Barton L. Riberich, SE, President, Uni-Systems, LLC 4600 Lake Road, Minneapolis, Minnesota, 55422 Tel. 763-536-1407,

It is known that kinetic architectural elements have been integrated into stadia as far back as the days of the Roman Coliseum. The Romans incorporated retractable sun shades, elevator floors, and trap doors into the Coliseum for the same reasons that we use kinetic architecture in modern stadia. These adjustable mechanized structures adapt to changes in climate, need, or purpose and provide awe inspiring elegant solutions to what appear to be nearly impossible engineering challenges. Today there are over 30 modern sports venues around the world that incorporate retractable roofs and other large mechanized structural elements. These venues are still somewhat novel and they generally evoke a reaction of awe and amazement as they gracefully translate and rotate building elements weighing thousands of tons.

Since the days of the Roman Coliseum, popularization of retractable roof stadiums has been somewhat slow. This is illustrated by the fact that over two thirds of the retractable roof sports venues that are in operation around the world today have been built and put into operation within the last ten years. During the same ten year period, only a few fixed roof stadiums were designed and built, further illustrating the increasing popularity of retractable roof stadiums worldwide. Furthermore, in addition to retractable roofs, recent years have seen an increase in the integration other large mechanized elements into sports venues. There are now a few retractable playing surfaces in service at soccer and American football stadiums around the world. Large mechanized walls, mechanized seating sections, and mechanized entertainment features are becoming more common in new sports venue construction as well.

Why has the integration of large mechanized elements into modern stadia been so slow in coming and why is it so popular now? Many factors contributed to the scarcity of large mechanized elements in the sports venues of decades past. These include the following:

• The design, manufacturing, and installation of large mechanized elements do not fit seamlessly into the conventional core competencies of the sports venue design and construction industry. There are no design standards, building codes, estimating guides, or standard specification sections available to help address the most challenging aspects of planning, designing, and building a retractable roof facility.

• Some of the first few attempts at incorporating retractable roofs into modern sports venues were plagued with significant cost overruns, significant schedule delays, leaky facilities, and mechanically unreliable systems. These projects were considered problematic at best, and created a general feeling within the sports venue construction industry that retractable roofs and other large mechanized features were costly and potentially risky.

• Patron’s needs, facility operating requirements, and markets have changed over the years. In decades past, the cost benefits associated with retractable roofs were not as great as they are today and the benefits that did exist were not fully recognized until some successful operating examples were in service for a period of time. Many of these historical barriers that had held back the wide spread construction of retractable roof sports venues have since disappeared or been broken down by the pioneers of retractable roof stadia. These trail blazing owners, operators, architects, engineers, and constructors led the way past some of those initial setbacks to construct and operate some of the most successful and highly revered sports venues in the world. Design and construction teams learned how to effectively incorporate mechanization design and supply into their planning, designing, and constructing processes. Technology has improved to make operating systems more cost effective, easier to operate, and more operationally reliable. In North America, the lessons learned from some of the problems experienced on those initial attempts have been taken to heart such that the last several retractable roof stadiums constructed have been delivered on budget, delivered on schedule, and put into service with nearly 100 percent operational reliability from opening day forward. A more detailed discussion of the lessons learned and methodologies proven to be successful will follow in later sections of this paper.

The increased desire of owners and operators to integrate large mechanized features into stadia has been the direct result of the operating success of those first attempts. Even some of those initial projects that were difficult and costly to build or that suffered through initial operational problems have persevered to become operationally reliable systems that produce increased revenues and operational flexibility for their owners. The Association for Retractable Roof Operators Worldwide (ARROW) has published a paper titled, Retractable Roofs in Sports Stadiums…..Money Well Spent that outlines the operational advantages and increased revenues associated with retractable roof sports facilities.

The Milwaukee Brewers increased their fan attendance at ballgames by nearly 50 percent during the first six years of operation of Miller Park when compared to the last six years of operation at their previous ballpark. Attendance increased even though their winning percentage was lower at the new ballpark. Miller Park has an impressive retractable roof and a set of 70 foot tall operable outfield walls. Mike Brockman, Facilities Manager at Miller Park, says the following regarding the growth in fan attendance, “We think that increased fan comfort and guarantee of an event is a big part of this growth. We have hosted over 600 events without a weather related delay or cancellation over six years.”

University of Phoenix Stadium, in Glendale Arizona, is a publicly owned and operated multipurpose facility that is the new home of the Arizona Cardinals. The facility opened in the summer of 2006 and incorporates not only a retractable roof, but also the first retractable playing surface in the National Football League (NFL). This facility has become the benchmark for multipurpose facilities. It is able to host an NFL game on a natural grass playing surface, and then, immediately following the completion of the game, the 19 million pound playing surface is driven outside to expose a completely functional trade show floor.  This operational flexibility allows much more utilization of the facility and thus produces increased revenues. Ted Ferris, CEO of the Arizona Tourism and Sports Authority, states, “If you want your stadium to be truly multipurpose, then you must invest in these mechanized systems to make the facility work for you.”


University of Arizona Retractable Roof and Field
University of Arizona Retractable Roof and Field

The improvements in the design and construction of large mechanized elements, coupled with a better understanding of the operational benefits associated with these features, has fueled the most recent demand for the use of retractable roofs and other large mechanized structural elements in stadia. It is now up to our engineering community to meet this demand by providing safe, cost effective, and reliable systems.

Delivering an Architectural Icon & an Engineering Feat

Although civil engineers have successfully designed and constructed large mechanized structures such as mechanized bridges and water control structures thoughout our history, the task of designing and constructing multimillion pound retractable roofs, operable glass walls, and retractable playing fields is outside of the norm for most of us. So, what should be done when challenged with such endeavors? All available precedents, historical data, operating examples, technical guidance, and personal experiences should be drawn upon. The stadia projects that have successfully incorporated large mechanized elements can provide a set of best practices. Some of those best practices are listed and discussed below:

• The owners, operators, architects, structural engineers, mechanization consultants, building officials, and constructors should work together as early as possible to define design criteria and operating parameters of the mechanized elements. Although there is not much building code guidance on these matters, there is now a wealth of historical precedents and several design firms experienced in these processes. This effort produces important basic design criteria such as operating times (typically 5 to 20 minutes), operating wind speeds (typically 50 miles per hour), stopping times or deceleration requirements (varies depending on safety considerations and structural capabilities), life cycle duty (varies depending on intended usage), etc.

• The mechanization design and all of its implications must be an integral part of design from the beginning of schematic design forward. It could be catastrophic to assume that a performance specification can be written, and the drive and control systems can be integrated into the structure as a design/build element during the construction phase. Instead, the mechanization consultant should be included on the design team from day one of design.

•The initial roof weight and operating load estimates should be conservative.

Contingencies on operating and static condition loads should be included throughout design of the mechanical systems; 20 percent should be included in early design and 10 percent toward the end of the design phase. A holistic design approach should be taken. The structural engineer must have an intimate understanding of the control sequences, the braking characteristics, and the forces imposed on the structure by the drive system under all conditions. The structural engineer should expect the number of loading conditions that must be evaluated to be several times greater than what would be expected for a static structure. The mechanization consultant must have an intimate understanding of the stiffness of the structure, all externally applied loads, deflections under load, control sequences, fabrication tolerances, and construction tolerances as these things affect drive loads and wheel loads. The controls engineer must have a complete understanding of the behavior of both the structural and mechanical systems.

Wherever practical, a delivery method should be used that provides a single point of responsibility for the successful design and delivery of the mechanized element. This is often accomplished with some form of design/build contract for the mechanized element, where the contractor is selected early in the design process and the mechanization design team is included on the building design team. A clear scope of work should be defined to delineate areas of responsibility in detail for all parties involved.

Thrust release mechanisms should be integrated into retractable roof stadiums to minimize the need for tight construction tolerances and eliminate unnecessarily punishing thrust loads on the mechanization components. In general, moving part components cost more than static components. Therefore it is usually cost effective to minimize the loads directed through moving parts using built in hinges, linkages, or slide bearings such that the retractable roof span is a determinant structure.

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Thrust Release Mechanisms
Thrust Release Mechanisms

• The effects of the changing support conditions as the retractable roof structure moves through its full range of motion over its supporting structure should be evaluated. This includes effects of support structure construction tolerances, structural displacements, potential differential foundation settlements, and thermal movements. Either the retractable roof structure should be flexible…