Introduction
L’Ambiance Plaza in Bridgeport Connecticut, USA was a construction project for a
16 story residential complex. The design comprised of two towers. The structure would have a steel frame shell with each individual floor being constructed of post tensioned concrete slabs. The concrete slabs were deigned to be 7 inches in depth.
An interesting feature of the design was the decision to construct the concrete slabs at ground level and subsequently hoisting the slabs into position before they were welded to the steel columns. Youtz and Slick devised this technique in the late
1940’s.
Lateral restraint in the structure was provided by shear walls in each of the towers.
For the top two levels of the building, the only lateral restraint came from the rigid joint between columns and slabs
On April 23 1987, the incomplete structure (fig. 1) was overloaded and collapsed, killing 28 of the workers. Following the failure, numerous investigations took place, outlining various theories as to why the structure collapsed. A brief overview of these theories will be set out in this report.
(Fig.1 Illustrated by Rachel Martin)

Structural Failure
With all of the different expert views as to why the structure collapsed, there is a common theme. That is, that the load path to distribute applied loads during construction was flawed. Many believe that the failure of shear heads lead to the overloading of the structure.
One study reported how a lateral force was applied to the structure in order to plumb the building. This extra force, coupled with the fact that evidence concluded that a wedge was improperly fixed to the level 12 and roof slabs at column E3 resulted in the excessive force that caused the failure. The critical failure occurred when the level
12 and roof slabs fell on to the package of slabs for levels 9-11. According to a surviving member of the wedging team not all of the fixed wedges had been welded to the column (fig 2) which would seem to give this theory some foundation
(Fig 2. Illustrated by Rachel Martin)
Another theory of how the structure failed was put forward by Failure Analysis
Associates. They reviewed how the tower would react to both lateral and torsion loading. Using computer modelling, they showed that in the time where the slabs were being welded to the column, rotation is possible and therefore is unstable. Their study concluded how the lateral loading to plumb the west of the structure would be sufficient to cause collapse
Also, questions were asked about the suitability of the welding process adopted during construction. The Occupational Safety and Health Authority discovered that welds between header bars and channels at column E3.8 had failed. N S Moreton was hired to inspect a number of welds at shearheads. They found that not even half of the welds met welding standards. On top of this, it was discovered how welds at these locations would be required to resist substantial loads even for adequate welds.

Conclusions
Following the towers in Bridgeport collapsing, a number of valuable lessons can be learned from the disaster to prevent further failures.

Constant lateral support should be supplied to the structure during construction until the building is structurally complete

The use of vertical supports to slabs in the form of cribbing (ACROS) until welds are complete

To prevent the packages of floors from moving horizontally during the process, sway bracing can be deployed.

Appropriate responsibility should be assigned to the correct parties to ensure no sloppy construction occurs.

Higher practices of safety should be used during construction to protect the lives of valuable workers
References
 http://911 research.wtc7.net
 “Investigation of L’Ambiance Plaza Building Collapse” by C.G Culver and
R.D Marshall
 L’Ambiance Plaza Collapse Bridgeport, Connecticut, April 23 1987 by Rachel
Martin