilevered out to each side of it. Instead of
the beam collapsing down in the middle, as had been feared, its center remained in place and
its two ends collapsed downward. The assumption had been that failure would be precluded
by the addition of the third support, but that assumption overlooked the fact that changing the
design of the support introduced a potential new mode of failure. Any change to any
structural system changes the system and introduces new failure modes.
Fig. 3. Failure modes for a marble column in storage as a beam
5. Railroad Mania
Although Galileo made an oversimplifying assumption in his analysis of the cantilever
beam, his method of analysis represented a breakthrough in taking into account both the
geometry and strength of a structure. However, two centuries later the state of structural
analysis as practiced by engineers remained primitive and only capable of attacking
relatively simple problems. During the period of railway mania in Britain, when new routes
were being developed at a frantic pace, designers often had to rely upon experiments and
empirical formulas.
Crossing the Menai Strait in Wales with a railroad bridge provides an example in which
failure played a key role. The strait had been crossed by a suspension bridge for two decades
when it came time in the mid-1840s to carry railroad trains across it. However, the
suspension bridge had a light and flexible deck, which was deemed inadequate for the weight
of locomotives. A new bridge dedicated to rail traffic was proposed nearby. The engineer
Robert Stephenson conceived of a bridge with spans of the order of 450 feet that would be
built up of wrought-iron plates riveted together to form massive tubes. Exactly what form the
tubes would take was determined by scale-model experiments, in which the tubes were
loaded to failure. The results of these experiments not only provided a means for selecting a
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rectangular cross section for the full-size tubes but also where the model tubes broke or
buckled identified points of weaknesses and thereby allowed for iterations toward a robust
design. Increasingly larger tubes were tested, and an empirical formula for their strength was
developed from the data. However, as with civil engineering structures generally, the
ultimate model was the full-size structure itself, which when in place was proof-tested under
the weight of a string of locomotives. The Britannia Bridge passed the test and was declared
a structural success.
Fig. 4. Britannia Tubular Bridge
The tubular bridge form was short-lived, however. Only a half dozen or so were built
worldwide, in part because although successful structurally they were environmental failures.
With the tracks laid inside the tube, the smoke that the coal- and wood-burning locomotives
emitted created a dirty and suffocating experience for passengers. Furthermore, in the heat of
summer, the interior of the wrought-iron tubes grew oppressively warm and uncomfortable.
Finally, the cost of labor and materials involved in riveting wrought-iron plates into a tubular
structure made it uneconomical. The death knell for the tubular bridge design was sounded
with the appearance of alternative truss-like designs, which were not only less costly but also
provided a more comfortable passenger experience.
However, it was the catastrophic failure of bridges that was to plague the railroad
industry and greatly influence the evolution of structural types used. Some early railroad
bridges had been simple cast-iron beams, but since these were suitable for use only up to
about a 50-foot span, longer spanning bridges had to be of another design. One early bridge
type was the trussed girder, which typically consisted of three cast-iron girders bolted end-toend and post-tensioned with wrought-iron bars, and this was the type that was employed in
the Dee Bridge, which in 1847 collapsed under a train, killing five passengers. The accident
led to an inquest and an investigation by a royal commission, which looked into the use of
iron in railway bridges. There was considerable contemporary debate about what caused the
accident, which was attributed to the brittle fracture of cast iron. It was over a century and a
half later, when the forensic engineer Peter Lewis revisited the matter, that fatigue crack
propagation was identified as the most likely cause.
Another landmark railroad-bridge failure was that of the Tay Bridge, which crossed the
river near Dundee, Scotland. During a storm in 1879, the bridge’s high girders fell suddenly
and claimed seventy-five lives. This accident was investigated by a royal commission, which
concluded that the bridge was poorly designed, poorly constructed, and poorly maintained.
High winds were believed to have blown the highest and longest spans of the truss bridge
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and the train driving through it into the river. Peter Lewis also revisited this accident and, by
digitizing and blowing up details of some contemporary photographs of the accident scene,
was able to demonstrate convincingly that this failure too was ultimately caused by fatigue
failures of lugs by which critical bracing was attached to the columns that formed the piers.
Fig. 5. Dee Bridge failure of 1847
Thomas Bouch, the engineer who designed the Tay Bridge, had also been commissioned
to design another bridge sixty miles south on the same railroad line. His design for a bridge
across the Firth of Forth near Edinburgh, was for a suspension bridge. However, following
the collapse of his Tay Bridge, Bouch was replaced as engineer for the new crossing. Instead
of a suspension bridge, the Forth Bridge was designed and built as a cantilever, with record
spans of 1,710 feet. The bridge, being designed as it was in the wake of the Tay disaster,
appeared to some to be overdesigned, but it has served the railroad well since it was
completed in 1890. The practice of designing and building a very substantial and different
looking structure in the wake of an accident with which it is associated is common.
A third landmark railroad-bridge collapse occurred in 1907 near Quebec, Canada. With a
span of 1,800 feet, this bridge was to have been the longest cantilever in the world. Its chief
consultant and de facto chief engineer was Theodore Cooper, and the record-setting span was
to be the crowning achievement of a long and distinguished career. However, while the
bridge was being built Cooper did not visit the construction site and relied upon reports from
inexperienced junior engineers to keep him apprised of progress. The engineer who carried
out the design calculations had underestimated the weight of the structure, and some key
compression members began to show signs of being overloaded as steel continued to be
added to bridge. These warning signs were not communicated to Cooper until it was too late;
the bridge collapsed and claimed the lives of seventy-five construction workers. It was
redesigned as a more substantial and geometrically more easily analyzed structure and a
century later remained the longest cantilever span in the world.
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Fig. 6. Quebec Bridge under construction in 1907, just before the cantilever collapsed
6. Suspension Bridges
The modern suspension bridge dates from the early nineteenth century, when iron began
to be used for the structure’s chains and cable wire. At the time, the traffic using such bridges
consisted of pedestrians, horses, and wagons, and thus was relatively light. The roadway
suspended from the chains or cables was accordingly made light and flexible. Although such
decks were frequently destroyed during wind storms, replacing them was not especially difficult or expensive. With the development of the railroads, however, such flexible decks made
the suspension bridge form unsuited to the heavier loads. Especially in Britain, as discussed
above, this led to the design of such unusual bridge structures as the tubular Britannia.
Fig. 7. John Roebling’s 1854 Niagara Gorge Suspension Bridge
The American engineer John Roebling did not dismiss the suspension bridge form out of
hand. He studied whatever literature he could find available on the many suspension bridge
failures that had occurred in the early nineteenth century and distilled from them the rather
basic conclusion that storms were the enemy of suspension bridge decks, a conclusion he
published in 1841. Starting with a definitive knowledge of the destructive force that was the
root cause of the failures, Roebling came up with a scheme for designing suspension-bridge
decks that could survive storms. He reasoned that the deck should be sufficiently heavy so
that it had inertia to stay in place against gusting winds. He also insisted that the deck should
have sufficient stiffness so that it would maintain its shape in the wind. And, he specified
that the deck should be anchored via cables to relatively immovable objects like the bridge
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towers or canyon walls. Roebling successfully applied his principles to the design of the Niagara Gorge Suspension Bridge, which in 1854 became the first suspension bridge to carry
railroad trains.
Roebling’s insistence that his bridges be designed with weight, stiffness, and stays
produced such successful structures as the bridge across the Ohio River at Cincinnati and, his
masterpiece, the Brooklyn Bridge. Completed posthumously in 1883 with an almost 1,600foot main span, the Brooklyn Bridge provided a successful model for subsequent suspension
bridge designers to emulate. And they did so, but they did not heed Roebling’s specification
of weight, stiffness, and stays whereby his successes were achieved. New York’s
Williamsburg Bridge, which was completed in 1903, had a span five feet longer than its
neighbor, the Brooklyn Bridge, but the Williamsburg had none of the Brooklyn’s architectural grace. The Williamsburg Bridge has all-steel towers of no distinction and a stiffening
truss that is overly heavy structurally and visually. Most importantly, the Williamsburg
Bridge was designed and built without stay cables. This omission influenced all subsequent
suspension bridge designs, which were also built without the cables.
In the early decades of the twentieth century, large suspension bridges continued to be
built with wide roadways stiffened by substantial trusses. But in 1931 a bridge with a span
almost twice as long as any of its immediate predecessors set a new aesthetic standard. The
3,500-foot span of the George Washington Bridge, which connected New York City and
New Jersey across the Hudson River, was wide and therefore heavy, but it was only about 10
feet deep. Its designers argued that the mass of the suspension cables and wide deck provided
sufficient stiffness so that no truss was necessary. This new standard influenced bridges
designed and built in the later 1930s. Unfortunately, those bridges were located not in
populous cities but in remote areas, where traffic did not demand wide multi-lane roadways.
The bridges were thus designed with not only shallow but also narrow decks. It followed that
the decks were light. Thus, in the course of less than a half century, Roebling’s specification
of weight, stiffness, and stays had been totally forgotten and ignored. The bridges built in the
late 1930s had decks that were light, flexible, and unstayed.
Fig. 8. George Washington Bridge, as built 1931
These newer bridges began to exhibit unanticipated and undesirable behavior under
certain wind conditions. Their decks began to undulate. There was disagreement among
engineers not only about what was causing the undulations but also about how to retrofit the
bridges to check them. It was in this climate that the Tacoma Narrows Bridge was designed
and constructed in the northwestern U.S. state of Washington. The bridge opened in 1940
and exhibited the increasingly familiar undulations of a slender roadway, which drivers had
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come to find amusing. However, after about four months of such behavior, some checking
cables slipped and the bridge deck began to execute torsional vibrations. After about four
hours of these uncontrollable movements, the roadway broke apart and fell into the water.
Fig. 9. Tacoma Narrows Bridge, November 1940
The investigative committee that studied the Tacoma Narrows Bridge failure came to
conclusions strikingly similar to those that John Roebling had reached almost exactly
a century earlier. It is shortsighted of engineers to think that the newer analytical tools that
they use in designing or the advanced materials that are used in building modern structures
make the history of those structures irrelevant. The wind that destroyed suspension bridge
decks in the early nineteenth century was effectively the same wind that attacks them today.
And the rational approach that John Roebling took to dealing with those destructive forces is
as relevant today as it was in his time. Plain thinking about design, especially with regard to
obviating failure, is never obsolete.
7. Buildings, Towers, and Walkways
Building tall has been a structural objective since ancient times. The evolution of pyramids, cathedrals, and towers attest to this. The Eiffel Tower was the first man-made structure
to reach the nineteenth century’s goal of 300 meters and was also the last great wrought-iron
structure. That material’s successor, steel, made it possible to pursue ever higher goals, as
embodied in the skyscrapers of the early- and the super-tall buildings of the later-twentieth
century. As these structures reached higher, the wind became as challenging to their design
engineers as it did to bridge engineers. Innovative structural systems incorporating tubular
concepts and damping devices enabled super-tall structures to be achieved more or less
economically.
The New York World Trade Center towers were designed in such a climate, and they
stood as the tallest buildings in the world until the Sears Tower was completed in Chicago in
1974. The Twin Towers continued to be the tallest in New York City, until they were
attacked in 2001. The possibility of an airplane striking one of the towers was considered
while they were being designed. After all, a B-25 bomber had struck the Empire State
Building in 1945. While it may not have taken that incident to make the designers of the
Twin Towers consider an airplane impact as triggering a possible failure mode, it certainly
made it less easily dismissed. The structural integrity of the towers was analyzed under the
impact of a Boeing 707, but the analysis did not proceed beyond the mechanical implications. Structural engineers of the time did not take into account the effects of an ensuing fire.
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In 2001 it was, of course, the fire that softened what structural columns remained and thus
led to the collapse of the tubular structures.
Designing super-tall structures that can withstand airplane impacts and subsequent fires
may be a new challenge for structural engineers, but even much more modest structures can
cause engineers to overlook key failure modes. This was certainly the case with the design of
some elevated walkways in the Kansas City Hyatt Regency Hotel, which opened in 1980.
Three walkways were designed to be suspended by steel rods from the atrium roof. Two of
the three were to be hung one above the other, to serve the hotel’s second and fourth floors
and allow convention goers to move freely between the tower containing hotel rooms and the
convention center without having to pass through the lobby below. The support detail as
originally conceived was for a single long rod to pass through a box beam supporting the
upper walkway and continue through a similar box beam supporting the lower walkway.
At each level, the load of the walkway was to be transferred to the rod by means of a washer
held in place by a nut.
At some point during construction, the recommendation was made to replace each single
long rod with two shorter rods, one anchored in the roof and terminating just beneath the box
beam of the upper walkway, and the second extending from just above the box beam of the
upper walkway to just beneath the box beam of the lower walkway. In all cases the walkway
loads would be transferred to the rods through washers held in place by nuts screwed onto
the threaded ends of the rods. In changing the connection detail from a single long rod to two
sorter ones, a second hole had to be drilled through the box beam of the upper walkway. The
second hole was offset from the first. The walkways were constructed with this revised detail
and they stood in place for about a year. However, in July 1981, when the lobby was filled
with people attending a dance and many people looked down from the elevated walkways,
the two with the modified support detail fell suddenly onto the lobby floor. The final death
toll was 114, and many more people were injured. At the time, it was the worst structural
accident in U.S. history.
The cause of the failure was soon apparent. A rod hanging from the roof still had its
washer and nut intact on the threaded end of the rod, indicating that the box beam that it had
supported had been pulled over the washer-nut assembly. The deformed box beam on the
floor also showed this to be the case. Had the single long rod not been replaced by the two
shorter ones, the walkways would likely still be in place, and 114 people would not have lost
their lives. In changing the support design from one rod to two, the load that the washer-nut
assembly under the top walkway’s box beam had to bear was doubled, since now the lower
walkway was hung from the upper one. As originally designed, the load of each walkway
was transferred directly to the steel rod, and so each washer-nut assembly had only to bear
the load of its walkway. Evidently no one involved in the design or redesign of the support
detail caught this elementary mistake.
The story of the Hyatt Regency walkways collapse is strikingly similar to the story that
Galileo told almost three-and-a-half centuries earlier. Recall that the marble beam was
originally simply supported, but a third support was added under the middle of the beam to
prevent it from collapsing there. The change in support detail changed the entire structural
system and introduced the possibility of a new mode of failure, which did eventually occur.
By analogy the elevated walkways as originally designed were supported by a single rod, but
it was replaced by two, thereby changing the structural system and introducing a new mode
of failure. Historical case studies should not be viewed as just quaint stories that show how
foolish and unsophisticated previous generations of engineers must have been. The stories
demonstrate lapses in logic and thoroughness, which can and do happen even in an age of
mathematical and computer models.
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Fig. 10. Hyatt Regency Hotel elevated walkways support detail, as-built and as-designed
8. Conclusion
Structural failures have always occurred, and they are likely to continue to occur unless
the historical record of mistakes and oversights in design is seen to remain relevant even as
technology and technique advance. Case histories of failures provide insights into the nature
of design and design error. They also expose the limitations of design and reveal how lapses
in logic and judgment can corrupt the design process. As much as the products of design
today may appear to be different from those of ancient times, the creative human design
process by which they are achieved is fundamentally the same as it has always been. Though
concrete and steel may have replaced stone and timber as our basic building materials,
nothing has replaced the human factor in the design of structures. Advanced mathematical
and computational tools may enable us to calculate more quickly and accurately, but they do
not necessarily help us appreciate the changeless nature of the design process.
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