Keywords: fault-tree, fuzzy logic, L'ambiance Plaza collapse, building collapse, construction failure.

Risk is an unavoidable in the construction industry, due to its unprecedented nature. However, practitioners can reduce the risk involved in the construction by employing appropriate tools for decision-making. Analysing construction failure cases permits practitioners and students to develop their skills in planning for unexpected occasions in the field that they cannot develop without years of on-the-job experience.

This paper presents a computer program developed for evaluation of the slab connections in lift-slab construction technique using a fault tree. The program aims to reduce construction risk by permitting the user to learn from historical construction failures. The findings of several different investigations on L'ambiance Plaza collapse are used in the development and testing of this program. The catastrophic effect generated by the collapse of L'ambiance Plaza appeared to be related to three features: poor workmanship, design deficiency, and lack of control mechanisms. By using this assumption, a generalized fault tree is employed in the program, which provides graphical display of solutions for the decision makers.

L'ambiance Plaza which totally collapsed during installation of the wedges connecting the slabs of the ninth, tenth, and eleventh floors on April 1987, killing 28 people, has had a significant influence on the development of this program. L'ambiance Plaza was designed to be a residential building in Bridgeport, Connecticut, consisting of twin towers (approximately 19.2 m x 33.5 m) with 13 floors of apartments and 3 floors of parking. The lift-slab method was selected for the construction of the slabs; in which the slabs were poured on the ground and lifted by the help of hydraulic-jacks in packages of three to their temporary and/or final positions. These slabs were then welded to the steel columns following post-tensioning. The computer program used the same structure and construction technique, referring to different investigations carried out by different parties, in order to provide an evaluation of slab connections.

The theories from previous investigations are evaluated in order to achieve reasonable assumptions for the possible failure of similar construction projects. According to the NBS (National Bureau of Standards), the main reason for failure was the deformation of the overloaded steel angle. T-T (Thornton-Tomasetti Engineers) suggested that the wedges of the 12th floor and roof package were not stable, while SSE (Schupack Suarez Engineers, Inc.) blamed the design defects of post-tensioning tendons. OSHA (Occupational Safety and Health Administration) found substandard welds (17 out of 30) that also lacked proper attention to detail. Finally, FaAA (Failure Analysis Associates, Inc.) developed a 3D-model that indicates vulnerability of the building to lateral forces before the process of welding of the slabs to the columns using wedges.

Because all the studies had different conclusions for the reason(s) and mechanism(s) of the collapse, a simplified-fault-tree is developed assuming the failure of slab as the top event. Each subsequent event contributes to the failure by its enabling events, triggering events, and events that follow. After the evaluation of simplified-fault-tree and evaluation of previous studies, the main assumption is stated: the combination of poor workmanship and design deficiency with lack of control mechanisms lead the whole structure to collapse in seconds. This assumption is used for the development of an extended-fault-tree from the initial fault tree. Moreover, evaluation of the minimal cut sets of this fault-tree indicates, "shear head loss of support" as the most critical event. For research purposes, this part of the fault-tree is isolated from the other branches and modified by eliminating some unique events and adding more generalized events for the development of program so that it can be used for similar projects.

The program consists of two main parts. The first part provides the necessary literature for the user. By entering "Description of the Collapse," the user can find theories about how L'ambiance Plaza collapsed or by entering "Description of the Program," the user can be informed of the following topics: program development, fuzzy logic, fault-tree analysis. The second part is be used for evaluation of conditions and events by employing fuzzy concepts and calculation methods. The user can evaluate enabling events and triggering events of the final fault-tree individually. The results are provided both as matrix and graphical display for the interpretation by the user. The user can evaluate the probability of failure for the condition of "substandard welding process given workmanship" when a certain condition and its probability of failure are given by using modens-ponens deduction technique. The results are displayed graphically for two different models: Baldwin's model and the angular model. The user also can evaluate the entire fault-tree for shear head loss of support with assigned effectiveness values.

The L'ambiance Plaza collapse is one of the worst construction failures in construction history. Construction companies should take the necessary precautions to avoid construction failures so as to avoid the dire consequences of such failures. The use of this and similar tools for decision-making and process control is expected to reduce the chance of making such mistakes over and over again.

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