Structural Engineering, Special Inspections & Temporary Works Peer Review Consultant
New York, USA
Oxford Properties (Base Building & Special Inspections);
Turner Construction (Temporary Works Peer Review)
Adamson Associates Architects (Executive Architect);
COOKFOX Architects DPC (Design Architect)
Future Green Studio
1,300,000 ft² (120,000 m² )
Targeting LEED® Platinum & Well Certification
550 Washington Street was constructed in the 1930s as St. John’s Rail Terminal, the depot for the rail tracks that are today’s High Line. The building’s redevelopment will create a new, high-performance health and wellness commercial office building described as a workplace of the future.
The structure is a first-of-its-kind application of precast segmental bridge technology to vertical core construction in buildings.
We used a high-level of slag in the concrete. This meant we had a high percentage of recycled content in the mix, greatly reducing embodied carbon emissions for cement manufacturing.
The project optimizes human comfort with radiant heating and cooling (with hot water coming from the District Energy building two blocks away) while minimizing the need to condition outdoor air thanks to a dedicated outdoor air system (DAS). The system supplies outdoor air to the space at a low level and low velocity, known as displacement ventilation, which efficiently removes unwanted heat and contaminants as the supplied air rises through the space.
The nodes of the 55-metre truss consist of five piles of 100-mm-thick plate. The largest node weighs the same as some battle tanks at 12 metric tonnes. The two diagonal web members, which support the largest compression forces of 33 and 38.5 meganewtons, comprise four piles of 100-mm solid plate. These were welded using partial joint penetration welding along the edges in the shop, requiring high pre-heating and careful post-heating to minimize potential weld-induced stress concentrations and indicators. The site welding required for connecting the truss web members to the largest node took approximately 150 man hours to complete over multiple days, with induction heaters running 24 hours a day.
The library’s entryway is designed to resemble a Chinook arch cloud formation. The three-storey, 18-metre-tall trusses feature architecturally exposed web members, with the largest members comprising four built-up piles of 4” thick plate. The largest truss spans 55 metres, is curved, and supports another truss spanning 30 metres.
The library sits over Calgary’s busiest light rail transit (LRT) line. This presented many unique structural challenges. The curved shape of the LRT line helped achieve the library’s distinctive shape by mirroring the curve along its longitudinal axis. Cost-effective structural solutions involving long-span floor framing and long-span, mega-trusses provided large, column-free spaces at the main entrance and oculus.
Each truss was too long, too tall, and too heavy to ship as one piece. As such, each piece was individually shipped and erected 10 metres above grade and temporarily braced back to the concrete cores that were advanced first. A temporary bracing system was used to stabilize each truss as additional floors were cast. Welded in shop along the long edges in partial joint penetration, the truss pieces required careful pre- and post-heating to minimize potential weld-induced stress and steel cracking. Two-end dialog b members support the largest compression forces of 33 and 38.5 meganewtons.
Meet the Team
Europe’s Largest Urban Wetlands
The original design of St. John’s Terminal offered a rare opportunity for a significant vertical overbuild without reinforcing the existing columns or foundations.
The building was originally designed for a typical live load of 300 psf (pounds per square foot) to accommodate trucks at the ground floor and warehouse space above, while level 2 was designed for rail loads. The columns and caisson foundations were also originally sized to accommodate a future addition.
As live loads for commercial office space are typically in the 50-100 psf range, our team designed the renovated building as a 12-story structure, almost tripling the original building height with only the lateral system needing reinforcement.
As multiple undersized beams had to be replaced, the structural team worked closely with the contractor to ensure that the means and methods were the most economical and feasible, while ensuring that the replacements would not adversely impact the new building envelope components that would be installed overtop.
The extensive decay to the timber framing required much of it to be replaced in-situ, including corrections to poorly executed carpentry work found once the walls were fully revealed. Some of the decay was so severe that tools were not required to remove the affected components. (The team could remove them by hand.)
The 1.3 million ft² building will include commercial, retail, and event space, including an auditorium and conference center. A new nine-level addition will be constructed on top of a renovated existing three-story podium structure, for a total of 12 stories plus a penthouse plant and service space.
The main entrance to the building celebrates the structure’s original use, showcasing the structure of the rail bed. The building’s green rooftops and recessed terraces on multiple floors will offer views of the Hudson River and New York City skyline.
BSc(Hons) CEng MIStructE
550 Washington Street Redevelopment
The building’s proximity to the shoreline creates a high risk of flooding. The original cellar slabs were designed to resist hydrostatic uplift; however, revised FEMA flood maps with a higher design flood elevation required the construction of new pressure slabs over the existing pressure slabs.
In addition, the ground floor of the building was raised by up to 2 feet from its original elevation to mitigate the risk of flooding at grade.
Once St. John’s Terminal was decommissioned for rail, it was converted into an office building that was well equipped for internet and 24/7 occupancy. This interim use required the construction of a large vault, approximately 35 feet by 100 feet, to house electrical transformers and network protectors.
Unfortunately, the vault was in a disruptive location for the new office building use. Entuitive worked with the construction manager and the client to devise a jacking scheme whereby the entire 900-ton vault was lowered as one unit by two floors and shifted horizontally, solving the architectural programming challenge and permitting an early “power on” scenario over conventional construction scheduling.
To improve the office space planning for the new use of the building, Entuitive extended the floor spaces rather than using the closely spaced columns of the original floors supporting the rail loading. The building design includes an articulated façade system with independent exterior terrace areas adjacent to key assembly spaces for the tenant.
The End User
Reviewing archived drawings and testing existing materials was a critical first step toward understanding the existing building’s capacities. Through close collaboration with the architect, MEP engineer, owner, and contractor, Entuitive proposed solutions that maintained and exposed a significant amount of the structure.
Entuitive also carried out a significant portion of the concrete special inspections for the project, maintaining a full-time presence at the precast plant in New Jersey, producing the wall segments as well as precast stairs in the core.
Vice President, Structures
Just inside the main entrance at the north end of the building, a pair of dedicated glass-enclosed elevators and a high-end feature staircase allow tenants and visitors direct access from the first floor up to the second, third, and fourth floors of the building. The entire stair structure is fabricated with architecturally exposed structural steel (AESS) and finished in accordance with standards for ‘Showcase Elements,’ the most stringent category for AESS.
The stair is an elegant combination of steel plates, slender steel rod hangers, and terrazzo finishes. The design makes efficient use of materials, minimizing weight and bulk while still meeting stringent design criteria, including limits on vertical and lateral accelerations induced by occupants’ footfalls.
The Precast Concrete Cores
Although the existing building was sufficiently robust for gravity loads, most low-rise buildings from this era were not designed explicitly for wind or seismic loads. In order to bring the new, taller building in compliance with the current New York City Building Code, and to accommodate vertical transportation and mechanical requirements for floor plates up to 120,000 ft², we designed two independent cores.
Entuitive was engaged directly by the construction manager to provide a third-party review of all temporary structural work used in the project, including hoisting, scaffolding, shoring and formwork, perimeter netting and pedestrian protection. In addition, work included support for the two large tower cranes needed to erect the 40-ton precast concrete segments. These Favco M1280D cranes are the largest tower cranes in New York City and have been used only a handful of times in the history of the city, including for the erection of the World Trade Center Transportation Hub.
Due to the large lateral loads imposed on the existing structure by the cranes, Entuitive designed temporary steel bracing to the building for the construction phase, before the new concrete cores could be relied upon for full lateral resistance.
Originally conceived as cast-in-place, the client challenged the design team to develop an innovative solution with precast, post-tensioned concrete elements, similar to the methodology used in segmental bridge construction and also used on the Manhattan West platform, a previous Entuitive project.
Over 200 factory-built, precast C-shaped wall units were stacked vertically without any physical connections at the horizontal joints except through the vertical high-strength bars prestressed to clamp the units together. Horizontal monostrands were used for connections at vertical joints. The segments are 30 feet long, matching the width of the core, and vary from 9’-8” to 12’-10” in height; the heaviest individual segment weighs approximately 45 tons.
In the cellar, a conventional cast-in-place concrete “starter core” is supported on new and existing caissons, while a new pressure slab also acts as the elevator pit slabs. Steel beams link the precast segments at each floor level and support the precast elevator lobby slabs. The precast walls feature an architectural board-form finish that will be exposed to view in the completed building.
The precast concrete cores benefitted the project by reducing the structural construction schedule by two months, thereby reducing the cost of the general conditions for the project and facilitating an earlier occupancy by the building’s tenant. Other benefits included reduced onsite welding, improved worker safety, maximized use of repeatable forming systems, and the opportunity for the walls to be exposed as part of the interior architecture.
The landings are finished in terrazzo, set on a stiffened steel landing structure with concrete infill and the treads are also capped with a terrazzo finish. The plate material varies in thickness from 5/8” for the landings and risers/treads, to 3” in thickness for the heavy inside ballustrade, which supports the stair between landings. The 3” thick inner balustrade, which is approximately 72” in height between landings, reduces to 9” in depth under the landings, and ultimately to 5 ½” in depth where it extends beyond the landing to 1 ½” dia rods that support the landings from the existing structure.
In addition to meeting all necessary strength and deflection requirements, the stair was designed to meet stringent vertical and lateral acceleration requirements, without the need for any supplemental damping systems.
On-site special inspections included the erection and post-tensioning of the precast segments and conventional cast-in-place concrete. Although special inspection work in New York City is often contracted out to a separate third-party agency, having the Engineer of Record perform this role helps the project run more efficiently: the inspectors are intimately familiar with the structural design and can make the necessary decisions to avoid delays when clarifications or changes are needed.
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The construction of 550 Washington includes demolition, major renovation, and new construction over a site with a three-block footprint and a bridge over an active roadway.
Coordinating all the temporary works to enable construction and protect the public is critical to the safe and successful logistics of the project.