1679 report

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,

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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.

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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).

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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).

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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

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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


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.

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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

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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

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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

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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.

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• 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.

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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?

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L'AMBIANCE PLAZA - BRIDGEPORT, CN - 1987


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).

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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).

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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


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:565, 000 lb

ActualDesign


Allowable moment:9,490 lb-ft




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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