68
WHY
BUILDINGS
FALL DOWN
Hartford Arena Roof Collapse
On the evening of January 17, 1978, Horace Becker was staying at
the Hartford, Connecticut, Sheraton Hotel in a room facing the
Civic Center Arena. As he retired, he looked out the window and
saw snow falling heavily for the second time that week. In the middle
of the night (it was actually 4:15A.M.) he was awakened by what
sounded like a "loud cracking noise," which continued for some
time. Startled into a fully awakened state, he looked out the win­
dow again and saw, diagonally across the street from the hotel, the
northwest corner of the arena roof rise and the center sink with a
whooshing sound. Within seconds the windows of his room had
started to shake, and thinking that a plane had crashed on the
building, Mr. Becker dropped to the floor. When the noise stopped,
4.10
Hartford Center Roof: after the Collapse
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4.11
Hartford Center Space Frame Diagram
he looked out again and saw the other three corners of the arena
also pointing skyward. Like a four-cornered hat, the 2.4 acre (9.7
ha) roof of the arena had settled down in the center, throwing up a
cloud of debris in the air and tossing pieces of roof insulation down
on the parking deck immediately below Mr. Becker's room (Fig.
4.10).
That same night roofs fell in two other Connecticut towns. Three
days later, after a third heavy snowfall, the roof of the auditorium
at C. W. Post College on Long Island collapsed (see p. 42). In fact,
throughout that winter hundreds of roofs fell under the weight of
unusually heavy snowfalls, but none was as dramatic as the Hart­
ford collapse. Had the roof fallen six hours earlier, many of the five
thousand fans watching a basketball game might have been killed
or injured. Luckily, the fourteen hundred tons of twisted steel, gyp­
sum roofing panels, and insulation fell on ten thousand empty seats.
The arena roof (Fig. 4.11) measured 300 by 36 0ft. (91x 110m)
and was constructed as a space frame, 2 1 ft. ( 6.4 m) deep, a struc­
ture consisting of top and bottom square grids of horizontal steel
bars with joints, or nodes, 30ft. (9 m) on center connected by diag­
onal bars between the horizontally staggered nodes of the upper
and lower grids. The resulting space frame looked like a series of
linked pyramidal trusses. The 30ft. (9m) long top horizontals were
braced by intermediate diagonals, and the main diagonals were
braced at their midpoints by an intermediate layer of horizontal
bars.
The top horizontal bars of most space frames perform a double
function: They support the roofing panels, and they act as upper
structural members of the space frame. In the Hartford roof, how­
ever, the roofing panels were supported on short vertical posts above
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4.12
Typical Pyramid Module with Posts Supporting Roof
Panels
the top nodes of the space frame (Fig. 4.12). The designers claimed
two advantages for this scheme: (1) If the height of the posts was
varied, the roof could be sloped to provide positive drainage inde­
pendently of the original level and the deflections of the top bars
of the space frame, and (2) the top bars of the frame would not be
subjected to bending stresses from roof loads.
Additionally, three unusual concepts characterized the design
of the Hartford roof: (1) The frame's top horizontal bars were con­
figured in the shape of a cross built up of four steel angles (Fig.
4.13). (Unfortunately the cross is not a particularly efficient section
for a compression element because it bends and twists under rela­
tively small stresses and hence buckles more easily than if the same
amount of material were used in a tube or an I bar shape.) (2) A
truss node is usually the theoretical point where the center lines of
all the bars connected at the node intersect, but in the Hartford
frame the top horizontal bars intersected at one point, and the
diagonal bars at another, somewhat below the first. Thus the forces
transmitted between diagonal and horizontal bars caused bending
stresses in these bars (Fig. 4.13). (3) The overall space frame roof
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was supported on four enormous pylon legs located 45ft. (13.7 m)
inboard of the four edges of the space frame, rather than on bound­
ary columns or walls (Fig. 4.11).
In spite of the unusual aspects of its design the frame appeared
to be sturdy. For five years, it withstood the harsh Hartford weather
before suddenly failing on that winter night although it gave many
hints of impending danger, surprisingly ignored by architects,
engineers, builders, and inspectors.
Vincent Kling, a well-known Philadelphia architect, was engaged
in 1970 as architect of the proposed Civic Center, and he hired the
Hartford office of Fraoli, Blum & Yesselman, Engineers to design
the structure of the arena. Early in the design phase the engineers
proposed a unique roof structure that they thought would save half
a million dollars in construction cost but that required a complex
computer analysis to check its safety. The city gladly granted the
additional fee for this money-saving analysis, which proved to
everybody's satisfaction the innovative structural scheme was safe.
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Diagonals Eccentrically Connected to Cross-Shaped
Chords
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72
WHY
BUILDINGS
FALL DOWN
A year later the construction documents were completed, the proj­
ect was put out to public bid, and the construction of the roof
structure was awarded to the Bethlehem Steel Company of Beth­
lehem, Pennsylvania. Gulick-Henderson, an inspection and testing
agency, was engaged to ensure correct execution of the design.
A unique aspect of the construction procedure concerned the
method of erection. Instead of the frame's being assembled in place
almost 100ft. (30m) above the ground, a costly, time-consuming,
and somewhat dangerous procedure, it was completely assembled
on the ground. Not only was the structure bolted together, but the
heating and ventilation ducts, the drain pipes, and the electrical
conduits, as well as the service catwalks, were assembled while the
structure sat on the ground. Only a badly timed painter's strike
prevented the structure from receiving its final coat of light gray
paint before erection. Assembly of the roof frame, begun on the
floor of the arena in February 1972, was completed by July of that
year. It was during this short assembly time that the engineers were
notified by the inspection agency of a suspicious and excessive deflec­
tion of some nodes, but soon the roof was ready to be lifted, and it
began to move up slowly.
The lifting process was completed in two weeks by means of
hydraulic jacks fixed to the top of the four pylons. It was an
impressive and awe-inspiring sight to see a roof the size of a foot­
ball field rising slowly upward, day after day, in preestablished
steps. A concerned citizen who witnessed the operation questioned
the capacity of such an immense structure to withstand the forces of
wind and snow but was reassured by the engineers that he had no
reason to worry.
In January 1973 the roof, in its final position but not yet bur­
dened by the weight of the roof deck, was measured to have a
deflection at the center twice that predicted by the computer
analysis. When notified of this condition, the engineers expressed no
concern, explaining that such discrepancies had to be expected in view
of the simplifying assumptions of the theoretical calculations. The
contractor installing the fascia panels covering the space frame at
the top of the four facades claimed that the actual boundary deflec­
tions of the structure were so random that when he tried to mate
the prepunched holes in the two pieces of steel of the space frame
and the facade panels and to insert bolts, he encountered such dif­
ficulties (because the holes did not line up) that he had to weld
rather than bolt the joint.
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73
By mid-1974, after the roof was completed, another technically
minded citizen expressed concern about the large dip he had noticed
in the roof that he believed might indicate an unsafe structure.
Once again the engineer, this time joined by the contractor, assured
the city that there was no reason for concern. Finally, in January
1975, a few days before the official opening of the center, a coun­
cilwoman made it public that a construction worker had told her
the actual deflection of the roof was almost twice the predicted
value. In light of the earlier assurances, this "political" concern
was not even referred to the engineers, but independent measure­
ments taken three months later by the city confirmed the anoma­
lous deflections.By this time such statements were probably treated
as rumors based on earlier allegations.
Five years later the roof collapsed.
Within days of the collapse experts had been retained by the
city to address the issue of the responsibility and possibly the cul­
pability of contractors, architects, and engineers (who engaged their
own experts to protect their respective interests). This army of
experts crawled like ants over the wreckage for weeks, looking for
clues to the cause of the disaster, while the city announced: "We'll
build a new structure.... It will be bigger and better, and it will
have a different kind of roof."
The first question explored by the experts concerned the weight
of snow and ice that had accumulated on the roof the night of Jan­
uary 17. Accurate measurements showed that the actual weight
(the live load) of the accumulated snow from the two storms pre­
ceding the collapse was about half the live load specified by the
code; the weight of the roof (the dead load), was also checked and
turned out to be 25 percent greater than that assumed in the design.
However, the sum of the dead and live loads was less than the total
load assumed in the design.In any case, the code safety factor should
have easily taken care of even such an accidental overload.
Attention was then directed to the configuration of the actual
structure in comparison with the mathematical model postulated
by the designers. The structural model had assumed that all the
top chord bars were braced laterally by the inclined secondary
diagonals, and this was the case in the interior of the space frame
where diagonals form a pyramid (Fig. 4.12). But along the frame
edges, the diagonals and top bars were in the same inclined plane;
hence buckling out of this plane was not prevented. The top bars
were free to bend outward, or buckle, in a direction perpendicular
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74
WHY BUILDINGS FALL DOWN
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4.14
Buckling ofTop Horizontal Chord
to that plane (Fig. 4.14). Prevention of buckling would have required
the outer top horizontals to be four times stiffer than the typical
interior top horizontals because the outer horizontals had twice the
unbraced length. Since the top horizontals were the same size as
the interior horizontals, they were doomed to buckle.
The question remained: Why did the roof survive for five years?
To answer this puzzling question, a study was made of the pro­
gressive failure of the roof. A computer model of the roof structure
with correct buckling lengths and stiffnesses of all bars was "loaded"
in steps, searching for the value of the load at which the first bar
would buckle. This load was conservatively evaluated (by ignoring
the springlike restraint offered by the actual connections) to be 13
percent below the total load actually on the roof on the day of fail­
ure. Loading of the model was increased further to explore what
happened after the first bar buckled. When a member of a frame
buckles, it transfers its load to adjacent bars that most of the time
cannot carry the extra load and then also buckle. The failure of
additional bars transfers their load progressively to new bars until
the roof cannot carry any greater load and begins to collapse. This
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75
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collapse live load was found to be only 20 percent above the actual
measured snow and ice load and to cause a progressive inward
folding pattern of the roof similar to that observed after the failure.
Since no real structure is quite as perfect as the equivalent com­
puter model, the actual failure of the Hartford Civil Center roof
must have taken place at a load somewhat above that causing the
buckling of the first bar and below that calculated as the ultimate
collapse load. Progressive collapse can start as the result of even a
minor deficiency unless redundancy is introduced as a matter of
structural insurance, and it is sad to note that the addition of less
than fifty bars to brace the top outer horizontals to a frame con­
sisting of almost five thousand bars would have made the Hartford
roof safe by preventing bar buckling.
Once the wreckage was cleared away, the firm of Ellerbe Archi­
tects of Minnesota began planning for a new arena. True to the
promise of the city fathers, it was bigger, seating four thousand
more spectators than the old one. Its roof was simpler, with two
ordinary parallel vertical trusses sitting on the same four pylons
raised up 12ft. (3.6 m) to fit the grander facility (Fig. 4.15). Second­
ary trusses were framed into these primary trusses at six locations,
and tertiary trusses framed into the secondary ones, resulting in a
grid of trusses bearing a family resemblance to the original roof.
The revamped coliseum began to take shape sixteen months after
the collapse and by the spring of 1980 was ready to receive its first
guests, the fans of a local hockey team that had been homeless for
over two years.
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Section through Original and Enlarged Arena
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