'Steeling' the Show

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Elevators are a near necessity in most publicly accessible buildings. They augment stairwells by dispersing traffic flow. They also support accessibility and provide additional means of egress. With advances in fire-rated glazing, designers have shifted from using strictly opaque materials to planning open, bright and code-compliant elevator shafts. However, unlike other elements of the built environment, elevators generally require larger load tolerances to accommodate full cars, machinery, and dynamic forces. These higher tolerances help maintain precise alignment of vertical elements.

These considerations can make designing and engineering modern and visually stunning elevators challenging, especially when taking into account visibility into and out of elevator cars. When an elevator design includes glass curtain walls or other glazing assemblies, project teams can utilize steel sub-frames to meet both fire-rated as well as dynamic and static load requirements. And when these sub-frames are roll-formed, they can be specified with narrower profiles to maximize the glazing area and allow the use of a variety of cover caps without significantly increasing framing width. This can support a cohesive design aesthetic between fire-rated and non-rated assemblies in a wide range of applications.

Steel sub-frames were used in two prominent elevator shafts in New York City: The Fulton Center Transit Hub and the Empire State Building. These projects exemplify how steel can be at the center of a successful elevator shaft design—literally and metaphorically. But before discussing them directly, it is important to detail what these structures may need to be safe and code-compliant.

First and foremost, as a vital component in a means of egress system, elevators shafts are often required to meet local building code requirements for fire and life safety. As these requirements can vary significantly between locations, project teams are encouraged to consult with local fire- and life-safety codes and to clarify any ambiguities with an Authority Having Jurisdiction (AHJ). That said, model building codes, like the International Building Code (IBC), provide an adequate baseline for the discussion of these structures.

According to Chapter 7 of the 2024 edition of the IBC, shaft enclosures must be constructed as fire barriers, which is a wall or other kind of continuous membrane designed and tested to limit the spread of fire—a full definition can be found in the National Fire Protection Association’s Life Safety Code (NFPA 101). Commonly, for elevator shafts that connect less than four stories, a fire-resistance rating of 60 minutes is required. Shafts larger than four stories will often need a 120-minute fire-resistance rating. To meet these requirements, structural components, including glazing and framing systems, will need to defend against fire, smoke, and radiant heat for the duration specified in the building codes for their size and occupancy.

In addition to meeting requirements for fire and life safety, as structural elements of the built environment, elevator shafts and their components must meet all applicable design loading criteria, including wind, flood, and seismic loads, as defined by local codes. Likewise, these structures must also have the strength to hold the machinery, elevator cars and their maximum load weight without deformation to ensure proper functioning. They must also meet requirements for vertical and lateral deflection tolerances. These requirements can vary significantly between projects based on local building codes, location of the elevator shaft within the building, its height, its motor and car, capacity, and other considerations—all found in Chapter 16 of the 2024 edition of the IBC.

Although aluminum frames can meet these requirements, they may require more material, supplemental supports, and ancillary fire defense systems. Steel frames, on the other hand, can minimize the need for these additional systems. Steel frame systems can pass ASTM E330-97 test standards, demonstrating a resistance to damage from a uniform structural load of +/- 125 pounds per square foot (PSF). Additionally, there are systems ranked for use in hurricane zones that have been certified to multiple standards that determine their resistance to wind loads, cyclic wind pressure, impact, design pressure, and more.

In terms of fire-resistance, two factors differentiate steel from aluminum: melting point and thermal conductivity. Depending on the alloy, steel generally has a melting point between 1,370C and 1,540C while aluminum’s is around 660C. Considering temperatures can reach up to 1,000C in typical commercial building fires, steel’s higher melting point helps reduce risk of deformation and melting, which is crucial for ensuring elevator components remain functional to help occupants evacuate during a fire emergency.

Steel also has a lower thermal conductivity than aluminum—approximately 50-60 Watts per meter-Kelvin (W/mK) compared to 205-237 W/mK. This means steel transfers less heat over a given time range than aluminum. With a higher resistance to radiant heat, steel framing helps egress paths remain traversable as occupants evacuate and first responders arrive. Both the higher melting point and lower thermal conductivity of steel contribute to a full glazing system’s ability to meet ASTM E119 (Standard Test Methods for Fire Tests of Building Construction and Materials) and UL 263 (Fire-resistance Ratings) as required by fire- and life-safety building codes.

In addition to fire-resistance, steel also offers a modulus of elasticity of over 29 million pounds per square inch (PSI), nearly three times that of aluminum’s 10 million PSI. As a stiffer material, steel framing can accommodate design loading criteria, including both static and dynamic loads, as well as the extra weight of fire-rated glazing. And the material can do this without requiring larger framing profiles since it does not require any fire resistive interlayers or multiple bulky secondary supports. This helps project teams create a close visual match to adjacent non-rated systems.

The material data paints a picture of what fire-rated steel frames can contribute to a project from an engineering perspective. But their project value extends beyond their data. The Fulton Center Transit Hubs’ central elevator clearly demonstrates how strong-but-narrow frames and large spans of transparent glass can create a design-centered solution to an engineering challenge.
Due to the design’s influence on the Fulton Center’s means of egress system and New York’s particular code requirements for this occupancy type, the upper level of the transit hub required fire-rated glazing curtain wall assemblies while the lower level did not. The central elevator stretches between the fire-rated glazing assemblies on the upper level to the non-rated ones on the lower level. As a part of the Fulton Center’s means of egress system and as a vertical shaft enclosure that connects less than four stories, the entire elevator shaft was built in compliance with fire barrier requirements, including its openings. The glazing assemblies used in the elevator shaft are fire-resistance rated for 120 minutes, exceeding the minimum requirements for vertical shaft enclosures that connect less than four stories.

The project team specified roll-formed, fire-rated steel frames to provide both fire-resistance ratings and sufficient strength to maintain safe operating conditions in normal and emergency situations. Because these systems meet and exceed the project’s fire- and life-safety requirements without bulkier frames, they also support a cohesive design. While the narrow-profile fire-rated frames created a close visual match to non-rated systems, their strength allowed large spans of uninterrupted glass, which contributed consistent vertical mullion spacing between systems.

Both the profile size and spacing supported an open, light-filled design that remains consistent between levels—and as visitors move between levels. Whether it opens to showcase a fire-rated curtain wall system on the upper floor or a non-rated one on the lower, the elevator shaft helps maintain a seamless aesthetic across the entire built environment.

While slender steel frames can satisfy code requirements for design load as well as fire and life safety, their strength can also support larger lites of fire-rated glazing. Often, fire-resistive rated glazing can weigh between 0.54 kilograms per square meter (kg/m²) and 0.91 kg/m² while non-rated glass typically ranges from 0.27 to 0.38 kg/m². At approximately two to four times the weight of non-rated glass, fire-rated glazing requires strong framing systems. Since curtain walls, windows, doors, and other openings in elevator shafts are typically required to be fire-resistive rated for 60 to 120 minutes, if fire-rated glazing is used in these applications, it will skew toward the heavier end of the spectrum, which in turn impacts the strength needed for other components.

In elevator shafts that incorporate fire-rated glazing, steel frames provide the necessary physical strength to accommodate spans of glass large enough to meet accessibility requirements for elevator openings and car dimensions. They can do this without always requiring vertical mullions between the corners of an elevator shaft. This allows uninterrupted spans of glass between corner frames.

While this was a design benefit for the Fulton Center Transit Hub’s central elevator, it was almost a design necessity for the elevator that brings visitors to the Empire State Building’s 102nd floor Observatory Deck.

The elevator to the Empire State Building’s Observation Building offers a top-tier view of the city that never sleeps. Like in the Fulton Center Transit Center application, the material performance data of these frames support the project’s design goals. In this elevator, 120-minute fire-resistive-rated glass, held by narrow-profile fire-rated frames and wrapped in stainless steel cladding, form an interior elevator surround.

The strength of these frames allow uninterrupted spans of glass between corner mullions. This gives visitors jaw-dropping, 360-degree views of the city skyline. According to Anthony E. Malkin, Chairman, President, and CEO of Empire State Realty Trust, this is a crucial design feature as “the interior curtain walls help maintain purity of view, ideal for tourists looking for an unrivaled view from 1,250 feet above New York City.”

Likewise, because of the system’s slender framing profiles, it could also support stainless steel cladding without significantly increasing its profile size. As a result, the system creates a close visual match to adjacent non-rated systems in terms of framing dimension and blends the elevator opening seamlessly with the Observatory Deck's interior through material choice. The cohesive design aesthetic made possible by steel’s material strength limits visual distraction to both frame and augment the aerial views of Manhattan.

In addition to multiple cladding options, narrow-profile steel frames can incorporate cover caps without significantly increasing the framing profile size to maintain a close visual match with non-rated systems. Cover caps are non-structural components that cover a framing system’s subframe to subtly change its exterior shape and finish. They offer designers a variety of finish options and profile shapes to meet a wider range of aesthetics without compromising the performance capabilities inherent to the subframe.

For instance, an elevator shaft that uses fire-rated steel sub-frames can include custom H-channel cover caps that match the finish color of adjacent framing systems and allude to the art deco architectural styles prominent in New York. The shape and material of these cover caps extends beyond this example—from box and contour shapes to a broad swath of finish options, including wood veneer. Because these caps fit over the fire-rated steel components, they allow more design versatility while maintaining steel’s strength and ability to defend against fire, smoke, and radiant heat.

Elevators used to be limited to small cars within shafts enclosed by opaque materials, visually cutting them off from the rest of the building and their surroundings.

As The Fulton Center Transit Hub and The Empire State Building demonstrate, fire-rated glass and fire-rated steel frames have revolutionized elevator shaft design. Providing significant strength, fire-resistance and aesthetic variety, steel framing systems readily accommodate design load criteria as well as fire and life safety requirements for most projects without compromising the visual goals of the built environment.

But buildings encompass more than just elevators. Steel-framed glazing assemblies can support design goals and code requirements for stairwells, fire-rated curtain walls, door systems, and non-rated architectural systems in both interior and exterior applications. With such versatility, project teams can specify steel-framed glazing systems in multiple areas of the built environment to create a coherent and modern design that meets the most stringent performance requirements.

In this way, steel framing systems contribute to both building functionality and occupant experience—whether they are used for an elevator enclosure, an exterior curtain wall, or anything in between. ■

About the Author

Devin Bowman is General Manager of Technical Glass Products (TGP) and AD Systems. With over 20 years of industry experience, Bowman is actively involved in advancing fire- and life-safety codes and sits on the Glazing Industry Code Committee (GICC). (Devin.Bowman@allegion.com)