L’AMBIANCE PLAZA
Introduction:
L’Ambiance Plaza in Bridgeport Connecticut was a 16 story residential project. The building design
was a lift slab construction. This construction technique comprises of a steel frame shell and a 16
individual post tensioned concrete floor slabs lifted into position and then fixed to the steel frame.
The construction method on this project was to pour each individual slab at ground level and then hoist
the slabs into position. Once the slabs were lifted into position, they would be welded to the steel
columns.
The lateral restraint used in the structure was provided by shear walls in each of the two towers. In the
top two levels of the buildings, the lateral restraint came from the rigid joint between columns and
slabs. This joint between the columns and the slabs was welded joint, which took the lateral restraint.
The top two floors were constructed first which led to insufficient lateral restraint during the initial
stage of construction.
On the construction site of the L’Ambiance Plaza Connecticut in April 23 1987, the incomplete
structure of the two towers was overloaded and collapsed. The collapse of the two towers resulted in
28 construction workers being killed. The failure was the worst modern day disaster in Connecticut.
The disaster caused a major federal investigation to be conducted. The investigations looked into the
design of the building, the construction technique, and ultimate circumstances that led to the collapse of
the L’Ambiance Plaza.
Schematic of the L’Ambiance Plaza
Structural Failure:
At the time of the collapse, the structure was only half way completed. Many investigations were
carried out after the collapse but due to a general settlement ending litigation to the matter, many
investigations were incomplete.
Various investigators disagree on how the structure collapsed; there is however agreement that there
were many structural design and construction deficiencies in the project. Several of these deficiencies
could have been the triggering cause of the collapse.
Failure Theory’s:
There were 5 major deficiencies (according to Frank J. Heger in Engineers.com), found that may have
triggered the collapse:
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1.
2.
3.
4.
5.
Unreliable and inadequate temporary slab column connections that was to ensure frame
stability.
Overstressed concrete slab sections adjacent to two temporary floor slots for cast-in-place
shear walls.
Overstressed and excessively flexible steel lifting angles during frame stability.
Improper drape of post-tensioning tendons adjacent to elevator openings.
The NBS theory on overstressing and shear arm deformation.
NBS THEORY: (one of the mail theories behind collapse)
The NBS (National Bureau of Standards) led a full investigation into the incident.
They found many problems with the construction one main cause being improper design of certain
lifting collars, to be the most probable cause of the collapse.
The NBS investigation concluded that the failure occurred most likely at the building’s most heavily
loaded column E4.8 or the adjacent column E3.8 as a result of a lifting assembly failure. The
shearhead reinforces the concrete slab at each column. When the shearheads and lifting angles were
lifting a total package of three 320-ton slabs, they were dangerously close to their maximum capacity.
The lifting capacities of the two types of jacks used during construction were too small for the 960-ton
package being lifted
NBS did tests on the shear heads in question. The shear head and lifting angle tended to twist as the
loads approached 80 tonnes because although strong enough, they were not rigid enough. The excess
force deformed the lifting angle allowing the jack rod (a max. of 89tonnes or 150tonnes) and lifting nut
to slip out of the lifting angle and hit the column causing the structure to collapse.
(Drawing by Rachel Martin- Google images)
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Conclusion:
While the vast majority of structural projects are completed without any accidents or injuries, the
history of concrete construction has been plagued by several major failures, which resulted in large
death tolls. These have been caused by unnecessary risks which were taken pertaining to unreasonable
construction schedules, temporary material weaknesses, design flaws, and as is most relevant to the
L’Ambiance Plaza, the improper sequencing of operations.
The sequence of operations is fundamentally critical to structural safety in many projects. Prestressed
concrete structures require particular care in following the proper procedures, whether pre-tensioning
work at the precast plant or post-tensioning work at the construction site. The design engineer makes
certain assumptions about sequencing, and if these are not communicated clearly to those responsible
for field operations, the results can be catastrophic.
Errors in sequencing contributed to the totality of the tragic 1987 collapse of the L'Ambiance Plaza
housing project. Twenty-eight construction workers were killed when failure occurred in the slablifting assembly. At the time of collapse, the building relied on shear walls for stability, but
construction of the shear walls was lagging behind the lifting operation.
Failure has often been defined as ‘an unacceptable difference between expected and observed
performance.’ In modern times, with the level of knowledge that we have, there is no excuse for such
an occurrence. The improper sequencing which caused the L’Ambiance disaster may be attributed to a
non-traditional construction technique, and this can lead to the possibility of confusion unless the
various parties involved have their tasks perfectly defined and controlled. This highlights the
importance of competent project management and is one of the main reasons why in recent times the
appointment of a Construction Manager has become increasingly popular. With one hierarchal figure
whose sole purpose is to oversee the various parts of the construction sequence, there is a much lower
risk of an error being made. ‘Design-build’ contracts are also becoming more popular, whereby the
same company is responsible for both the design and construction of the project. This results in better
communication as everyone is aware of what the other members of the team are doing. This too can
lead to a safer construction environment and a higher build quality.
References: - 911research.wtc7.net/wtc/analysis/compare/lambiance
- www.911myths.com/html/progressive_collapse.html
- eng.uab.edu/cee/faculty/ndelatte/.../L'Ambiance Plaza/ambiance.htm
- “Investigation of L’Ambiance Plaza Building Collapse” by C.G Culver and R.D
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Marshall