A R C H I T E C T
U R E & D E S I G N
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Among stadia owners and operators around the world there is an increasing desire for functional, reliable and cost-effective mechanised natural grass playing surfaces. Bart Riberich tells us how an owner can have their turf – and move it too! |
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Among stadia owners and operators around the world there is an increasing desire for functional, reliable, and cost effective mechanised natural grass playing surfaces. The need for high quality natural grass playing surfaces and the economic requirement for multi-purpose use of facilities are the two primary factors contributing to the increasing interest in mechanised playing fields. Although the concept of reconfiguring venues for a variety of sporting and entertainment events is not new, the notion of moving an entire natural grass playing surface in and out of a stadium is a relatively new and bold endeavour. However, if the system is designed and constructed properly, an owner can have their turf – and move it too! Operating Parameters & Design Criteria Several functional requirements of the mechanised playing field must be established before any detailed design can begin. The primary function of any mechanised field is of course, to deliver a high quality playing surface. This has become increasingly important to owners and managers in response to their desire to minimise injuries to valuable athletes. Thus, the mechanised field system design must start with, and be designed around, a quality turf system. The turf system requires a minimum depth of about 30 cm. The turf systems are highly evolved and include the turf itself along with a substrate system and utilities. The entire system is designed to tightly control the delivery and/or removal of water, air, and chemicals. The pan structure of the mechanised field must be designed to contain and accommodate the entire turf system and associated utilities. The dimensions of the playing surface to be moved must be established as well as the path over which the field will be transported. Typical mechanised field dimensions are 60 to 90 metres wide by 100 to 140 metres long. Keeping the field dimensions to a minimum and the travel path straight and short can effectively minimise the construction costs of the field.
Design criteria such as loading requirements can also have a significant impact on construction costs. Loading requirements for vehicle loads, half-time entertainment stages, etc. should be established early in the design process by the owners, operators, and architects. It is most practical to limit the loading on the field to small vehicles and restrict the use of large trucks on the field. |
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A reasonable operating speed must be established. One of the primary purposes of mechanising the natural grass playing field is to minimise the time and labour required to move the field in or out of the stadium. A reasonably short time of operation is important for purposes of reconfiguring the venue as well as managing the playing surface. For instance, it may be beneficial to move a playing surface into a covered stadium to protect it from approaching severe weather or to manage the amount of rain or sunlight it receives. In order to achieve these goals the system must be easy to operate and must have a reasonable operating speed. That being said, the trade off is that the power required to move the field is directly proportional to the speed with which it is required to move. One hour has been established as the target for the total time of operation for one stroke of the system on past projects. This has been determined to be a reasonable compromise of speed and power requirements. Each team of owners, operators, and designers must establish the time of operation that best serves their specific application. Stadium Design Issues Before proceeding with design of the mechanised field itself, there are several stadium design issues that must be acknowledged and addressed. The first, and most obvious, of these is that there needs to be an area outside the stadium to which the field can be moved and stored. This may sound like a trivial issue, but site space is almost always in high demand. Also, in many cases stadiums are designed with the playing surface below nominal outside grade to facilitate circulation, etc. If the design requires that the playing surface be below grade, then a depressed field storage area must be developed. The field storage area must be provided with utilities and vehicle access required for care and maintenance of the turf. (See Figure 1 for an illustration of a typical mechanised field arrangement.)
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A second, and also obvious, issue that must be addressed is that there must be an opening in the stadium perimeter through which the field can be moved. If there is no requirement for seating at this perimeter opening, then there is no cost premium. Since many venues require 360 degree seating and circulation, the field must pass under sections of concourse and seating. These areas of the stadium must span over the width of the mechanised field. Where several levels of seating and concourse must span this opening, the structural cost premium can be about $3,000,000. If only one level of seating spans the opening, or if the end of the stadium can be left entirely open, there is little or no cost premium. Also, in most cases an operable field door must be provided to seal the opening in the perimeter created for the field. Field doors are typically one to two metres high and 60 to 90 metres wide. The cost premium associated with these field doors can be assumed to be between $100,000 and $200,000. The stadium floor must be designed to accommodate the mechanised field. When steel wheels or slide pads are used to move the field, there must be steel rails set in the floor. If water or air bearings are used to move the field, the concrete floor must be level and finished appropriately in the pathways of the bearings. Regardless of the bearing system used, the floor must be designed to take the bearing loads. Wheels, pads, and air/water bearings can concentrate very high loads (100 to 200 kN) in small areas. Control joints, floor drainage, and other floor features must be designed around the field bearing pathways. For multi-purpose facilities, the floor must meet these design criteria and also be functional as a convention floor. In addition to the issues identified above, there are many features and details that must be incorporated into the stadium design to successfully integrate the mechanised field. Designers must develop edge conditions that provide seamless and safe transitions from the stationary structure to the mechanised field. These details have included overlapping edges, fold-down bridging plates, etc. Vehicle and personnel access must be provided to the field when it is in place as well as to the stadium floor when the field is outside of the stadium. This requires some kind of adjustable ramp access. Also, at some point along the travel path of the field, the designers should provide an inspection and maintenance trench. This feature is important to provide access to the bottom side of the mechanised field for normal inspection, maintenance, and repair functions. The trench should be configured to provide access to all areas of the bottom side of the field system as the system is moved over the trench. Drag , Float or Roll The first design decision that must be made is the determination of the bearing system on which the field will be moved. The most basic bearing system that can be chosen is a system of bearing pads and ways, or rails, along which the field would be simply pushed or pulled. Although this concept is simple, it has one major disadvantage. The force required to push or pull the mechanised field over the ways is extraordinary. Even when using ultra low friction polymer bearing pads, the very lowest static coefficient of friction that the designer should expect to achieve is about .10. Although tests done under laboratory conditions might indicate that a lower coefficient of friction could be achieved, experience under field conditions dictates that a minimum of .10 should be used for design. Therefore, for a mechanised field system that can easily weigh 5000 tons, a drive force of over 500 tons must be developed. At a travel speed of about 7.5 cm per second and an overall drive efficiency of 85 percent, this system requires about 430 kW to overcome this friction. Worse yet, this friction value can vary wildly with the surface condition of the bearing pads and ways. In most cases this is an impractical system due to the extremely high drive forces and power requirements to move the system. In some applications, it may be most practical to float the field in and out of the stadium on air or water bearings. Air and water bearing systems pump pressurised water or air under the field until the field floats on a film of air or water. These systems are reliable and have some undeniable advantages, but they are very expensive relative to the other bearing alternatives available. If air or water bearings are used, large water pumping or air compressing systems are required. In the case of water, a reservoir and collection system is also required. It is also worth noting that these systems can be somewhat messy and/or noisy to operate. Air and water bearings have several advantageous characteristics. They provide a low friction bearing. They operate over a smooth concrete floor and do not require any steel rails or ways. The entire air or water bearing system may be 7.5 to 15 cm thick, so they are a very low profile system. Since the field floats on these bearings, it is free to move in any direction and thus it has the ability to ‘go around square corners’, which is a distinct advantage over other bearing systems. The cost of air and water bearing systems is at least double that of other systems, so they should only be used in applications where one of their advantageous characteristics is required. Wheels of Steel In most cases the most practical means to move a field in and out of a stadium is to roll it with steel wheels on steel rails. If the wheel system is appropriately designed and manufactured, this system is the most economical and it offers high reliability, low friction or rolling resistance, quiet operation, and low power requirements. For the same 5,000 ton field discussed earlier, the total rolling resistance and wheel-bearing friction may be about 50 tons rather than the 500-ton drive force requirement associated with the low friction slide pads. At a travel speed of 7.5 cm per second, and an overall drive efficiency of 85 percent, this system requires about 43 kW to overcome this rolling resistance and wheel bearing friction. Please note that additional power would be required for drag or friction of horizontal guides as well as some small allowance for incidental grade deviation. Accounting for all sources of resistance, it would take about 60 kW to move the 5000-ton field with steel wheels, over level steel rails, at a speed of 7.5 cm per second. The steel wheel/rail system has three characteristics that can be disadvantageous in some applications. The wheels themselves occupy some depth and therefore increase the overall depth of the system. Wheels can add 45 to 65 cm to the overall system depth when compared to slide pads or air/water bearings. Wheeled systems are most cost effective in applications where only linear motion is required and the field is not required to move around corners. Also, wheels require steel tracks on which to roll. Although the tracks can be flush with the floor, tracks are required. If these features can be tolerated in the design, it is generally most cost effective to roll your field in and out of the stadium with steel wheels on steel rails. Field Pan Design The structural frame and shell, in which the turf is confined and on which the bearing and drive system are mounted, is referred to as the field pan. Some general design criteria for the field pan are as follows;
For budgeting purposes, one should expect the fabrication and construction of the pan system to cost from $150 to $250 per square
metre. (See Figure 2 for an illustration of a pan framing system.) |
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Wheel and Design Drive Nominal operating wheel loads are normally in the range of 10 to 20 tons, and systems as large as mechanised fields require hundreds of wheels to support them. Therefore, a cost effective wheel design is critical to the overall economy of the system. Uni-Systems has developed a very cost effective design for hardened steel wheel assemblies that bolt directly into the pan framing system. This system is easily manufactured, shipped, and installed at the job-site. Most of the wheels can be flangeless. However, at least one row of the wheels must be double flanged to provide horizontal guidance for the system, keeping the field on track as it moves. Now that the field has wheels, the designer can choose from a wide variety of methods by which to move, or drive, the field. However, the most reliable, convenient and economical system is to drive some of the field wheels with AC electric motors. It only takes about 10 to 20 percent of the total number of wheels to provide enough traction to drive the field.
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These drive wheels can be chosen to be at the perimeter or other locations that facilitate ease of maintenance and repair. Power can be supplied to the moving field via flexible cord or a power feed rail system. (See Figure 3 for a schematic illustration of a typical drive wheel assembly.) For budgeting purposes, one should expect the cost of manufacturing and supplying the wheels and drive system to be between $1,500,000 to $2,500,000 for a 5000-ton field. Rail Design Generally speaking, most mechanised field layouts will produce wheel or pad layouts with a grid of about six to ten metres. Therefore, in the direction parallel with the field movement, there must be a set of rails or ways every six to ten metres apart. These rails must be supported by thickened slabs or independent rail foundations. These rails can be flush with the floor. However, every system requires some kind of guidance rail or rails. The most economical design for guidance rails is to determine a location where a kerf can be provided at each side of the head of the rail to accommodate guide wheel flanges. For a typical system the rail and foundation cost premium can be about $2,500,000.
Although there is a cost premium of several million dollars associated with the mechanisation of playing fields, they satisfy a very specific and demanding set of functional requirements. As the demand for high quality playing surfaces and multi-purpose configurations for sports venues increases; the need for functional, easy to operate, reliable, and cost effective mechanised natural grass playing surfaces will also increase. If those systems are designed
and constructed properly, the owner can have their turf – and move it too! Bart L. Riberich is PE Vice President of Engineering,
Uni-Systems. |
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Bart Riberich is a seasoned veteran at turning concepts into engineered solutions. His practical engineered solutions focus on achieving client’s design parameters within the specified budget and time frame. His knowledge of manufacturing and construction techniques result in value added solutions at the lowest overall cost. Bart began his professional career at the respected engineering firms Fluor Daniel, Inc. and Black and Veatch. Since joining Uni-Systems in 1994, Bart has focused his attention on the dynamic loads imposed on moving structures. The design of all Uni-Systems’ projects is overseen by Bart, who is heavily involved with the concept development and detailed design of most projects. One of Uni-Systems’ greatest design challenges is to cost effectively integrate high precision mechanical and electrical systems into large structural systems, which, by nature, are heavily loaded, flexible, and constructed with generous tolerances. One of Bart’s specialties is designing cost effective systems that meet this challenge and successfully blend the coarsely built structures with precision built machinery. Bart developed the torque tube truss system that has been used on the most recent “Uni-Dock”™ systems and has been the leader of the design teams for Uni-Systems’ ballpark and stadium work. |
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laying surfaces in the sporting world have always received a considerable amount of attention from the likes of players, owners and facility managers. As of late the humble playing surface has also been receiving more attention from stadium architects and structural and mechanical engineers. Mechanised playing fields, where the entire field can be moved out of the stadium as a unit, are a result of this attention.
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