Introduction
On April 23 rd
1987, the partially erected structural frame of the L’Ambiance Plaza, a 16 story apartment tower being constructed in Bridgeport, Connecticut totally collapsed, killing 28 construction workers. There were numerous investigations carried out by a number of engineering organisations as to why the building collapsed. However many of the investigations were not completed because a general settlement ended litigation of matters related to the disaster. There are a lot of disagreements between various engineering firms about the precise mechanism that triggered this collapse, there is however agreement that there were many structural design and construction deficiencies in the project. Several of these deficiencies could have been the triggered causing the collapse.
The Structure
L’Ambiance plaza was to be a 16 story structure with 3 parking levels and 13 apartment levels. It was composed of two offset rectangular towers, separated by a construction joint at a central elevator lobby. The structural steel frame had steel columns and 178mm thick post tensioned concrete flat plate floors. The flat plate floors were to be constructed by the “lift slab system”. This method consists of fabricating all of the floor slabs as a stack at ground level, then elevating the slabs up into their final positions using a hydraulic lifting apparatus. The floors were lifted in stacks of three which would have had a large weight. Steel channel shear collars were provided at all column locations for load transfer and lifting operations.
Lateral resistance in the completed structure was provided by two shear walls in each direction in each tower. These extended to two levels below the roof. For the top two levels, lateral resistance relied on frame action with rigid joints between steel columns and post-tensioned slabs.
The building was founded on spread footings intended to be on rock. Ground level was up to 2 stories higher on the north side than on the south side. The unbalanced lateral earth pressures were supported by the building foundation walls and the shear walls.
The design used unbounded plastic sheeted post-tensioning tendons in each direction.
Each column had a hydraulic jack at its top connected to a pair of lifting rods extending down to collars in the floor slabs. The floors were permanently attached to the columns once they were lifted into place.
Collapse
The two adjacent towers of the building were being cast in a stack at ground level and then jacked up along steel columns into their final position. At the time of collapse, slabs
3, 4, and 5 of the east tower had been placed into final position and slabs 9,10 and 11 of the west tower had just being lifted. A 12 ton hydraulic jack was being used to plumb the two towers when both towers suddenly and completely collapsed.

Theories as to why structure collapsed

The fact that a lateral load had been applied to plumb the west building , and clear evidence of an improperly installed wedge into the 12/roof slab package at column
E3, engineers conclude that wedge roll out and severe overstressing of the remaining wedge at this location constituted the trigger mechanism. According to this scenario progression of the collapse involved the 12/roof slab package dropping onto the top of the 9/10/11 package at column E3, and ejection of the lifting nuts from under the shearhead at level 9 due to the resulting overload conditions. The deformed shape of column E3 is taken as supporting evidence for this collapse scenario.
 The accident highlighted the deficiencies of the lift slab construction technique. It was found that some of the slabs had warped due to the heavy weight of the subsequent slabs on top of them.
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Sheer walls were required to stabilize the building against lateral loads. The plans called for the installation of the sheer walls within three floors of the elevated stack of slabs. However, at the time of the collapse the sheer walls had been built up to only about five levels below the stack.
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Variations in control of lifting operations, particularly the failure to engage one of the slabs at one column during one of the slab lifts in the east tower.
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Defective welding of permanent column-to-slab connections and column-to-column connections, including both field and shop welds.

Conclusion
Although the exact cause of the collapse has never been determined the building had a number of deficiencies any one of which could have triggered the collapse. The question really is which one of these deficiencies failed first in turn triggering the rest of the failures and causing the collapse of the two towers. One of the major issues though is the serious failure of American practice for designing and constructing major buildings to protect public safety. If nothing else the construction industry has learned from the
L’Ambiance tragedy and can prevent disasters like this from happening in the future. The disaster demonstrated some important flaws in the in the American construction system and assuring public safety along the way. Some of these are:

Slab design was severely flawed and the responsibility for the design was fragmental
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Building authorities accepted an incomplete design on record drawings ( the authorities did not require design documents showing designs for the temporary lifting conditions)
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The ACI was deficient because sections relating to two-way slabs with unbonded post-tensioning systems do not limit the use of such systems to slabs having approximately the regular rectangular bays used in tests to determine design requirements

Building regulations do not require adequate consideration of structural safety related to construction methods and means.
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Also the Connecticut state building code which incorporates the B0CA code did not include any reference to ANSI Standard A10.9 which covers safety requirements in construction including specific requirements for the lift-slab construction method, so compliance with this standard was not subject to enforcement by city building authorities.
If nothing else this disaster 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.