Achieving Column-Free Platforms—Design and Construction of Large Span Station Mezzanines on the Second Avenue Subway Project

Phase 1 of the Second Avenue Subway in Manhattan, New York, is nearing completion. There are two mined cavern stations and one cut-and-cover station in this phase. All three stations have reinforced concrete mezzanines of similar geometry, with a large single span beam and slab system. This configuration allows for the provision of unobstructed column-free spaces at platform level and large central penetrations for vertical transportation. Each mezzanine is integral with the varied station shell types, providing intermediate lateral support, but each mezzanine was built under separate construction contracts. These factors required complex modelling of each system. The contrasting behavior of each station shell type and their respective construction staging results in significant differences in forces applied to the mezzanines, and in detailing, for both station types. Maintaining a consistent architectural feel between all stations required a unique structural design approach. This paper will discuss the design process, construction methodology and construction sequencing for each of the mezzanines, and review the similarities and differences in the design forces and subsequent structural design details for each. Special design considerations for large spans will also be explored. As construction of the mezzanines is now complete, the paper will review the final construction methodologies adopted and how these impacted the design. Attendees will gain an insight into the design of intermediate permanent lateral supporting systems in underground structures with large penetrations, and an understanding of the degree to which construction techniques and construction contract packaging can drive the design and detailing of structural members.