New Millennium Building Systems

Unique challenges surface in natatorium design

Successful natatorium building designs address corrosion, acoustics, aesthetics—and budget

Natatorium and closeup of indoor pool with diving board

Indoor pools, or natatoriums, are popular because of their ability to be used year-round and to neutralize unpredictable weather conditions. Ironically, however, an indoor facility housing hundreds of thousands of gallons of water creates its own problem-causing “weather,” introducing unique challenges for building design teams and contractors alike.

While keeping budget and appearance in mind, a natatorium design needs to consider two main issues. Humid, corrosive environments that demand careful considerations for mechanical systems and exposed building materials. Controlling acoustics and ambient noise, as these spaces act as giant echo chambers.

The degradation of 304 stainless-steel door hardware illustrates the corrosive environment of a natatorium, likely is due to the mechanical system not properly evacuating moisture.

A structural steel column exhibits rust as a result of an ineffective paint system and a floor with no slope that allows chlorinated pool water to accumulate at the column’s base.

Steel is the answer

There should be no apprehension in using steel in a natatorium’s structural system. When implemented correctly, a steel design will facilitate efficient, long-lasting construction. The resulting structure requires nominal maintenance and achieves attractive aesthetics—all while minimizing costs.

Steel has clear and substantial advantages over two most common alternatives: concrete and wood. Concrete is substantially heavier and—unless trying to achieve a fire rating—impractical. Maneuvering personnel, materials, and equipment around a deep, Olympic-size pool favors a lighter, easier-to-handle material. Concrete, whether pre-cast or cast-in-place, also requires secondary acoustical treatments. A popular option in standard construction to address acoustics is to use panels comprised of cementitious wood fibers.

The large panels are difficult to maneuver into position for the contractor, thus damaging the corners is always a possibility due to the brittleness of the material. While providing acoustical value, it is reduced with each coat of paint. In the humid environment or during construction, the panels can trap water for extended periods of time.

Cementitious wood fiber panels

Cementitious wood fiber panels continue to trap water days after a rain event during construction.

Wood stick-built construction is seldom used but worth commenting on for its lack of spanning capabilities. As an organic material, it is a poor choice for a structural system in a continuously humid environment. For all of the failures associated with wood-framed indoor pools, the hospitality industry continues to design in this manner. The result, continual dedicated resources to maintain. In many cases these pools are not used often, and many hotel chains have gone away from providing indoor pools and spas due to the constant maintenance.

Savvy owners are concerned with the long-term viability of their indoor pool. In many instances, these are publicly funded recreation centers that are community amenities. With high-caliber coating systems, structural steel is the resounding winner of optimizing quality and cost. Steel joists and long-span acoustical steel deck roof construction creates open, spacious pool environments.

Long-span steel deck maximizes the distance between load-bearing members. Spans start at 8 feet and can exceed 30 feet (as opposed to the typical 5 feet on center for standard roof deck). The key is having a structural layout that maximizes the efficiency of the deck and joists, directing the loads to columns or walls. Strategically selecting load-bearing members to reduce overall tonnage of the steel package keeps a project’s cost down.

Steel joists are light and flexible. They can literally be designed into most any architectural shape: arch, bowstring, scissor, gable, to name a few. Steel joists use a fraction of the steel that wide-flange beams or tube steel requires, making them a more cost-effective structural member. The open webbing of a steel joist also allows ductwork, mechanical or electrical components to pass through, further increasing the finished ceiling height and opening the space.

Coating prevents corrosion

Substrate and surface treatments are the backbone of joist and deck design for natatoriums, addressing the project’s primary concern: corrosion.

For joists, there are two prevailing corrosion-resistant treatment paths. In the first option, joists are bare steel, not receiving a factory primer. The fabricator is responsible for surface-preparation in accordance with the Society for Protective Coatings to ensure proper coating adhesion. The manufacturer or a reliable third party may do this. This may be SSPC-SP6 Commercial Blast Cleaning or SSPC-SP7 Brush-off Blast Cleaning. The second choice specifies the joists to receive a primer, affording time to install since they can sit on the job site with the protection of the primer. Using this method, it is important to verify the compatibility of the top coat with the primer. The paint manufacturer likely requires surface preparation SSPC standard for adhesion. Field-applied joists and deck should receive the same topcoat paint.

In a caustic environment, steel deck should be specified as G90, a thicker galvanization than a typical G60 substrate. As with joists, deck surface treatments vary, but the overarching goal is the same: a robust finish coating system to protect the steel.

The design specifications may call for powder coating as a solution. In this instance, the bare galvanized deck may require an SSPC-SP1 before receiving the powder coat. Powder coating is a process by which the already-fabricated steel deck is electrically charged. A dry paint is then statically applied in a powder form and baked.

A middle route is to have the finish topcoat applied in the field. The procedure may go as following: pre-treating the galvanized steel, then the deck manufacturer applies an oven-cured epoxy coil coat (primer) prior to shipping to the job site. In the field, one or more coats of a hydrophobic (designated for natatoriums) paint is applied by others—brands such as Tnemec and Sherwin Williams Macropoxy. Tnemec 115, for example, is a dryfall paint that is ideal for this environment and coats the perforations in the acoustical deck.

Another option, one that is most recommended, calls for the deck manufacturer to spray-apply the first coat of natatorium paint. This scenario involves G90 deck with an oven-cured epoxy coil coat, along with a polyester coating on the unexposed back side, that then receives a thick mil Tnemec 161. Once installed, the deck and joists receive a field-applied topcoat to ensure maximum performance.

The last method is to eliminate field painting altogether. Instead of field-applying the topcoat, the deck manufacturer delivers the materials to the job site ready to install. The acoustical deck panels are protected with thick paper interleafing (craft paper has been proven to be superior to strippable film that will easily scratch as panels are slid off during unloading).

No matter the application method, coatings must be considered in relation to the accompanying roofing system, fasteners and deck attachments (securing to structure and side laps)—all could impact the coating in some way. Deck panels are stitched along their sides by either welds or screws (side-lap attachment). If pre-finished, welding in the field is not recommended, as welds may permanently mar the deck. Attaching to structural members can be accomplished by either welds or shot-pinning. Again, if the deck is pre-finished, welding is not recommended. (It should be noted that welds may be unavoidable because of seismic considerations or particular jurisdictional standards.) Field-applying the topcoat is the most popular strategy because structural engineers tend to favor welded attachments.

In*Situ Architecture’s design of Westside Natatorium in El Paso, Texas, minimizes the use of wood and improves acoustics with Versa-Dek® LS ES Acoustical.

Northern Arizona University’s Aquatic and Tennis Complex in Flagstaff, Ariz., designed by Sink Combs Dethlefs architecture, features 3.5” Versa-Dek® LS Acoustical.

Roofing, mechanical systems are key

The roof-ceiling assembly is vital to the long-term success of a project. In projects calling for a flat roof, a roofing membrane with rigid insulation over polyiso board and steel deck is the most common assembly. Corrosion-resistant roof fasteners, either stainless-steel or coated, are crucial. Architectural deck derives its aesthetic value from its ability to conceal roof fasteners while standard-profile deck or deck is punctured sporadically, leaving the ceiling littered with unsightly fasteners blading through, exposed. Versa-Dek® and Deep-Dek® Cellular (acoustical or non-acoustical) conceal all roof fasteners, creating a clean, lineal appearance.

Indoor pools, regardless of which structural system is used, also need to address acoustics. Secondary materials such as the aforementioned cementitious wood fibers can address ambient noise (noise is defined as unwanted sound), but the most efficient solution is starting with acoustical deck. Whether Versa-Dek® or Deep-Dek®, utilizing the cavity within the deck for acoustical batts gains the natatorium a noise reduction coefficient (NRC) well over .65 (a typical classroom NRC achieved with acoustical tile). In an interior-aggressive environment, PVC-wrapped acoustical batts that are clear, plastic-encapsulated fiberglass strips that lay in the deck cavity should be used. This mitigates the potential for humidity or condensation affecting the fiberglass elements. Standard acoustical batts are yellow; there are formaldehyde-free versions if the project requires avoiding products on the Red List. These “eco-batts” are dark brown and do not sacrifice the NRC rating.

The combination of Versa-Dek® 3.5 LS Acoustical and the Versa-Wedge® hanging device allows designers to maximize facility height.

De Moines Area Community College’s Student Activities Center and Trail Point Aquatics & Wellness Center features steel on steel and the Versa-Wedge® hanging device that work together to create a clean, lineal appearance. The project was designed by RDG.

Accessories are integral to final design

Accessories are the final consideration but are no less important to a cohesive design. The dovetail profile of Versa-Dek® allows for a hanging device called Versa-Wedge®. Design of a natatorium should limit the quantity of deck penetrations, and Versa-Wedge® achieves this while allowing lighting, fire suppression and mechanical systems to be integrated.  

With Versa-Wedge®, which is movable or adjustable within the deck’s dovetail flutes, the design team can elegantly suspend components from the ceiling, avoiding clunky unistrut assemblies. This system can be applied to pitched roofs up to 5 percent as well. Other hanging devices maintain the clean aesthetic of Versa-Dek® even in extreme roof pitches and seismic areas.

In addition to Versa-Dek®, Deep-Dek® Cellular Acoustical is another viable solution for natatorium design. It features the longest spanning capabilities along with exceptional NRC values. Liner panels are shop-welded steel and add to the structural capacity of the deck. Deep-Dek® Cellular and Cellular Acoustical can simple-span upwards of 30 feet. In this scenario, low-voltage conduit for lighting and branch fire suppression lines may be placed into the deck cavity. It is possible to suspend lighting and mechanical ductwork from the underside of the deck.

Deep-Dek® 6 Cellular Acoustical or Deep-Dek® 7.5 Cellular Acoustical features access panels to service systems in the deck’s cavity.

Deep-Dek® 7.5 Cellular Acoustical can span up to 30 feet.

Building a better steel experience