Press Clippings
Civil Engineering
August, 2000

Glass Nautilus
The new pavilion constructed at the base of Seattle's Space Needle preserves the integrity of the structure's original design while respecting the strictures imposed by landmark status.
By: Greg Varney, P.E., and Todd St. George

Since its debut as the focal point of the 1962 World's Fair, Seattle's Space Needle has stood as a symbol of the region's achievements and hopes for the future. While the observation deck and revolving restaurant atop the giant 518 ft (158 m) steel tripod have long been the structure's main attractions, a recently completed glass-enclosed pavilion at the base of the Needle is now attracting considerable attention. The 15,000 sq ft (1,394 m2) pavilion, with its sweeping concentric ramp, is the centerpiece of a $20-million package of improvements that also includes an enlarged basement, landscaping, reconfigured roadways, and a revamped observation deck. The improvements are designed to better accommodate the more than 1 million annual Space Needle visitors, many of whom wait hours for the 43-second elevator ride to the top.

The improvement project was designed by Callison Architecture with KPFF Consulting Engineers, both based in Seattle, and construction was performed by the Bellevue, Washington, office of McCarthy Building Companies, which is headquartered in St. Louis.

Like an immense transparent nautilus, the pavilion encircles the three-sided base of the Needle and guides visitors up a spiraling ramp to the second-floor elevator level. As it winds around the base of the Needle, the ramp narrows from a width of 32 ft (10 m) at the plaza (grade) level to 12 ft (3.7 m) at the bridge connecting with the elevator platform. The ramp accommodates as many as 400 people, who then line up for the two capsule-shaped elevators that carry them to the top-25 at a time-and offers a succession of dramatic views of downtown Seattle. Video screens positioned along the ramp provide information of interest. (Restaurant visitors use a separate elevator waiting area at the plaza level.) Upon their return, visitors exit through a new retail area.

Gary Wakatsuki, Callison's principal in charge of design, says that original sketches of the Needle by architect Victor Steinbrueck depicted a ramp circling the base of the tower. While Steinbrueck's sketches provided a conceptual basis for the design of the new pavilion, the design team had to accommodate the strictures of a 1998 decision that officially conferred landmark status on the structure. Any improvements to the Needle had to respect the historic and aesthetic nature of the original structure. The nautilus design remains true to Steinbrueck's concept and satisfies the requirement imposed by the landmark designation. The pavilion features a fully exposed, self-supporting steel frame structure that encircles the massive base of the Needle with glass paneling to preserve the views of the legs. The pavilion is a freestanding atrium, its legs located between the ramp and the core of the pavilion.

The slope of the spiraling ramp was a determining factor in the geometry of the entire pavilion. The maximum ramp slope allowed under the Americans with Disabilities Act is a 1 ft (0.3 m) rise for every 20 ft (6 m) of length. This meant that the pavilion's ramp would have had to be at least 400 ft (122 m) long to reach the elevator boarding level, which is 18 ft (5.4) above grade. The team's circular ramp configuration allows a slightly gentler slope of 1 to 22 ft (0.3 to 6.7 m). However, the ramp design required that the perimeter of the pavilion extend 12 to 14 ft (3.7 to 4.2 m) outside the base of the Needle's legs, increasing the difficulty of keeping the legs visible. The pavilion also had to remain structurally independent of the Needle so as not to induce additional loads on it. To further enhance the visual impact of the pavilion, the architect avoided using vertical walls. Instead, the pavilion's outer wall angles out at the bottom. The inner wall forms a truncated cone that leans toward the center of the building and shelters the elevator platform.

Ruling out view-obstructing columns at the pavilion's perimeter, the design team devised a cantilevered support system for the glass roof and walls that stands just outside of the base of the Needle-but inside the pavilion itself. A Vierendeel truss moment frame spans the major structural bents. Braced frames supporting the ramp inside the perimeter of the Needle's base provide additional lateral stability. Engineers drew their inspiration for the Vierendeel truss system from curving highway bridges in the Seattle area. The truss is essentially a box with a frame or a slab on each face-there are no diagonal elements. The combined brace frames create an element with exceptional torsional stiffness, which is the key to the structural support of the ramp.

The ramp winds a total 378 degrees around an inside radius of 57 ft 9 in. (17.6 m). For the first 138 degrees, the ramp is outside the pavilion (with 72 degrees covered by canopy). The remaining 240-degree ramp section is a bridge with four spans between five supporting bents at spacings that alternate between 48 and 72 degrees. Each bent is formed with four columns, 30 in. (760 mm) beams, and back-to-back 8 in. (200 mm) channel sections for bracing.

Most of the pavilion's supporting elements are made of architecturally exposed structural steel, and many are specially shaped and scaled for esthetic considerations. Rods with a diameter of 1 in. (25 mm) are positioned diagonally to form bracing in the ramp roof that is welded to the tops of the ramp beams to create a diaphragm for transferring lateral forces while maintaining the transparent appearance of the ramp roof. Other examples where structural framing sizes were specified by the architect include consistent column sizes in the ramp (with 16 in. [400 mm] wide flanges), double-channel braces in the braced frames, and tapered steel beam sections at the second floor to create minimal beam depths at the edge of the slab. The structure's complex configuration required the construction crews to place temporary shoring under the ramp until the steel erection and welding were completed.

Every component inside and outside the pavilion, from ducts to downspouts, is on view. Even the mechanical systems are in plain sight. Connection details as well are designed to complement the visibility of the superstructure. In several cases the design team and the constructor worked together to produce preconstruction mock-ups of details. This aesthetic attention to detailing includes consistently aligned bolt heads in beam connections, smoothly ground exposed welds, smoothly ground full penetration welds at beam splices to eliminate splice plates, and gusset plates with 90-degree edges for vertical and horizontal connections.

The continually changing dimensions and load characteristics of the ramp mean that no two sections have the same load conditions. Every column, beam, and cross brace is unique. In addition, because of the integration of the load-bearing systems, the design team found it important to consider gravity and lateral loads together. Plan views of the structural model used during the design depict beam and column elements in yellow and roof and floor elements in red.

The Needle itself was designed for live loads of 100 psf (488 kg/m2) and wind loads of 80 mph (129 km/h); it sways approximately 1 in. (25 mm) for every 10 mph (16 km/h) of wind. Seismic concerns are also an important lateral load consideration: Seattle is in seismic zone 3, and the Needle survived a tremor measuring 6.5 on the Richter scale in 1965. However, the unique loading characteristics of the pavilion made seismic design analysis counterintuitive.

Dynamic analysis of vibration impacts on the pavilion showed that the first several dynamic modes of the structure are not translational (side-to-side), as would normally be expected, but vertical, creating a very slight springlike movement of the ramp. The dynamic analysis showed that the pavilion structure is far stiffer than that of a typical building because of the redundancy in the gravity framing and lateral load-resisting elements.

The team also used these vertical dynamic modes to assess the dynamic characteristics of the ramp to limit footfall vibration. The results showed that the structure is not resonant with footfalls and is sufficiently stiff without additional support.

Because the pavilion envelope is mostly glass, deflection was a principal design criterion. The steel framing carries both the gravity and wind loads of the glass. The ramp deflection was also a determining factor for the minimum number of joints required in the glass system. The architect wished to limit the number the joints to 3-degree segments. Through the use of the 20 ft (6 m) diagonal tension rods forming an X pattern along the roof of the ramp, the design team limited live-load deflection ratios in the ramp to L/1,000, or 7/8 in. (22 mm) deflection over a 72 ft (24 m) span, thereby meeting the requirements of the glass supplier and the architect.

The Needle's landmark designation made it possible for the design team to receive an exemption from typical code requirements that would have required fire-retardant material to be applied to the Needle's steel legs as they pass through the pavilion. The landmark status also allowed the team to gain a variance to Seattle building code limits on exposed glass, allowing the pavilion walls to be nearly 100 percent glass (as opposed to the typical requirement of 40 percent or less). The glass is tinted to maximize energy efficiency. The pavilion's structural elements are dark gray to distinguish them from those that are part of original Needle construction, which are lighter in color. The project also includes a 9,000 sq ft (836 m2) basement under the pavilion. The basement provides 3,700 sq ft (344 m2) of additional space for Needle administration and operations, thereby allowing more aboveground space to be used for retail and dining. The Needle's original 6,000 ton (5,443 Mg) footing is in the shape of a Y to support each of the three legs. These footings and the 18 ft (5.5) of soil above them provide lateral stability to resist wind and seismic loads on the Needle. Therefore, the basement design involved balancing the need for additional space with the engineering requirement to maintain the stability of the Needle.

Nestled between the Needle's south and northeast legs, the new basement is shaped like an L and extends to the north. The 11 ft (3.3 m) tall basement is sandwiched between grade level and the top of the existing Needle footings 18 ft (5.5 m) below. The new basement is outside of the footprint of the original footings to preserve the soil mass above the footings. When soil above the footings needed to be temporarily removed during construction, crews placed an equal weight of concrete there to maintain the load.

The columns of the new pavilion are incorporated as spread footings in the structure of both the new and existing basements. Columns outside the footprints of either basement are on shallow spread footings. The new footings are located amidst or around existing utility lines for the Needle that had to remain in service throughout construction. The engineering team and the constructor collaborated to develop a construction sequence that allowed the Space Needle to remain open during construction. Crews completed the new basement first and then created a temporary ticket booth outside the old pavilion (itself a temporary structure from 1962). A tunnel built out of shipping cargo containers served as a visitor passageway through the construction area as crews erected the pavilion structure and completed finish work. The Space Needle's new pavilion opened in May 2000, in time for the busy summer tourist season.

Modeling Geometric Complexity

The geometric complexity of the Space Needle's new nautilus shell pavilion exceeds that of typical rounded buildings. As the curving ramp rises at a consistent slope of 1 to 22 ft (0.3 to 6.7 m), the outside radius changes continually, while the inside radius of the ramp is a constant 57 ft 9 in (17.6 m). In addition, the glass walls feature reversing slopes relative to the rest of the building.

To manage these geometric complexities, the team developed a unified design approach that defined the structure's configuration in polar coordinates instead of the Cartesian coordinate system used for most buildings. The polar coordinate system describes locations in degrees of clockwise rotation, radial distance from a center point, and degrees above a horizontal plane.

In addition, under a standard design approach, gravity-bearing and lateral-load-bearing systems would have been modeled separately. However, the need to stiffen the ramp against deflection and stabilize the entire pavilion against seismic motion required that both load-bearing systems work together to achieve the necessary stability.

Unfortunately, most available design software either failed to adequately accommodate the polar coordinate system and the algorithmic formulas that had to be written to define it or did not address unified load modeling. However, the SAP 2000 finite-element program, produced by Computers and Structures, Inc., of Berkeley, California, was able to meet the team's needs.

With the unified model, the project team was able not only to complete the design analysis and calculations, but also to create visual models of the pavilion's key architectural features and structural components from any angle desired.

Greg Varney, PE, of KPFF Consulting Engineers, Seattle, was project manager for the Space Needle Plaza; Todd St. George was the project engineer.

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