Presidential Award for Engineering Design and Construction

"It’s an engineering and architectural marvel that redefines immersive entertainment. It combines a stunning LED-clad exosphere with groundbreaking steel innovations to create the largest spherical structure in the world. Detailing in two dimensions is already complicated, but bringing to life this three-dimensional geometry was even more commendable. This project exemplifies the kind of creativity and collaboration that will set the standard for future wonders like this one." -- Nima Balasubramanian, AIA, NOMA, Director of Architecture, AISC, 2025 IDEAS² Awards Judge

The largest spherical structure in the world is a display of groundbreaking steel innovation that has helped redefine the immersive entertainment experience. Sphere, the next-generation music and performing arts venue just east of the Strip in Las Vegas, is a 516-ft-diameter semi-spherical building rising 366 ft above ground. It encloses a bowl-shaped theater for 17,600 guests seated beneath a domed roof and a suspended media plane.

Sphere visitors first encounter the exosphere, the venue’s outer latticed grid shell composed of steel pipe sections and cast steel connecting nodes and covered with 580,000 sq. ft of programmable LED lighting. Starting from a traditional geodesic arrangement--a “Bucky dome,” as envisioned by architect Buckminster Fuller--Severud Associates structural engineers employed parametric design and optimization to determine the sphere’s lightest tessellation. The engineers also found significant benefits with cast steel nodes, a crucial project component.

Inside the venue, the lower seating bowl framing, stage and proscenium, and back-of-house components are concrete bearing on piles and mats. The upper seating framing and perimeter concourses are structural steel in a barrel-shaped arrangement. All seats are supported on precast concrete stadia, and 10,000 of them are immersive and include haptic systems. An optimized steelframed dome tops off the theater.

The project also includes a separate collar structure with the loading dock and other back-of-house facilities, plus a 1,200-ft serpentine pedestrian bridge framed with steel box trusses that connects the venue to a convention center. A bridge building provides a transition to the venue. The bridge and collar buildings are isolated from the exosphere.

Steel Solutions--and Conversions

The exosphere shape is critical to Sphere’s aesthetics and its exterior LED displays. A sphere is one of nature’s most stable shapes, but wind load, thermal expansion, and acoustics made it a completely isolated structure. Constructing a free-standing sphere in any other material would have been essentially impossible.

Sphere’s multi-sensory theater experience requires a huge column-free space, clear sightlines to the stage, and an expansive media plane. Maintaining the proper focus for 17,600 guests made the shape of the venue--a sphere within a sphere--a natural choice. As with the exosphere, structural steel was the best material to carry the roof and rigging loads efficiently without adding significant dead load.

Formwork for a concrete roof dome would have been extensive, costly, and time-consuming and would have interfered with other trades. The roof dome framing was prefabricated in sectors--including intermediate framing--that were easily erected and required only one center temporary support tower. A 10-in.- thick concrete slab on metal deck provides permanent stability and acoustic damping.

The venue’s exterior walls (immediately behind the exosphere) are barrel-shaped to maximize internal space, and the resulting double-curvature was easily framed with curved hollow structural section mullions and girts. The mullions are supported laterally by the cantilevered slabs at each concourse level and vertically only at the third level. Slip connections allow the other floors to deflect independently without inducing loads in the wall framing.

Steel fabrication’s flexibility and its ability to coexist with other materials were critical to Sphere’s construction. The ground through fifth floors are framed with concrete slabs, beams, and columns, while the rest of the floors are steel.

Early in the pre-construction phase, Severud flipped the sixth floor and above from concrete to steel. The switch allowed the steel contractor to start detailing and fabricating at about the same time as the superstructure concrete contractor. When the concrete contractor approached the fifth floor, the steel contractor was prepared to start erection as soon as the concrete was placed. That quick transition saved about six months on the overall schedule and facilitated all following work by eliminating several levels of shoring.

The seating is supported by raker beams carrying the precast concrete stadia. Four concrete shear wall cores combine with concrete walls that wrap around the stage to provide lateral support for the entire venue.

Diurnal temperature ranges are as high as 100° in the summer and vary from one side of the structure to the other. The exosphere is self-supporting and isolated from the rest of the building; it rests on its own ring of pile caps and grade beams. The separation allows the exosphere to expand and contract in or out up to 2 in. without restraint from the venue within, which is insulated and less thermally variable.

Castings Bring Savings

The project team’s solution for the lightest constructable and transportable tessellation was 14 horizontal latitudes of continuous ring members and crisscrossing diagonal geodesic elements, continuous between the pilesupported grade beam at its base and a latitudinal ring near the crown. The topmost framing, known as the Oculus, is framed radially. This configuration resulted in a slightly higher tonnage, but at a lower cost because it could be erected with minimal shoring.

The team next studied node connection details, bringing in CAST CONNEX to advise on cast steel nodes. Two designs based on welded plates were compared to castings with flanges, with a focus on cost and schedule. Extensive analysis revealed that built-up plate nodes presented daunting constructability issues.

Cast steel nodes, though, offered significant advantages in material optimization, improved tolerances, and reduced construction risk. They resulted in a 40% reduction in weight compared to built-up nodes because they did not need stiffener plates and other appurtenances. Further, they occupy a quarter of the surface area, which afforded significant savings in the exosphere’s three-part, high-performance weatherproof coating.

All the castings are essentially identical, eliminating concerns about fabrication tolerances. Grid shell structures are sensitive to angular variations at the nodes and length variations of the members. Cast nodes with CNC machining of the flanges increased precision, while bolted end-plate connections allowed shim packs to accommodate variations in the length of the members, which were fabricated slightly short. The cast nodes’ geometry was an order of magnitude smaller than that of built-up nodes.

The system provided greater overall geometry control during erection, which reduced construction risk by simplifying erection and minimizing the potential for out-oftolerance errors, resulting in further cost savings.

Dome and Concourses

The steel-framed top dome is 400 ft in diameter, and it’s designed to optimize depth, radial arches, and compression rings. Pairs of adjacent half-arches, intermediate framing, and a temporary tie rod were prefabricated into units and lifted into place between the perimeter columns and a temporary center shoring tower.

The flat roof of the concourses is incorporated into the dome framing and acts as a tension ring to resist the dome’s thrust; only vertical forces are delivered to the supporting columns under gravity load. The connections to the columns allow the dome roof to move radially without restraint under most conditions. The dome’s shape and location within the exosphere minimize wind load; therefore, it is only under seismic load that the columns are engaged laterally.

The double ring of perimeter columns supports the concourse levels. A combination of slabs, metal deck, and diagonal braces provide the strength and rigidity necessary to collect lateral loads from the entire venue (except the self-supporting exosphere) and deliver them to the cores and stage walls.

Two composite steel and concrete girders span across the stage to transfer roof columns and support the fly tower. The increased stiffness--roughly three times what it would be with concrete-only or steel-only designs--keeps deflections small and mitigates concerns about possible adverse impacts from the augured pile foundation’s differential settlement.

All dome field connections were bolted--there was no field welding. Fabrication and erection tolerances were expected to be on the order of 3 in., but fabricator W&W|AFCO delivered the dome to about 1 in. of ideal geometry. The 160,000-sq.-ft media plane required tolerances down to 1⁄8 in.

A steel roof dome and grillage system allowed for adjustments at several stages needed to create a nearly spherical surface. The grillage, which supports rigging and catwalks, hangs only from the dome to avoid bridging systems that behave differently. It transitions from the dome geometry to the media plane configuration while maintaining the 1-in. tolerance. Erecting the primary framing that hangs from the grillage involved jacks to adjust elevation and reduce tolerance to ½ in. The secondary framing included attachment clips that allowed ¼-in. precision. LED tile connections to the secondary framing created a spherical surface within 1⁄8 in. of theoretical.

Construction Considerations

A staged construction analysis showed the exosphere could be cambered by adjusting member lengths. As erection progressed, the framing settled into proper alignment. The first ring cantilevered from the foundation. Once fully bolted, a ring could support the next ring’s framing up to the Oculus, which was erected in a single unit. Finite element modeling also achieved a higher level of confidence in the roof dome’s structural behavior.

Additionally, multiple daily surveys aimed to predict structural displacements and assess conformance to tolerances at regular intervals. Consequently, large framing sections were pre-deformed to fit the structure’s surveyed shape. Large racking frames introduce forces into the structural members and balance the additional deflections to meet the LED systems’ stringent tolerances.

Structural systems were chosen with contractor availability and expedited construction in mind. Some systems were changed pre-pandemic and early in the pandemic to keep the project moving. Sphere’s successful and timely completion relied heavily on its structural steel components and their integration with other trades.

A Venue for the Future

Sphere’s commitment to sustainability began with its design and construction. Parametric structural optimization, a cutting-edge algorithm-based design approach that seeks reduction in materials and costs as a goal by iterating structural properties such as the number, arrangement, type, and depth of structural members, led to significant tonnage reductions for the exosphere and the venue’s roof dome.

All concrete substituted supplementary cementitious materials for up to 20% of standard cement and used reinforcement made from nearly 100% recycled steel. The structural steel framing contains over 90% recycled material.

Sphere is also committed to sustainability in its operation and hopes to set a new standard for environmentally responsible energy use by entertainment venues. The venue intends to source approximately 70% of its electricity from solar power facilities and is pursuing a long-term agreement with its local electric utility.

Sphere’s exterior and interior lighting is composed entirely of LED systems that are among the most energy-efficient lighting available today. Its advanced heating and cooling systems avoid wasteful reheating. The central plant is comprised of high-efficiency chillers and condensing boilers and is also used to cool distributed kitchen equipment loads. The facility’s data center conforms to state-of-the-art efficiency standards, using hot-aisle containment and directing cooling where needed.

Although Sphere is composed of distinct structural systems, they are all interconnected to form a cohesive, balanced, efficient, and elegant superstructure. Sphere aimed to create a fully immersive experience and elevate in-person entertainment to new levels. The $2.2 billion facility took five years to construct and opened in September 2023.

Owner: Sphere Entertainment Co., New York
General contractor: MSG LV Construction, LLC, Las Vegas
Architect: Populous, New York
Structural engineer: Severud Associates Consulting Engineers, PC, New York
Steel team:
Fabricator/erector: W&W | AFCO Steel, Oklahoma City *AISC full member; AISC-certified fabricator and erector*
Detailer: Pro Draft, Inc., Surrey, B.C. *AISC associate member*
Bender/rollers: Chicago Metal Rolled Products, Chicago *AISC associate member*; Max Weiss Company, Milwaukee *AISC associate member*
Casting supplier: CAST CONNEX, Toronto *AISC associate member*
Erection and construction engineer: Stanley D. Lindsey & Associates, Ltd. (SDL), Atlanta