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Failure Case Studies in Civil Engineering Education
By Rachel Martin
ABSTRACT: Engineering failure case studies should be incorporated into the
undergraduate curriculum. In light of the already overcrowded course load, incorporating
failure case studies into already existing engineering classes provides the most sensible
solution. One of the largest problems with implementing this plan is that there are not
sufficient failure case studies and teaching aids available. This paper presents case
studies of four major structural failures: (1)Hyatt Regency Walkway Collapse, (2)
Tacoma Narrows Bridge Collapse, (3) L'Ambiance Plaza Collapse, and (4) Hartford
Civic Center Arena Collapse. The design, construction, and causes of failure are
presented for each case. Then the cases are examined from legal, technical, procedural,
and ethical standpoints. These case studies can be used as resources in the incorporation
of failure case studies into undergraduate courses.
INTRODUCTION
Knowledge of engineering's failures is just as important as knowledge of its
successes. Unfortunately, the engineering profession tends to highlight its magnificent
successes while trying to forget its catastrophic failures. A success illustrates what
engineering can make possible, while a failure demonstrates its limits. It takes numerous
successful structures to ensure the quality of a design or a construction method. One
failure, however, can discredit an entire design or building technique. Because of this
reality, the information that each failure has to offer should be carefully studied and
applied to all future designs. As a result similar failures, as well as their tragic
consequences, can be avoided.
Because of their importance, failures should be incorporated into engineering
education early on. Unfortunately, undergraduate engineering students receive little
exposure to engineering failures in college. This approach to engineering education not
only leaves students ignorant and unprepared for what they will face after college, but it
also teaches them to devalue the importance of failures in their future careers (Delatte,
2
July 1997). This may be one of the reasons that an 1983 survey of ASCE section and
branch presidents found that engineering failures are all too common (Bosela, 1993).
Since undergraduate engineering students already face an overcrowded
curriculum, rather than requiring a new class covering failure case studies, these case
studies can be incorporated into existing classes throughout a student's college career.
Not only will this approach capture the students' interest by showing how their classes
relate to engineering, but it will also inspire them to learn more about the history of the
profession. In addition, it teaches them the importance of continued learning throughout
one's professional career. Finally, failure case studies provide a perfect opportunity to
discuss ethical concerns, another neglected topic in engineering education, in real life
situations, as well as serving as a constant reminder of the repercussions of careless
engineering (Delatte, Spring 1997).
According to a 1987 survey conducted by the Committee of Education of the
Committee on Forensic Engineering of the American Society of Civil Engineers, 63.2%
of schools indicated that they would consider teaching a course on failure case studies if
the appropriate materials were available. This clearly demonstrates the need for case
study material and teaching aids to encourage the incorporation of failure case studies
into the engineering curriculum (Rendon-Herrero, May 1993). This paper presents four
case studies of structural failures: the Hyatt Regency walkway collapse, the Tacoma
Narrows Bridge collapse, the L'Ambiance Plaza collapse, and the Hartford Civic Center
Arena Collapse. Each case study
1. Summarizes of the design, construction, and collapse of the structures.
2. Examines the causes of the failure as well as the legal ramifications, if any.
3. Explores the technical, procedural, and ethical concerns present, focussing on
how the failure could have been avoided and how to prevent similar failures in
the future.
These failure case studies can be integrated into engineering classes to introduce new
topics, as example problems, as homework problems, or even as the topics of a short
research paper.
3
HYATT REGENCY WALKWAY COLLAPSE - KANSAS CITY, MO - 1981
Design and Construction
In July of 1980, the Hyatt Regency opened to the public after four years of design
and construction. A 40-story tower, an atrium, and a function block, housing all of the
hotel’s services, combined to form this impressive building. Three walkways suspended
from the atrium’s ceiling by six 32-mm-diameter tension rods each spanned the 37-m
distance between the tower and the function block. The 2nd floor walkway, directly below
the 4th floor walkway, was suspended from the beams of the 4th floor walkway, while the
3rd and 4th floor walkways hung from the ceiling (Feld and Carper, 1997).
The erection of this hotel, however, was not as picture perfect as the final product.
During construction, the atrium roof collapsed as a result of inadequate movement in the
expansion joint and improper installation of a steel-to-steel concrete connection.
Concerned about the building’s structural integrity, the owner hired another engineering
firm to investigate the collapse and check the roof design. The consulting structural
engineering company also rechecked all of the connections and found nothing to cause
alarm. Construction resumed and the hotel opened a little less than 2 years later (Roddis,
1993).
Collapse
Ruins of the Hyatt Regency Walkway
On the evening of July 17, 1981,
between 1500 and 2000 people
inundated the atrium floor and the
suspended walkways to see a local radio
station’s dance competition (Feld and
Carper, 1997). At 7:05, a loud crack
echoed throughout the building and the
2nd and 4th floor walkways crashed to the ground killing 114 people and injuring over 200
others. It was the worst structural failure in the history of the United States (Levy and
Salvadori, 1992).
4
Causes of Failure
Upon investigation, the National Bureau of Standards (NBS) discovered that the
cause of this collapse was quite simple: the rod hanger pulled through the box beam
causing the connection supporting the 4th floor walkway to fail. Because of lack of
redundancy, this failure caused the collapse of both of the walkways.
3rd floor hanger rod andcrossbeam assembly
4th floor beam Hanger rod, washer, andsupporting nut
Originally, the 2nd and 4th floor walkways were to be suspended from the same
rod (as shown in fig-1) and held in place by nuts. The preliminary design sketches
contained a note specifying a strength of 413 MPa for the hanger rods which was omitted
on the final structural drawings. Following the general notes in the absence of a
specification on the drawing, the contractor used hanger rods with only 248 MPa of
strength. This original design, however, was highly impractical because it called for a nut
6.1 meters up the hanger rod and did not use sleeve nuts. The contractor modified this
detail to use 2 hanger rods instead of one (as shown in fig-2) and the engineer approved
the design change without checking it. This design change doubled the stress exerted on
the nut under the fourth floor beam. Now this nut supported the weight of 2 walkways
instead of just one (Roddis, 1993).
Analysis of these two details revealed that the original design of the rod hanger
connection would have supported 90 kN, only 60% of the 151 kN required by the Kansas
City building code. Even if the details had not been modified the rod hanger connection
would have violated building standards. As-built, however, the connection only
supported 30% of the minimum load which explains why the walkways collapsed well
below maximum load (Feld and Carper, 1997).
5
Original As-Built
Fig-1 Fig-2
Legal Repercussions
While Kansas City did not convict the Hyatt Regency engineers of criminal
negligence due to lack of evidence, the Missouri Board of Architects, Professional
Engineers, and Land Surveyors was not as timid. It convicted the engineer of record and
the project engineer of gross negligence, misconduct, and unprofessional conduct in the
practice of engineering. Both of their Missouri professional engineering licenses were
revoked, and they lost membership to ASCE. Also the billions of dollars in damages
awarded in civil cases brought by the victims and their families dwarfed the half million
dollar cost of the building (Roddis, 1993).
Technical Concerns
Neither the original nor the as-built design for the hanger rod satisfied the Kansas
City building code making the connection failure inevitable. If, however, the building
design had contained more redundancy this failure may not have resulted in the complete
collapse of the walkway. Kaminetzky (1991) suggests two much stronger design
alternatives for the connectors. The toe-to-toe channels used in the Hyatt Regency
provided for weak welding which allowed the nut to pull through the channel/box beam
assembly initiating the collapse. A back-to-back channel design using web stiffeners
when necessary (fig-3) or the use of bearing crossplates in conjunction with the toe-to-toe
channels (fig-4) would have made the connection much stronger, making it much more
difficult for the nut to pull through (Kaminetzky, 1991).
6
fig-3fig-4
Procedural Concerns
The Hyatt Regency walkway collapse highlighted the lack of established procedures
for design changes as well as the confusion over who is responsible for the integrity of
shop details (Roddis, 1993). The legal repercussions experienced by the Hyatt engineers
established the engineer of record's responsibility for the structural integrity of the entire
building including the shop details. It is important for all parties to fully understand and
accept their responsibilities in each project (Feld and Carper, 1992). Certain procedural
changes could help prevent similar collapses.
• The engineer of record should design and detail all nonstandard connections.
• All new designs should be thoroughly checked.
• All of the contractor's modifications to design details should require written
approval from the engineer of record (Kaminetzky, 1991).
Ethical Concerns
During the trial the detailer, architect, fabricator, and technician all testified that
during construction they had contacted the project engineer regarding the structural
integrity of the connection detail. Each time he assured them that the connection was
sound claiming to have checked the detail. In reality he had never performed any
calculations for this design at all. Neglecting to check the safety and load capacity of a
crucial hanger even once shows his complete disregard for the public welfare (Rubin and
Banick, 1987). Ethical engineers should check and recheck their work in order to be able
to properly assure the public of a building's structural integrity (Delatte, 1997). Also, the
7
high number of fatalities resulting from the walkway's collapse raises the questions of
whether the factor of safety required for a building should be proportional to the possible
consequences of it collapse (Kaminetzky, 1991).
TACOMA NARROWS BRIDGE - TACOMA, WA - 1940
Design and Construction
Tacoma Narrows Bridge
On July 1, 1940, the Tacoma
Narrows Bridge, connecting Seattle to
Tacoma with nearby Puget Sound Navy
Yard, opened to the public after two
years of design and construction. Its
2,800-ft. mainspan connected two 420-
ft. towers from which cables were
draped (Levy and Salvadori, 1992). Even though it was the third longest bridge in the
world, Tacoma Narrows was much narrower, lighter, and more flexible than any other
bridge of its time. With a 39-ft wide and 8-ft deep concrete deck, it accommodated two
lanes of traffic quite comfortably while maintaining a sleek appearance. This appearance
was so important to the bridge’s designer, Leon Moisseiff, that he designed it without the
use of stiffening trusses, leaving Tacoma Narrows with 1/3 the stiffness of the Golden
Gate and George Washington bridges. Tacoma Narrows light appearance, however, was
no illusion. Its dead load was 1/10 of that of any other major suspension bridge. These
unique characteristics coupled with its low dampening ability caused large vertical
oscillations in even the most moderate of winds. This soon earned it the nickname,
"Galloping Gertie," and attracted thrill seekers from all over (Feld and Carper, 1997).
Table-1 compares the properties and deflections of the five long suspension bridges in
1941.
8
Table-1*
Golden Gate GeorgeWashington
TacomaNarrows
SanFrancisco
Bay
Bronx-WhiteStone
Length ofcenter span
4200 ft 3500 ft 2800 ft 2310 ft 2300 ft
Length ofside spans
1125 ft 650 ft 1100 ft 1160 ft 735 ft
Ave wt ofcenter span
21,035 lb./ft 31,590 lb./ft 5,700 lb./ft 18,740 lb./ft 11,000 lb./ft
# and type ofgirders
2 trusses 2 chords 2 pl. girders 2 trusses 2 pl. girders
Depth ofgirders
25 ft 36 ft 8 ft 28 ft 11 ft
Girder'smoment of
inertia
88,000 in2ft2 168 in2ft2 2,567 in2ft2 156,000in2ft2
5,860 in2ft2
Wind forceon floor and
cables
1,330 lb./ft 1,500 lb./ft 620 lb./ft 1,545 lb./ft 920 lb./ft
Width ofwind truss
90 ft 106 ft 39 ft 66 ft 74 ft
Wind truss’smoment of
inertia
1,236,000in.2ft2
481,100in.2ft2
95,000in.2ft2
743,000in.2ft2
410,000in.2ft2
Relativevertical
rigidity atquarter point
2.3 5.0 1.0 4.0 2.6(without stays)
3.0(with stays)
Relativetorsionalrigidity
4.2 14.1 1.0 4.9 3.0(without stays)
4.4(with stays)
*Information on this table and these graphs is taken from the May 8, 1941 edition ofEngineering News-Record
9
Fig-1 Comparative torsional rigidity offive long suspension bridges
Fig-2 Comparative vertical rigidity of 5long suspension bridges
While these undulations could be quite unnerving to motorists, no one questioned
the structural integrity of the bridge. Leon Moisseiff was a highly qualified and well-
respected engineer. Not only had he been the consulting engineer for the Golden Gate,
Bronx-Whitestone, and San Francisco-Oakland Bay bridges, but he had also developed
the methods used to calculate forces acting on suspension bridges (Levy and Salvadori,
1992). Even though the Tacoma Narrows Bridge adhered to all of the safety standards
and its oscillations were not considered a threat, Prof. F. B. Farquharson began
researching ways to reduce its motion at the University of Washington. By studying how
different winds affected a highly accurate model of the Tacoma Narrows Bridge and
testing new devices on it, Farquharson was able to propose helpful modifications to the
bridge. After proving successful on the model, 1 9/16 -in. steel cables attached a point on
each side span to 50-yd concrete anchors in the ground. Unfortunately these cables
snapped a few weeks later proving to be an ineffective solution (Ross, 1984). They,
however, were reinstalled in a matter of days. In addition to these cables, center stays and
inclined cables, which connected the main cables to the stiffening girder, were installed.
Finally, an untuned dynamic damper, similar to the one that had proved quite successful
10
in curtailing the torsional vibrations of the Bronx-Whitestone Bridge, failed immediately
after its installation in the Tacoma Narrows Bridge. It was discovered that the leather
used in this device was destroyed during the sandblasting of the steel girders before they
were painted rendering it useless (Levy and Salvadori, 1992). Farquharson also
discovered that proper streamlining would almost completely stop the bridges disturbing
movements. The bridge collapsed before this knowledge could be applied (Ross, 1984).
Collapse
Ruins of the Tacoma Narrows Bridge
At 7:30 A.M. on November 7,
1940, Kenneth Arkin, the chairman of
the Washington State Toll Bridge
Authority, arrived at the Tacoma
Narrows Bridge. While the wind was not
extraordinary, the bridge was undulating
noticeably and the stays on the west side
of the bridge which had broken loose were flapping in the wind. Just before 10:00 A.M.
after measuring the wind speed to be 42 mph, Arkin closed the bridge to all traffic due to
its alarming movement, 38 oscillations/minute with an amplitude of 3 ft (Levy and
Salvadori, 1992). Suddenly, the north center stay broke and the bridge began twisting
violently in two parts. The bridge rotated more than 45° causing the edges of the deck to
have vertical movements of 28 ft and at times exceed the acceleration of gravity (Ross,
1984). Two cars were on the bridge when this wild movement began: one with Leonard
Coatsworth, a newspaper reporter, and his cocker spaniel and the other with Arthur
Hagen and Judy Jacox. All three people crawled to safety (Levy and Salvadori, 1992). A
couple of minutes later the stiffening girders in the middle of the bridge buckled initiating
the collapse. Then the suspender cables broke and large sections of the main span
dropped progressively, from the center outward, into the river below. The weight of the
sagging side spans pulled the towers 12 ft towards them and the ruined bridge finally
came to a rest (Feld and Carper, 1997). The bridge’s only fatality was Coatsworth’s
cocker spaniel. Due to the fact that Prof. Farquharson was present that day studying the
11
bridge, its collapse is well documented, photographed, and recorded on film (Levy and
Salvadori, 1992).
Leonard Coatsworth described the collapse by saying,
"Just as I drove past the towers, the bridge began to sway violently from side to
side. Before I realized it, the tilt became so violent that I lost control of the car... I
jammed on the brakes and got out, only to be thrown onto my face against the curb."
"Around me I could hear concrete cracking. I started to get my dog Tubby, but
was thrown again before I could reach the car. The car itself began to slide from side
to side of the roadway.
"On hands and knees most of the time, I crawled 500 yards or more to the
towers... My breath was coming in gasps; my knees were raw and bleeding, my hands
bruised and swollen from gripping the concrete curb... Toward the last, I risked rising
to my feet and running a few yards at a time... Safely back at the toll plaza, I saw the
bridge in its final collapse and saw my car plunge into the Narrows (The Tacoma
Narrows Bridge, 1999)."
Causes of Failure
The Federal Works Agency (FWA) investigated the collapse of the Tacoma Narrows
Bridge and found the following:
• The bridge was well designed and well built. While it could safely resist all static
forces, the wind caused extreme undulations which caused the bridge’s failure.No one
realized that Tacoma’s exceptional flexibility coupled with its inability to absorb
dynamic forces would make the wild oscillations which destroyed it possible.Vertical
oscillations were caused by the force of the wind and caused no structural damage.
• The failure of cable band on the north end, which was connected to the center ties,
probably started the twisting motion of the bridge. The twisting motion caused high
stresses throughout the bridge, which lead to the failure of the suspenders and
collapse of the main span.
• A suspension bridge was the most practical choice for the site.
• The supervision of and workmanship on the bridge was exceptional.
12
• Rigidity against static forces and rigidity against dynamic forces cannot be
determined using the same methods.
• Efforts were made to control the amplitude of the bridge’s oscillation.
• Subsequent studies and experiments are needed to determine the aerodynamic forces
which act on suspension bridges.
In other words, the FWA concluded that because of Tacoma Narrow’s extreme
flexibility, narrowness, and lightness the random force of the wind that day caused the
torsional oscillations that destroyed the bridge. Table-1 compares Tacoma Narrow’s
flexibility, width, and weight to that of four other long suspension bridges of its time. The
FWA believed that wind induced oscillations approached the natural frequencies of the
structure causing resonance (the process by which the frequency on an object matches its
natural frequency causing a dramatic increase in amplitude). This explains why the
relatively low speed wind (42 mph) caused the spectacular oscillations and destruction of
the Tacoma Narrows Bridge (Engineering News-Record, 1941).
The FWA’s theory, however, is not the only explanation. Many people believe
that this explanation overlooks the important question as to how wind, random in nature,
could produce a periodic impulse. One explanation proposed by von Kármán, an
aeronautical engineer, attributed the motion of the bridge to the periodic shedding of air
vortices which created a wake known as a von Kármán’s street. This wake reinforced the
structural oscillations eventually causing the collapse of the bridge. The problem with
this theory is that the calculated frequency of a vortex caused by a 42 mph wind is 1
Hertz while the frequency of the torsional oscillations of the bridge measured by Prof.
Farquharson was 0.2 Hertz (Petroski, 1991). Another explanation proposed by Billah and
Scanlan admits that vortices associated with the Kármán vortex street were shed but did
not affect the motion of the bridge. Another kind of vortex, one associated with the
structural oscillation itself, having the same frequency as the bridge was also created. The
resonance between the bridge and these vortices caused excessive motion destroying the
bridge (Billah and Scanlan, 1991). While these three theories differ in their opinions as to
what exactly caused the torsional oscillations of the bridge they all agree that the extreme
flexibility, slenderness, and lightness of the Tacoma Narrows Bridge allowed these
oscillations to grow until they destroyed it.
13
Technical Concerns
The Tacoma Narrows Bridge collapse showed engineers and the world the importance
of dampening, vertical rigidity, and torsional resistance in all suspension bridges (Ross,
1984). Once the threat of twisting was realized there are many ways that the disaster of
Tacoma Narrows could have been averted. Making any one of the following adjustments
could have prevented the collapse:
• Use open stiffening trusses which would allow the wind free passage through the
bridge
• Increase the width to span ratio
• Increase the weight of the bridge
• Dampen the bridge
• Use an untuned dynamic damper to limit the motions of the bridge
• Increase the stiffness and depth of the trusses or girders
• Streamline the deck of the bridge (Levy and Salvadori, 1992)
Procedural Concerns
The Tacoma Narrows Bridge collapse highlighted the importance of failure case
studies in engineering education. Between 1818 and 1889, the wind destroyed or
seriously damaged ten suspension bridges (Petroski, 1994). Most of these bridges, like
Tacoma Narrows, had small width to span ratios, ranging anywhere from 1/72 to 1/59.
They also experienced severe twisting right before collapse as Tacoma Narrows did
(Levy and Salvadori, 1992). In 1826, a hurricane partially destroyed the Menai Straits
Bridge in eastern England. The deck experienced 16-ft oscillations before it broke (Feld
and Carper, 1997). Thirty-eight years later in 1854, the bridge over the Ohio River at
Wheeling, West Virginia also collapsed due to wind. A witness's description of this
collapse stated:
For a few minutes we watched it with breathless anxiety, lunging like a ship in the
storm. At one time, it rose to nearly the height of the towers, then fell, and
twisted and writhed and was dashed almost bottom upward. At last there appeared
to be a determined twist along the entire span, about one half the flooring being
14
nearly reversed, and down went the immense structure from its dizzy height to the
stream below, with an appalling crash and roar.
A description of the Tacoma Narrows Bridge collapse would be quite similar.
Many other bridges suffered a similar fate (Levy and Salvadori, 1992). In fact, it
was not until the success of John Roebling’s suspension bridges that they became widely
accepted. Through his understanding of the importance of deck stiffness and knowledge
of past failures, Roebling was able to make suspension bridges accepted as strong railway
bridges (Feld and Carper, 1997). Soon, however, the success of the suspension bridges
completely overshadowed the failures of the last century. Once again, suspension bridges
evolved towards the longer sleeker designs forgetting the cornerstone of their success,
wind resistance (Petroski, 1994).
Ethical Concerns
In the face of new technology, how do we balance public welfare and progress? If
Moisseiff had designed a bridge similar to the ones which had already proven their
stability, Tacoma Narrows Bridge would never have collapsed costing thousands of
dollars and endangering many lives. It would also have been significantly more
expensive. On the other hand, if engineers had never tried innovative techniques,
suspension bridges may never have been built at all. At the time of their introduction, no
one believed that a suspension bridge could safely accommodate trains. Roebling,
however, took a gamble, pushed the limits of the current technology, and built a
suspension bridge that he believed could safely support rail traffic. Luckily he was
correct, and suspension bridges soon became widely accepted (Petroski, 1985). Moisseiff
also took a gamble, trying to create a longer, sleeker, less expensive bridge, by pushing
the limits of technology. He, however, was not as lucky, and what could have been a
breakthrough in technology turned into a catastrophic failure. Every time engineers push
the limits of technology they risk a similar loss, sometimes even a loss of life. How much
is too much? When is a possible advance worth a risk to public safety? What can the
engineering profession do to make the implementation of new technology safer? Do our
current peer review and building code committee processes adequately protect public
safety?
15
L'AMBIANCE PLAZA - BRIDGEPORT, CN - 1987
Design and Construction
L'Ambiance Plaza was planned to be a sixteen-story building with thirteen
apartment levels topping three parking levels. It consisted of two offset rectangular
towers, 63 ft by 112 ft each, connected by an elevator. Seven-inch thick posttensioned,
concrete slabs and steel columns comprised its structural frame (Cuoco, 1992).
Posttensioning overcomes the tensile weakness of concrete slabs by placing high strength
steel wires along their length or width before the concrete is poured. After the concrete
hardens, hydraulic jacks pull and anchor the wires compressing the concrete (Levy and
Salvadori, 1992).
Floor Plan of L'Ambiance Plaza
The lift-slab method of construction, patented by Youtz and Slick in 1948, was
utilized in the construction of this building. Following this technique, the floor slabs for
all sixteen levels were constructed on the ground, one on top of the other, with bond
breakers between them. Then packages of two or three slabs were lifted into temporary
position by a hydraulic lifting apparatus and held into place by steel wedges. This
hydraulic lifting apparatus consisted of a hydraulic jack on top of each column with a pair
of lifting rods extending down to lifting collars cast in the slab. Once the slabs were
positioned correctly, they were permanently attached to the steel columns. Two shear
walls in each tower were to provide the lateral resistance for the completed building on
all but the top two floors. These two floors depended on the rigid joints between the steel
columns and the concrete slabs for their stability. Since the shear wall played such an
indispensable role in the lateral stability of the building, the structural drawings specified
16
that during construction the shear walls should be within three floors of the lifted slabs
(Heger, 1991).
Collapse
Ruins of L'Ambiance Plaza
At the time of collapse, the building was a little
more than halfway completed. In the west tower,
the ninth, tenth, and eleventh floor slab package
was parked in stage IV directly under the twelfth
floor and roof package. The shear walls were about
five levels below the lifted slabs (Cuoco, 1992).
Status of Construction at Time of Collapse
*Figure by Rachel Martin based on information from Cuoco, 1992
The workmen were tack welding wedges under the ninth, tenth, and eleventh floor
package to temporarily hold them into position when they heard a loud metallic sound
followed by rumbling. Kenneth Shepard, an ironworker who was installing wedges at the
time, looked up to see the slab over him "cracking like ice breaking." Suddenly, the slab
fell on to the slab below it, which was unable to support this added weight and in turn
fell. The entire structure collapsed, first the west tower and then the east tower, in 5
seconds, only 2.5 seconds longer than it would have taken an object to free fall from that
height. Two days of frantic rescue operations revealed that 28 construction workers died
17
in the collapse, making it the worst lift-slab construction accident. Kenneth Shepard was
the only one on his crew to survive (Levy and Salvadori, 1992).
Causes of Failure
An unusually prompt legal settlement prematurely ended all investigations of the
collapse. Consequently, the exact cause of the collapse has never been established. The
building had a number of deficiencies; any one of which could have triggered the
collapse. The question, however, remains which one of these failed first, triggering the
rest of the failures and ultimately total collapse. There are five competing theories as to
the trigger.
Theory 1: National Bureau of Standards (NBS) - An overloaded steel angle welded to a
shearhead arm channel deformed, causing the jack rod and lifting nut to slip
out and the collapse to begin (Korman, Oct 29, 1987).
Theory 2: Thornton-Tomasetti Engineers (T-T) - The instability of the wedges holding
the twelfth floor and roof package caused the collapse (Cuoco, 1992).
Theory 3: Schupack Suarez Engineers, Inc. (SSE) - The improper design of the
posttensioning tendons caused the collapse (Poston, Feldman, and Suarez,
1991).
Theory 4: Occupational Safety and Health Administration (OSHA) - Questionable weld
details and substandard welds could have caused the collapse (McGuire, 1992).
Theory 5: Failure Analysis Associates, Inc. (FaAA) – The sensitivity of L’Ambiance
Plaza to lateral displacement caused its collapse (Moncarz, Hooley, Osteraas,
and Lahnert, 1992).
Theory 1
The NBS investigation concluded that the failure occurred at the building’s most heavily
loaded column E4.8 or the adjacent column E3.8 as a result of a lifting assembly failure.
The shearhead reinforces the concrete slab at each column, transfers vertical loads from
the slabs to the columns, and provides a place of attachment for the lifting assembly. It
consists of ]-shaped steel channels cast in the concrete slab leaving a space for the lifting
angle. The lifting angle has holes to pass the lifting rods through. These rods are raised by
the hydraulic jacks on the columns above them (Levy and Salvadori, 1992).
18
Lifting Assembly
Shortly before the collapse the workers lifted the 9th, 10th, and 11th floor
package to its final position and began tack-welding the steel wedges into place. They
used a jack on top of the column E4.8 or E3.8 to slightly adjust the position of the slab
overloading the lifting angles. When the shearheads and lifting angles had lifted the
package of three 320-ton slabs, they were dangerously close to their maximum capacity,
so adding even the smallest of loads could strain them. One of the reasons was that the
lifting capacities of the two types of jacks used were too small for the 960-ton package
being lifted. The regular jacks have a maximum load of 89 tons, while the super jacks
have a maximum load of 150 tons. NBS also tested the shearhead and lifting angle and
found that they tended to twist as the loads approached 80 tons because although strong
enough, they were not rigid enough. The excess force deformed the lifting angle allowing
the jack rod and lifting nut to slip out of the lifting angle and hit the column with 75,000
lb of force. This accounts for the loud noise that Kenneth Shepard heard and the
indention found in that column. After this initial slip, the jack rods and lifting nuts in the
entire E line progressively slipped causing the ninth floor slab to collapse, initiating the
collapse of the entire building (Korman, 1987).
19
Failure Sequence
Theory 2
Thornton-Tomasetti Engineers (T-T) concluded at the end of their investigation
that the instability of the wedges at column 3E caused the 12th floor/ roof package to fall
initiating the collapse. Unlike the NBS investigation, their investigation found that all the
wedges supporting the 9th/10th/11th floor package were mounted prior to the collapse
and that that column had no indentations on it. They, however, did find abnormal tack
welds on the wedges which supported the 12th floor/roof package, a large deformation on
the top edge of the west wedge of this set, and indentations on the underside of the level 9
shearhead. The shallowness of the indentations indicated that, while both lifting nuts
slipped out, they were not heavily loaded at the time. Their investigation also found that
the shearhead gaps on columns 3E and 3.8E (0.628 in) were much larger than the gaps on
the rest of the building (0.233 in- 0.327 in) and other buildings built with the lift-slab
technique (0.250 in - 0.375 in). In addition to these abnormally large gaps, the shearheads
used on these two columns did not have cut outs in their lifting angles to restrict relative
shifting, and were installed eccentrically. Finally, until a wedge is completely welded into
place it depends on friction to hold it. Normally, this is sufficient. The large shearhead
gaps on columns 3E and 3.8E and the presence of hydraulic fuel on these wedges,
however, would have demanded an extremely high friction coefficient to hold the wedges
in place.
On the day of collapse, the lateral load from the hydraulic jack exerted on the
heavily loaded wedges caused the west wedge to roll. Then the local adjustments to slab
elevations caused the remaining wedge to roll out initiating the collapse of the 11th
floor/roof package and the west tower. Forces transmitted through the pour strips or the
horizontal jack, or the impact of the debris from the west tower triggered the east towers
collapse (Cuoco, 1992).
20
Theory 3
General layout of posttensioning tendons*each line represents 1-5 monostrand tendons which are shown in grey
SSE analyzed the structural behavior of a typical west tower floor slab under ideal
conditions with regards to the unusual layout of the posttensioning tendons. The tendons
in the east tower follow a typical two-way banded posttensioning tendon layout. In this
layout the vertical tendons distribute the weight of the slab to the east-west column lines
which in turn distribute the weight to the columns. The west tower, however, deviates
from this pattern. At column 4.8E the tendons split in two, both diverging from the
column line. In the west tower the vertical tendons still distribute the slab's weight to the
column line. In line E, however, there are no tendons to carry this weight. This setup
violates the American Concrete Institute (ACI) building-code. Also, the design details of
the posttensioned floor slabs do not show the location of the shear walls or the openings
for the walls at columns 11A, 8A, and 2H. The design did not take these opening into
account. Detailed finite element analysis showed that tensile stresses along column line
E, east of column 4.8E, exceeded the cracking strength of the concrete. Therefore once a
crack began, it would immediately spread to column 4.8E. In addition under ideal lifting
conditions, column 2H demonstrated unsuitably high compressive and punching shear
stresses (Poston, Feldmann, and Suarez, 1991).
Theory 4
OSHA found that the header bar-to-channel welds on one side of the 9th floor
shearhead at column E3.8 had failed. The use of one-sided square-groove welds for the
header bar-to-channel connection was dubious from the start, since they were not
prequalified joints according to the American Welding Society. Because their penetration
21
was not known, their strength could not be determined. OSHA hired Neal S Moreton and
Associates to examine 30 welds around the shearheads at column E3.8 at the 7th, 8th, and
10th floors. They found only 13 of the 30 welds acceptable; the other 17 were
substandard. The questionable weld details and the substandard welding coupled with
drawings that indicated that the welds would undoubtedly experience forces that they
could not resist all point to weld failure as the trigger of the collapse (McGuire, 1992).
Theory 5
The FaAA studied the tower’s torsional instability and reaction to lateral loading
to understand its collapse. When the concrete slabs are temporarily resting on the wedges,
the connection is rotationally stiff, but as soon as the slab is lifted off one of the wedges
into its final position it can rotate freely from the column. Once the wedges are fully
welded into their final position the connection becomes rigid again. In the absence of
lateral loading, the tower is completely stable.
Wedged Slab-to-Column Connection
Lateral loading and displacement, however, can cause the slab to lift off one of its
wedges causing the structure to become laterally flexible. The FaAA used 3-D computer
modeling (ANSYS) and nonlinear stability modeling to study this phenomenon. Their
investigation and analysis lead them to the conclusion that the towers’ sensitivity to
lateral displacement caused its collapse. While the FaAA acknowledges that another
mechanism could have triggered the lateral displacement, they believe that lateral jacking
provided sufficient displacement to initiate the collapse ((Moncarz, Hooley, Osteraas, and
Lahnert, 1992).
22
Legal Repercussions
A two-judge panel mediated a universal settlement between 100 parties closing
the L’Ambiance Plaza case. Twenty or more separate parties were found guilty of
"widespread negligence, carelessness, sloppy practices, and complacency." They all
contributed, in varying amounts, to the $41 million settlement fund. Those injured and
the families of those killed in the collapse received $30 million. Another $7.6 million was
set aside to pay for all of the claims and counter claims between the designers and
contractors of L’Ambiance Plaza. While this settlement kept hundreds of cases out of
court and provided rapid closure to a colossal collapse, it also ended all investigations
prematurely, leaving the cause of collapse undetermined (Korman, Nov 24, 1988).
Technical Concerns
While buildings constructed by the lift-slab method are stable once they are
completed, if great care is not taken during construction they can be dangerous. The
following measures can be taken to insure lateral stability and safety during construction.
• During all stages of construction, temporary lateral bracing should be provided.
• Concrete punching shear and connections redundancies should be provided in the
structure (Kaminetzky, 1991).
• Cribbing (temporary posts which support the concrete slab until it is completely
attached to the column) should be used.
• Sway bracing (cables which keep the stack of floors from shifting sideways) should
be used. This was required, but not used in L’Ambiance Plaza (Levy and Salvadori,
1992).
Due to the terms of the settlement, many of the technical lessons that could have been
learned from this incident have been lost forever.
Procedural Concerns
The L’Ambiance Plaza collapse highlighted several procedural deficiencies.
Responsibility for design was fragmented among so many subcontractors that several
design deficiencies went undetected. If the engineer of record had taken responsibility for
the overall design of the building or a second engineer had reviewed the design plans
23
these defects probably would have been detected (Heger, 1991). Also, standardized step-
by-step procedures for lift-slab construction should be established to ensure the safety of
the construction workers. A licensed professional engineer should be present during
construction to ensure that these guidelines are followed (Kaminetzky, 1991).
Ethical Concerns
While L’Ambiance Plaza was designed to be safe once it was completed, during
construction it had a considerably lower factor of safety. This is all too common in the
construction industry today (Heger, 1991). Canon 1 of the American Society of Civil
Engineers (ASCE) Code of Ethics states, "Engineers shall hold paramount the safety,
health and welfare of the public and shall strive to comply with the principles of
sustainable development in the performance of their professional duties" (ASCE Code of
Ethics, 1998). This includes the safety of construction workers. Building regulations do
not sufficiently consider structural safety during construction and should be changed to
require a high standard of safety during construction as well as after a building’s
completion. In the absence of such regulations, however, an ethical engineer must always
consider the safety of the workers (Heger, 1991).
HARTFORD CIVIC CENTER ARENA COLLAPSE - HARTFORD, CN - 1978
Design and Construction
In 1970 Vincent Kling agreed to be the architect for the Hartford civic center.
Shortly thereafter he hired Fraoli, Blum, and Yesselman, Engineers (F,B,&Y) to design
the arena. In order to save money, F,B,&Y proposed an innovative design for the 300 by
360 ft. space frame roof over the arena. The proposed roof consisted of two main layers
arranged in 30 by 30-ft grids composed of horizontal steel bars 21-ft apart. 30-ft diagonal
bars connected the nodes of the upper and lower layers, and, in turn, were braced by a
middle layer of horizontal bars. The 30-ft bars in the top layer were also braced at their
midpoint by intermediate diagonal bars.
24
Space Frame Roof
Section of the Space Frame Roof
This design departed from standard space frame roof designing procedures in five ways.
1. The configuration of the four steel angles did not provide good resistance to buckling.
The cross-shaped built up section has a much smaller radius of gyration than either an
I-section or a tube section.
2. The top horizontal bars intersected at a different point than the diagonal bars rather
than at the same point, making the roof especially susceptible to buckling.
3. The top layer of this roof did not support the roofing panels; the short posts on the
nodes of the top layer did. Not only were these posts meant to eliminate bending
stresses on the top layer bars, but their varied heights also allowed for positive
drainage.
4. Four pylon legs positioned 45-ft inside of the edges of the roof supported it instead of
boundary columns or walls (Levy and Salvadori, 1992).
25
5. The space frame was not cambered. Computer analysis predicted a downward
deflection of 13-in at the midpoint of the roof and an upward deflection on 6-in at the
corners. These deflections were taken into account (ENR, Jan 26, 1978).
Because of these money-saving innovations, the engineers employed state of the art
computer analysis to verify the safety of the building.
A year later construction began. To save time and money, the roof frame was
completely assembled on the ground. While it was still on the ground the inspection
agency notified the engineers that it had found excessive deflections in some of the
nodes. Nothing was done. After the frame was completed, hydraulic jacks located on top
of the four pylons slowly lifted it into position. Once the frame was in its final position
but before the roof deck was installed, its deflection was measured to be twice that
predicted by computer analysis, and the engineers were notified. They, however,
expressed no concern and responded that such discrepancies between the actual and the
theoretical should be expected (Levy and Salvadori, 1992). When the subcontractor
began fitting the steel frame supports for fascia panels on the outside of the truss he ran
into great difficulties due to the excessive deflections of the frame. Upon notification of
this problem, the general contractor "directed the subcontractor to deal with the problem
or be responsible for delays." As a result the subcontractor coped some of the supports
and refabricated others in order to make the panels fit, and construction continued (ENR,
April 6, 1978). The roof was completed on January 16, 1973 (Feld and Carper, 1997).
The next year, a citizen expressed concern to the engineers concerning the large
downward deflection he noticed in the arena roof, which he believed to be unsafe. The
engineers and the contractor once again assured the city that everything was fine (Levy
and Salvadori, 1992).
Collapse
On January 18, 1978 the Hartford Arena experienced the largest snowstorm of its
five-year life. At 4:15 A.M. with a loud crack the center of the arena's roof plummeted
the 83-feet to the floor of the arena throwing the corners into the air. Just hours earlier the
arena had been packed for a hockey game. Luckily it was empty by the time of the
collapse, and no one was hurt (Ross, 1984).
26
Causes of Failure
Hartford appointed a three-member panel to manage the investigation of the
collapse. This panel in turn hired Lev Zetlin Associates, Inc. (LZA) to ascertain the cause
of the collapse, and to propose a demolition procedure (Ross, 1984). LZA discovered that
the roof began failing as soon as it was completed due to design deficiencies. A
photograph taken during construction showed obvious bowing in two of the members in
the top layer. Three major design errors coupled with the underestimation of the dead
load by 20% (estimated frame weight = 18 psf, actual frame weight = 23 psf) allowed the
weight of the accumulated snow to collapse the roof (ENR, April 6, 1978). The load on
the day of collapse was 66-73 psf, while the arena should have had a design capacity of at
least 140 psf (ENR, June 22, 1978). The three design errors responsible for the collapse
are listed below.
• The top layer's exterior compression members on the east and the west faces were
overloaded by 852%.
• The top layer's exterior compression members on the north and the south faces were
overloaded by 213%.
• The top layer's interior compression members in the east-west direction were
overloaded by 72%.
In addition to these errors in the original design, LZA discovered that the details omitted
the midpoint braces for the rods in the top layer. The exterior rods were only braced
every 30-feet, rather than the 15-feet intervals specified, and the interior rods were only
partially and insufficiently braced at their midpoints. This significantly reduced the load
that the roof could safely carry. The table below compares some of original details to
actual designs used in the building, demonstrating the reduction in strength that these
changes caused. Connection A was typically used on the east-west edges of the roof,
while connection B was used on the north-south edges. Most of the interior bars used
connection C, while a few used connection D. The key difference between the original
and the as-built details is that the diagonal members were attached some distance below
the horizontal members, and thus were unable to brace the horizontal members against
buckling.
27
Connection A Connection B Connection C Connection D
OriginalDesign
Allowable force:160,000 lb
Allowable moment:0
Allowable force:185,000 lb
Allowable force:625,000 lb
Allowable force:565, 000 lb
ActualDesign
Allowable force:15,440 lb
Allowable moment:9,490 lb-ft
Allowable force:59,000 lb
Allowable force:363,000 lb
Allowable force:565,000 lb
Original vs. Actual Design*Drawings by Rachel Martin based on information from ENR, April 6, 1978
The most overstressed members in the top layer buckled under the added weight
of the snow, causing the other members to buckle. This changed the forces acting on the
lower layer from tension to compression causing them to buckle also. Two major folds
formed initiating the collapse (ENR, April 6, 1978). These were not the only errors that
LZA discovered. Listed below are other factors which contributed to, but could not have
caused, the collapse.
• The slenderness ratio of the built-up members violated the American Institute of Steel
Construction (AISC) code provisions.
• The members with bolt holes exceeding 85% of the total area violated the AISC code
(ENR, June 22, 1978).
• The spacer plates were placed too far apart in some of the four-angle members
allowing individual angles to buckle.
• Some of the steel did not meet specifications.
• There were misplaced diagonal members (Feld and Carper, 1997).
AssumedBrace
AssumedBrace
NoBrace
NoBrace
28
Loomis and Loomis, Inc. also investigated the Hartford collapse. They agreed
with LZA that gross design errors were responsible for the progressive collapse of the
roof, beginning the day that it was completed. They, however, believed that the torsional
buckling of the compression members, rather than the lateral buckling of top chords,
instigated the collapse. Using computer analysis, Loomis and Loomis found that the top
truss rods and the compression diagonals near the four support pylons were approaching
their torsional buckling capacity the day before the collapse. An estimated 12 to 15 psf of
live load would cause the roof to fail. The snow from the night before the collapse
comprised a live load of 14 to 19 psf. Because torsional buckling is so uncommon, it is
often an overlooked mode of failure (ENR, June 14, 1979).
Hannskarl Bandel, a structural consultant, completed an independent investigation
of the collapse for architect's insurance company. He blamed the collapse on a faulty
weld connecting the scoreboard to the roof. This opinion conflicts with the opinions of all
the other investigators (ENR, June 24, 1979).
Legal Repercussions
Six years after the collapse, all of the parties reached an out-of-court settlement.
While this was beneficial to the parties involved, it robbed the engineering world of the
precedents that such a case could set. The issue of who ultimately holds responsibility
for the structural integrity of a project when the tasks are divided among numerous
subcontractors or if anyone does was never resolved (Feld and Carper, 1997).
Technical Concerns
The engineers for the Hartford Arena depended on computer analysis to assess the
safety of their design. Computers, however, are only as good as their programmer and
tend to offer engineers a false sense of security. The roof design was extremely
susceptible to buckling which was a mode of failure not considered by that particular
computer analysis and, therefore, left undiscovered (Shepherd and Frost, 1995). A more
conventional roof design would have been much stronger. Instead of the cruciform shape
of the rods, a tube or I-bar configuration would have been much more stable and less
vulnerable to bending and twisting. Also, if the horizontal and diagonal members
29
intersected at the same place it would have reduced the bending stresses in these
members. Finally, the failure of a few members would not have triggered such a
catastrophic collapse if the structure had been designed and built with more redundancy
(Levy and Salvadori, 1992).
Procedural Concerns
The Hartford Arena contract was divided into five subcontracts coordinated by a
construction manager. Not only did this fragmentation allow mistakes to slip through the
cracks, but it also left confusion over who was responsible for the project as a whole.
Even though the architect recommended that a qualified structural engineer be hired to
oversee the construction, the construction manager refused saying that it was a waste of
money and that he would inspect the project himself. After the collapse he disclaimed all
responsibility on the grounds that a design error had caused the collapse. He believed that
he was only responsible for insuring that the design was constructed correctly and not the
performance of the project. It is important for the responsibility for the integrity of the
entire project to rest with one person. Fragmented responsibility leaves no one with a
sense of or a real concern for how everything will work together making errors more
likely to go undetected.
As a result of the construction manager's refusal to hire a structural engineer for
the purpose of inspection, no one realized the structural implications of the bowing
structures. This collapse illustrates the importance of having a structural engineer,
especially the designer, perform the field inspection. The designer understands the
structure that is being built and would best be able to recognize the warning signs of bad
design and rectify them before they grow to catastrophic proportions.
Finally, the Hartford department of licenses and inspection did not require the
project peer review of the arena design that it usually did for projects of this magnitude.
If a second opinion had been required, the design deficiencies responsible for the arena's
collapse probably would have been discovered. Peer reviews are an essential safety
measure for all high capacity buildings and structures experimenting with new design
techniques (Feld and Carper, 1997).
30
Ethical Concerns
The excessive deflections apparent during construction were brought to the engineer's
attention multiple times. The engineer, confident in his design and the computer analysis
which confirmed it, ignored these warnings and did not take the time to recheck its work.
An ethical engineer would pay close attention to unexpected deformations and investigate
their causes. They often indicate structural deficiencies and should be investigated and
corrected immediately. Unexpected deformations provide a clear signal that the structural
behavior is different from that anticipated by the designer. Also this collapse raises the
important question of whether the factor of safety should be increased for buildings with
high occupancy. Should the impact of a possible failure be taken into account in
determining the factor of safety (Kaminetzky, 1991)?
CONCLUSIONS
Failure plays an important role in engineering practice. Through failure analysis
engineers can learn to avoid similar technical errors allowing them to build stronger, safer
structures. Since failure analysis plays such an integral role in a good engineer's
professional career, it only makes sense that, in college, engineering students should be
taught how to analyze engineering failures, as well as their importance to any engineer's
professional life. In light of an already overcrowded undergraduate engineering
curriculum, integrating failure case studies into already existing engineering classes is the
most logical solution. This approach gives students a better idea of the obstacles that will
face them after college, in addition to demonstrating how the theoretical ideas taught in
their classes are actually applied by engineers. The only real obstacle that lies in the way
of increased failure awareness at an undergraduate level is the absence of adequate
resources, such as well-developed failure case studies and appropriate illustrations. This
paper provides professors and students with four failure case studies that can be
integrated into undergraduate classes.
31
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Recommended