The Arizona Football Cardinals are still searching for a winning season, but their fans already have witnessed one of the sporting world’s engineering marvels – the team’s new stadium.
Opened just in time for last year’s season, Cardinals Stadium in Glendale boasts some real engineering victories, most notably a unique retractable roof that bi-parts at the 50-yard line and rests over the end zone in the open position. In addition, an 18.9-million-lb playing field can be driven in and out of the stadium at the touch of a button, depending on whether the venue is used for sport or a convention.
Behind these innovations is a Minnesota engineering company called Uni-Systems, which has earned a respected reputation for designing movable structures, a specialty that it refers to as “kinetic architecture.” Among the company’s many creations: numerous retractable roofs for sports stadiums and arts venues, mammoth docking stations for aircraft, elaborate storage and retrieval systems, the Wall St. Ferry Terminal's moveable walls and even a giant skydiving simulator in Arizona.
Each of these projects presents its own mix of challenges, which calls for a blend of mechanical, electronic and controls solutions. At Cardinals Stadium, for example, Uni-Systems engineers devised a patented cable-driven system for safely transporting the roof panels along a sloped path.
Just how did they do it? Design News put that question to two of the key members of the Uni-Systems engineering team on the Cardinals Stadium project: Mike Becker, lead mechanical engineer and Lennart Nielsen, senior electrical designer. Here's a look at what happens when mechatronics meets kinetic architecture:
Design News: Your company practices “kinetic architecture.” Just what does that mean?
Mike Becker: Kinetic Architecture is the study of mechanizing and moving building elements to modify the form or function of the venue. In its simplest form, it is bringing movement to architecture. We are positioned uniquely in our field, since we combine the disciplines of structural, mechanical and electrical engineering. We are often placed in a building between two structural engineers -- often one for the moving roof or wall, and one for the foundation. It is our job to make sure that the loads that are produced by the mechanization and transferred by the mechanization are understood by both engineers. Additionally, our approach to working on a construction project is unique, in that we stay with the project through all stages: concept, schematic design, fabrication, installation and follow-on maintenance.
Why did the general contractor, Hunt Construction, decide on your firm for this project? And what was the chief design challenge versus other stadium roof projects that you designed in the past?
Becker: Hunt liked our proven track record of completing successful projects. They also were very concerned about using a firm that would limit their risk on the project. The first obstacle is controlling the roof on a slope. The roof always wants to move to a state of low potential energy. We designed a 480-hp system to move the two roof panels and maintain control of the roof. The system also includes eight cable drums or winches that move each roof panel. That adds up to a total of 16 cable drums on the project. Each cable drum is powered by four 7.5-hp motors that wind up one 1 ½” diameter cable. Synchronizing these drums was the real challenge.
Quick Facts: Cardinal Stadium Roof Design
• Two retractable roof panels weighing 1,100,000 lb each (550 tons)
• Each Roof panel’s dimensions: 185 ft long x 285 ft wide x 16 ft deep
• 8 cables (1.5-inch diameter) connect each retractable roof panel to the
stadium structure at the 50 yard line (over one half mile of cable used)
• 32 motors (7.5 hp) power each roof panel (480 total hp for stadium)
• 16 crane wheels (36-inch diameter) support each retractable roof panel
• 595,000 lb of mechanization equipment used to make the roof move
• 257.5-ft span of retractable roof over stadium bowl
• Two rail lines with 175 lb/yd crane rail (over one quarter mile of rail used)
• Maximum travel speed: 25 ft/min or one quarter mph
• Total travel time: 11.5 min (10 min run time, plus 1.5 min of slow speed for
What are the key mechanical components that provide the solution to this design challenge?
Becker: The key drive components included: an ABB variable frequency drive, Bonitron regenerative unit, Leeson electric motors, Kebco spring set brake, Emerson Power Transmission planetary gearbox and custom designed open gearing final drive. I was concerned about the proven track record of the vendors, most of whom we had success with on past projects. The motor/Variable frequency drive (VFD) combination needed to be rated for full torque at low speed for both the start-up period and the slow-down period at the end of travel. I selected the planetary reducer for its inherent redundancy in gear mesh, compact design, back-driving efficiency and robustness. The spring set brake is the lifeline of the system in the event of a control failure. I needed a brake I could rely on any time I needed it. We have also worked with Kebco on coatings for corrosion control and sensor feedback that let the control system know when the pressure plate of the brake moves.
From an electronics/controls standpoint, what were the biggest challenges you faced on this project?
Lennart Nielsen: It was without a doubt having a roof panel parked on a slope and being held by eight steel cables! We knew that the moment we released all the motor brakes we were committed to reel in or pay out the cables in perfect unison. Since the opposing sides of each roof panel had to be kept in alignment and the four cables in each roof quadrant had to be in alignment with each other, we had no choice but to use variable speed drives. We wanted to be able to “start” the motors against their brakes each time we were ready to move. We did this to prove both that each brake was capable of holding the load and also that the motor could develop the necessary torque before committing to release the brakes.
Only one supplier, ABB, would guarantee that their drive could develop 100% torque at 0 hertz. This would allow us to perform this test and to avoid any back sliding on startup due to insufficient torque at low frequencies. The ABB ACS800 drives are equipped to communicate with each other via fiber optic in a master-follower network with one master per roof quadrant and seven followers. This arrangement removes the intense VFD data traffic from the main ProfiBus network. Other key elements in the system include: GE Fanuc VersaMax I/O nodes, Turck proximity switches and encoders, and the GE Fanuc Series 9030 PLC, which referees the roof panels overall position and run/stop state. The PLCs on the two roof panels communicate wirelessly with a third PLC in the control room. This PLC keeps track of the relative roof positions and acts as a go-between for the GE Fanuc Cimplicity HMI program that runs on the control room PC. The roof panels cannot be allowed to get more than 10 feet out of relative alignment due to the stress they would otherwise put on the building structure.
What were the major design tools that you relied on for the project?
Becker: We used AutoCAD to communicate our designs with the architect and generate a number of drawings. For complex, detailed mechanical assemblies, we turned to SolidWorks for its ease of use and depth of functions. I used COSMOSworks for finite element analysis, and it works seamlessly with SolidWorks. It is also really easy for engineers to pick up and use effectively. Throughout the project, I kept the calculations in MathCAD. This displays the equations and results plainly and has easy annotating capabilities.
How about special tests employed on the project?
Nielsen: We tested apair of cable drum drives in a mockup, lifting a 90,000 lb concrete weight to simulate the load the cable drums would see on the roof. The test was done to check ABB’s claims and was performed in close cooperation with ABB personnel. As for other tools, we relied primarily on AutoCAD, the NEC, and spreadsheets. We also used specialty routines designed in MathCAD to test different scenarios.
How much interaction did you have with engineering staff from the general contractor and key vendors on the design of the roof system?
Becker: For about six months, I traveled to Phoenix every other week to meet with Hunt (the general contractor), Walter P. Moore (the structural engineer), HOK (architect) and Schuff Steel (the steel erector). These design meetings were vital for making sure that all parties understood the intricacies of the design. These considerations included: load transfer between elements, unusual control scenarios, construction tolerances, construction schedule, roof sealing issues, ADA issues related to field fit up, testing, and commissioning time schedule. You name it; we discussed it.
Looking to the future stadium projects, can you comment briefly on the new challenges you face in the roof design for the new Indianapolis Colts stadium?
Nielson: Just the sheer quantity of motors and drives is daunting: exactly twice the quantity and horsepower that what was used in Phoenix! And on top of that, we are dealing with not just two rails per roof panel but five, all of which has to be kept, if anything, in closer coordination than at Cardinals stadium. Here we are not just talking skew from side to side but also potentially uneven stresses along the face of the roof panels. We are again using ABB ACS800 drives for the job, and this time with the integrated regenerative package where we used stand-alone regenerative drives at the Cardinals stadium. This should give us a tighter integration and more diagnostic feedback.
Regardless of the project, do customers come to Uni-Systems because of your ability to provide a solution that integrates many technologies?
Becker: Our clients come to us for several reasons. The most important reason is that we provide a complete package to the owner. The owner or architect will come to us with an idea, and we will help them develop that into a real engineering solution that includes structural, mechanical, electrical, controls, programming and operator interface. When trying to determine the source of a problem, the owner doesn’t want to deal with several firms that are all pointing fingers at each other. They expect us to understand the complete system and be responsible for every aspect of it. At the end of the project, we supply the owner with a state of the art system that moves a large structure quietly and efficiently at the touch of a button. The operator also has a graphical interface that gives him all the feedback about the operation. And if he encounters a fault, the system will lead him through trouble-shooting the fault, including showing pictures of the location and component that is at fault. We give the owner a complete set of operation and maintenance manuals in an interactive electronic form. At the end of the day, customers come to us for a total package that works.