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: gmeador@bps-ok.org or inquiry@meador.org
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.
Page 2 of 34
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 3 of 34
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
Page 4 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 5 of 34
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.
Page 6 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 7 of 34
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.
Page 8 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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?
Failure By Design by Granger Meador (failurebydesign.info)
Page 9 of 34
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
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 11 of 34
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.
Page 12 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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)
Page 13 of 34
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
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 15 of 34
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.
Page 16 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 17 of 34
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
Page 18 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 19 of 34
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
Page 20 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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
Failure By Design by Granger Meador (failurebydesign.info)
Page 21 of 34
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.
Page 22 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 23 of 34
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.
Page 24 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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
Failure By Design by Granger Meador (failurebydesign.info)
1950 bridge deck design
Page 25 of 34
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.
Page 26 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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
Failure By Design by Granger Meador (failurebydesign.info)
Page 27 of 34
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.
Page 28 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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.
Failure By Design by Granger Meador (failurebydesign.info)
Page 29 of 34
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.
Page 30 of 34
Failure By Design by Granger Meador (failurebydesign.info)
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
Failure By Design by Granger Meador (failurebydesign.info)
Page 31 of 34
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
Page 32 of 34
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
Failure By Design by Granger Meador (failurebydesign.info)
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used for non-profit educational purposes in accordance with the “fair use” provisions of copyright law
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