Failure by Design Hyatt Regency

Failure By Design (Two Case Studies of Bridge Design Failure)

by Granger Meador Physics Teacher, Bartlesville High School Bartlesville, OK failurebydesign.info

FAILURE BY DESIGN (Two Case Studies of Bridge Design Failure)

by Granger Meador Published by the Author Version 2.0 Contact information: Granger Meador Physics Instructor Bartlesville High School 1700 SE Hillcrest Drive Bartlesville, OK 74003-7299 1-918-336-3311 FAX 1-918-337-6226 Email: [email protected] or [email protected] WWW: http://failurebydesign.info

This work is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

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Failure By Design by Granger Meador (failurebydesign.info)

PURPOSE Disasters are inherently intriguing to students, and can motivate them to engage in high levels of analysis. My goals in having my own 11-12th grade physics students analyze the Hyatt Regency hotel walkways collapse and the failure of the Tacoma Narrows bridge include: • • • •

develop students’ analytical and deductive skills in determining the possible and actual failure modes of a structure illustrate for students how engineers and architects must utilize physics in designing safe structures have students use their knowledge of force loading and vectors to explain the flaw in the design change of the Hyatt walkways introduce concepts of wave motion and aerodynamics to explain the vibrations and failure of the Tacoma Narrows bridge

I have attempted to include enough information in this document to allow other teachers to deal with each disaster at varying levels of sophistication. Teachers’ own instructional goals and knowledge of their students’ level of conceptual and mathematical development will dictate how much time and effort they wish to devote to these topics. The Hyatt disaster is introduced as a puzzle to be solved, to increase student interest and motivation. Evidence is presented for students to debate and analyze in small groups or as a class. Students can use the evidence to develop and evaluate speculative conceptual solutions, which are then judged against additional evidence gathered after the accident. I have also included any additional data I could find for optional quantitative analysis. Students are asked to suggest possible design improvements, and the societal aspects of the design failure and the repercussions of the accidents are also presented. The Tacoma disaster is too complex to expect most introductory physics students to analyze its failure. Instead, a qualitative description of its behavior is used to illustrate some aspects of wave motion and point out how a large structure’s aerodynamic behavior cannot be safely neglected. Students are again asked to suggest possible design improvements which are compared to the changes made when the bridge was reconstructed. Many physics texts identify resonance as a reason for the Tacoma disaster; in recent years some engineers have argued that resonance was not involved, but more complex modes of self-excitation. The reader is referred to the technical discussions on the CD-ROM for more information.


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ONLINE PRESENTATION In addition to this handout, there is online presentation at http://failurebydesign.info with graphical images, animations, and movies. The online presentation is adapted from a PowerPoint presentation, and you can progress through it in your web browser.

TACOMA NARROWS VIDEOTAPE The public domain and online clips of the Tacoma Narrows disaster are of limited quality and duration. For the greatest dramatic effect, teachers are urged to consider purchasing a high-quality color video from a respected source. The American Association of Physics Teachers offers a videotape with three segments: Tacoma Narrows Bridge Collapse by Franklin Miller, Jr. (1963), The Puzzle of the Tacoma Narrows Bridge Collapse by R.G. Fuller, D.A. Zollman, & T.C. Campbell (1982), and the combined footage (1998). As of March 2008 the videotape and accompanying user’s guide was available for $48.95 for AAPT members, $37.00 for student members, and $61.00 for non-members. The user’s guide includes a number of suggested activities where students analyze the videotape to obtain quantitative data. Catalog Number: Description:

VT-20 Twin Views of the Tacoma Narrows Bridge Collapse

American Association of Physics Teachers www.aapt.org One Physics Ellipse College Park, MD 20740-3845 PHONE 301-209-3311 The incredible footage of the Tacoma disaster used in the above videotape was shot by a Tacoma camera store owner. His shop is now run by his son-in-law, who also has a videotape and DVD for sale. As of March 2008 the DVD price was $65.00 and the videotape price was $49.00 from: http://www.camerashoptacoma.com

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THE COLLAPSE OF THE HYATT REGENCY HOTEL WALKWAYS IN KANSAS CITY Hallmark Cards, Inc. is headquartered in Kansas City, Missouri. By 1968, the area surrounding its headquarters south of downtown suffered from urban blight, and the company began the massive “Crown Center” 85-acre redevelopment project. As part of the project, a Hyatt Regency Hotel was designed and constructed from 1976 to 1980. Unfortunately, the hotel had design and construction problems which led to a horrific disaster one year after it opened. The hotel has three main sections: a 40-story tower, a function block, and a connecting atrium area. The atrium’s dimensions are 145 ft (44.2 m) by 117 ft (35.7 m), and 50 ft (15.2 m) high. For guest convenience, three walkways were constructed along the width of the atrium to connect the tower and function block on the second, third, and fourth floors of the structure. The walkways were suspended from the ceiling so that the main floor would not be obstructed by support columns. The third floor walkway, which connected to a ballroom, was wider than the others and suspended from the roof on its own set of rods. The fourth floor walkway, which led to the health club and sports area, was also suspended from the roof by rods. The second floor walkway was suspended by rods from the fourth floor walkway, as seen in the architectural rendering at right.

The Disaster: On July 17, 1981 the hotel had been open for a year and a local radio station was holding a dance competition. By 7 pm the atrium was crowded with between 1500 and 2000 people, with many spectators observing from the walkways. At 7:05 pm a loud crack echoed through the building and the second and fourth floor walkways collapsed, killing 114 people and injuring over 200 others.


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Hyatt Puzzle What part(s) of the walkways failed? Given the overall design as constructed, what parts of the support structure could have failed to cause both the second and fourth-floor walkways to collapse? What evidence would you look for to decide which part actually failed? The fourth-floor walkway was suspended from the roof by three sets of hanger rods. The rods passed through longitudinally welded box beams and were capped with washers and nuts which held up the walkway by pressing on the underside of the box beams. The second-floor walkway was suspended from the fourth-floor walkway by another set of hanger rods. These rods were held up by washers and nuts pressing on the top surface of the 4th-floor box beams. The rods passed through box beams for the second-floor walkway and were again capped with washers and nuts pressing against the underside of the 2nd-floor box beams. Long I-beams running down the sides of each walkway were suspended from the box beams by angle brackets. At right is a closeup view of the construction of the 4th-floor walkway and its connections to the hanger rods.

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Hyatt Structural Details Walkway length: Walkway width: Hanger rod diameter: Hanger rod length: Inset for 2nd-floor rods: I-beams: Box beams:

Walkway floor:

117 feet (35.7 m) approx. 18 feet (5.49 m); 4th and 2nd-floor walkways were 15.6 in (0.397 m) narrower than separate 3rd-floor walkway 1.26 in (32 mm) as built, each approx. 20-23 ft (6-7 m); 4th-floor connection was 6.1 m below roof 4 in (102 mm) inward along axis of box beam 16-inch (40.6 cm) deep (W16x26 steel: wide flange steel beam of 16-in depth, weighing 26 pounds per foot) two 8-in (20.3 cm) deep rectangular channels welded toeto-toe (MC8x8.5: flanged channel steel beams of 8-in depth, weighing 8.5 pounds per foot); Jensen (2000) assumed capacity of 115 kips or 115,000 pounds of force concrete, 3.25 in (8.26 cm) thick, approx. 150 lb/ft3

Hyatt Possible Conceptual Solutions/Failure Modes These are presented to assist you in leading a student discussion. Likely failure modes: 1)

The hanger rods from the roof to the fourth-floor walkway snapped or the roof connection failed.

Examination of the accident scene would show the upper rods broken off, most likely near the roof where the load in each rod would be maximized.

2)

The nuts under the fourth-floor box beams stripped free.

3)

The fourth-floor box beams gave way at the ends, allowing the washers and nuts underneath them to punch through.

The accident scene should reveal nuts stripped from the hanger rods, but few if any punctures of the nuts through the fourth-floor box beams.

The accident scene should reveal washers and nuts still on the rods hanging from the roof, with punctures through the box beams.

4)

A combination of the above failure modes occurred.

One would expect a mix of the above physical evidence in the debris.


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Unlikely failure modes: 5)

The fourth-floor box beams gave way, but not at the ends, allowing the washers and nuts above them to punch through.

The physics of the situation makes this highly unlikely: the washer/nuts under the beam carry a much greater load than the washer/nuts above the beam. This would, however, be able to cause the full failure of the fourth-floor walkway if the punctured box beams separated sufficiently along the welds to allow the washers and nuts under the beam to escape.

6)

The nuts above the fourth-floor box beams stripped free.

7) 8) 9)

The nuts below the second-floor box beams stripped free. The washers and nuts below the second-floor box beams pulled through those beams. The hanger rods for the second-floor walkway snapped.

10)

The I-beams or angle brackets on one of the walkways failed.

This scenario suffers from the same defect as the one above.

Each of the above three scenarios would likely only lead to the failure of the secondfloor walkway, rather than both the second and fourth-floor walkways. However, if one side of the second-floor walkway failed, sufficient stress or torque could be placed on the upper walkway to make it collapse.

This would have allowed one section of a walkway to collapse, but the remaining sections would likely have survived.

Evaluation of these possibilities: Possible failure modes 1, 2, and 3 will likely be the ones most frequently raised in discussion. There is no direct evidence given of the relative load-bearing strength of the hanger rods, nuts, or box beams. The actual solution must be ascertained from an examination of the accident scene. There is, however, an important clue we have not yet revealed: a design change in the fourth-floor connection. This change would make the placement of the washers and nuts above and below the welded seams on the box beams the culprit.

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Hyatt Clue: The Design Change To help one pick from the alternatives, a valuable clue is a design change that occurred. The original conceptual design did not have one set of hanger rods going to the fourth-floor walkway and then another set from it to the second-floor walkway. Instead, originally there were to be long hanger rods attached to the roof and passing through the box beams under the fourthfloor walkway and continuing down through the box beams under the second-floor walkway. The schematic at right shows the original design for the connection between the hanger rod and the fourth-floor walkway. This original design called for extremely long hanger rods, which were difficult to manufacture and install. Another problem was that the design did not use sleeve nuts, but somehow threads for a nut had to appear 6.1 m along each rod. This would have either required a specially manufactured rod with extruding threads at that location, or a rod with threading 6.1 m down its length from one end. The steel company subcontracted for the hanger work proposed a simple design change which was approved by the structural engineers for the project. The change consisted of replacing each single long hanger rod with a pair of rods offset on the fourth-floor walkway’s box beams, as seen earlier and shown again at right. This allowed for easy installation of a nut and washer below the beam to support the fourth-floor walkway, and another nut and washer above the beam supporting a hanger rod for the second-floor walkway. This greatly reduced the expense and complexity of the construction.

Original design

As-built

The size of the washers and nuts and the design of the box beam were NOT changed when this change in the 4th-floor connection was made.

Now which part(s) do you think most likely failed?


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Hyatt Conceptual Solution What actually happened was that the washers and nuts below the fourth-floor box beams punctured those beams, as shown by the photographs on this page (and the final parts of the video animations on the accompanying CDROM).

Fourth-floor hanger rods intact, with nuts and washers in place

Close-up of fourth-floor box beam, showing how upper hanger rod pulled completely through (hanger rod from fourth to second floor is still in place)

The design change was conceptually flawed, as it doubled the load on the upper box beam. A useful analogy is that of two monkeys hanging from a rope: the design change is equivalent to tying one monkey onto the other with a second rope, making the first monkey’s grip on the rope treacherous.

Close-up of intact nut and washer on upper hanger rod

The actual failure began with the center rod connection on one side of the 4thfloor walkway, and then all of that side’s connections failed. (The top west rods at the accident scene were bent west while the east rods were not distorted.) Page 10 of 34


Hyatt Quantitative Solution A common misconception is that the design change halved the number of people the walkways could support. In fact, the change had a much more dramatic impact on the number of people that could be supported. Knowing that the walkways as built could barely support themselves and with some data from the National Bureau of Standards, we can calculate the probable impact of the design change: An investigation by the National Bureau of Standards reported: • • •

The Kansas City Building Code required a minimum support value of 151,000 newtons for each hanger rod/box beam connection in the original design. A test showed that each connection could, on average, support only 90,000 newtons. This was only 60% of the required minimum for the original design. The design change doubled the load on the fourth floor box beam, so the code would have called for a support value of 302,000 newtons at the fourth floor connections, yet as-built those connections could still only support 90,000 newtons.

Q:

Assume that the two walkways themselves each weighed 220,000 N. Each of the six 4th-floor hanger rod/box beam connections could support 90,000 N, as could the six 2nd-floor connections. How many 150 lb (667 N) people could the walkway connections support a) as built? b) as originally designed? c) if built with the original design, but meeting code?

A.

a)

AS-BUILT: (6 * 90,000 N) – (2 * 220,000 N) = 100,000 N remaining capacity (we have to subtract the weight of BOTH walkways from ONLY the 4th floor total connection strength, since as-built those connections carried both walkways) 100,000 N / 667 N = 150 total people on both walkways (video at time of collapse showed about 80 people on the 4th-floor walkway)

b)

AS ORIGINALLY DESIGNED: (6 * 90,000 N) + (6 * 90,000 N) – (2 * 220,000 N) = 640,000 N remaining capacity (as designed the 2nd and 4th-floor connections were independent of each other) 640,000 N / 667 N = 960 total people on both walkways (which is over SIX TIMES more than the as-built design)

c)

IF BUILT TO ORIGINAL DESIGN, BUT MEETING CODE: (6 * 151,000 N) + (6 * 151,000 N) – (2 * 220,000 N) = 1,372,000 N remaining capacity 1, 372,000 N / 667 N = 2057 total people on both walkways (which is almost FOURTEEN TIMES more than the as-built design)

Thus one can see that if the design change had not been made, the walkways would likely still be intact to this day, giving no indication that they did not meet the conservative building code. It also turned out that the hanger rods did not meet specifications: the preliminary design sketches for the walkways called for hanger rods with a strength of 413 megapascals. This was omitted from the final structural drawings and the contractor specified rods with a strength of only 248 megapascals. However, their weakness played no role in the catastrophe.


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Hyatt Quick Demonstration The doubling of the load on the fourth-floor box beams due to the design change can be illustrated with some very simple equipment: 3 spring scales 2 equal weights support One spring scale will represent the load at the roof connection for a hanger rod. The other scales represent the load on the second and fourth-floor box beams. Original design analogy: Hang the first scale from the support. Hang both of the remaining scales from the first scale. Hang the weights from those scales. The two “box beam” scales will read identical amounts, while the “roof connection” scale will read approximately twice what the other scales read.

As-built analogy: Now shift one of the lower scales so that it hangs from the other lower scale. Now the lower “box beam” scale reads the same amount as before, but the upper “box beam” scale reads twice that, almost as much as the top “roof connection” scale.

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Hyatt Consequences There were a number of warnings during its construction that the Hyatt hotel project was troubled. The atrium roof collapsed during construction, fortunately during a weekend so that no one was injured. After the walkways were up, there were reports that construction workers found them unsteady under heavy wheelbarrows, but the construction traffic was merely rerouted and the walkway design not subjected to intense scrutiny. After the terrible tragedy, the Kansas City Star newspaper hired consulting engineers to review the evidence and identify the cause of the accident. Within four days of the failure, the front page of the Star showed drawings pinpointing the cause. The investigative reporting of the accident won a Pulitzer Prize that year. Within days of the tragedy, the remaining walkway for the third floor was dismantled and removed in the middle of the night, despite protests from the city mayor. The hotel owners argued it was a hazard, but attorneys for the victims objected. Engineers studying the tragedy were disappointed they did not have the chance to use the remaining walkway to test theories that the tempo of dancers on the walkways may have contributed to the collapse (similar to the vibration problems of the Tacoma Narrows Bridge). After a twenty-month investigation, the U.S. attorney and county prosecutor announced they had no evidence of a federal or state crime in connection with the collapse. But two months later the state attorney general charged the design engineers with “gross negligence” and the U.S. Department of Commerce eventually concluded that the steel fabrication contractor (Havens Steel Company) was not at fault. The design engineers had approved the original design that did not meet code as well as the design change that weakened the structure. During a 26-week administrative trial the detailer, architect, fabricator, and technician on the project 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 when in reality he had never performed any calculations for this design at all. The Missouri Board of Architects, Professional Engineers, and Land Surveyors eventually convicted engineer of record Jack D. Gillum and project engineer Daniel M. Duncan of gross negligence, misconduct, and unprofessional conduct in the practice of engineering. Their Missouri (and later Texas) professional engineering licenses were revoked, and their company (Gillum-Colaco or G.C.E. Inc.) lost its certificate of authority as an engineering firm. As of September, 2000 Gillum was an Adjunct Professor of Civil Engineering at Washington University in St. Louis, Missouri. The tragedy eventually led to what has been reported as over $140 million in damages awarded in civil cases brought by the victims and their families. These amounts dwarfed the half million dollar cost of the building. Failure By Design by Granger Meador (failurebydesign.info)

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Hyatt Prevention This accident could have been prevented by a better design, better design procedures, and ethical engineer behavior. Better Design: There are a bewildering number of design details in a massive project like the Hyatt Regency, and it is too easy when one presented with the accident as a puzzle to think that the problems in the design were obvious. But there were some severe deficiencies in the walkway design. The original design, with its extremely long hanger rods and awkward nut threading, was difficult to implement, and it did not meet building codes. Engineers have suggested possible improvements: • add additional hanger rods and box beams to reduce the load on each connection • use larger washers to lower the pressure on each box beam’s welded face • keep the toe-to-toe channels in the box beams, but add bearing crossplates to transfer part of the load to the sides of the box beam (see upper diagram) • rework the box beams to have back-to-back channels with web stiffeners to prevent buckling (see lower diagrams) • add support pillars from the floor as an alternate load path So how was the Hyatt atrium rebuilt? Noted engineering author Henry Petroski reported, “Today the Hyatt Regency lobby in Kansas City is spanned by a single walkway resting on stout columns sitting on the solid floor (1985, p. 93).” Better design procedures: In this disaster, the firms involved did not have clear procedures for approval of design changes. Procedures should ensure that the engineers recalculate loads and safety factors when a support structure is redesigned. In this case, there was also a procedural failure when the shop detail specifying the required rod strength was not included in the final drawings. Ethical engineers: Martin (1999a) criticized the project engineer: “Neglecting to check the safety and load capacity of a crucial hanger even once shows his complete disregard for the public welfare.” Page 14 of 34


ACTIVITY: Balsa Bridge Building

courtesy of Pitsco (www.pitsco.com)

Objective: Design and engineer a bridge using 1/8 inch x 1/8 inch balsa wood to hold as much mass as possible over a 12-inch span. Construction: 1. The overall width of the bridge may not exceed 3 inches. 2. The overall length of the bridge may not exceed 15 inches. 3. The bridge shall allow a 1-inch wide x 3/4-inch thick board to pass through it over the roadbed. 4. The bridge shall have no structures below the abutments that support the bridge. 5. The bridge shall allow a 3/8-inch bolt to pass through the center of the bottom of the bridge unobstructed (for testing). 6. The bridge shall be constructed entirely of 1/8-inch x 1/8-inch balsa wood. 7. Any common adhesive may be used at the joints of the wood members. 8. Adhesives may be used only at joints. 9. Wood joints may be notched if desired. 10. The mass of the bridge may not exceed 40 grams. Competition: 1. The mass of the bridge will be determined before testing. 2. All bridges will use the same test device. 3. The bridge will be tested using a 1-1/2-inch wide x 10-inch long x 3/4-inch thick wood block with a hole in its center for the testing mechanism. 4. The bridge is placed on the bridge tester, with the span set at 12 inches. 5. The teacher will attach the testing mechanism to the bridge. 6. When the testing mechanism is ready, the student will begin to add sand to the bucket at the rate he or she chooses. 7. There will be a 10-minute time limit on adding sand to the bucket. 8. The student will continue to add sand to the bucket until the bridge collapses, and the bucket falls. 9. The mass of the sand will be measured. 10. The bridge supporting the greatest load is the winner. 11. In the event that the bucket is completely filled without breaking for more than one bridge, then, of those bridges, the one that has the least mass wins. Variations: • Vary the length of the span of the bridge from 8 inches to 24 inches. • Assign dollar amounts to supplies (wood and glue), and judge the bridges on cost efficiency (cost per pound of load held). Products for this competitive event can be found in the Pitsco Competitive Events Theme Catalog.


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ACTIVITY: Toothpick Bridge Building

courtesy of Pitsco (www.pitsco.com)

Objective: Design and engineer a bridge using toothpicks (or Blunt End Structure Sticks) to hold as much mass as possible over a span of 6 inches. Construction: 1. The overall width of the bridge may not exceed 3 inches. 2. The overall length of the bridge may not exceed 9 inches. 3. The bridge shall allow a 1-inch wide x 3/4-inch thick board to pass through it over the roadbed. 4. The bridge shall have no structures below the abutments that support the bridge. 5. The bridge shall allow a 3/8-inch bolt to pass through the center of the bottom of the bridge unobstructed (for testing). 6. The bridge shall be constructed entirely of toothpicks (or Blunt End Structure Sticks). 7. Any common adhesive may be used at the joints of the wood members. 8. Adhesives may be used only at joints. 9. The mass of the bridge may not exceed 20 grams. Competition: 1. The mass of the bridge will be determined before testing. 2. The bridge will be tested using a 1-1/2-inch wide x 4-inch long x 3/4-inch thick wood block with a hole in its center for the testing mechanism. 3. The bridge will be placed on the bridge tester, with the span set at 6 inches. 4. The teacher will attach the testing mechanism to the bridge. 5. When the testing mechanism is ready, the student will begin to add sand to the bucket at the rate he or she chooses. 6. There will be a 5-minute time limit on adding sand to the bucket. 7. The student will continue to add sand to the bucket until the bridge collapses and the bucket falls. 8. The mass of the sand will be measured. 9. The bridge supporting the greatest load is the winner. Variations: • Vary the length of the span of the bridge. • Assign dollar amounts to supplies (toothpicks and glue) and judge the bridges on cost efficiency (cost per pound of load held). • Model the bridge after a local or famous bridge. Products for this competitive event can be found in the Pitsco Competitive Events Theme Catalog.

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THE COLLAPSE OF THE TACOMA NARROWS BRIDGE A costly yet less tragic accident than the Hyatt walkway collapse was the dramatic demise of the Tacoma Narrows Bridge in 1940. The collapse is well remembered, thanks to color film footage shot by a local camera store owner. The general design problems of the bridge were identified long ago, but the precise technical explanation for its collapse is still a subject of debate.

The first suspension bridge to connect the Olympic Peninsula with the mainland of Washington State, the bridge linked Seattle to Tacoma with the nearby Puget Sound Navy Yard. The Washington State Toll Bridge Authority spent over $6 million on the project. Construction began on November 23, 1938 and concluded on July 1, 1940. The bridge would collapse four months later, on November 7.

Koughan (1996) notes, “Suspension bridges work on essentially the same principle as a clothesline. This type of bridge fundamentally consists of cables anchored to the earth at their ends and supported by towers at intermediate points. From these cables a floor or ‘deck’ is suspended.” These bridges are inherently more flexible than other designs, and require bracing to reduce vertical and torsional motions. The inadequate design of the Tacoma Narrows bridge would subject it to vertical and torsional motions of incredible and destructive magnitudes.


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Bridge Deck Design One cause for the inadequate design was cost-cutting imposed when one of the funding sources, the Federal Public Works Administration, balked at the $11 million design from the state’s Toll Bridge Authority. A $7 million budget was eventually secured and a radically narrow, shallow-deck design was put forward by the respected bridge designer Leon S. Moisseiff. This aesthetically pleasing design substituted the usual deep open trusses with 8-foot tall steel Ibeams along the sides of the deck. The bridge was the third-longest in the world at the time, with a 2800 foot center span, but was only 39 feet wide with two lanes of traffic and two sidewalks.

Bridge deck under construction; note solid Ibeams along sides

Open trusses under the deck of the reconstructed Tacoma Narrows Bridge

Overhead, side, and cross-sectional views of the bridge and its deck bracing

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Attempts to Reduce Vibration The bridge began exhibiting unusually large vertical oscillations (transverse waves) during construction. Consulting engineer F.B. Farquharson of the Department of Civil Engineering at the University of Washington was brought in to study the bridge’s behavior and recommend solutions. An accurate scale model of the bridge was constructed and tested in a wind tunnel for $20,000. This led to several attempts to correct the bridge’s behavior:

1.

2.

3.

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, although they 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 at the middle of the bridge, were installed. Finally, untuned dynamic dampers, similar to one that had helped curtail torsional vibrations of Moisseiff’s Bronx-Whitestone Bridge in New York, failed immediately after their installation in the Tacoma Narrows Bridge. It was discovered that the leather Center cable stays used in the devices was destroyed during the sandblasting of the steel girders before they were painted, rendering them useless.


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Galloping Gertie Prior to the day of its collapse, the bridge only exhibited vertical oscillations. The transverse modes of vibration had nodes at the main towers, with from 0 to 8 nodes between the towers. The maximum double amplitude (crest to trough) was about 5 ft in a mode with 2 nodes between the towers, with a frequency of 12 vibrations/min. This mode would startle motorists as cars ahead of them would disappear from view and then reappear later. The bridge bounce led a wit to call it “Galloping Girdie” for its motion and its side girders. This was corrupted into the sobriquet “Galloping Gertie.” Some motorists would drive across the bridge and even stop midway to enjoy the bouncing sensation, while others made long detours to avoid it. The most frequently observed vibration was one with no nodes between the towers, a double amplitude of up to 2 feet, and a frequency of 8 vibrations/min. Measurements made before the bridge failed indicated that higher wind speeds correlated to higherfrequency vibration modes. But there was no significant correlation between wind speed and vibration amplitude: winds of 3 or 4 mph could create motions of several feet, while at other times the bridge remained motionless in winds as high as 35 mph.

The Collapse A midnight storm on November 7, 1940 probably weakened the K-bracing under the bridge deck, since a lone observer reported the bridge’s amplitude of vertical vibration increased. By early morning the wind speed was 40 to 45 mph, and the bridge was undergoing large vertical oscillations. By 9:30 am the span was vibrating in 8 or 9 segments at a frequency of 36 vibrations/min and a double amplitude of about 3 feet. Traffic was shut down, but two cars with three passengers were trapped on the bridge when it suddenly began to vibrate torsionally (twist) around 10 am. The passengers, gripping the concrete curbs, crawled to safety. Unfortunately a frightened dog in one car refused to budge and had to be left behind. Farquharson was on the scene studying the bridge. At one point, he walked along the torsional nodal line along the center of the roadway to study the center stays and, incidentally, unsuccessfully attempt to retrieve the dog, which perished in the collapse.

The road deck tilted alarmingly by torsional vibrations; note the center stripe which served as a nodal line of little vibration

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The twisting vibration was in 2 segments between the towers with a frequency of 14 vibrations/min, later changing to 12 vibrations/min. The amplitude of torsional vibration built up to an amazing 35E each direction from horizontal, amid sounds of cracking concrete. This vibration occurred when the north bridge cable loosened in its collar, which was tied to the deck girder by diagonal stays. One stay broke and the other’s cable clamp slipped, allowing the cable to slip back and forth. This allowed the destructive torsional vibration to build up uncontrollably. By 11 am, the vibration was simply too much and the center span broke apart, crashing into the water below. The side spans remained intact, although they sagged about 45 ft. The two towers each sagged shoreward 25 ft at their tops, buckling them. The main cables were intact, except for a 42-inch section in the center of the north cable where the collar scraped; 500 of the 6308 strands of No. 6 galvanized cold-drawn steel wire were ruptured.

Sagging side span Buckling of one tower after collapse Frayed north cable


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Tacoma Puzzles: Why did this bridge oscillate so much? The Tacoma Narrows bridge was a victim of poor aerodynamics. The long, narrow, and shallow deck with its solid I-beam sides was too flexible to survive the winds of the canyon in which it was situated. Most suspension bridges used deep, open trusses which allowed wind to flow through relatively unimpeded. Designers were used to making sure that a bridge could handle static loads, but the dynamic forces of winds were seldom considered at the time of this bridge’s construction. In fact, subsequent to this failure large bridges and buildings were routinely checked for aerodynamic stability, often incorporating wind tunnel testing of scale models. The Federal Works Agency investigated the failure after the collapse, and included on its commission Theodore von Karman, a noted aeronautical engineer. The review stressed that the bridge had met accepted engineering criteria of the time, having been built to accepted safety factors for static loading. It did not cast blame on Leon Moisseiff, but accepted that the bridge failure was the result of design limits being stretched into previously unexplored areas. The commission focused on three possible sources for the destructive dynamics of the bridge: aerodynamic instability producing self-induced vibrations, periodic eddy formations, and random turbulence. Many textbooks today attribute the collapse to resonance. Resonance is when a system begins to oscillate with a large amplitude as it is acted upon by periodic impulses of a frequency approximately equal to one of the natural frequencies of oscillation of that system. However, an analysis by Billah and Scanlan (1991) showed that the bridge’s behavior was not due to simple resonance. Mathematical models of its motion are better explained by more complex selfexcitation mechanisms. Commission member von Karman had proposed that the wind blowing across the bridge deck created turbulent vortices. This process of “vortex shedding” would have created alternating high and low pressure regions on the lee side of the bridge, causing it to oscillate.

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Billah and Scanlan agree that von Karman’s explanation is suitable for the vertical or transverse oscillations the bridge exhibited until the day of failure. But they argue that the twisting or torsional oscillations cannot be attributed to resonance or von Karman’s vortices. Instead, they argue that the north cable slipped back and forth, changing the angle of the bridge deck in the wind. This change in the “angle of attack” would have interacted with the wind: the wind supplying power and the torsional oscillation itself and the resulting changes in deck angle tapping into that power.

Why did this bridge fail when other, longer bridges didn’t? The extreme flexibility of the Tacoma Narrows bridge arose from its shallow deck and narrow width in comparison to its long span. A comparison to other bridges of the time is revealing: Bridge (Location; Designer; Year Opened)

Deck Depth / Span Length

Ratio of Depth to Span

Deck Width / Span Length

Ratio of Width to Span

Tacoma Narrows (Puget Sound, WA; Moisseiff; 1940)

8 ft / 2800 ft

1 : 350

39 ft / 2800 ft

1 : 72

Bronx-Whitestone (Long Island Sound, NY; Moisseiff; 1939)

11 ft / 2300 ft

1 : 209

74 ft / 2300 ft

1 : 31

Golden Gate (San Francisco, CA; Strauss; 1937)

25 ft / 2400 ft

1 : 168

89 ft / 2400 ft

1 : 47

George Washington (New York; Ammann; 1931)

36 ft / 3500 ft

1 : 97

106 ft / 3500 ft

1 : 33

The lower depth-to-span and width-to-span ratios of the other bridges made them much less vulnerable to both vertical and torsional deflections, as shown by the graphs on the following page.


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Comparative vertical deflections (in feet) of the five-longest suspension bridges in 1940, with data showing the slight improvement made in the Bronx-Whitestone Bridge when center stays were added

Comparative torsional deflections (in % tilt of floor) of the five-longest suspension bridges in 1940

What design changes might have prevented this failure? Martin (1999) identified a number of design changes that could have saved the original Tacoma Narrows Bridge: • • • • • •

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 Use an untuned dynamic damper to limit the motions of the bridge (the dampers on the bridge did not work) Increase the stiffness and depth of the trusses or girders Streamline the deck of the bridge

The concept of deep stiffening trusses was applied in retrofitting several other bridges. Moisseiff’s Bronx-Whitestone bridge had a truss added above its side I-beams in 1946. The famous Golden Gate Bridge in San Francisco had a $3.5 million retrofit to stiffen it as well.

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Several design changes were made when a new Tacoma Narrows Bridge was constructed, using the original bridge’s piers. Scale models of the new $18 million bridge were tested in wind tunnels at the University of Washington during its design, and it was a four-lane structure with a 60-ft wide deck and 25-ft deep stiffening open trusses. It has been a successful design for over 50 years.

1940 bridge deck design


1950 bridge deck design

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Analysis of the specifications on this page reveals that the new bridge had a depth/span ratio of 1:112, versus the 1:350 ratio of the failed design. The new design also boasted a width/span ratio of 1:47, versus the 1:72 ratio of Galloping Gertie.

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The Latest Tacoma Narrows Bridge In 2007 another suspension bridge opened at Tacoma Narrow, adjacent to the 1950 structure. There had been hours of daily congestion on the 1950 bridge, leading to thoughts of building another bridge. After seven years in early development, the political arena and the courts, and another five years in design and construction, the new Narrows bridge opened to traffic on July 16, 2007. It was built parallel to and south of the 1950 Narrows Bridge, and carries four 11-foot-wide lanes of eastbound traffic toward Tacoma. The left lane is a high-occupancy-vehicle (HOV) lane, the two center lanes are general purpose lanes open to all traffic, and the right lane is an "add/drop" lane that extends across the bridge to the Jackson Avenue eastbound exit. In addition, the bridge has a 10-foot right shoulder for disabled vehicles, and a 10-foot barrier-separated bicycle/pedestrian lane. -Washington State Department of Transportation


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Wave Demonstrations Slinky Wave Demos Simple but effective demonstrations of wave motion can be performed with a Slinky, two people, and a short length of string. While a regular metal or plastic Slinky will work, you will obtain better results using an extra-long Slinky, such as the one shown in the photograph. You can obtain such springs from science education supply houses. Transverse Waves: A transverse wave vibrates perpendicularly (at right angles) to the wave travel (water waves are a good example). To demonstrate, have two students each take one end of a Slinky and stretch it out along the floor (the waves will be more apparent this way). Have one student move his or her end of a plastic or metal Slinky back and forth (left and right, like a snake crawling), perpendicular to its stretched length. The other student must hold his or her end of the Slinky still. A series of transverse waves will be generated. Transverse Wave Reflection: A wave striking the boundary of a more dense medium will partially reflect, and the reflection will be inverted. This is seen in the above demonstration, where one end of the Slinky is held by a student, whose grip creates a more dense medium for the wave energy. However, a wave striking the boundary of a less dense medium will have an erect reflection. This can be demonstrated by tying some string onto one end of the Slinky and having the student hold the string rather than the Slinky. The string is a less dense medium for wave travel, so waves sent toward the student holding the string will reflect right-side-up. Standing Waves: When a series of wave pulses are reflected off a more dense medium, standing waves can be generated. These distinctive waveforms have places where the medium does not vibrate at all, called nodes, and other places where the medium vibrates the most, called antinodes. When the students are demonstrating transverse waves (without using a string), standing waves with varying numbers of nodes and antinodes can be generated by having the student moving the Slinky vary the rate at which he or she continually moves it back and forth.

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Longitudinal Waves: A longitudinal wave vibrates parallel to (in the same direction of) wave travel (sound waves are a good example). This kind of wave was NOT exhibited in the movies of the Tacoma Narrows Bridge. To demonstrate, have one student grasp and draw toward himself or herself several coils of a stretched metal Slinky and then release the coils. The other student must hold his or her end of the Slinky still. A longitudinal wave pulse will be generated and travel down the length of the Slinky. Longitudinal Compressions and Rarefactions: Longitudinal waves can be composed of compressions, where the parts of the medium (coils of the Slinky) are closer together than normal, or rarefactions, where the parts of the medium are farther apart than normal. In the above demonstration, the students created compressional longitudinal waves. A rarefactional longitudinal wave can be produced by stretching a segment of the Slinky and then releasing it. The stretched area (rarefaction) will then travel along the length of the Slinky.


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The Wave Machine (from Ronald Edge’s String and Sticky Tape Experiments) A fancy Shive wave machine like the one in the photograph will exhibit torsional waves. But you can build your own cheap “wave machine” with straws and sticky tape, as described below. You will need about half a meter of sticky tape, and twenty or so straws. Turn each end of the tape over about one centimeter, and stick them on the desk, or other flat surface as shown in the upper figure. Now place the straws about 1 cm apart crosswise with their centres on the tape as shown in the lower figure. Pick up the ends of the tape, and attach one end to the lintel of a door, or other suitably high place where the chain of straws is free to oscillate. Now sharply tap the bottom straw, and a torsional pulse will travel up the machine and be reflected at the top. Since the top is fixed, the pulse will be of opposite sign on reflection, descending. This device can be used to display almost all the properties of one dimensional transverse traveling and stationary waves. For example, increasing the tension by hanging a weight at the bottom will speed up the waves, loading the ends of the bottom half of the straws (which can easily be done by inserting paper clips into each end of a straw) will be like a "dense medium"- with a lower velocity and reflection at the interface with the light straws. Moving the bottom straw to and fro with the correct period will produce standing waves.

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IMAGE CREDITS for this document used for non-profit educational purposes in accordance with the “fair use” provisions of copyright law

Page 1 1

Description Hyatt walkway collapse Tacoma Narrows bridge

2 4 5

Author photo CD-ROM clipart Hyatt hotel exterior

5

Architect’s walkway rendering Crowd in Hyatt atrium

5 5 5 6 6 9 9 10 10 10 10 12 14 14 17 17 17 18 18 18 18 19 19 20

Source (see reference list for details) L. Lowery, 1999, online image: 11th.gif Institute for Structural Analysis, 1997, extracted image from online movie: http://www.cis.tugraz.at/ifb/img/others/tacoma/tacoma1.mov G. Meador, photo by Ken Dolezal, ISD 30, Bartlesville, OK Corel WordPerfect Clipart Collection Crown Center, Inc., http://www.crowncenter.com/crownhyatt.html, online image: big_hyatt.jpg H. Petroski, 1994, book illustration: p. 59

Exponent, Inc., extracted image from online movie: hyatt_lg.mov Overhead view of collapse R. Martin, 1999a, online image: hyatt1.gif Floor-level view of collapse L. Lowery, 1999a, online image: 10th.gif Schematic of both walkways G. Meador, original to this document H. Petroski, 1994, book illustration: p. 61, Close-up of 4th floor connection as modified by G. Meador for this document Schematics of original and H. Petroski, 1994, book illustration: p. 61 as-built designs Lower schematic C.E. Harris & M.J. Rabins, 1992, p. 157 comparing the designs L. Lowery, 1999, online image: 5th.gif Close-up of 4th floor beam th L. Lowery, 1999, online image: 13th.gif 4 floor hanger rods Close-up of hanger rod L. Lowery, 1999, online image: 4tha.gif Monkey analogy G. Meador, original to this document Demo diagrams G. Meador, original to this document I beam diagrams R. Martin, 1999a: online images: hyatt7.gif & hyatt8.gif Web stiffener Hanley-Wood LLC, 2000: online image: squash2.gif as modified by G. Meador for this document Bridge shot from shore B. Lou, undated, online image: new.jpg Map of Washington state Expedia, 2000, online map as modified by G. Meador for this document Aeriel photo USGS, Old bridge deck R. Andradne, 2000, online image: img10.gif New bridge trusses K. Rogers, 1997 Old deck design J. Koughan, 1996, online image: figB2_jk.gif, as modified by G. Meador for this document Bridge schematics J. Koughan, 1996, online image: figB2_jk.gif Old bridge view J. Koughan, 1996, online image: figB2_jk.gif, as modified by G. Meador for this document Center cable stays D. Smith, 1974, online image: tac12.gif, as modified by G. Meador for this document Titled bridge deck D. Smith, 1974, online image: tac07.gif


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IMAGE CREDITS (continued) Page 21 21 21 21

Description Twisting bridge Bridge collapse Sagging side span Frayed cable

21

Buckled tower

22

Vortex shedding diagram

23

Vortices from bridge tilt

24 24 25 25 25

Vertical deflection graph Torsional deflection graph Old bridge side view New bridge side view Old deck design

25

New deck design

25

Buckled old deck

25 26 26 26 27

New bridge trusses Old bridge shot from shore New bridge Bridge specifications Two bridges, profile view

27

Two bridges, top view

28

Slinky on lecture table

28 28 29 30

Transverse waves Nodes and antinodes Longitudinal waves Shive wave machine

30

Wave machine, upper diagram Wave machine, lower diagram

30

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Source (see reference list for details) D. Smith, 1974, online image: tac06.gif D. Smith, 1974, online image: tac09.gif D. Smith, 1974, online image: tac10.gif D. Smith, 1974, online image: tac12.gif, as modified by G. Meador for this document D. Smith, 1974, online image: tac14.gif, as modified by G. Meador for this document B. Tan, et al., 1998, online image: diagram.gif, as modified by G. Meador for this document K. Billah & R. Scanlan, 1991, Fig. 6. Vortex pattern over rotating deck section. J. Koughan, 1996, online image: fig1_jk.gif J. Koughan, 1996, online image: fig2_jk.gif D. Smith, 1974, online image: tac08.gif S. Scott, undated, online image: narrows.jpg Underwater Atmospheric Systems, undated, online image: bridge71.jpg Underwater Atmospheric Systems, undated, online image: bridge72.jpg D. Smith, 1974, online image: tac13.gif, as modified by G. Meador for this document K. Rogers, 1997, online image: tnb27pic.gif B. Lou, undated, online image: new.jpg M. Ketchum, undated, online image: Tacoma-320x500.JPG R. Andradne, online image: img31.gif Washington State Dept. of Transportation, online image (2007a) Washington State Dept. of Transportation, online image (2007b) P. Groutt, 1996, online image from: http://jedlik.phy.bme.hu/~hartlein/physics.umd.edu/deptinfo/fa cilities/lecdem/g3-24.htm G. Meador, 1997, online image: transver.gif G. Meador, 1997, online image: nodes.gif G. Meador, 1997, online image: longit.gif P. Groutt, 1996, online image from: http://jedlik.phy.bme.hu/~hartlein/physics.umd.edu/deptinfo/fa cilities/lecdem/g3-01.htm R. Edge, 1998, online image: Image101.gif R. Edge, 1998, online image: Image102.gif


REFERENCE LIST

used for non-profit educational purposes in accordance with the “fair use” provisions of copyright law

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REFERENCE LIST (continued) Lou, B. (undated). Bridge and resonance [Online]. Available: http://instruction.ferris.edu/loub/media/BRIDGE/Bridge.htm [2001, February 4]. Lowery, L. (1999). Engineering ethics: The Kansas City Hyatt Regency walkways collapse [Online]. Available: http://lowery.tamu.edu/ethics/ethics/hyatt/hyatt1.htm and http://lowery.tamu.edu/ethics/ethics/hyatt/hyatt2.htm [2001, January 28]. Martin, R. (1999a). Hyatt Regency walkway collapse [Online]. Available: http://www.eng.uab.edu/cee/REU_NSF99/hyatt.htm [2001, January 28]. Martin, R. (1999b). Tacoma Narrows bridge collapse [Online]. Available: http://www.eng.uab.edu/cee/REU_NSF99/tacoma.htm [2001, January 28]. Meador, G. (1997). Activity #2: Slinky waves [Online]. Available: http://www.teachingtools.com/SlinkyShindig/activ2.html [2001, February 8]. Moore, K.S. (1999). Large amplitude torsional oscillations in a nonlinearly suspended beam: A theoretical and numerical investigation [Online]. Available: http://www.math.lsa.umich.edu/~ksmoore/ [2001, February 4]. Neyman, P. (undated). Torsion wave representation [Online]. Available: http://sps1.phys.vt.edu/~pat-man/LiNC/movies/torsion.mov [2001, February 5]. Petroski, H. (1985). To engineer is human: The role of failure in successful design. New York: St. Martin’s Press. Available for purchase at: http://www.amazon.com/exec/obidos/ASIN/0679734163 Petroski, H. (1994). Design paradigms: Case histories of error and judgment in engineering. New York: Cambridge University Press. Available for purchase at: http://www.amazon.com/exec/obidos/ASIN/0521466490 Pitsco. (undated). Bridge building competition [Online]. Available: http://www.pitsco.com/p/CCbridges.htm and http://www.pitsco.com/p/rulesdoc.pdf [2001, February 13]. Rogers, K. (1997). The Tacoma Narrows bridge disaster [Online]. Available: http://137.142.19.40/seconded/second/Kent/Kent.html [February 8, 2001]. Russell, D. (2000). Vibration and wave animations [Online]. Available: http://www.kettering.edu/~drussell/Demos.html [2001, February 4]. Scott, S. (undated). Tacoma Narrows bridge [Online]. Available: http://people.mn.mediaone.net/sscott2/Text_Files/gertie.html [2001, February 8]. Smith, D. (1974, March 29). A case study and analysis of the Tacoma Narrows Bridge failure [Online]. Unpublished manuscript, Carleton University, Department of Mechanical Engineering, Ottawa, Canada. Available: http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/DSmith/photos.html [2001, February 7]. Tan, B.T., Thompson, M.C., & Hourigan, K. (1998). Simulated Flow around Long Rectangular Plates under Cross Flow Perturbations [Online]. International Journal of Fluid Dynamics, 2(1). Available: http://sibley.mae.cornell.edu/IJFD/1998_vol2/paper1/paper1.html [2001, February 8]. Underwater Atmospheric Systems. (undated). Tacoma Narrows bridge [Online]. Available: http://www.nwrain.com/~newtsuit/recoveries/narrows/narrows.htm [2001, February 8]. United States Geological Survey. (1992, May 13). Tacoma, Washington, United States [Online image]. Available: http://terraserver.microsoft.com/image.asp?S=14&T=1&X=168&Y=1635&Z=10&W=2 [2001, February 10]. Washington State Department of Transportation. (2008). SR 16 - New Tacoma Narrows Bridge [Online]. Available: http://www.wsdot.wa.gov/projects/sr16narrowsbridge/ [2008, March 6]. Washington State Department of Transportation. (2007a). NTPAW Tacoma Narrows Bridge Tour [Online image]. Available: http://www.flickr.com/photos/wsdot/1129855059/ [2008, March 6]. Washington State Department of Transportation. (2007b). View from the Top [Online image]. Available: http://www.flickr.com/photos/wsdot/829349869/ [2008, March 6].

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Failure by Design Hyatt Regency

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