Edward M. Depaola, PE, And Fortunato Orlando, PE 2023-12-18 21:59:46
New York City’s largest steel frame is a futuristic corporate headquarters built on a constraint-filled site.
BY SEEKING CONSOLIDATED OFFICES for its New York workforce, financial giant JPMorgan Chase embarked on a design that would eventually feature New York’s largest steel frame.
The company decided the most logical location for the sole New York City office it desired was its existing headquarters at 270 Park Avenue, and the choice made sense. The full-block site is deep in the heart of Midtown Manhattan, bounded by Madison Avenue to the west, Park Avenue to the east, and East 47th and 48th Streets to the south and north, respectively. It has the necessary footprint for a high-occupancy office building and boasts numerous connections to public transportation and other urban amenities.
The idea had one hurdle, though. A rather significant one.
The existing 50-story building, originally constructed for Union Carbide and completed in 1960, was intended for only 3,500 employees, a quarter of JPMorgan Chase’s desired maximum. Furthermore, the building’s close column spacing and low ceiling heights were not conducive to modern office layouts even after several interior upgrades. Studies concluded that renovation and overbuilding would be costly and impractical.
The conclusion was clear: If JPMorgan Chase wanted to house up to 14,000 New York City employees at 270 Park Avenue, it had to demolish and replace the existing building. And that undertaking carried complications of its own, inherent challenges of building in ultra-dense Manhattan aside.
The most significant site challenge was the Park Avenue side’s base. All but the western quarter of the 200-ft by 400-ft site sits over tracks and platforms of Grand Central Terminal, which in turn bears over portions of the recently opened Grand Central Madison. The tracks run north and south and are typically paired with a platform between them—an east-west spacing of about 60 ft—leaving only the gap between adjacent tracks for support. The structure supporting the trainshed roof occupied much of that gap, and the Union Carbide building’s foundation further congested it.
All told, a teardown and rebuild was a daunting, complex task. But a skilled design team navigated its hurdles to construct a 94,000-ton steel building without significantly disrupting or altering the train tracks and stations below.
The design team, led by architects Foster + Partners and structural engineers Severud Associates, worked closely with the Metropolitan Transportation Authority (MTA), which operates the Metro-North Railroad and Long Island Rail Road tracks below the Park Avenue side of the site, to determine acceptable areas where new structure could be erected. They started by examining the narrow space between dynamic envelopes, which are often no more than 48 in. wide. Additionally, they had to work around existing power, signal, and other utilities that could not be interrupted or relocated without prohibitive cost. Only a few lines of potential bearing were identified, and along them, only a handful of locations were deemed feasible.
The west quarter of the site—roughly 200 ft by 100 ft along Madison Avenue—is much less obstructed than the Park Avenue side. It has more direct support on bedrock, which gave the designers greater flexibility. They concluded that the building’s elevator and service cores should be located at that end. The architects, though, desired an externally symmetrical design and chose column locations that mirrored the supports located along the east side.
The Tabletop
Collecting the gravity and lateral loads from the tower columns—spaced at 40 ft east to west and up to 66 ft north to south—and delivering them to the selected points required an extensive transfer system. The design team studied several schemes for it before arriving at a two-story-high arrangement of steel transfer girders and sloping super-columns that came to be known as the Tabletop.
Along the north and south elevations, groups of four adjacent exterior columns slope inward and toward each other to create three fan-shaped sub-structures. Longitudinally, two 25-ft-deep plate girders span the length of the building to transfer the interior columns. The girders, which also carry the second and third floors, are each supported by V-shaped columns. Together, the girders and columns form two vertical planes down the center of the structure. At the east end, two super-columns slope from the building’s mid-point to the base of the easternmost fan columns. Additional vertical and near-vertical columns at the west end integrate the girders with the core framing.
The diagonals are box sections up to 48 in. wide by 34 in. deep, formed from plate up to 8 in. thick and capable of carrying up to 30,000 kips of axial load each. Despite their size, their sparse arrangement and length—on the order of 100 ft—make them appear graceful, with an obvious sense of the smooth flow of force from top to bottom. The Tabletop’s 86-ft height allows for an open and spacious lobby that affords views through the building from Park to Madison Avenues.
The office tower springs from the Tabletop and seems to float above the lobby. Architecturally, it is a vertical “book,” the pages of which are composed of nine rectangular extrusions of the longitudinal column bays. The east- and west-most column bays terminate at the 17th floor, creating roof setbacks. The next column bays drop off at the 38th, 52nd, and 58th floors. From the 58th floor to the peak at 1,388 feet, the building is only 40 ft wide.
Starting in the cellars, the elevators are in the western quarter of the footprint to avoid bearing on the underground trainshed. At the 14th floor, the shafts shift eastward to the center of the floor plate in a more traditional layout. Here, the multiple-floor transfer, known as the sky lobby, creates a livable space at the center of the building with a conference center and recreational facilities. Moving up the building, the low-and mid-rise elevators terminate as the floor plate reduces in size.
Foundation and Lateral System
The trade-off of having relatively few support points is an elegant building with tremendous demands on its foundation. The loads at each super-column base are almost 100,000 kips, while the area available at each support is only about 60 sq. ft. Access was constrained by the underground location and the MTA’s desire to protect the trainshed roof and minimize track outages. Steel base plates up to 33 in. thick accommodate the loads, but below ground level, a concrete solution was needed to create a path to bedrock.
The concrete had to be stronger than the 14,000-psi mix successfully used on Severud’s One Vanderbilt Avenue project nearby. Higher strength concrete mixes were not readily available, so the design and construction team developed one themselves. After many iterations, they crafted a mix that, by the end of the project, had an average strength of nearly 20,000 psi and a maximum strength of 23,000 psi.
The lateral system of the office tower is simple yet elegant. In the east-west direction, concentric and eccentric braced frames, moment-resisting frames, special braced-moment frames, and outrigger trusses are spread between two column lines. In the northsouth direction, concentrically braced frames combined with outrigger trusses and macro bracing on the east and west facades provide the necessary lateral stiffness and strength. The macro braces form a diamond shape on the face of each building setback, creating distinctive façade elements while providing supplemental lateral bracing.
Capping it off, an opposing pendulum tuned mass damper sits on the 55th floor and is suspended from the 57th floor to assist in the control of building accelerations. The 280-ton damper will keep wind-induced accelerations within the desired comfort level for one- and ten-year events.
The north-south lateral system delivers loads directly to the supporting concrete walls, even those located between train tracks. With few closely spaced supports, however, the uplift forces generated are extremely high, on the order of thousands of kips. The foundation walls are anchored using 13.5-in. diameter caissons, drilled into bedrock from the lowest track level and post-tensioned with three No. 32 Grade 75 threaded bars.
Anchoring the column bases two levels above, though, required a more innovative solution.
The traditional approach is to install prestressing tendons from the caisson cap or the bottom of the wall and post-tension them from the ground level. The design team chose this route, but was concerned that if a dead-end anchor failed during prestressing or in service, it might not be repairable or replaceable at track level.
Instead, engineers devised an ingenious modification: U-shaped ducts for the tendons, with both openings at ground level, that dip to the bottom of the caisson cap to engage the rock anchors. Tendons of 27 strands each were fished through the ducts, passed through holes in the massive column base plates, and tensioned with a jack at each end, working simultaneously.
In the east-west direction, the train tracks and platforms prevent the transfer of lateral load from ground level to foundation for most of the building’s length. Consequently, the ground floor slab is used as a gigantic drag strut to deliver the lateral forces from the columns bearing over the trainshed to the western portion of the foundation. The 16-in. thick slab has a concrete strength of 10,000 psi and is post-tensioned with four groups of four tendons aligned with the column bases. The tendons are composed of 55 strands each and required specialized jacks from France to be tensioned.
Vibration Control
The building shares its foundation with two dozen railroad tracks, therein subjecting it to intense vibration with each passing train. At minimum, several trains travel the tracks underneath the building every hour, and even more during rush hours. That was not the only vibration challenge. JPMorgan Chase desired stricter perceptibility limits for its building’s occupants, choosing values normally used for residential uses.
Severud engineers worked with wind tunnel and micro-climate consultant RWDI to tackle the vibration demand, starting by developing a vibration monitoring protocol and an initial forcing function. Using an analysis model of the Union Carbide building, the trial function was applied and its response predicted. The trial accounted for the subgrade modulus of the supporting rock, the travel path of vibrations from the track support framing to the building foundation and up through the building columns, and the proportion of dead load participating in the response.
Analysis predictions were then compared to measured responses in the building. Using the observed measurements, the forcing function was recalibrated accordingly and re-applied to the analysis model. After more than 150 iterations, a forcing function that produced responses in good agreement with measured vibrations was established and applied to an analysis model of the new building.
Tabletop Erection
As design of the building—and the Tabletop in particular—developed, construction manager AECOM Tishman brought in steel contractor Banker Steel and erector NYC Constructors, both DBM Global companies, to address fabrication, logistic, and erection issues and to ensure that the steel component of the project remained economical, constructable, and timely.
The Park Avenue site’s central Midtown location presented another significant challenge: Access for material delivery and storage, cranes, and other construction equipment. In fact, loading restrictions barred contractors from using Park Avenue for this phase of the project. The options were Madison Avenue and two narrow side streets.
After performing an analysis of the load capacity of the existing roadways, AECOM Tishman and Banker Steel devised a logistics and erection plan. It accommodated and maintained access to public transportation and other necessary traffic, and diverted pedestrians. The plan also included delivery and storage areas on 47th and 48th streets that allowed the roads to remain open to other vehicles. The plan’s most critical feature, however, was its methodology for erecting the Tabletop.
The Tabletop’s two longitudinal transfer girders, located roughly at the third points of the building’s width, and the two lines of fan columns create three natural east-west traffic lanes through the site. Banker Steel envisioned placing crawler cranes in the north and south lanes, which could pick up and place members delivered via the center lane. Temporary elevated runways were designed and installed starting at Madison Avenue—where material deliveries are allowed—and progressing to the east. A temporary protection platform, with a capacity of 600 lb per sq. ft, was also installed over the entire building footprint to protect the trainshed below and support shoring loads.
Working independently, the cranes erected the fan columns and their temporary supports. Working in tandem, they erected the transfer girders—the segments of which exceeded a single crane’s capacity. Framing between the girders was also erected as the cranes moved eastward, creating a stable sub-structure. An additional advantage to this erection scheme was that could start while demolition of the Union Carbide building and construction of the foundation walls were still in progress.
By the time the cranes reached mid-block, demolition of the existing building had been completed and the cranes continued east to Park Avenue. From there, they backed out the way they came in, erecting the framing between the fan columns and transfer girders and setting the four tower cranes that would erect the remainder of the building. The temporary runways were also removed as the crawler cranes made their exit. The elegance and efficacy of this erection scheme contributed to the building topping out ahead of schedule.
Tabletop Connections
Collaboration between Severud, AECOM Tishman, and Banker Steel—based on close relationships strengthened during their work together on One Vanderbilt Avenue—also led to significant improvements in connection designs. This was especially critical for the Tabletop, where at each of the selected support points, up to five massive members converge at a single point.
The traditional approach is to create nodes using welded plates, but that presented daunting constructability issues, mainly due to large, multi-pass welds and the likelihood of heat distortion. The potentially unfavorable aesthetics of the exposed connections were an additional liability.
Instead, the team proposed nodes fabricated from forged steel to reduce fabrication issues and better accommodate the three-dimensional stress field acting on the nodes. It is a simple but brute force solution—the forgings are essentially huge blocks of solid steel—and also a sophisticated one. Using the results of advanced finite element modeling that determine detailed 3D stress distributions, metallurgists chose an appropriate alloy and proscribed a process of heat and mechanical treatments to fabricate weldable nodes that safely transmit high stresses delivered to the node at several locations and in different directions.
Once forged, the nodes were milled using CNC equipment to generate multiple bearing surfaces with small tolerances, thereby reducing the risk of fit-up issues in the field. Milling was also used to smooth the exposed surfaces. Another advantage of forgings is that there are no extraneous plates or angles that might interfere with the architect’s desired appearance.
The forgings solved several problems, but their size—the largest are up to 12 ft wide by 7 ft high by 4 ft thick and weigh up to 75 tons—created another set of challenges. The first was transportation. Trailers with up to 19 axles were needed to move the nodes by road and keep within load limits. Trucks had to wait at the George Washington Bridge until the middle of the night to cross into Manhattan before being escorted to the Madison Avenue site entrance.
Another consideration was heat capacity. Welding such massive sections requires pre-heating that reduces temperature differentials between the weld and base materials to minimize residual stresses and potential crack formation. The forgings acted as heat sinks and took several hours to warm up. While manageable in the shop, controlling preheat was difficult in the field, especially in winter. In some cases, stubs of the fan column sections were shop-welded to the nodes to move the field-welded section away from the node and reduce pre-heat demands on site. In all cases, heavy insulating blankets were used to prevent the members from cooling too quickly.
Plate Girders
The plate girders that form the backbone of the Tabletop are the building’s largest single elements. Spanning the 360-ft length of the building, the girders are 25 ft deep—the full height of the second floor. Their flanges are 5 ft wide, with plates that vary from 4 to 8 in. thick. The webs vary from 2 to 6 in. thick. Their total weight is on the order of 1,800 tons each. Again, close coordination between Severud and Banker Steel produced fabrication and connection details that facilitated shipping and erecting these gigantic elements.
As a starting point, the girders were divided depth-wise into three stacked sections. Each 8-ft, 4-in. deep section could be shipped upright, allowing greater flexibility in their lengths, and then bolted to each other in the field using 30-in. wide intermediate flange plates. Banker Steel suggested locations for vertical splices, picked with the total tonnage in mind and the allowance of stiffener plates, connection material—including forgings, in some instances—web openings, and other appurtenances.
Severud reviewed the splice locations and adjusted them based on stresses in the affected areas, with a goal of using bolted connections wherever possible. When the process was completed, each girder had been divided into about 20 individual pieces, which varied in weight from 86 to 136 tons. Due to their heft, these pieces were subject to the same shipping methodology as the forged nodes. However, the weight of some sections exceeded the George Washington Bridge’s load limits and were instead ferried across the Hudson River by barge.
The Future of the Building
JPMorgan Chase envisions 270 Park Avenue as a model for the 21st century workplace with its sustainability features such as all hydro-electric infrastructure, zero net operational emissions, and best-in-class air quality. The building will also feature intelligent, sensor-based controls, efficient water usage and storage, and high-performance glazing. The construction employed a high proportion of low-carbon materials, including substitution of ground glass pozzolans for up to 47% of the cement in all structural concrete except the 16,000-psi mix. Remarkably, 97% of the demolished existing building was reused, recycled, or upcycled.
The building also serves as a model for how modern workplaces can be designed and constructed. Collaboration among all parties from the earliest stages of design development through construction allowed everyone to focus on critical issues and continually refine and improve the building. This was especially true of the structural steel component, where teamwork by the structural engineer and steel contractor simplified fabrication and erection of a complex structure with accompanying cost and schedule savings.
New York’s Largest Steel Frame, By the Numbers
Entire Building Steel 94,000 tons
Field Welds 85 tons
Field Bolts 1 million
Metal Deck 2.6 million sq. ft
Headed Studs 400,000 (field-welded)
Connections 20% of overall tonnage
Tabletop Steel 15,600 tons
Shop Welds 250 tons
Temporary Steel 4,000 tons
Heaviest Member 136 tons (segment of plate girder)
CJP Weld Thickest Material 8 in.
Length 248 in.
Field Weld Largest 1,050 lb (Tabletop Column)
Time to Complete 6 weeks (two welders)
Largest Connection 1,026 high-strength bolts (4-sided column-beam-diagonal connection)
Owner
JPMorgan Chase & Co.
Development Partner
Tishman Speyer
Design Architect
Foster + Partners
Executive Architect
AAI Architects
Structural Engineer
Severud Associates
Consulting Engineers, PC
Construction Manager
AECOM Tishman
Steel Team
Fabricator
Banker Steel
Erector
NYC Constructors
Edward M. DePaola (edepaola@severud.com) is president and CEO, and Fortunato Orlando (forlando@severud.com) is associate principal, both with Severud Associates. Chet McPhatter, president of Banker Steel, Ritchie Bhaskaranand, vice president of Banker Steel, and Barry King, president of NYC Constructors, also contributed to this article. Andrew Mueller-Lust, a former principal of Severud Associates, contributed to the writing.
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