Introduction

University of Phoenix Stadium in Glendale, Arizona, USA is one of the first in a second generation of US mul-tipurpose stadium venues that serve both the primary sports tenant and the community that funded the stadium construction. Built as the first per-manent home of the Arizona Cardi-nals National Football League (NFL) franchise, the stadium also hosts the Arizona Sports and Tourism Author-ity (AZSTA) as an equal cotenant charged with utilizing the venue facili-ties beyond the eight NFL game days each year. Central to the design pro-gram was the ability for the building to be functional, flexible, and rapidly adaptable to the needs of both tenants, while demonstrating a world-class ar-chitectural design.

Enabling the rapid adaptability of the stadium are two key kinetic structures. The 46 000 sq.m, 213 m span roof structure includes a 9000 sq.m movable roof that retracts to create an opening over the playing field. In addition, the playing field is mounted inside a giant movable tray structure that glides out of the south end of the stadium to re-veal a state-of-the-art trade show floor. This article describes the structural de-sign and construction of the roof and field structural systems.

Keywords: long-span roof; retractable roof structures; steel construction; ki-netic architecture; operable pitch; roof lift erection method; strand jacking; vibration.

Roof Design

A key aspect of the formal architectural concept (Fig. 1) was that the roof mass presents a relatively low-rise form that did not visually dominate the unique building skin below. These require-ments translated to a 16 m allowable rise over an eave perimeter span of 250 m, a ratio unacceptable to realize traditional dome action economically. At the same time, the design team was

challenged to develop an opening for the retractable roof that was only just large enough to give the feel of out-door play with the roof open in order to control the roof mechanization costs and retain a slender profile of the mov-able roof panels.

The final roof solution (Fig. 2) created a roof opening of 74 m × 110 m framed by two large lenticular trusses span-ning 213 m along the long direction of the field to four large inset structural concrete columns. This solution was driven by the need to provide a stiff

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The Retractable Roof and Movable Field at University of Phoenix Stadium, ArizonaMark C. Waggoner, Principal, Walter P Moore, Austin, Texas, USA

Fig. 1: University of Phoenix stadium exterior view

Fig. 2: University of Phoenix stadium roof structural system

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line of support directly beneath the movable roof panels and the suitabil-ity of the lenticular form to the archi-tectural constraints. The upper 16 m of the 26,5 m total lenticular form depth could stick up above the stadium eave to form the roof surface, while the re-mainder could be tucked beneath the roof, yet stay above the spectator sight-lines.

The use of discrete, linear spanning primary elements versus two-way sys-tems that require the entire roof form to be completed on shores also proved advantageous for constructability. At an early stage of the design, the roof erector proposed to build the primary trusses and all framing between them on the ground and jack the entire as-sembly into place. The entire struc-tural design was subsequently worked around this roof erection concept, in a unique and rewarding collaboration between engineer and contractor.

Roof Structural System

In deference to the similarity of their lenticular form to I.K. Brunel’s 1859 Royal Albert Bridge, the primary span-ning roof elements were dubbed “Bru-nel Trusses”. Of course, these elements do not act at all like a traditional truss, but rather a self-resolving superposi-tion of an arch and a catenary tension element separated by compression struts. Only light diagonal rod x-bracingis required in the webs to resist un-balanced loads. As a result, the large, complex, and unsightly gusset plate con-nections typically associated with mas-sive trusses are avoided entirely. The top chord of the Brunel truss consists of a 4,5 m square box truss fabricated of wide-flange shapes and laced with double angle members. Each top chord segment is joined to the next through simple end plate compression bear-ing plates. A pair of Hollow StructuralSection (HSS) struts arranged in a V configuration connect the top and bot-tom chords. The bottom chord tension element consists of a pair of jumbo wide flange shapes spliced with rela-tively simple bolted connections.

Due to various unbalanced loading cases from wind loads and potential retractable panel locations, each of the x-bracing rods could experience load reversals and thus required significant prestress. Conventional in-line pre-stressing methods would have been time consuming to accomplish in the erected position of the trusses. Because

the planes of x-bracing existed on each face of the truss, it was possible to in-troduce a single horizontal connecting rod at the central crossing point of the rod pairs. With the main rods installed straight to their fixed length, an induced catenary action was created by tension-ing the single “draw-in” rod to 3,5 kN, simultaneously producing 23 kN of pre-stress in each of four sets of rod pairs.

The remainder of the fixed roof con-sists of relatively conventional sec-ondary trusses of welded wide-flange shapes that span between Brunel truss-es and to the exterior stadium perim-eter. A panelized roof surface between secondary trusses consists of custom joist girders at 10 m centers support-ing conventional joists and metal deck. Sized deeper than necessary for their 20 m span, the stiffness of the joist girders was utilized as the primary sta-bility system for the secondary trusses through torsional bracing, eliminating the need for costly horizontal bracing systems in the roof plane.

Lateral loads on the roof structure result from conventional unbalanced and drag wind loads as well as the im-pact and braking requirements of the retractable roof panels. The Brunel Trusses are hard connected to both supercolumns along their longitudinal axis in order to share the large brak-ing and impact forces. Restraint forces from temperature changes are readily handled through vertical deformation of the Brunel Truss combined with horizontal flexibility of the cantile-vered supercolumns. Perpendicular to the Brunel Trusses, each secondary truss is hard connected to the adja-cent bowl structure. Because of their pitch, the secondary trusses develop thrusting action under applied load. A series of staged lateral connections during erection allowed elimination of thrusting loads that would be undesir-able for the bowl concrete frames from roof dead load, while the thrust forces from imposed roof loading were read-ily transferred.

Retractable Roof Panel Structure and MechanizationThe retractable portion of the roof consists of two 55 m wide × 79 m span panels that bi-part about the center of the field. Each panel rides along the sloping top surface of the Brunel Truss in a departure from past US retract-able roofs, which have all previously ridden on flat rails. Polytetrafluoroeth-

ylene (PTFE)-coated fiberglass ten-sioned fabric forms the panel cladding, allowing a significant amount of light into the building.

Each retractable roof panel consists of eight trusses riding on four carriers at each panel support line. Taking a cue from the Brunel Trusses, the retractable trusses are also lenticularly shaped, but use a vierendeel system to resist unbalanced loads. The truss chords consist of 355 mm square rolled HSS shapes, with closely spaced vertical HSS shape web members. Relatively simple moment connections create the vierendeel system. Adjacent trusses are joined by horizontal bridging ele-ments at top and bottom chords, which connect to horizontal trusses between the truss pairs at each end of the panel. Diagonal members in a vertical plane were intentionally avoided in order to present a regular orthogonal grid of framework that avoids the cluttered appearance of most long-span systems with repetitive trusses (Fig. 3).

The retractable roof mechaniza-tion system is shown in Fig. 4. Each pair of retractable trusses sits atop a single two-wheeled carrier, allowing predictable load distribution without a mechanical suspension system. The two 1 m diameter wheels at each car-rier ride along a continuous 175 lb hardened crane rail. Of the four car-riers along each panel edge, two are powered and two are idler carriers. A cable drum is mounted to the pow-ered carriers and rides with the panel as it moves along the sloping rail. Two wire rope cables, one on each side of the rail, connect from each cable drum to a fixed attachment point at the center peak of the Brunel Truss-es. To open the roof, the cables sim-ply unwind from the cable drum at a controlled rate as the weight of the structure drives the panel downhill. The cables are wound back onto the drum to close the roof.

Roof Erection

At the stadium jobsite both Brunel Trusses, the fixed trusses that span between them, and the operable roof panels were assembled on the ground, roughly 37 m below their final posi-tion. The ability to assemble the largest components of the roof nearly on the ground with simple scaffold and man-lift access greatly speeded assembly and created an extremely safe working environment for the ironworkers.

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that also allows easy access from the exterior for maintenance. This system requires only 76 horsepower to move the 10 500 t field in just 1 h. Horizontal rollers at the guide rail keep the en-tire field tray tracking in a straight line into the building. These rollers ride in a groove in the event slab, which is filled in by kerf covers when the sta-dium is outside of the building. Each roller is fixed horizontally to the side of the rail head through spring loaded assemblies, which allow distribution of peak loads between guide rollers. Extensive structural analysis of the interaction of the rigid in-plane tray structure with potential misalignment of the central guide rail was conducted to determine the guide roller design loads.

Because the weight of the field tray had been optimized to allow an economical and fast-operating mechanization sys-

Fig. 3: Retractable roof structure

Because the erection intent was known at the outset of the project, the effects of the roof lift in the distribution of the internal dead load stresses was able to be considered through a nonlinear staged construction structural analysis that explicitly modeled the exact erec-tion sequence. The temporary condi-tions during the roof lift and subsequent erection steps were also considered early in the process. Such measures in-cluded a separate wind tunnel test of only the roof lift assembly.

Stability of the system both during and after the lifting process was provided by a giant temporary x-brace that was created at the mid-span of the roof to function as a torsional brace. A tempo-rary laced box strut connected the bot-tom chords, while special temporary connections were made to allow four of the operable roof trusses to serve as a strut connecting the top chords. Tem-porary strand cables formed the cross bracing.

The 6000 t roof lift was successfully completed over five days in late February 2005 (Fig. 5). The roof was lifted by two strand jacks installed atop each of the four supercolumns. At its peak, the strand jack system lifted the roof at 6 m/h. Movement and strain behavior of the roof were monitored throughout the lift using an extensive array of wireless monitoring equipment. Following completion of the roof lift, the remainder of the roof was erected through more conven-tional crane lifts from the exterior of the stadium.

Movable Field Tray Design

In studying various options for provid-ing a natural grass playing field that

could be converted to a convention floor in a single day, it became obvious that despite a greater initial cost a mechanized playing field provided the only solution to achieve the venue flexibility desired at the stadium. A 71 m × 123 m movable field tray was developed that slid from its position inside the stadium along 13 steel rail lines to sit outside the south end of the stadium where the grass could grow in the ample Arizona sun (Fig. 6). A rela-tively simple structural grid supports the weight of the soil and turf through a series of twin composite steel beams spanning to single wheel boxes. Twelve rail lines are embedded flush with the event slab, while the central rail is upset and serves to guide the tray into the stadium.

38 drive wheels were located along each outer rail, providing a well-proportioned traction drive system

Fig. 5: Roof lift

Fig. 4: Retractable roof mechanization system

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Fig. 6: Movable field tray

tem, the steel structure that supported the surface was susceptible to transmit-ting undesirable vibrations. The players themselves would never be bothered by the floor vibrations while playing since their bodies are already experiencing accelerations far in excess of what the floor might cause. However, the station-ary coaches and officials, as well as the players lined up for a play with a man in motion, could be distracted if the field bounced noticeably. Though the field could never provide the same feel as playing on solid ground, it had to be sufficiently stiff that it did not become a distraction to the game.

Though extensive design guidance ex-ists for evaluation of typical building systems for vibration performance, no published information was available that might set out acceptable perfor-mance for a professional sports floor. While established analytical methods could be extended to reasonably pre-dict accelerations, the acceleration acceptance limits that exist for other building types could not be applied.

A series of tests on mock-ups of the proposed system were conducted to establish an acceptable design. Each mock up was instrumented with accel-erometers and subjected to both simu-lated football activity as well as more controlled excitation through a shaker

device. The results provided a valuable assessment of the dynamic response and damping characteristics including the drainage, soil, and turf media. Of most benefit though was the ability to demonstrate actual performance to the team representatives that would in the end use the field. This testing process pointed to an optimum combination of structural stiffness, drainage system depth, and soil depth that provided a superior kinematic experience for the players.

Conclusion

The adaptability made possible by the retractable roof and playing field has enabled University of Phoenix Stadi-um to host over 180 events in its first year of operation. With the ability to host a trade show the day after an NFL game, the stadium has already influ-enced the design of future venues by demonstrating how well-engineered kinetic structures can greatly enhance revenue generating capacity with relatively modest initial investments. Developments in kinetic structure technology for movable roofs, walls, stands, and other major structural ele-ments will allow future venues to build on the example set at University of Phoenix Stadium through even more robust venue flexibility.

SEI Data Block

Owner:Arizona CardinalsArizona Sports and Tourism Authority

Architects:Eisenman Architects, New York, NY, USAHOK SVE, Kansas City, MO, USA

Structural Design:Walter P Moore, Austin, TX and Tampa, FL, USA

Mechanization:Uni-Systems, Minneapolis, MN, USA

Field Vibration Consultant:Dr Tom Murray and Dr Mehdi Se-tareh, Virginia Tech University, Blacks-burg, VA, USA

Construction Manager:Hunt Construction Group, Phoenix, AZ, USA

Roof Steel Fabrication and Erection:Schuff Steel, Phoenix, AZ, USA

Roof Fabric:Taiyo Birdair, Amherst, NY, USA

Steel (t): 7700

Total cost (USD million): 425 (hard cost)

Service Date: August, 2006

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