The Kemper Arena roof collapse
Introduction:
In 1973, the Kemper Arena, Missouri was built for the
Kansas City Kings basketball team.
It sat 17,000 people
and was built on the site of the old Royals baseball
stadium. People hailed the stadium as a work of art and an
engineering feat. Many people revered the Kemper Arena
because it was groundbreaking and exotic. The American
Institute of Architects credited the arena with an award in
1976 and confirmed its importance as a monument by holding
in it its 1979 national conference.
What happened:
On June 4th, 1979, a downpour of rain with a 120 km/h
storm hit Kansas City. After structural failures detailed
further on was determined that approximately 4,000 m2 of
roof collapsed. The air pressure, increased by the rapidly
falling roof caused some of the walls to blow out.
Ironically, thousands of architects there for the American
Institute of Architects Convention had been sitting in the
arena only 24 hours before.
Technical reasons:
The Kemper Arena roof design had the feature of using
the roof as a temporary reservoir in large downpours. The
city drainage system would benefit from the limiting of the
rate of water that drained from the roof.
However, the
roof of the arena had only eight drains that were 12.5 cm
in diameter and placed 5 cm above the base of the roof.
The maximum downpour codes for Kansas City required 55 of
these drains even though such downpours only happened every
10 years.
The water pooling on the roof, the 120 km/h wind and
the suction of the water up onto the northern part of the
roof cause by the wind were not sufficient to produce a
dangerous accumulation of water on the southern portion of
the roof [where the collapse occurred]. Therefore, a
ponding effect was the main cause of the collapse. Ponding
happens when water covers stiff horizontal surfaces. As the
water accumulates, the middle of the roof begins to sag or
deflect. This causes more water to accumulate and thus more
of a deflection, continuing until the roof becomes
unstable. To tell whether a roof is unstable the critical g
is calculated. If this number is greater than one (the unit
weight of water), then the roof is stable, however, if this
value is less than one then the roof is unstable.
Therefore, the formulae used to calculate ponding in this
case determined a g of 0.627.
This means that the water
level at the drains on the roof was 23 cm deep.
Four vertical stiffeners, a base plate, and plastic
composite plate and four high strength bolts that held the
truss and the hanger together connected the hangers and the
trusses. The composite plate added to the roof while the
rest of the assembly ensured rigidity. The engineers
estimated that during the six years preceding the failure
these connections were subjected to at least 24,000
oscillations,
which
in
turn
introduced
oscillating
variations in the initial tension of the bolts. These
oscillations caused the metal to fatigue and therefore fail
at lower load values than initially designed for. In
addition, the steel used for the bolts, specifically A490,
is not resilient under variable loads. This particular
aspect was overlooked by the design team because the
coefficient of safety of the bolts under design loads
appeared to be sufficiently high.
Conclusions:
The mistake in designing the roof of the arena was the
type of bolt used. The bolt used on the stadium roof was
the A490. They are a high strength fastener used only for
tensile strength. However, these bolts had dynamic loading
on them from the gusting wind. This weakened the bolts by
placing repeated stress on them. It was reported that the
A490 high strength steel bolts lack ductility and only
maintain tensile strength for static conditions. Engineers
concluded this after the bolts were tested under dynamic
stresses.
The test showed that the bolts failed at only
one third of their capacity after tightening and loosening,
simulating dynamic stress, five times.
The other major oversight involves the number of
assembly hangers involved. The 42 hangers had to support
3,000 tons of weight and a wind displacement. The pooling
effect and the metal fatigue of the bolts caused hangar one
to fail. Then, this caused the surrounding hangers, 2, 3,
and 4 to fail because they were unable to carry the extra
weight of the roof. This inevitably caused a domino effect
across all of the hangers on the south end of the roof –
disproportionate collapse.
References:
http://aurora.wells.edu/
Levy, Mario Salvadori. Why Buildings Fall Down.