Excellence in Adaptive Reuse

"Top of the Rock embodies the adaptive reuse that’s possible with steel. It’s not easy to rig an amusement ride at the top of an existing 90-year-old building. The ride captures the spirit of steel and the ironworkers who have brought it to life through the ages to celebrate the connection between a community and the structures that define it." -- Jeremy Loebs, Business Development Executive, Schuff Steel, 2025 IDEAS² Awards Judge

An iconic New York City building added attractions to its popular rooftop observation area, and the project’s new steel elements facilitated disruption-free work and success on a tight rooftop job site.

The Top of The Rock on the 67th, 69th, and 70th floors of 30 Rockefeller Plaza in midtown Manhattan is a popular tourist destination, and the owners wanted to bring new interactive experiences to the observation deck portion. One is called The Beam and allows patrons to recreate the famous "Lunch Atop a Skyscraper" photograph from the 1930s. The other, the Skylift, is a telescoping platform that rises 30 ft above the 70th floor and rotates 360° to provide panoramic city views. The rides lift out of the roof deck and, when not operating, look like natural observation deck features.

The observation deck could not close during construction of both rides and the frame adjustments to accommodate them, and work could not interrupt the building’s elevators despite a lack of freight elevator access to Top of The Rock. Luckily, choosing steel for the additions and modifications allowed structural engineer Gilsanz Murray Steficek (GMS) to work more efficiently with Rockefeller Center, which was constructed with a steel frame. The range of available steel shapes and sizes allowed for creative use of new sections and reinforcement, and any other material choice for a ride that pays tribute to a photo of ironworkers constructing Rockefeller Center would have felt wrong.

Not So Spacious

Space and access constraints were the building’s biggest hurdle. The 67th and 69th floors are roughly 60 ft wide and the 70th floor is only about 20 ft wide, with elevator machine rooms and other base-building mechanical systems occupying nearly all the space along the core.

Keeping the observation deck open meant that most work occurred between midnight and 8:00 a.m. when it was closed. While portions of the deck could close during the day, jobsite space on the deck never exceeded 2,000 sq. ft.

Early design phases brought frequent coordination calls and design submissions to the ride manufacturer to understand its restrictions and requirements. Gantry cranes on the roof were used to install both rides, and nearly every cubic inch of space was accounted for and needed in the Skylift motor room and the suspended pits for The Beam.

GMS had the building’s original 1932 erection drawings and an incomplete set of drawings from the 2004 renovation project that included substantial observation deck work. There were numerous instances where the conditions on site did not match any drawings, leading to probes of existing conditions to finalize the design. Steel’s design flexibility was crucial for incorporating new information and the resulting design tweaks.

Building up The Beam

The backbone of The Beam’s support system is a new W36×150 that spans approximately 28 ft between two existing girders, which needed extensive reinforcing. When the ceiling below the 69th floor was removed, the team discovered a 30 in. by 18 in. web opening had been cut into the west existing girder. The web opening was at a high-shear area and did not comply with proportioning and spacing recommendations in AISC Design Guide 2: Steel and Composite Beams with Web Openings.

The W36 would connect approximately 6 in. from the nearest edge of the opening, and it could not be moved without moving the location of The Beam, which was not an option. A surplus of utilities--including approximately 12 antenna cables owned and operated by NBC, the New York Police Department, and the FBI--were crammed within the web opening.

GMS created extensive finite element analysis models to explore all options to avoid moving the cables. None were viable, though, so the solution was to cut and reroute the antenna cables, partially reduce the web opening size, and add a doubler plate.

Skylift Substructure

The Skylift’s substructure has four static guideposts connected to the 70th-floor framing at the 12, 3, 6, and 9 o’clock positions. The project phasing relied on GMS completing the base-building work at the motor room and roof opening and turning over a turn-key space that would accept the Skylift equipment. The location, elevation, and configuration of the base-building structure at these four connection points along the 70th floor were hyper-critical. The new beam at the 3 o’clock position was the last project hurdle.

The new Skylift motor room is a former gift shop that occupied half a column bay, and at first, the team had limited access to survey details concealed by finishes. To stay on schedule, Skylift fabrication began before the gift shop was gutted. Gutting revealed the space’s east-west dimension was approximately 1 ft smaller than initially thought based on prior renovation drawings and initial surveys. Recovering the lost space by setting the existing structure 3 ft lower cost nearly all the extra pit floor space allotted for tolerances. It also meant the new beam along the 70th-floor east edge would be partially within the elevator machine room in the other half of the bay.

The two new girders that support the Skylift are each 28 ft long, weigh about 2 tons, run east-west between existing columns, and are set directly below the original 69th-floor framing. Originally, the girders would frame into the existing columns’ centerline, but that would put the east half of the girders partially within the 900-ft-tall elevator shaft. Moving them outward from the shaft would encroach more within the elevator machine room, which didn’t have space for rigging and installation.

Four disconnect panels were along the demising wall’s north end, and critical electrical cabinets were along the south end. In that condition, the new beam would pass directly behind the disconnect panels and inches above the electrical cabinets. Furthermore, existing east-west beams into the Skylift motor room would have to be shored, cut, and reconnected to the new beam after the roof slab above the motor room was demolished. In response, all steel work was scheduled in coordination with a separate elevator upgrade project because it included temporarily relocating the electrical cabinets.

GMS worked with the contractor to develop a shoring sequence and reconnection detail that would fit into the constantly changing conditions. The final detail is like an inverted beam hanger where the vertical straps are in compression, a design that allowed all work to occur on one side of the demising wall without impeding building operations.

Piecing Things Together

Given the lack of freight elevator access, each 28-ft-long girder had to be delivered to the site as four pieces, each about 7 ft long. Each piece traveled to the 67th floor in a recently refinished passenger elevator and was rigged up to a temporary work platform, which was built above the elevator staging area via an access hatch purposefully set where the Skylift installation team could later rig up materials from 67 to 70.

Once on the work platform, a system of carts on channel tracks maneuvered each 1,000-lb girder piece to its staging area on the west side of the work platform, and then the girder pieces were fit up and CJP-welded on a workbench hung from the 69th-floor framing. After the splice was completed, inspected, and ultrasonically tested, the workbench doubled as a track for pulling the girder to the east to allow for the next piece to be fit up in an assembly line-like fashion.

After all segments were spliced, shelf angles were welded to the column faces so the girder could be jacked into final position. The connections at the girders’ eastern ends were designed to require access only from one side and required partial installation and inspection before the girder was pulled into its final location.

Both rides opened to the public in December 2023 and are a testament to steel’s adaptability, even in the face of inconvenient jobsite constraints, and also demonstrate that steel frames can be adjusted later in their lifespan to serve a purpose never even considered during their initial design decades earlier.

The Top of the Rock project would have been easier to execute if the observation deck could have closed for a few months to accommodate blowing out existing bays and bulkheads and replacing them with a purpose-built frame, but it wasn’t an option. Deliberate coordination, creative engineering, and working with an adaptable material resulted in a project whose original structure, designed for one purpose, could be elegantly altered for a completely new one.

Owner: Tishman Speyer, New York
General contractor: Gilbane Building Company, New York
Architect: Montroy DeMarco Architecture LLP, New York
Structural engineer: Gilsanz Murray Steficek, New York
Attraction consultant: THG Creative, Pasadena, Calif.
Steel team:
Fabricator/detailer: North American Manufacturing Corp., Maspeth, N.Y. *AISC full member, AISC-certified fabricator*
Erector: Maspeth Welding, Inc., Maspeth, N.Y.