Evaluation of L'Ambiance Plaza Posttensioned Floor Slabs

Abstract

This paper summarizes a structural evaluation of the posttensioned floor lift slabs of L'Ambiance Plaza, Bridgeport, Connecticut. The evaluation has been conducted to investigate, among other factors, the effect on slab stresses of the unorthodox horizontal splay of posttensioned tendons around column 4.8E and the effect of shear wall openings that existed during the lifting operation, for ideal lifting conditions (slab perfectly level), and for scenarios less than ideal. Initially, hand computations are made using simplified beam models to examine two areas of the west‐tower slab design that appeared particularly suspicious. Those computations reveal that a more refined stress analysis is warranted. To investigate the slab stresses more precisely, linear elastic finite element analyses of a typical west‐tower floor slab are conducted. Results from the analyses indicate that the anomalous layout of the posttensioning tendons in the west tower could be considered a possible cause that triggered the failure or, at a minimum, was clearly a factor in the propagation of the collapse after it had begun. The inclusion of nominal amounts of auxiliary bonded mild reinforcement (structural integrity steel) in the bottom of the slabs, continuous through the column strips, might have confined the slab failure, assuming the column supports and jacks could sustain the redistributed load.