The Space Needle. Photo: ©Nic Lehoux

Arup provides engineering services for strengthening and retrofitting the landmark.

By Clayton Binkley, P.E., S.E., PEng; and Kristen Strobel, P.E.

Built for the 1962 World’s Fair, the Space Needle is a Seattle landmark. The Century Project, a restoration and modernization of the building’s 25,000-square-foot top house, is intended to prepare the 50-year-old icon for the next 50 years. Arup provided structural, mechanical, electrical, and plumbing engineering services — working in collaboration with Olson Kundig, Seneca Group, Hoffman Construction, and more than 50 other partners to realize the project.

Conceived in 2013, the project has developed during the last five years into a complete refresh of the top house — the occupiable top portion of the Space Needle. The top house starts at the 500-foot level and continues up to the mezzanine, observation, mechanical, and elevator machine room levels.

Section through the Space Needle’s top house. Image: Arup

Design

The architectural design is driven by the goal of enhancing the guest experience and modernizing the building. Key architectural elements include new floor-to-ceiling glass at the 500-foot and 520-foot observation levels to provide views out to the city beyond, replacing the existing rotating floor with The Loupe — a new glass rotating floor — and addition of a new stairway feature that connects all three public levels of the top house to improve guest circulation.

Arup’s structural engineering design work implements retrofits in support of the architectural interventions — strengthening the structure to support the increased weight of new structural glass and creating new openings to allow new stairs to pass through the existing structure. The structural design also brings the existing structure up to modern building codes through a seismic retrofit of the existing lateral system. During construction, additional retrofits were designed to repair structural elements that had deteriorated over time.

Existing structural design

The top house’s existing structure consists of two lines of vertical structure: a steel braced-frame core and a perimeter ring of 12 canted steel columns that sit atop the tower’s legs and are connected at each level by a ring beam. At each floor level, a composite steel floor structure spans between the core and perimeter structure.

At the 500-foot level, plate fin girders cantilever from the ring beam to support the revolving glass floor. At the mezzanine level, a story-deep plate ring girder sits atop the canted steel columns. Its top and bottom flanges form tension and compression rings, respectively, that resist chord forces from the story-deep steel trusses that cantilever out beyond the ring girder. These triangular trusses support the observation deck and give the Space Needle its distinctive shape.

The roof of the Space Needle is formed by sloped wide flange beams. The lateral system of the top house consists of the central truss core and a perimeter system aligned with the canted columns. The existing perimeter system includes a concrete shear wall at the 500-foot level, steel plate ring girder at mezzanine level, and steel braced frames at observation and mechanical levels.

Oculus stair

Oculus stair. Photo: ©Nic Lehoux

One of the key aspirations of the renovation was to improve the guest experience by creating new stairs connecting the public spaces. A new elliptical stair connects the 500-foot level to the mezzanine and observation levels. The bent plate stair cantilevers off concealed supports, making it appear to float above the glass floor below.

Under typical loading, the stair is stiff to minimize vibrations but incorporates movement joints to prevent it from contributing to the building’s primary gravity and lateral systems. Movement joints at the top of each post and cutting across the top treads allow for differential movement between the floor levels under gravity load. Under seismic loads, the top connection of the stair incorporates a shear pin that is calibrated to break away, thus protecting the floor framing supporting the stair. The stair is designed to remain stable (albeit more flexible) without the top connection intact.

To install the stair, one sixth of each floor was cut away to create the opening for the stair. This required demolition of existing primary floor framing that served to stabilize the plate ring girder and form a key part of the overall stability of the structure. Due to the challenges associated with temporarily shoring the structure, Arup designed the structure to allow the new primary girders that frame around the opening to be installed prior to the demolition of the existing girders, incorporating some of the existing framing connections to maintain work points. This approach maintained the stability of the structure through constructions without requiring temporary bracing or shoring.

Trusses

New stairs at the observation level. Photo: ©Nic Lehoux

At the observation level, each of the 48 trusses that cantilever out to support the exterior observation deck required retrofit. The primary drivers for retrofit were the increased weight of the new glass barrier and benches, creation of new stairs that transition between the interior and exterior of the observation level, and repair of corrosion and water intrusion discovered during construction.

The existing trusses were very light, constructed from tees and angles. This made them very sensitive to increased loading from the new structural glass. To increase the capacity of the existing structure, plates were added to the existing tees, transforming them into wide-flange shapes.

The Space Needle’s 500-foot level. Photo: ©Nic Lehoux

At each of the three new stairs that connect the exterior observation deck to the interior observation level, the geometry of the stairs necessitated that the top chord of the existing trusses be removed and lowered. Arup designed the new truss members and connections to be installed around the existing truss to eliminate the need for shoring or tiebacks that would disrupt the building’s operation.

After new elements were installed, the old chords were cut away with minimal residual deflection due to load transfer. The construction sequence minimized disruptions to the observation level, maintaining guest access during the initial installation of the retrofits below. The contractor could then demolish the stairs in sequence, closing only a small area of the observation deck at any time.

Seismic retrofit

A seismic retrofit of the top house was carried out using ASCE 41 procedures. In general, the existing structural members were sufficiently robust, but as expected for a building of this era, many connections between existing components were deficient and required retrofit.

Throughout the building, the diaphragm was strengthened by adding new studs to the steel floor framing and welding continuity plates to the connections of collectors back to the core and the ring beam elements. At the 500-foot level, the existing perimeter lateral system was a 100-foot-long concrete shear wall. While the wall itself had sufficient capacity to resist the seismic demands, the top and bottom connections were inadequate and would have required costly retrofit. Three shorter steel-plate shear walls were installed in lieu of retrofitting the existing wall, minimizing construction cost and improving circulation through the space.

Conclusion

Arup is proud to have collaborated on the renovation of this unique structure. By carefully tailoring the structural interventions to the existing structure, Arup helped to bring this icon into the future while maintaining beloved aspects of the Space Needle.


Clayton Binkley, P.E., S.E., PEng, is an associate structural engineer and Kristen Strobel, P.E., is a structural engineer, both in Arup’s Seattle office.