Capitol Crossing Platform
LERA was the structural engineer for a 1.2 million-sf multi-level podium being built over the active I-395 highway. The structure consists of both structural steel and reinforced concrete members, and the foundations consist of shallow spread footings and deep drilled shafts. The platform over highway uses structural steel transfer girders with precast concrete panel and cast-in-place topping slab to support the buildings above. The multi-phase project involves restoring the site to grade level and filling in the highway overpass, which reconnects and redefines the east end of downtown Washington through the creation of three new city blocks.
The platform supports a 7-acre, 2.2 million-sf mixed-use development containing four office buildings and one residential structure with ground floor retail, underground parking and a 150-unit residential building in Washington’s Central Business District. LERA developed multiple alternative framing systems for the platform and optimized the final design, including considerations for redundancy, fire protection and constructability.
Five levels of basement are located between the highway and several existing buildings on 3rd Street. To meet the multiple challenges of excavating adjacent to the highway and buildings, the foundation wall system is an optimized combination of slurry wall with temporary tie backs along the highway, and conventional cast-in-place reinforced concrete wall with a tied back H-pile, bracket pile and wood lagging support of excavation on the remaining three sides of the site.
The southwestern portion of the basement is located immediately adjacent to an existing masonry church with shallow foundations. To minimize the potential for damage to the church, LERA designed an innovative two-level mat footing foundation system that avoided the need for deep foundations adjacent to the church.
The platform construction meets the design requirements of both the Federal Highway Administration and the District of Columbia.
Property Group Partners
Skidmore, Owings & Merrill (SOM)