Air Structures... PRESSURE ON THE COURT:
What Owners and Managers Need to Know About Air Structures
Air structures, often referred to as "bubbles", are a common sight at tennis facilities during the winter season. Many players have probably been inside one at one time or another. Yet for owners and managers considering installing one over their courts, this sort of casual familiarity rarely prepares them for the arduous task of planning for and choosing an air structure. Building manufacturers represent a wealth of information about air structures, but each of them will be offering their own unique approach to building design and installation, making across the board comparisons between brand names difficult. Landscape architects, architects, and engineers can provide more objective advice, though they are usually less experienced in the design of air structures or the specific needs of tennis. As a result, decision makers often find themselves in the difficult position of having to educate themselves without really knowing for sure what is best for their facility. The purpose of this article is to provide owners and managers with a basic understanding of air structures so that they can more effectively and confidently implement a successful project.
Air Structures: What are they?
An air structure is technically defined as a high-strength fabric membrane building which derives its structural integrity and stability from controlled internal air pressure. This internal air pressure supports the enclosing fabric membrane in the same way columns and beams would support the walls and roof of a conventional building. In an air structure, however, there is no differentiation between walls and ceiling as the fabric membrane forms one continuous envelope anchored to the ground at its perimeter edges. Despite the uniqueness of their structural systems, air structures are not an experimental form of technology. The basic concept was developed and refined in the 1950s for military and industrial use and they have been in wide-spread use since the early 1960's. To date, there have been more than 10,000 air structure installations world-wide, including applications for the enclosure of missile storage facilities, warehouses, construction sites, swimming pools, football fields, hockey rinks as well as all types of tennis courts.
Since pressure within an air structure must be maintained continuously for the building to remain erect, air structures should be viewed as a system of mutually operating components. An air structure for tennis typically includes five major components:
1. Fabric enclosure
2. Building anchorage system
3. Air supply system
4. Controlled means of access and egress
5. Lighting system.
Knowing a little bit about each of these elements is critical to be able to evaluate an air structure project proposal effectively. You should start by purchasing a copy of the Air Structures Design and Standards Manual (ASI-77) published by the Industrial Fabric Association of St. Paul, MN (612) 222-2508 (or visit their website at www.ifai.com). This manual defines all of the standard terms and minimum specifications required for the design, fabrication, installation and maintenance of air structures. Though the Industrial Fabric Association book is comprehensive, it does not address the specific needs of tennis court enclosures. Here are some basic items to consider about each of the five basic building components:
1. Fabric Enclosures
The fabric enclosure is the largest part of an air structure. It must provide maximum resistance to wind and internal inflation pressure, but at the same time it must be easy to maintain, provide some insulating capacity, and be designed to optimize player experience. Tennis air structure fabric membranes are usually multi-layered. The exterior fabric is usually a 24 to 32 ounce material. This material can be treated with a number of different coatings to prevent ultra violet degradation of the material, accumulation of snow, or discoloration from air pollution. Exterior fabrics come either as a translucent or opaque material. Translucent fabrics allow natural light to enter the structure reducing the need for lighting of the interior. However, since most buildings are used during the winter, the light passing through the translucent membrane is often uneven and lasts only for a few hours a day. While translucent fabrics allow for some solar gain during the winter season, reducing heating costs somewhat, it is very expensive to cool an air structure during warmer periods in the early spring or late fall. Opaque fabric offers better control of interior lighting because it can act as a reflective surface for interior fixtures. Opaque fabrics also allow for more consistent temperature control.
The interior fabric membranes are usually 12 to 18 ounce material. The interior fabric traps air between the interior of the building and the exterior fabric insulating the building. Multiple interior layers can provide added insulation value to the membrane, reducing heating costs and the size of the mechanical system. The inner-most fabric is usually called the liner and should be fabricated with an integral contrasting color to a height of 10 to 12 feet to provide a backdrop which improves ball visibility from the court. The remainder of the liner should be a highly reflective white color to maximize lighting.
One final point to consider about the design of the membrane is how it is sectionized or broken down into parts. Most air structure membranes are field assembled from multiple pieces because the weight of one piece can exceed several thousand pounds. You can dictate to the building manufacturer the size and weight of each section of the building and these are shipped on individual palates which can be easily moved and stored on site.
2. Building Anchorage Systems
Unlike conventional building foundations, which support load bearing walls, air structure foundations are designed primarily to withstand building uplift forces due to wind. It is not unusual for the stress along the base of an air structure to exceed 1,500 pounds per linear foot. Most tennis air structures use a continuous concrete grade beam around the base of the building. The structure is attached to the grade beam by either a continuous metal clamp bolted into the concrete or a continuous channel embedded into the concrete into which the fabric is inserted. In either attachment scenario, careful consideration must be given to how the grade beam and attachment hardware will function both when the structure is in place and when it is not. As a rule, grade beams should be designed flush with the court surface. Since all outdoor courts must be constructed with a slope so the can drain, the grade beam should follow the slope of the court. If the grade beam projects above the level of the courts at the edges, it will pose a hazard to players. All attachment hardware should be mounted flush with the grade beam. Any projecting objects should be removable. All sockets or channels should have some means of enclosure so that players will not get their feet caught in any depressions when the air structure is not in place.
Some building systems utilize an additional anchorage system called "stress relief." A stress relief system is a fabric webbing or metal cable system which spans the exterior surface of the structure strapping it more securely to the foundation, reducing the stress on the fabric membrane. Stress relief systems should be set into special sleeves fabricated into the exterior fabric to prevent abrasive wear against the surface of the fabric membrane. All manufacturers utilize stress relief systems around doorways and other protrusions of air structure walls. Some manufactures also utilize stress relief to span the entire structure. Stress relief, like the rest of an air structure has to be specifically designed by the manufacturer.
3. Air Supply Systems
The inflation system supplies air to pressurize the fabric membrane. Inflation system components must be specially sized for each project in order to optimize cost. An inflation system should consist of at least two blower (fan) systems, one primary and one auxiliary, each of which has adequate capacity to maintain full inflation pressure in the event one blower fails. The heating system, which is coupled with the primary blower(s), should be an indirect fired unit. Usually all blowers operate during inflation, but for normal operation, the primary unit(s) runs to maintain the internal pressure while the other unit(s) provides an emergency backup. The auxiliary blower(s) is designed to engage automatically if the primary blower becomes inoperative. In terms of cost effectiveness, for example, using one blower for both the heating and primary air supply will require a sizable fan system. Instead, the use of two smaller fan systems, one for the primary blower and one for the heating system will require less energy and cost less to operate over the long term, since the heater fan does not need to run continuously. The heating of the building can also be made more effective by installing small fans at the apex of the interior ceiling to re-circulate warm air back down to the court level. It is also useful to equip the building with a pressure alarm system which will alert the operator to potential problems before they become serious. It is a very simple matter to hook up the building pressure gauges to a remote alarm system to alert the building operator of a critical drop in building pressure. Typically, alarms are wired to the main desk of a club or to a central alarm system capable of automatically telephoning the building operators.
The best method of delivering air to the building is by a ground vault supply system. This is a below grade concrete air duct passage system that delivers air to the building and returns it to the blowers via covered duct openings in the surface of the tennis court. The duct openings should be located along the side of the building at the net line to minimize distraction to players. Duct openings should be spaced a minimum of six feet apart in order to prevent short circuiting the air flow between the supply and return duct openings. The air delivery system should never blow across the courts or directly at players. Feeding air into the structure through the wall via an above grade duct system makes it highly susceptible to being crushed by snow or ice sliding off the dome of the structure.
The use of air structures over fast-dry courts requires a special air supply system design. Because fast-dry courts are irrigated, the interior humidity levels will naturally be higher. If too much humidity is present in the air within an air structure, it will condense on the inner walls and drip on to the courts, pitting the surface. Excessive moisture also leads to unsightly and odoriferous mildew growth on the structure fabric. To avoid these problems, the building should be designed with a manual vent system and an air supply system capable of replacing the interior air at least three to four times an hour depending upon the size and condition of the courts. The use of a sub-surface irrigation system can also help minimize humidity problems my minimizing the amount of water required to keep the courts in optimal playing condition.
4. Building Access and Egress Equipment
Doorways for air structures should be planned around the idea of controlling their use, thereby minimizing air pressure loss. The total number of required doorways and emergency exits will vary depending on local building code requirement and specific facility needs. In general, a revolving doorway is used for the primary entryway. The door panels should be designed as large as possible since most tennis players usually carry large bags. A 36" wide door panel should be considered a minimum, but a 48" or 54" panel is more comfortable. Since all new buildings in the United States are required to provide handicap access, provision should be made for easy access to the building through a 34" minimum width single panel door as well. Often one of the emergency exits can serve this purpose if properly located. Since handicap access doors are not used frequently, air leakage which occurs during use is not of concern. If handicap access will be more frequent however, a double door air lock system should be provided. Emergency exits can be served by single panel doorways. A double 48" doorway should be provided if access for the building will be required for maintenance equipment. All doors should be outward opening and automatic closing against inflation pressure. Emergency exits should be equipped with panic hardware and should have lighted emergency exit signs as determined by local codes.
5. Lighting Systems
There are two basic lighting systems for tennis court air structures: indirect, pendant mounted (hung systems and indirect, pedestal (floor) mounted systems. Pendant mounted lighting systems are the most trouble free. The lights are hung from a cable attached to the structure and directed up towards the ceiling. In a building with a white interior liner, they provide uniform, glare free lighting. Pendant systems, however, usually have a remote ballast system which must be located in an enclosed structure either adjacent to the air handling system or within a vented closet in a nearby building. Floor mounted systems are somewhat more problematic. The lights must be mounted on towers which must be located between courts either at the net line or at the rear walls of the court. This makes them a potential hazard to players. This positioning is also less efficient in providing uniform lighting, resulting in unsightly bright and dark spots across the structure ceiling. Floor mounted systems come either with integral ballasts or remote ballasts. Another draw back to the floor mounted fixtures is that they must be demountable in order to take the structure up and down. Floor mounted fixture footings located within the court areas when the air structure is not in place make them a further liability for player injury. With lighting systems it is best to bury all electrical conduit within the concrete grade beam. Junction and wiring pull boxes should be flush mounted within the grade beam or court surface, again to prevent player injury.
Planning and Design Considerations
Air structures, like other buildings, are designed to the specific requirements of their site. Meeting site requirements usually includes three aspects:
1. complying with local codes
2. considering the programming of the building on the site
3. making site improvements accommodate the structure.
The building will have to meet local building, fire safety, and zoning requirements. Meeting these codes and getting all of the necessary permits for your project should be considered before you begin.
In terms of programming, you need to think about how the air structure will be used. Building location and access should be carefully considered in terms of its relationship to parking areas, roadways, other buildings, and potential utility connections. It is undesirable for players to go outdoors from locker rooms and lounge areas to get to an air structure. This is inconvenient for players and makes it difficult to monitor who is entering and exiting the building. Typically, temporary covered and heated passageways are built for use during the air structure season. These can be designed to appear as attractive architectural elements when not in use with the air structure. Air structures can also be directly attached to either temporary or permanent buildings. When proximity to an existing building with lounges and locker rooms is not possible, you might want to rent a temporary portable building to serve as lounge area for players and supervisory staff.
As far as site improvements are concerned, having enough space for an air structure is perhaps most important. The building should be at least 123' wide (preferably 125') to allow for full court width and an access walkway along the rear of the courts. The building length should provide a minimum of 10' between the building wall and the sideline of the nearest court (12' preferred). The spacing between courts should be no closer than 12', with a minimum of 18' between courts if divider netting is used. Adequate room must be provided for the mechanical system and fuel storage tanks, if required.
Mechanical areas are best located along the side of the building as close to the net line as possible minimize below grade air vault runs. Air structures should be set back from adjacent buildings and air structures by a minimum of 5'(10'preferred). This will prevent snow from piling up against the structure and to allow for easy removal. The installation of a grade beam requires the modification of standard court fencing designs to prevent snow build up as well. When a perimeter grade beam is installed, the fence posts can be cast directly into the concrete, saving the need for separate footings for each fence post. The typical 10' height fence fabric must be divided into two horizontal panels so that the lower 5' of fence can be removed when the air structure is in place, preventing the snow from being trapped by the fence fabric. This will require the addition of middle rails for all 10' height fencing. Gate openings must be located where each of the building doorways are located. Also, a double 5' panel gate will be required in the fencing to allow for fork lift access to deliver the air structure to the courts each year. Finally, adequate room should be provided to allow for the drainage of storm water away from the building perimeter. If necessary, a storm drainage system should be installed to prevent flooding of the structure during heavy rain or snow storms.
Since no rigid framing members are required, air structures represent an inexpensive method for temporarily enclosing large areas. As with any facility improvement, however, the total life cost of a project should be considered from the start. This includes both the initial costs and the operational costs. Air structures have two main initial cost items: the building itself and the site modifications required to accommodate the building. Costs generally range from $8.00 to $10.00 per square foot for the building alone, depending on the overall size and configuration of the building. This translates into roughly $58,000 to $72,000 per court. Site costs vary more widely depending upon factors such as the condition of the site, the ability to access the site with construction equipment, labor and material costs within your geographic area, and contractor familiarity with air structure construction. The operational costs of air structures are perhaps the more important considerations over the long term. Operational costs include the cost for the continuous operation of the blowers while the building is erect, heating and cooling costs, lighting costs, routine maintenance and repair of the structure, the cost for annual erection and dismantling of the air structure, as well as storage costs if the building itself is not stored on site in an existing building. Air structure fabric membranes typically have a life expectancy of 10 to 12 years. The other components such as the anchorage and mechanical systems should he considered as permanent investments. It is a simple task, therefore, to project the anticipated life cost of a proposed facility once you decide how long you plan to operate the facility.
Air structures represent an exciting opportunity for year round tennis for players who live in areas with less than friendly winter climates. They also represent exciting revenue-producing opportunities for owners and operators if properly implemented. By properly planning beforehand and asking the right questions, there is no reason to be afraid of a little bit of pressure on the court.
by Andrew R. Lavallee, ASLA and Sheldon Westervelt, P.E.
Published in Tennis Industry
For More Information Contact:
Global Sports & Tennis Design Group, LLC
P.O.Box 6224, Fair Haven, NJ 07704