The ARC & the PARK

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Situated on a challenging site in a dense, urban area, West Hollywood Aquatics and Recreation Center (ARC) embodies many of the technical and constructability challenges common to urban infill projects. Designers were challenged to create space where there was very little, bending the program around existing structures, over a roadway, onto rooftops, and into a public park. The result is a unique, vertically stacked building that meets the needs of a diverse community and creates a new center for social and recreational life in West Hollywood. Designed by LPA Design Studios and completed in 2021, the project involved a host of innovative structural engineering strategies, including a long-span rooftop pool deck suspended over basketball courts, a new multistory building on top of an existing parking structure, a sky bridge over a public road and a three-story grand stair that cascades into West Hollywood Park.

The ARC comprises three separate structures totaling 138,000 square feet, each with its own set of structural engineering challenges. A four-level Recreation Center—which includes indoor and outdoor community recreation and wellness spaces, a multi-purpose gymnasium, and two pools (competition and recreational)—occupies an area tightly constrained by roadways and an existing building. On its roof, aquatic facilities are supported above a 100 foot by 160 foot column-free gym by a two-way trussed steel space frame system. Across El Tovar Lane, a public roadway that runs through the project site, a new two-story Community Center sits on top of an existing concrete parking structure. Bridging the two buildings, a multi-level sky bridge spans across El Tovar Lane to connect the new rooftop pools and an existing tennis complex to a raised observation and recreation deck on the roof of the Community Center. These interconnected outdoor environments and the ARC’s main entrance are linked to the open space of West Hollywood Park by a three-story outdoor Grand Stair.

Central to the design was the city’s desire for an inclusive amenity that reflected the diverse West Hollywood community. Designers went to great lengths to create a facility “for everyone.” Community engagement was an impassioned, years-long process with more than 16 user groups and many arts organizations and neighborhood associations engaged. Meeting the diverse, competing needs of the community motivated the design team to explore innovative solutions and do more with less.

LPA’s integrated design process was critical throughout, with structural engineers playing a leadership role in the creative process. The firm’s multi-discipline approach enabled the integrated team to develop collaborative solutions that may not have emerged if architecture and engineering had worked independently.

Aquatics have long been a part of West Hollywood’s identity. The previous “aquatic park” was a cultural and recreational hub of the community and an anchor to West Hollywood Park. The need to accommodate aquatics on the tight site led to the most significant structural engineering challenge of the project: suspending a rooftop pool deck above a column-free gymnasium.

Designers envisioned a structural system capable of suspending an Olympic-sized swimming pool and a second, large recreation pool above a 100-foot x 160-foot gym without the aid of columns. Adding to this weighty challenge, the support structure had to fit a strictly limited depth, bounded by the mandatory clear height of the basketball and volleyball courts below and the height of existing adjacent rooftop tennis courts that are accessible from the pool deck.

Every available inch of depth is utilized below and between the pools to support 400,000 gallons of water, a the pool deck amenities and an unreducible, 100 pounds per square foot live load. Two elements make up the support system for the pool: a 100-foot x 100-foot two-way steel trussed space frame, which spans to exterior concrete bearing and shear walls, and a 15-foot-deep x 4-foot-wide box truss tucked into the 100-foot-long slot between the two large pools. The bi-directional space truss creates a 5-foot, 3-inch deep geometric web of steel that minimizes deflection as the pools are filled and drained.

Structural design considerations included vertical deflection and related horizontal deformations associated with varying volumes of pool water as well as the effects of the large volume of liquid mass on the structure during seismic events. The steel rooftop structure supporting the two pools was structurally continuous, with most of the connections of individual members fully welded. To understand the deformation of the structural framing, designers studied various loading conditions representing different combinations of water levels in the pools. This included scenarios where both pools were full, only the Olympic pool was full, only the recreation pool was full, and both pools were empty. Each condition was also checked with and without live loading at the pool deck. The design team used the results of these analyses to determine that strategically cambering the two-way space truss at four control points would ensure that, when fully loaded, the structural steel would not encroach into the clear height required for club, high school and college basketball in the courts below. Designers used ETABS, by Computers and Structures Inc., as the primary analysis tool for both gravity and seismic analysis.

LPA’s structural engineers worked closely with Sinanian Development, the general contractor, Groupe ADF, the steel fabricator, and DCCI, the steel erector, to develop details for the two-way, steel-trussed space frame and optimize the fabrication and erection process. It was important to work closely with the contractor team to find creative solutions to reduce field welding of the two-way truss system. Further complicating the erection was the limited laydown space and tight access to the site. As such, shipping an entire length of a truss line was not feasible, Therefore, two shorter, parallel truss lines of the two-way truss system were shop-assembled, including the chord and web members of the perpendicular truss. This allowed for a significant reduction of onsite welding. These truss pairs were then lifted into place and welded together to create one direction of the two-way truss span. The chord and web segments of the perpendicular trusses were then welded in the field, completing the two-way truss system.

Providing pedestrian access from the ARC to the adjacent West Hollywood Park, across a public roadway, was a key programmatic and user-experience challenge of the project. A popular gathering space that has served as home base for many large-scale community events and festivals, including celebrity affairs hosted by Vanity Fair and the Elton John AIDS Foundation, the park was conceived as part of the cohesive recreational experience of the ARC.

Designers wanted community members to be able to walk directly from the newly renovated West Hollywood Park, up the Grand Stair, to the rooftop above the Community Center, across the wide, multi-level skybridge and into the pool deck and sports courts on the fifth-floor roof. This relied on the flawless execution of two distinct structural innovations.

First, the three-story sculptural Grand Stair connects the Park and the Community Center rooftop while creating a new gathering spot and icon for the city. The 70-foot-long structure bears on the top deck of the Community Center but is seismically independent, relying on a large, horizontally cantilevered steel truss to anchor the base of the grand stair to an elevated, structural concrete plinth founded in the park. While the top of the grand stair structure receives vertical support where it connects to the rooftop deck of the Community Center building, it is designed to allow up to 9 inches of horizontal movement in any direction.

Second, a multi-level skybridge over El Tovar Road completes the rise from the park to the pool deck, cantilevering 30 feet from the Recreation Center structure. Multiple structural levels share the cantilevered load to maintain necessary vehicular clearances below. The upper two framed levels rely on W30 and W40 cantilevered beams to support the skybridge from the ARC building, with no physical connection to the adjacent Community Center building with which it is functionally contiguous.

To allow for future flexibility in its use, the Community Center relies on special steel moment frames for seismic resistance, while the recreation center’s main seismic force-resisting system is the special concrete shear walls that surround the gymnasium. To achieve this, the more flexible moment-frame structure needed to be isolated from the rigid concrete shear wall building to ensure regular, predictable behavior in strong earthquakes. This isolation was achieved by adding seismic joints between the two buildings at each floor level. Seismic joints are located at the south side of the Community Center where the ARC building skybridge extends over El Tovar Road. Over 15 inches of seismic drift can be accommodated between the skybridge and the Community Center with seismic drift joints in floors, walls and ceilings. Close collaboration between structural engineers, interior designers, and architects was needed to ensure that the drift joint detailing carried through all building systems.

The ARC is an example of structural design going above and beyond to accommodate a community. By suspending two pools above a multi-sport gym, the design makes highly efficient use of the limited buildable space in the dense, urban site area. By making use of the rooftop and cantilevering structures to create connections between them, the design creates new spaces for West Hollywood’s vibrant social, arts, and culture scenes to thrive. And by connecting the ARC to the park, the project essentially adds square footage and program area to the only significant public green space in the city.

Overall, the project shows what can be accomplished on a tight urban site when structural engineering has a seat at the table as an integral part of the design team. ■

About the Authors

Bryan Seamer is Director of Structural Engineering at LPA Design Studios. With more than 25 years of experience in new construction, seismic assessment and rehabilitation of existing buildings, he uses emerging technologies and innovative design strategies to create safe, high-performing, cost-effective structural systems that are long-lasting, durable and resilient.

John Wilson is a Structural Engineering Managing Director at LPA Design Studios. From the earliest design phases through completion, John works closely with engineers and architects to achieve a unique blend of structure and architecture that will provide safe, efficient and unique designs for clients.