Tension fabric buildings offer superior resistance to waste鈥檚 corrosive attacks on facilities.
By Matt VanScoyoc
Everybody in the waste industry understands that their facilities can be at risk against corrosive elements, as caustic materials are constantly being hauled in and out daily. But this awareness does not mean that people are familiar with every option available to provide optimal corrosion protection for a building.
The secret has been out for decades that cladding a large structure with fabric is an economical choice for material recovery facilities, transfer stations, treatment plants, and more. It is also well understood that this solution significantly helps combat corrosion concerns, since the polyvinyl chloride (PVC) fabric material used is naturally resistant to corrosive elements.
Historically, however, even tension fabric buildings have possessed a vulnerability to corrosion, since the framing structure itself is made of metal. In response, leading fabric building manufacturers have kept innovating their offerings, beefing up both the structural frame and the coatings used to protect it in hopes of fully solving the corrosion problem in waste handling applications.
Solid Steel Beams
Hollow-truss frames were typically used in older style fabric structures. Even when the exterior surfaces of these trusses were treated to help prevent corrosion, a common problem was that corrosion could originate inside the tube and rust the frame from the inside out. This caused irreversible damage over time that was not even visible until it was far too late.
A major shift occurred when industry leaders developed a patented method for attaching tension fabric panels to a structural steel frame. By using solid steel I-beams in place of hollow tubes, one critical weak point was addressed, but more work remained to be done.
Getting Your Epoxy On
Corrosion is a particularly big issue for applications like sludge storage, composting, and bio recycling, as well as high humidity environments like those found inside wastewater treatment plants. For such situations, fabric building manufacturers initially used hot dip galvanizing to protect the structural I-beams against corrosion.
Galvanizing adds a thin 3.9-mil layer of zinc around the steel by immersing the entire steel member into a molten zinc bath. The purpose of that layer is essentially for it to be sacrificed over several years, with corrosion gradually eating the zinc away, rather than directly attacking the steel beneath. Galvanizing slows down the corrosion process as much as it can, but that protection will eventually degrade and leave the I-beams vulnerable.
Understanding that galvanizing protection disappears over time, manufacturers looked at alternatives that could create a true barrier between corrosive elements and the steel frame. The solution was epoxy coating, which could provide more permanent protection against corrosion, and subsequently increase facility longevity.
While epoxy would undoubtedly provide an upgrade, it had been considered cost-prohibitive due to the outsourcing involved in producing and shipping the coated steel. Suppliers began investing in onsite production facilities that included in-house steel beam fabrication and painting. This helped bring down the cost of offering epoxy-painted I-beams, and it also allowed companies to fully control the quality of their product.
Full Treatment
The epoxy treatment process begins by sandblasting the I-beam in a controlled environment to create a consistent blast profile on every inch of the steel. For the highest level of corrosion protection, the steel receives a commercial blast to fully clean the steel surface while removing any impurities or defects.
Without including this blast process, the steel could be susceptible to a filiform type of corrosion that develops underneath the protective coatings. This is an important difference from hot dip galvanizing, where the steel is usually just wiped down and then dipped鈥攏ot blasted first鈥攎eaning that any blemishes or potential contaminants are not being removed prior to applying the coating.
After blasting, the next step is adding a 3-mil layer of zinc that is applied with a paint method rather than dipping. In effect, this first paint layer offers protection that resembles the final product with hot dip galvanizing. Two separate 5-mil coats of epoxy are then layered over the zinc paint. The end result is a fully protective 13-mil barrier that prevents corrosive elements from ever contacting the steel.
Corrosion Tests
To determine the level of corrosion protection that epoxy coating provides to a steel beam, in comparison to hot dip galvanizing, manufacturers have put both methods through salt fog tests developed by the American Society for Testing and Materials (ASTM). Where some corrosion tests will try to emulate typical working conditions, the 2,000-hour salt fog test introduces constant and aggressive salt fog to materials that are expected to face the most corrosive waste handling applications.
In one such 2,000-hour test, a hot dip galvanized steel beam and epoxy-coated steel beam were placed in the same closed and controlled environment. The galvanized beam came out fully corroded and rusted over. The epoxy-coated steel had not corroded at all, outside of a thin line that was intentionally etched beforehand to see if the coating would help prevent rust from spreading into the rest of the piece; even in that intentionally damaged area, the corrosion did not penetrate below the epoxy coating.
Images courtesy of Legacy Building Solutions.
Structural Benefits
It is important to understand how revolutionary the integration of rigid-frame, structural I-beam engineering was for tension fabric buildings. Once one realizes how much more can be accomplished with this design approach, it is even easier to see why the suppliers who offer it go to great lengths to protect those steel beams from corrosion.
Rigid frame design brought a universally accepted engineering practice to fabric structures, which had lacked consistency and quality with web truss framing. It also provided the ability to totally customize a fabric building with unique features and the exact dimensions desired.
For material recovery facilities, compost covers and other waste applications, fabric-clad facilities can be built with large square footages at a lower cost than comparable alternatives. Engineered steel I-beams deliver superior structural strength, which allows for long clear span designs that provide more wide-open floor space for equipment, materials, and for load-in and load-out vehicles to operate.
The rigid frame design process always starts with a clean sheet, which allows the manufacturer and customer to work together from the beginning to optimize the project for the user鈥檚 specific needs. If the facility requires an overhead conveyor or other collateral loads that must be supported by the structural frame, engineers simply account for those loads in the design software to provide optimal I-beam depths and thicknesses precisely where required.
Another advantage for the waste industry is how easy a fabric building can be ventilated. Standard building designs incorporate low-cost passive systems using ridge and soffit vents that rely on the natural movement of warm air. Often this is all that is needed to address ventilation concerns from moisture and humidity. If fans or other supplemental ventilation are needed, those mechanical items can be applied as hanging loads on the steel frame.
Fantastic Fabric
Corrosion is not a concern when it comes to the fabric cladding itself. Whether using PVC or polyethylene (PE), architectural fabrics are not susceptible to corroding, making them a popular choice for waste structures and many other industries. Fabric also offers other operational advantages. Its translucency allows for natural daylighting inside a building. Unlike metal sheeting, fabric has thermally non-conductive properties, helping to keep building interiors warmer in the winter and cooler in the summer.
The type of fabric used can have a big impact on the building鈥檚 overall longevity. In the past, PVC was usually reserved for higher end projects due to its price point. However, leading suppliers have helped bring a high-quality PVC to market at a similar cost to PE. This advanced PVC features a high-strength woven fabric with additional primer and lacquer layers to enhance durability. In fact, it retains more than double the tensile strength of a standard PE fabric, thereby extending building life expectancy.
Defeating Corrosion
By introducing solid framing materials, superior coatings and improved fabric cladding technology, manufacturers have helped tension fabric buildings push back harder against the menace that is corrosion. Through custom engineering and applying the proper protective measures, today鈥檚 fabric structures provide a cost-effective and long-term solution for a variety of waste industry facility needs. | WA
Matt VanScoyoc is a Building and Project Design Consultant for Legacy Building Solutions. He has experience with all aspects of fabric building sales, design-build assistance and project management. He works with end users, contractors and government entities throughout solicitation, development, review and submission. Matt can be reached at [email protected]. For more information, visit .