Access and Protection at 1,250 Feet

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For many structural engineers, means and methods represent a nebulous cloud of scaffolding, shoring, cranes, and hoists that is swept into a corner labeled “by others.” In the traditional design-bid-build model, the general contractor is responsible for the design and coordination of all means and methods—the combination of temporary structures, sequences, and methodologies that answer the “how to” of getting a design built.

A consequence of this model is that the design of these temporary systems often cascades down a chain of subcontractors and their various engineering consultants. This presents obvious coordination challenges as the responsibility for design moves farther away from the owner and core design team (AOR/EOR). Often, and especially with new construction, coordination may center around timing (demolish this, then build that) or spatial considerations (the scaffold goes here, the crane over there) that can be effectively orchestrated by the general contractor.

However, working on existing buildings requires a higher degree of coordinated design, making the “too many cooks in the kitchen” model inefficient at best and unsafe at worst. When a project occurs at extreme heights, such as on the 102nd floor of the Empire State Building, the means and methods design becomes crucial, requiring an equal amount of time, energy, and collaboration as the final design itself.

This was the case when the Empire State Reality Trust embarked on a major refresh of the 102nd floor observation deck. The project, intended to enhance the visitor experience, called for the complete demolition of the existing envelope to make way for new floor-to-ceiling glass panels that would provide 360-degree views of the Manhattan skyline. A design-assist team consisting of Skanska USA, Plan B Engineering, and Greg Beeche Logistics (GBL) worked closely with the owner and building EOR, Thornton Tomasetti, to design and install an external access and enclosure system, dubbed “the cocoon.”

Over 30 tons of temporary steel and aluminum framing were assembled from custom, modular pieces at 1,250-plus feet above street level to create three exterior work platforms fully enclosed in high-strength fabric (galvanized steel mesh embedded in a weather resistant fabric). Once completed, the cocoon provided a contained work area that was approved by the New York City Department of Buildings (DOB) as an alternate means of protection under Chapter 33 of the New York City Building Code, which governs pedestrian safety in occupied spaces (Section 3307). This allowed the popular and lucrative 86th floor observation deck to remain open—a key mandate from ownership.

Underscoring the challenge of this temporary design was the fact that the installation and dismantling of the cocoon—permitted only between 2 a.m. and 7 a.m.—accounted for nearly two-thirds of the construction schedule.

Plan B was engaged by Skanska early in the planning process to envision an enclosure and access system that met the project needs. The system needed to:

The Tower Reconfiguration Project (TRP), a project previously designed by Plan B , provided proof of concept for the construction of a modular enclosure system at extreme heights. The TRP utilized triangular truss sections wrapped in high strength fabric (the same material eventually used for the cocoon) to create an enclosure and worker access to perform structural upgrades to the antenna spire. The TRP was located at the base of the spire, just above the ice shield (a circular, umbrella-like steel bracket that provides a protective “awning” above the exterior walkway on the 103rd floor). This enclosure was attached to the building at the ice shield and laterally tied to the antenna spire above, effectively functioning like a ground-based supported scaffold, albeit at an extreme height. Compared to the TRP project, the observation deck enclosure did not offer such readily available attachment points. However, the TRP project did represent a proof of concept for the construction of a modular enclosure system at extreme heights.

A laser scan conducted as part of the renovation documentation provided an accurate baseline of the building’s geometry. Historic structural drawings were useful but fragmented; over the years, the building’s more utilitarian upper levels were modified with little or no documentation. This necessitated multiple site visits, often in tight, claustrophobic spaces, to fully document all existing conditions. Each face of the building showed slight variations, requiring close attention when detailing the final system.

A previous restoration of the mooring mast glazing utilized a custom suspended scaffold hung from outriggers supported by the 104th floor ice shield. For that project, Plan B collaborated with Thornton Tomasetti to reinforce the ice shield framing and connections. The initial concept was to use this reinforced ice shield as the primary vertical support point for the cocoon. Plan B developed early schematic design loads for this option. However, after presenting this concept to Thornton Tomasetti, it was determined that the required reinforcements to the ice shield were deemed uneconomical. A second option of attaching brackets directly to the existing columns just below the ice shield proved to be the more practical solution.

The reinforced ice shield was nonetheless vital to the overall workplan. The upgraded ice shield incorporated modern OSHA rated lifeline support points that were used during critical exterior work—most notably the removal of the Alford radio antennas and installation of connections to the building columns.

On the 102nd floor, a series of 32 Alford radio antennas (formerly the largest combined FM station system in the world, circa 1965) were scheduled for removal. Knowing these attachment points would soon become available, the team conducted a series of probes and site visits to better understand the framing. Removing the antennas offered access to the building columns just below the work area—an ideal lateral tie point at the floor of the cocoon. The phased removal of the antennas was then integrated into the installation of the cocoon, a benefit to ownership by streamlining the two projects into one.

Such early investigations and vetting of ideas provided vital clarity for such a unique temporary design. Reducing the amount of design and existing condition unknowns allowed Skanska to solicit more accurate bids; and providing the specialty contractor with a viable basis of design for resolving the vertical and lateral loads streamlined the overall design development.

A small elevator used to transport VIP guests between the 86th and 102nd floors provided limited access to the work zone. This dictated an 8-foot maximum member length, corresponding to the diagonal clearance inside the elevator. From the 102nd floor, all equipment and material had to be carried by hand up a ladder to the 103rd floor. A hatch at the 103rd floor provided access to the “roof” of the building, the 104th floor, where the ice shield is located. All workers at this level were harnessed and tied off to pre-existing lifeline anchors on the ice shield. Additional radiation protection was needed to ensure workers were not exposed to excess radio wave energy from the active television and radio broadcast antennas directly above.

On the 90th floor, at the base of the mooring mast spire, were four small exterior platforms reinforced by Skanska and Plan B for the previous mooring mast project. As with that project, the platforms were identified early on as the only feasible staging area for assembling the cocoon. The north side platform was eventually modified into a partially enclosed “garage” with 8-foot-tall side walls, where the cocoon modules were assembled and rigged up to the work area.

Recognizing the coordination and pricing hurdles early on, Skanska engaged Plan B to develop performance drawings. The purpose of these drawings was three-fold:

A wind test was conducted by RWDI at its wind tunnel facility in Guelph, Ontario. Peak positive wind pressure was 74 psf and peak negative was -92 psf. The high strength fabric supplied by HAKI was previously tested for the TRP enclosure at Florida International University’s Wall of Wind. It was found to resist real wind speeds up to 140 mph. At the time of the project, the 2014 New York City Building Code modified the ASCE 7 wind speed up to 98 mph (ASD).

Keeping the 86th floor observation deck open to the public during the reclad project was a main priority for the owners. However, the construction variance approved by the city only applied to the completed cocoon system. This required the cocoon to be staged, assembled and then disassembled between the hours of 2 a.m. to 7 a.m. to maintain public use of the observation deck during the day and evening. This required modeling the structure in various forms of incompleteness to ensure a proper envelope was developed for loads imposed and member forces.

Construction loads included two platform levels rated for 25 psf and a floor rated for 300 psf to satisfy requirements of the protection variance granted by the DOB. A 23 psf snow load (adding 22 kips to the model) was applied at the roof, and an ice analysis added another 191 kips of vertical load.

Greg Beeche Logistics (GBL) was chosen to furnish and install the framework of the cocoon using its own modular aluminum double track (ADT) kit of parts developed in-house. Plan B peer-reviewed the final design package and collaborated with GBL engineers and drafters during the design development. GBL’s high level of fabrication precision allowed engineers to collaborate with drafters to identify and resolve detailing and design issues in a design-build style workflow.

Once the design was completed, Plan B led the loads-imposed review process with the Thornton Tomasetti and managed the lengthy DOB approval process. Plan B also prepared the site safety plan (SSP), which was tied directly to the previously mentioned approvals and variances.

The overall configuration of the cocoon consisted of 16 pairs (32 total) of vertical trusses, composed of ADT chords and steel HSS web members. These were assembled in modules on the 90th floor platforms and connected to brackets bolted to building columns on the 103rd floor.

Each module was an 8-foot-wide structural bay consisting of two vertical trusses laced together with bolted aluminum HSS3x3 web members to form a moment frame at every other bay around the donut-shaped structure (with the center of the donut being the building itself). The vertical truss web members were optimized by location along the span to reduce weight as much as possible.

Steel brackets on the 103rd floor were field bolted to structural columns by harnessed workers suspended from the ice shield. The brackets supported a 3D space frame composed of aluminum ADT chords and steel web members, with the top serving as a catwalk platform to provide access to tied- off workers around the exterior of the 103rd floor. The bottom of the cocoon contained another plan diaphragm and tie points to building columns to resolve wind loads. The overall assembly was considered by designers to be a hybrid frame/truss plan bracing system.

Each module provided two 2-feet 6-inch-wide work levels rated at 25 psf, using moment-framed ADT brackets. Small 2-inch to 4-inch aluminum HSS members laced the brackets together in plan to form intermediate diaphragms. Additional stiffness was provided by detailing nailers into the outriggers to fasten 0.5-inch BBOES plywood for the working deck.

A third work level was also provided at the floor of the enclosure. During the DOB approval process, the floor of the enclosure was required to be rated for 300 psf, the protection load then required for NYC sidewalk sheds for construction over 125 feet in height, under 2014 NYC Building Code. This required further customization of the ADT system for the increased load rating. The floor was custom cut to fit snug against the saw tooth profile of the mooring mast glazing.

The exterior cladding consisted of HAKI sheeting (PVC-coated polyester fabric) with an edge termination made of Keder beading wire sewn into the fabric. The beading was slotted into matching continuous aluminum tracks, which were restrained at 12 inches o/c vertically on the outer flange of the inner ADT chord with laser cut plates. The plates worked with the vertical pins connecting the ADT truss pairs to resist the catenary thrust of the HAKI fabric under wind loading.

The 103rd floor bracket, which supported 90% of the vertical loads, consisted of a combination of GBL’s modular ADT equipment and custom steel brackets. The bracket connections were bolted directly to eight existing building columns around the perimeter of the building. Although field drilling bolt holes at 1,250-plus feet is not ideal, it was considered favorable over the logistics of executing field welds and special inspections in such conditions . This led to an emphasis on bolted detailing as much as possible.

The brackets supported a space frame designed as a plan diaphragm at the roof of the cocoon. Since the cocoon had 16 vertical trusses but only eight building columns, the space frame was designed to span in two directions to deliver vertical and lateral loads directly to the building columns. As with the vertical trusses, the space frame consisted of aluminum ADT chords members and steel web members.

The top surface of the space frames created a 36-inch-wide catwalk for tied off worker access. A pair of W8x21 curved tracks sat on top of the space frame to support “the crab”—an electric winch built of ADT modules that could reach out beyond the façade. Another rail beam was underhung to the bottom of the space frame, supporting a small electric hoist used to move the new glass panels, weighing 450 pounds each, into position.

The crab’s primary purpose was to rig up modular sections of the enclosure assembled at the 90th floor exterior platforms. To ensure rigging stability, a mockup of the panels and crab was built to ensure the rigging aligned with the center of gravity of the completed module. The motorized crab could traverse along the curved W8 tracks, providing 360-degree rigging capabilities essential for the project.

Once the 103rd floor brackets, space frame and crab were installed, the 32 Alford antennas floors 102-103 were removed by workers in suspended scaffold rigs. Again, the crab proved invaluable as it supplied a four-point suspended support for the work, with 360-degree access to the façade. With the antennas removed, workers were able to access I6 (I-beam, 6 inches depth) vertical mullions that framed out the observation deck to provide a lower phase 2 tie point for the cocoon.

The lower tie points (“receivers”) consisted of built-up plates clamped with custom fabricated beam clamps to the existing I6 mullions that framed out the 102nd floor observation deck immediately above. Steel HSS 3x2 ties with a hammerhead end fixture were then installed from the exterior through existing façade openings left from the Alford antennas and connected with a single .75-inch diameter bolt. These ties lateral restrained the truss modules by providing support points for a horizontal diaphragm—essentially a circular trussed compression ring. The hammerhead end fixtures on the ties created moment connections at the main node points, with pinned truss web members spanning radially between ties. The truss web members consisted of 2-inch Sch 40 pipe struts with threaded end fittings that allowed for effective field fit up.

Modules were assembled on 90th floor garage platform by first building four separate vertical sections that were temporarily nested together within the tight confines of the 90th floor platform walls. The modules were then lifted using the crab and connected in top-down order. The modules were connected to the previously installed tie points at floor 103 via two 1-inch diameter pins and to each other via five 1-inch diameter x 24-inch long (vertically oriented) “module interface pins” to create a double spine. EPDM rubber seals were used at the gaps not filled by structure between the two spines). Once erected and tied in at floors 102 and 103, each module was a fully independent structure that was stable independent of its yet-to-be installed neighboring modules.

This was a project where means and methods were as critical as any facet of the new design. A collaborative effort allowed for the seamless removal of existing finishes, the installation of new glass panels and the dismantling of the cocoon from above—all while keeping the 86th floor observation deck operational throughout the project.

The enclosure set an early precedent that paved the way for the Engineered Enclosure System (EES) provisions now in the 2022 New York City Building Code. This system has quickly gained popularity as a strategy to minimize intrusive adjacent property protections. ■

About the Author

Robert Belardi, PE, is a 2011 Drexel University graduate in architectural engineering and a graduate of the SAHC program in Universidade do Minho, Portugal. He has worked at Plan B Engineering a decade, designing a wide range of access and stabilization systems for new and historic constructions