Atlantic City NJ Casino Collapse

Missing Connection to Wall May Have Weakened Garage, ENR November 17, 2003, reports on the ongoing investigation into the October 30 collapse of multiple concrete floor decks of an Atlantic City New Jersey casino under construction. Missing or inadequate connections between the decks and an adjacent bearing wall may be a significant factor contributing to the incident. Other possible factors that have been cited included inadequate shoring and inadequate curing time between deck concrete pours. According to the article, some workers reported seeing bowed or bent shoring columns prior to the collapse. Four workers were killed in the collapse.

This incident has also brought attention to the innovative construction system being used on this project, the filigree wideslab concrete construction system. The filigree concrete system is a composite system using both precast concrete formwork and cast-in-place concrete. It is one of a variety of permanent form systems, which aim to achieve higher economy through reduction in forming costs.

In the filigree system, formwork for slabs and beams are precast at a fabrication plant. They may be either prestressed or conventionally reinforced. Additional reinforcing in the form of shallow-height steel bar joists are cast into the precast units with the upper portion of these joists projecting out of the upper surface of the precast. When concrete is later poured on top of these units, the bar joists create a bond between the precast and the site-poured concrete. After delivery to the construction site, the precast units are lifted into place and supported on temporary shoring. Next additional reinforcing is placed above the formwork. Foam plastic blocks may also be added to the assembly. Where these blocks are placed, voids will be formed in the final pour where excess concrete is not needed, thereby lessening the overall quantity of concrete and reducing the weight of the structure. Finally, concrete is poured on site to complete the system.

In the completed assembly, the precast and sitecast concrete work as a single composite structural unit, bonded through the shallow-height joists described above. Bottom steel is contained within the precast units, and the remainder of the required reinforcing resides within the cast-in-place portions of the system. Variations on this system include flat plate construction over structural steel beams and girders, all-concrete flat plate construction, and concrete beam and slab construction (as illustrated above). Additionally, both one-way and two-way systems are feasible.

More information:
See the text for more about innovative formwork systems (pages 536 - 537), prestressing concepts (beginning on page 493), and one-way and two-way slab systems (pages 490 - 491).
Mid State Filigree Systems describes the advantages of the filigree concrete system, and provides diagrams illustrating variations on the system.
BOCA Research Report No. 96-14 describes the filigree system in additional technical detail.